ALBERT

Description: Impact of seawater Ca 2+ on the calcification and calcite Mg/Ca of Amphistegina lessonii Biogeosciences Discussions, 11, 17463-17489, 2014 Author(s): A. Mewes, G. Langer, S. Thoms, G. Nehrke, G.-J. Reichart, L. J. de Nooijer, and J. Bijma Mg/Ca ratios in foraminiferal tests are routinely used as paleo temperature proxy, but on long timescales, also hold the potential to reconstruct past seawater Mg/Ca. Impact of both temperature and seawater Mg/Ca on Mg incorporation in foraminifera have been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, is not fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg/Ca to seawater Mg/Ca and explains inter-species variability in Mg/Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg/Ca was manipulated by varying [Ca 2+ ] and keeping [Mg 2+ ] constant. Foraminiferal growth rates, test thickness and calcite Mg/Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg/Ca closest to that of ambient seawater. Calcite Mg/Ca is positively correlated to seawater Mg/Ca, indicating that not absolute seawater [Ca 2+ ] and [Mg 2+ ], but the telative ratio controls Mg/Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here we, however, suggest a transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).

Description: Fluxes of carbon and nutrients to the Iceland Sea surface layer and inferred primary productivity and stoichiometry Biogeosciences Discussions, 11, 15399-15433, 2014 Author(s): E. Jeansson, R. G. J. Bellerby, I. Skjelvan, H. Frigstad, S. R. Ólafsdóttir, and J. Ólafsson Fluxes of carbon and nutrients to the upper 100 m of the Iceland Sea are evaluated. The study utilises hydro-chemical data from the Iceland Sea time-series station (68.00° N, 12.67° W), for the years between 1993 and 2006. By comparing data of dissolved inorganic carbon (DIC) and nutrients in the surface layer (upper 100 m), and a sub-surface layer (100–200 m), we calculate monthly deficits in the surface, and use these to deduce the surface layer fluxes that affect the deficits: vertical mixing, horizontal advection, air–sea exchange, and biological activity. The deficits show a clear seasonality with a minimum in winter, when the mixed layer is at the deepest, and a maximum in early autumn, when biological uptake has removed much of the nutrients. The annual vertical fluxes of DIC and nitrate amounts to 1.7 ± 0.3 and 0.23 ± 0.07 mol m −2 yr −1 , respectively, and the annual air–sea uptake of atmospheric CO 2 is 4.4 ± 1.1 mol m −2 yr −1 . The biologically driven changes in DIC during the year relates to net community production (NCP), and the net annual NCP corresponds to export production, and is here calculated to 6.1 ± 0.9 mol C m −2 yr −1 . The typical, median C : N ratio during the period of net community uptake is 11, and thus clearly higher than Redfield, but is varying during the season.

Description: Technical Note: Artificial coral reef mesocosms for ocean acidification investigations Biogeosciences Discussions, 11, 15463-15505, 2014 Author(s): J. Leblud, L. Moulin, A. Batigny, P. Dubois, and P. Grosjean The design and evaluation of replicated artificial mesocosms are presented in the context of a thirteen month experiment on the effects of ocean acidification on tropical coral reefs. They are defined here as (semi)-closed (i.e. with or without water change from the reef) mesocosms in the laboratory with a more realistic physico-chemical environment than microcosms. Important physico-chemical parameters (i.e. pH, p O 2 , p CO 2 , total alkalinity, temperature, salinity, total alkaline earth metals and nutrients availability) were successfully monitored and controlled. Daily variations of irradiance and pH were applied to approach field conditions. Results highlighted that it was possible to maintain realistic physico-chemical parameters, including daily changes, into artificial mesocosms. On the other hand, the two identical artificial mesocosms evolved differently in terms of global community oxygen budgets although the initial biological communities and physico-chemical parameters were comparable. Artificial reef mesocosms seem to leave enough degrees of freedom to the enclosed community of living organisms to organize and change along possibly diverging pathways.

Description: Phytoplankton community structure in the North Sea: coupling between remote sensing and automated in situ analysis at the single cell level Biogeosciences Discussions, 11, 15621-15662, 2014 Author(s): M. Thyssen, S. Alvain, A. Lefèbvre, D. Dessailly, M. Rijkeboer, N. Guiselin, V. Creach, and L.-F. Artigas Phytoplankton observation in the ocean can be a challenge in oceanography. Accurate estimations of their biomass and dynamics will help to understand ocean ecosystems and refine global climate models. This requires relevant datasets of phytoplankton at a functional level and on a daily and sub meso scale. In order to achieve this, an automated, high frequency, dedicated scanning flow cytometer (SFC, Cytobuoy, NL), has been developed to cover the entire size range of phytoplankton cells whilst simultaneously taking pictures of the largest of them. This cytometer was directly connected to the water inlet of a~pocket Ferry Box during a cruise in the North Sea, 8–12 May 2011 (DYMAPHY project, INTERREG IV A "2 Seas"), in order to identify the phytoplankton community structure of near surface waters (6 m) with a high resolution spacial basis (2.2 ± 1.8 km). Ten groups of cells, distinguished on the basis of their optical pulse shapes, were described (abundance, size estimate, red fluorescence per unit volume). Abundances varied depending on the hydrological status of the traversed waters, reflecting different stages of the North Sea blooming period. Comparisons between several techniques analyzing chlorophyll a and the scanning flow cytometer, using the integrated red fluorescence emitted by each counted cell, showed significant correlations. The community structure observed from the automated flow cytometry was compared with the PHYSAT reflectance anomalies over a daily scale. The number of matchups observed between the SFC automated high frequency in situ sampling and the remote sensing was found to be two to three times better than when using traditional water sampling strategies. Significant differences in the phytoplankton community structure within the two days for which matchups were available, suggest that it is possible to label PHYSAT anomalies not only with dominant groups, but at the level of the community structure.

Description: Long term effects on regional European boreal climate due to structural vegetation changes Biogeosciences Discussions, 11, 15507-15547, 2014 Author(s): J. H. Rydsaa, F. Stordal, and L. M. Tallaksen Amplified warming at high latitudes over the past decades has led to changes in the boreal and arctic climate system, such as structural changes in high latitude ecosystems and soil moisture properties. These changes trigger land-atmosphere feedbacks, through altered energy partitioning in response to changes in albedo and surface water fluxes. Local scale changes in the arctic and boreal zone may propagate to affect large scale climatic features. In this study, MODIS land surface data are used with the Weather Research and Forecasting model (WRF V3.5.1) and Noah LSM, in a series of experiments to simulate the influence of structural vegetation changes over a Northern European boreal ecosystem. Emphasis is placed on surface energy partitioning and near surface atmospheric variables, in order to investigate changes in atmospheric response due to observed and anticipated structural vegetation changes. We find that a northward migration of evergreen needle leaf forest into tundra regions causes an increase in latent rather than sensible heat fluxes, increased near surface temperatures and boundary layer height. Shrub expansion in tundra areas has only small effects on surface fluxes. However, it influences near surface wind speeds and boundary layer height. Northward migration of mixed forest across the present southern border of the boreal forest has largely opposite effects on surface fluxes and the near surface atmosphere, and acts to moderate the overall mean regional effects of boreal forest migration on the near surface atmosphere.

Description: Spatial variability and hotspots of soil N 2 O fluxes from intensively grazed grassland Biogeosciences Discussions, 11, 15327-15360, 2014 Author(s): N. J. Cowan, P. Norman, D. Famulari, P. E. Levy, D. S. Reay, and U. M. Skiba One hundred N 2 O flux measurements were made from an area of intensively managed grazed grassland in central Scotland using a high resolution dynamic chamber method. The field contained a variety of features from which N 2 O fluxes were measured including a manure heap, patches of decaying grass silage, and areas of increased sheep activity. Individual fluxes varied significantly across the field varying from 2 to 79 000 μg N 2 O-N m −2 h −1 . Soil samples were collected at 55 locations to investigate relationships between soil properties and N 2 O flux. Fluxes of N 2 O correlated strongly with soil NO 3 − concentrations. Distribution of NO 3 − and the high spatial variability of N 2 O flux across the field are shown to be linked to the distribution of waste from grazing animals and the resultant reactive nitrogen compounds in the soil which are made available for microbiological processes. Features within the field such as shaded areas and manure heaps contained significantly higher available nitrogen than the rest of the field. Although these features only represented 1.1% of the area of the field, they contributed to over 55% of the total estimated daily N 2 O flux.

Description: Size-fractionated dissolved primary production and carbohydrate composition of the coccolithophore Emiliania huxleyi Biogeosciences Discussions, 11, 15289-15325, 2014 Author(s): C. Borchard and A. Engel Extracellular release (ER) by phytoplankton is the major source of fresh dissolved organic carbon (DOC) in marine ecosystems and accompanies primary production during all growth phases. Little is known, so far, on size and composition of released molecules, and to which extent ER occurs passively, by leakage, or actively, by exudation. Here, we report on ER by the widespread and bloom-forming coccolithophore Emiliania huxleyi grown under steady state conditions in phosphorus controlled chemostats (N : P = 29, growth rate of μ = 0.2 d −1 ). 14 C incubations were accomplished to determine primary production (PP), comprised by particulate (PO 14 C) and dissolved organic carbon (DO 14 C), and the concentration and composition of particulate combined carbohydrates (pCCHO), and of high molecular weight (〉1 kDa, HMW) dissolved combined carbohydrates (dCCHO) as major components of ER. Information on size distribution of ER products was obtained by investigating distinct size classes (

Description: Autonomous profiling float observations of the high biomass plume downstream of the Kerguelen plateau in the Southern Ocean Biogeosciences Discussions, 11, 17413-17462, 2014 Author(s): M. Grenier, A. Della Penna, and T. W. Trull Natural iron fertilisation from Southern Ocean islands results in high primary production and phytoplankton biomass accumulations readily visible in satellite ocean colour observations. These images reveal great spatial complexity with highly varying concentrations of chlorophyll, presumably reflecting both variations in iron supply and conditions favouring phytoplankton accumulation. To examine the second aspect, in particular the influences of variations in temperature and stratification, we deployed four autonomous profiling floats in the Antarctic Circumpolar Current near the Kerguelen plateau in the Indian sector of the Southern Ocean. Each "bio-profiler" measured more than 250 profiles of temperature ( T ), salinity ( S ), dissolved oxygen, chlorophyll fluorescence (Chl a ), and particle backscatter in the top 300 m of the water column, sampling up to 5 profiles per day along meandering trajectories extending up to 1000 km. Comparison of surface Chl a estimates (top 50 m depth; analogous to values from satellite images) with total water column inventories revealed largely linear relationships, suggesting that dilution of chlorophyll by mixed layer depth variations plays only a minor role in the spatial distributions observed by satellite, and correspondingly that these images provide credible information on total and not just surface biomass accumulations. Regions of very high Chl a accumulation (1.5–10 μg L -1 ) were associated predominantly with a narrow T – S class of surface waters, which appears to derive from the northern Kerguelen plateau. In contrast, waters with only moderate Chl a enrichments (0.5–1.5 μg L -1 ) displayed no clear correlation with water properties, including no dependence on mixed layer depth, suggesting a diversity of sources of iron and/or its efficient dispersion across filaments of the plume. The lack of dependence on mixed layer depth also indicates a limited influence on production by light limitation. One float became trapped in a cyclonic eddy, allowing temporal evaluation of the water column in early autumn. During this period, decreasing surface Chl a inventories corresponded with decreases in oxygen inventories on sub-mixed layer density surfaces, consistent with significant export of organic matter and its respiration and storage as dissolved inorganic carbon in the ocean interior. These results are encouraging for the expanded use of autonomous observing platforms to study biogeochemical, carbon cycle, and ecological problems, although the complex blend of Lagrangian and Eulerian sampling achieved by the floats suggests that arrays rather than single floats will often be required.

Description: On the use of the post-closure method uncertainty band to evaluate the performance of land surface models against eddy covariance flux data Biogeosciences Discussions, 11, 16911-16951, 2014 Author(s): J. Ingwersen, K. Imukova, P. Högy, and T. Streck The energy balance of eddy covariance (EC) flux data is normally not closed. Therefore, at least if used for modeling, EC flux data are usually post-closed, i.e. the measured turbulent fluxes are adjusted so as to close the energy balance. At the current state of knowledge, however, it is not clear how to partition the missing energy in the right way. Eddy flux data therefore contain some uncertainty due to the unknown nature of the energy balance gap, which should be considered in model evaluation and the interpretation of simulation results. We propose to construct the post-closure method uncertainty band (PUB), which essentially designates the differences between non-adjusted flux data and flux data adjusted with the three post-closure methods (Bowen ratio, latent heat flux (LE) and sensible heat flux ( H ) method). To demonstrate this approach, simulations with the NOAH-MP land surface model were evaluated based on EC measurements conducted at a winter wheat stand in Southwest Germany in 2011, and the performance of the Jarvis and Ball–Berry stomatal resistance scheme was compared. The width of the PUB of the LE was up to 110 W m –2 (21% of net radiation). Our study shows that it is crucial to account for the uncertainty of EC flux data originating from lacking energy balance closure. Working with only a single post-closing method might result in severe misinterpretations in model-data comparisons.

Description: Carbon, oxygen and biological productivity in the Southern Ocean in and out the Kerguelen plume: CARIOCA drifter results Biogeosciences Discussions, 11, 16877-16909, 2014 Author(s): L. Merlivat, J. Boutin, and F. d'Ovidio The Kerguelen Plateau region in the Indian sector of the Southern Ocean supports annually a large-scale phytoplankton bloom which is naturally fertilized with iron. As part of the second Kerguelen Ocean and Plateau compared Study expedition (KEOPS2) in austral spring (October–November 2011), one Carioca buoy was deployed east of the Kerguelen plateau. It drifted eastward downstream in the Kerguelen plume. Hourly surface measurements of p CO 2 , O 2 and ancillary observations were collected between 1 November 2011 to 12 February 2012 with the aim of characterizing the spatial and temporal variability of the biological Net Community Production (NCP) downstream the Kerguelen plateau, assess the impact of iron-induced productivity on the biological carbon consumption and consequently on the CO 2 flux exchanged at the air–sea interface. The trajectory of the buoy until mid-December was within the longitude range, 72–83° E, close to the polar front and then in the polar frontal zone, PFZ, until 97° E. From 17 November to 16 December, the buoy drifted within the Kerguelen plume following a filament carrying dissolved iron, DFe, for a total distance of 700 km. In the first part of the trajectory, the ocean surface waters are a sink for CO 2 and a source for CO 2 , with fluxes of respective mean values equal to −8 and +38 mmol CO 2 m −2 d −1 . Eastward, as the buoy escapes the iron enriched filament, the fluxes are in opposite direction, with respective mean values of +5 and −48 mmol O 2 m −2 d −1 . These numbers clearly indicate the strong impact of biological processes on the biogeochemistry in the surface waters within the Kerguelen plume in November-mid-December, while it is undetectable eastward in the PFZ from mid-December to mid-February. While the buoy follows the Fe enriched filament, simultaneous observations of dissolved inorganic carbon, DIC, and dissolved oxygen, O 2 , highlight biological events lasting from 2 to 4 days. Stoichiometric ratios, O 2 /C, between 1.1 and 1.4 are observed indicating new and regenerated production regimes. NCP estimates range from 60 to 140 mmol C m −2 d −1 . Based on the relationship between the time a water parcel has left the plateau and its iron content, we have highlighted that the main control on the value of NCP is the availability of iron in the upper water column, with the largest NCP occurring in waters that have recently left the plateau and presented the highest iron concentrations.

Description: Comparison of UV/Vis and FDOM sensors for in situ monitoring of stream DOC concentrations Biogeosciences Discussions, 11, 16855-16876, 2014 Author(s): G.-Y. Yoo, Y. Jeong, E.-J. Lee, J.-H. Park, and N.-H. Oh Optical measurements using ultra-violet/visible (UV/Vis) spectrophotometric sensors and fluorescent dissolved organic matter (FDOM) sensors have recently been used as proxies of dissolved organic carbon (DOC) concentrations of streams and rivers at high temporal resolution. Despite of the merits of the sensors, temperature changes and particulate matter in water can interfere the sensor readings, over- or under-estimating DOC concentrations. However, little efforts have been made to compare responses of the two types of the sensors in natural conditions. We conducted both laboratory experiments and in situ monitoring with a UV/Vis sensor and a FDOM sensor during the three storm events in the fall of 2012 and the spring of 2013 in a forest stream in Korea in order to compare their performance. Laboratory experiments using the Suwannee River natural organic matter, humic acid, and fulvic acid demonstrated strong linear relationships between both the sensor signals and measured DOC concentrations with R 2 ≥ 0.98. Although temperature compensation might not be needed for the UV/Vis sensor, it was sensitive to relativley small changes in turbidity. In contrast, the FDOM sensor was insenstive to relatively low turbidity while the FDOM sensor outputs decreased significantly as temperature increased, requiring temperature compensated FDOM (e.g. FDOM 20 for 20 °C) for in situ monitoring of DOC. The results suggest that both sensors can be employed as a~proxy for stream DOC concentrations after temperature and turbidity compensation in a forest stream where terrestrially derived humic-like materials are dominant components.

Description: Modeling the impact of riverine DON removal by marine bacterioplankton on primary production in the Arctic Ocean Biogeosciences Discussions, 11, 16953-16992, 2014 Author(s): V. Le Fouest, M. Manizza, B. Tremblay, and M. Babin The planktonic and biogeochemical dynamics of the Arctic shelves exhibit a strong variability in response to Arctic warming. In this study, in order to elucidate on the processes regulating the production of phytoplankton (PP) and bacterioplankton (BP) and their interactions, we employ a biogeochemical model coupled to a pan-Arctic ocean-sea ice model (MITgcm) to explicitly simulate and quantify the contribution of usable dissolved organic nitrogen (DON) drained by the major circum-Arctic rivers on PP and BP in a scenario of melting sea ice (1998–2011). Model simulations suggest that on average between 1998 and 2011, the removal of usable RDON by bacterioplankton is responsible of a ~26% increase of the annual BP for the whole Arctic Ocean. With respect to total PP, the model simulates an increase of ~8% on an annual basis and of ~18% in summer. Recycled ammonium is responsible for the PP increase. The recycling of RDON by bacterioplankton promotes higher BP and PP but there is no significant temporal trend in the BP : PP ratio within the ice-free shelves over the 1998–2011 period. This suggests no significant evolution in the balance between autotrophy and heterotrophy in the last decade with a constant annual flux of RDON into the coastal ocean although changes in RDON supply and further reduction in sea ice cover could potentially alter this delicate balance.

Description: Biogeochemical indicators of peatland degradation – a case study of a temperate bog in northern Germany Biogeosciences Discussions, 11, 16825-16854, 2014 Author(s): J. P. Krüger, J. Leifeld, S. Glatzel, S. Szidat, and C. Alewell Peatlands store a great proportion of the global soil carbon pool and can loose carbon via the atmosphere due to degradation. In Germany, most of the greenhouse gas emissions from organic soils are attributed to sites managed as grassland. Here we investigated a land-use gradient from near-natural wetland (NW) to an extensively managed (GE) to an intensively managed grassland site (GI), all formed in the same bog complex in northern Germany. Vertical depth profiles of δ 13 C, δ 15 N, ash content, C/N ratio, bulk density, as well as radiocarbon ages were studied to identify peat degradation and to calculate carbon loss. At all sites, including the near-natural site, δ 13 C depth profiles indicate aerobic decomposition in the upper horizons. Depth profiles of δ 15 Ndiffered significantly between sites with increasing δ 15 N values in the top layers with increasing intensity of use, indicating that the peat is more decomposed. At both grassland sites, the ash content peaked within the first centimeter. In the near-natural site, ash contents were highest in 10–60 cm depth. This indicates that not only the managed grasslands, but also the near-natural site, is influenced by anthropogenic activities, most likely due to the drainage of the surrounding area. However, we found very young peat material in the first centimeter of the NW, indicating recent peat growth. The NW site accumulates carbon today even though it is and probably was influenced by anthropogenic activities in the past indicated by δ 13 C and ash content depth profiles. Based on the enrichment of ash content and changes in bulk density, we calculated carbon loss from these sites in retrograde. As expected land use intensification leads to a higher carbon loss which is supported by the higher peat ages at the intensive managed grassland site. All investigated biogeochemical parameters together indicate degradation of peat due to (i) conversion to grassland, (ii) historical drainage as well as recent development and (iii) land use intensification.

Description: Sensitivity of the air–sea CO 2 exchange in the Baltic Sea and Danish inner waters to atmospheric short term variability Biogeosciences Discussions, 11, 16993-17042, 2014 Author(s): A. S. Lansø, J. Bendtsen, J. H. Christensen, L. L. Sørensen, H. Chen, H. A. J. Meijer, and C. Geels Minimising the uncertainties in estimates of air–sea CO 2 exchange is an important step toward increasing the confidence in assessments of the CO 2 cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO 2 flux along with its sensitivity to e.g. short-term temporal variability in wind speed, atmospheric mixing height and the atmospheric CO 2 concentration. With this study the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO 2 flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO 2 ( p CO 2 ) has been developed for this coastal area based on available data from monitoring stations and underway p CO 2 measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO 2 flux. Two model simulations were conducted – one including short term variability in atmospheric CO 2 (VAT), and one where it was not included (CAT). A seasonal cycle in the air–sea CO 2 flux was found for both simulations for all sub-basins with uptake of CO 2 in summer and release of CO 2 to the atmosphere in winter. During the simulated period 2005–2010 the average annual net uptake of atmospheric CO 2 for the Baltic Sea, Danish Straits and Kattegat was 287 and 471 Gg C yr -1 for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr -1 was found to be significant, and thus ignoring short term variability in atmospheric CO 2 does have a sizeable effect on the air–sea CO 2 exchange. The combination of the atmospheric model and the new p CO 2 fields has also made it possible to make an estimate of the marine part of the Danish CO 2 budget for the first time. A net annual uptake of 2613 Gg C yr -1 was found for the Danish waters. A large uncertainty is connected to the air–sea CO 2 flux in particular caused by the transfer velocity parameterisation and the applied p CO 2 climatology. However, the present study underlines the importance of including short term variability in the atmospheric CO 2 concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.

Description: Do climate factors govern soil microbial community composition and biomass at a regional scale? Biogeosciences Discussions, 11, 17729-17756, 2014 Author(s): L. Ma, C. Guo, X. Lü, S. Yuan, and R. Wang Soil microbial communities play important role in organic matter decomposition, nutrient cycling and vegetation dynamic. However, little is known about factors driving soil microbial community composition at large scales. The objective of this study was to determine whether climate dominates among environmental factors governing microbial community composition and biomass at a regional scale. Here, we compared soil microbial communities using phospholipid fatty acid method across 7 land use types from 23 locations in North-East China Transect (850 km x 50 km). The results showed that soil water availability and land use changes exhibited the dominant effects on soil microbial community composition and biomass at the regional scale, while climate factors (expressed as a function of large-scale spatial variation) did not show strong relationships with distribution of microbial community composition. Likewise, factors such as spatial structure, soil texture, nutrient availability and vegetation types were not important. Wetter soils had higher contributions of gram-positive bacteria, whereas drier soils had higher contributions of gram-negative bacteria and fungi. Heavily disturbed soils had lower contributions of gram-negative bacteria and fungi than historically disturbed and undisturbed soils. The lowest microbial biomass appeared in the wettest and driest soils. In conclusion, dominant climate factors, commonly known to structure distribution of macroorganisms, were not the most important drivers governing regional pattern of microbial communities because of inclusion of irrigated and managed practices. In comparison, soil water regime and land use types appear to be primary determinants of microbial community composition and biomass.

Description: Fossilized bioelectric wire – the trace fossil Trichichnus Biogeosciences Discussions, 11, 17707-17728, 2014 Author(s): M. Kędzierski, A. Uchman, Z. Sawlowicz, and A. Briguglio The trace fossil Trichichnus is proposed as an indicator of fossil bioelectric bacterial activity at the interface oxic – anoxic zone of marine sediments. This fulfils the idea that such processes, commonly found in the modern realm, should be also present in the geological past. Trichichnus is an exceptional trace fossil due to its very thin diameter (mostly less than 1 mm) and common pyritic filling. It is ubiquitous in some fine-grained sediments, where it has been interpreted as a burrow formed deeper than any other trace fossils, below the redox boundary. Trichichnus formerly referred to as deeply burrowed invertebrates, has been found as remnant of a fossilized intrasediment bacterial mat that is pyritized. As visualized in 3-D by means of X-ray computed microtomography scanner, Trichichnus forms dense filamentous fabric, which reflects that produced by modern large, mat-forming, sulphide-oxidizing bacteria, belonging mostly to Trichichnus -related taxa, which are able to house a complex bacterial consortium. Several stages of Trichichnus formation, including filamentous, bacterial mat and its pyritization, are proposed to explain an electron exchange between oxic and suboxic/anoxic layers in the sediment. Therefore, Trichichnus can be considered a fossilized "electric wire".

Description: Nitrate limitation and ocean acidification interact with UV-B to reduce photosynthetic performance in the diatom Phaeodactylum tricornutum Biogeosciences Discussions, 11, 17675-17706, 2014 Author(s): W. Li, K. Gao, and J. Beardall It has been proposed that ocean acidification (OA) will interact with other environmental factors to influence the overall impact of global change on biological systems. Accordingly we investigated the influence of nitrogen limitation and OA on the physiology of diatoms by growing the diatom Phaeodactylum tricornutum Bohlin under elevated (1000 μatm, HC) or ambient (390 μatm, LC) levels of CO 2 with replete (110 μmol L -1 , HN) or reduced (10 μmol L -1 , LN) levels of NO 3 - and subjecting the cells to solar radiation with or without UV irradiance to determine their susceptibility to UV radiation (280–400 nm). Our results indicate that OA and UVB induced significantly higher inhibition of both the photosynthetic rate and quantum yield under LN than under HN conditions. UVA or/and UVB increased the cells' non-photochemical quenching (NPQ) regardless of the CO 2 levels. Under LN and OA conditions, activity of superoxide dismutase and catalase activities were enhanced, along with the highest sensitivity to UVB and the lowest ratio of repair to damage of PSII. HC-grown cells showed a faster recovery rate of yield under HN but not under LN conditions. The finding that nitrate limitation and ocean acidification interact with UV-B to reduce photosynthetic performance of the diatom P. tricornutum implies that ocean primary production and the marine biological C pump will be affected by the OA under multiple stressors.

Description: Vegetation heterogeneity and landscape position exert strong controls on soil CO 2 efflux in a moist, Appalachian watershed Biogeosciences Discussions, 11, 17631-17673, 2014 Author(s): J. W. Atkins, H. E. Epstein, and D. L. Welsch In topographically complex watersheds, landscape position and vegetation heterogeneity can alter the soil water regime through both lateral and vertical redistribution, respectively. These alterations of soil moisture may have significant impacts on the spatial heterogeneity of biogeochemical cycles throughout the watershed. To evaluate how landscape position and vegetation heterogeneity affect soil CO 2 efflux ( F SOIL ) we conducted observations across the Weimer Run watershed (373 ha), located near Davis, West Virginia, for three growing seasons with varying precipitation (2010 – 1042 mm; 2011 – 1739 mm; 2012 – 1244 mm; precipitation data from BDKW2 station, MesoWest, University of Utah). An apparent soil temperature threshold of 11 °C at 12 cm depth on F SOIL was observed in our data – where F SOIL rates greatly increase in variance above this threshold. For analysis, F SOIL values above this threshold were isolated and examined. Differences in F SOIL among years were apparent by elevation ( F 4,633 = 3.17; p = 0.013) and by vegetation cover ( F 4, 633 = 2.96; p = 0.019). For the Weimer Run watershed, vegetation exerts the major control on soil CO 2 efflux ( F SOIL ), with the plots beneath shrubs at all elevations for all years showing the greatest mean rates of F SOIL (6.07 μmol CO 2 m -2 s -1 ) compared to plots beneath closed-forest canopy (4.69 μmol CO 2 m -2 s -1 ) and plots located in open, forest gaps (4.09 μmol CO 2 m -2 s -1 ) plots. During periods of high soil moisture, we find that CO 2 efflux rates are constrained and that maximum efflux rates in this system occur during periods of average to below average soil water availability. These findings offer valuable insight into the processes occurring within these topographically complex, temperate and humid systems, and the interactions of abiotic and biotic factors mediating biogeochemical cycles. With possible changing rainfall patterns as predicted by climate models, it is important to understand the couplings between water and carbon cycling at the watershed and landscape scales, and their potential dynamics under global change scenarios.

Description: Daily burned area and carbon emissions from boreal fires in Alaska Biogeosciences Discussions, 11, 17579-17629, 2014 Author(s): S. Veraverbeke, B. M. Rogers, and J. T. Randerson Boreal fires burn carbon-rich organic soils, thereby releasing large quantities of trace gases and aerosols that influence atmospheric composition and climate. To better understand the factors regulating boreal fire emissions, we developed a statistical model of carbon consumption by fire for Alaska with a spatial resolution of 500 m and a temporal resolution of one day. We used the model to estimate variability in carbon emissions between 2001 and 2012. Daily burned area was mapped using imagery from the Moderate Resolution Imaging Spectroradiometer combined with perimeters from the Alaska Large Fire Database. Carbon consumption was calibrated using available field measurements from black spruce forests in Alaska. We built two nonlinear multiplicative models to separately predict above- and belowground carbon consumption by fire in response to environmental variables including elevation, day of burning within the fire season, pre-fire tree cover and the differenced normalized burn ratio (dNBR). Higher belowground consumption occurred later in the season and for mid-elevation regions. Aboveground and belowground consumption also increased as a function of tree cover and the dNBR, suggesting a causal link between the processes regulating these two components of consumption. Between 2001 and 2012, the median fuel consumption was 2.48 kg C m -2 and the median pixel-based uncertainty (SD of prediction error) was 0.38 kg C m -2 . There were considerable amounts of burning in other cover types than black spruce and consumption in pure black spruce stands was generally higher. Fuel consumption originated primarily from the belowground fraction (median = 2.30 kg C m -2 for all cover types and 2.63 kg C m -2 for pure black spruce stands). Total carbon emissions varied considerably from year to year, with the highest emissions occurring during 2004 (67 Tg C), 2005 (44 Tg C), 2009 (25 Tg C), and 2002 (16 Tg C) and a mean of 14 Tg C per year between 2001 and 2012. Our analysis highlights the importance of accounting for the spatial heterogeneity within fuels and consumption when extrapolating emissions in space and time. This data on daily burned area and emissions may be useful for in understanding controls and limits on fire growth, and predicting potential feedbacks of changing fire regimes.

Description: Flux variations and vertical distributions of microzooplankton (Radiolaria) in the western Arctic Ocean: environmental indices in a warming Arctic Biogeosciences Discussions, 11, 16645-16701, 2014 Author(s): T. Ikenoue, K. R. Bjørklund, S. B. Kruglikova, J. Onodera, K. Kimoto, and N. Harada The vertical distribution of radiolarians was investigated using a vertical multiple plankton sampler (100–0, 250–100, 500–250 and 1000–500 m water depths, 62 μm mesh size) at the Northwind Abyssal Plain and southwestern Canada Basin in September 2013. To investigate seasonal variations in the flux of radiolarians in relation to sea-ice and water masses, time series sediment trap system was moored at Station NAP (75°00' N, 162°00' W, bottom depth 1975 m) in the western Arctic Ocean during October 2010–September 2012. We showed characteristics of fourteen abundant radiolarian taxa related to the vertical hydrographic structure in the western Arctic Ocean. We found the Ceratocyrtis histricosus , a warm Atlantic water species, in net samples, indicating that it has extended its habitat into the Pacific Winter Water. The radiolarian flux was comparable to that in the North Pacific Oceans. Amphimelissa setosa was dominant during the open water and the beginning and the end of ice cover seasons with well-grown ice algae, ice fauna and with alternation of stable water masses and deep vertical mixing. During the sea-ice cover season, however, oligotrophic and cold-water tolerant Actinommidae was dominant and the productivity of radiolaria was lower and its species diversity was greater, which might be associated with the seasonal increase of solar radiation that induce the growth of algae on the ice and the other phytoplankton species under the sea-ice. These indicated that the dynamics of sea-ice was a major factor affecting the productivity, distribution, and composition of radiolarian fauna.

Description: Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity Biogeosciences Discussions, 11, 16599-16643, 2014 Author(s): T. Schneider von Deimling, G. Grosse, J. Strauss, L. Schirrmeister, A. Morgenstern, S. Schaphoff, M. Meinshausen, and J. Boike High-latitude soils store vast amounts of perennially frozen and therefore inert organic matter. With rising global temperatures and consequent permafrost degradation, a part of this carbon store will become available for microbial decay and eventual release to the atmosphere. We have developed a simplified, two-dimensional multi-pool model to estimate the strength and timing of future carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes from newly thawed permafrost carbon (i.e. carbon thawed when temperatures rise above pre-industrial levels). We have especially simulated carbon release from deep deposits in Yedoma regions by describing abrupt thaw under thermokarst lakes. The computational efficiency of our model allowed us to run large, multi-centennial ensembles under various scenarios of future warming to express uncertainty inherent to simulations of the permafrost-carbon feedback. Under moderate warming of the representative concentration pathway (RCP) 2.6 scenario, cumulated CO 2 fluxes from newly thawed permafrost carbon amount to 20 to 58 petagrammes of carbon (Pg-C) (68% range) by the year 2100 and reach 40 to 98 Pg-C in 2300. The much larger permafrost degradation under strong warming (RCP8.5) results in cumulated CO 2 release of 42–141 and 157–313 Pg-C (68% ranges) in the years 2100 and 2300, respectively. Our estimates do only consider fluxes from newly thawed permafrost but not from soils already part of the seasonally thawed active layer under preindustrial climate. Our simulated methane fluxes contribute a few percent to total permafrost carbon release yet they can cause up to 40% of total permafrost-affected radiative forcing in the 21st century (upper 68% range). We infer largest methane emission rates of about 50 Tg-CH 4 year –1 around the mid of the 21st century when simulated thermokarst lake extent is at its maximum and when abrupt thaw under thermokarst lakes is accounted for. CH 4 release from newly thawed carbon in wetland-affected deposits is only discernible in the 22nd and 23rd century because of the absence of abrupt thaw processes. We further show that release from organic matter stored in deep deposits of Yedoma regions does crucially affect our simulated circumpolar methane fluxes. The additional warming through the release from newly thawed permafrost carbon proved only slightly dependent on the pathway of anthropogenic emission and amounts about 0.03–0.14 °C (68% ranges) by end of the century. The warming increased further in the 22nd and 23rd century and was most pronounced under the RCP6.0 scenario with adding 0.16–0.39 °C (68% range) to simulated global mean surface air temperatures in the year 2300.

Description: Distribution of black carbon in Ponderosa pine litter and soils following the High Park wildfire Biogeosciences Discussions, 11, 16799-16824, 2014 Author(s): C. M. Boot, M. Haddix, K. Paustian, and M. F. Cotrufo Black carbon (BC), the heterogeneous product of burned biomass, is a critical component in the global carbon cycle, yet timescales and mechanisms for incorporation into the soil profile are not well understood. The High Park Fire, which took place in northwestern Colorado in the summer of 2012, provided an opportunity to study the effects of both fire intenstiy and geomorphology on properties of carbon (C), nitrogen (N), and BC in the Cache La Poudre River drainage. We sampled montane Ponderosa pine litter, 0–5 cm soils, and 5–15 cm soils four months post-fire in order to examine the effects of slope and burn intensity on %C, C stocks, %N and black carbon (g kg −1 C, and g m −2 ). We developed and implemented the benzene polycarboxylic acid (BPCA) method for quantifying BC. With regard to slope, we found that steeper slopes had higher C : N than shallow slopes, but that there was no difference in black carbon content or stocks. BC content was greatest in the litter in burned sites (19 g kg −1 C), while BC stocks were greatest in the 5–15 cm subsurface soils (23 g m −2 ). At the time of sampling, none of the BC deposited on the land surface post-fire had been incorporated into to either the 0–5 cm or 5–15 cm soil layers. The ratio of B5CA : B6CA (less condensed to more condensed BC) indicated there was significantly more older, more processed BC at depth. Total BC soil stocks were relatively low compared to other fire-prone grassland and boreal forest systems, indicating most of the BC produced in this system is likely transported off the surface through erosion events. Future work examining mechanisms for BC transport will be required for understanding the role BC plays in the global carbon cycle.

Description: Evaluating the performance of commonly used gas analysers for methane eddy covariance flux measurements: the InGOS inter-comparison field experiment Biogeosciences Discussions, 11, 797-852, 2014 Author(s): O. Peltola, A. Hensen, C. Helfter, L. Belelli Marchesini, F. C. Bosveld, W. C. M. van den Bulk, J. A. Elbers, S. Haapanala, J. Holst, T. Laurila, A. Lindroth, E. Nemitz, T. Röckmann, A. T. Vermeulen, and I. Mammarella The performance of eight fast-response methane (CH 4 ) gas analysers suitable for eddy covariance flux measurements were tested at a grassland site near the Cabauw tall tower (Netherlands) during June 2012. The instruments were positioned close to each other in order to minimize the effect of varying turbulent conditions. The moderate CH 4 fluxes observed at the location, of the order of 25 nmol m −2 s −1 , provided a suitable signal for testing the instruments' performance. Generally, all analysers tested were able to quantify the concentration fluctuations at the frequency range relevant for turbulent exchange and were able to deliver high-quality data. The tested cavity ring-down spectrometer (CRDS) instruments from Picarro, models G2311-f and G1301-f, were superior to other CH 4 analysers with respect to instrumental noise. As an open-path instrument susceptible to the effects of rain, the LI-COR LI-7700 achieved lower data coverage and also required larger density corrections; however, the system is especially useful for remote sites that are restricted in power availability. In this study the open-path LI-7700 results were compromised due to a data acquisition problem in our data-logging setup. Some of the older closed-path analysers tested do not measure H 2 O vapour concentrations alongside CH 4 (i.e. FMA1 and DLT-100 by Los Gatos Research) and this complicates data processing since the required corrections for dilution and spectroscopic interactions have to be based on external information. To overcome this issue, we used H 2 O mole fractions measured by other gas analysers, adjusted them with different methods and then applied them to correct the CH 4 fluxes. Following this procedure we estimated a bias on the order of 0.1 g (CH 4 ) m −2 (8% of the measured mean flux) in the processed and corrected CH 4 fluxes on a monthly scale due to missing H 2 O concentration measurements. Finally, cumulative CH 4 fluxes over 14 days from three closed-path gas analysers, G2311-f (Picarro Inc.), FGGA (Los Gatos Research) and FMA2 (Los Gatos Research), which were measuring H 2 O vapour concentrations in addition to CH 4 , agreed within 3% (355–367 mg (CH 4 ) m −2 ) and were not clearly different from each other, whereas the other instruments derived total fluxes which showed small but distinct differences (±10%, 330–399 mg (CH 4 ) m −2 ).

Description: Phytoplanktonic response to contrasted Saharan dust deposition events during mesocosm experiments in LNLC environment Biogeosciences Discussions, 11, 753-796, 2014 Author(s): C. Ridame, J. Dekaezemacker, C. Guieu, S. Bonnet, S. L'Helguen, and F. Malien The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE "a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem" project. We simulated realistic dust deposition events (10 g m −2 ) into large mesocosms (52 m 3 ). Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1, -R2: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentration (Chl a ) were observed after a dry deposition event while a wet deposition event resulted in a rapid (24 h after dust additions), strong (up 2.4 fold) and long (at least a week duration) increase in PP and Chl a . We show that in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of NO 3 − (net increases up to +9.8 μM NO 3 − at 0.1 m depth) to the nutrient depleted surface waters due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO 3 − . By transiently increasing DIP and NO 3 − concentrations in P-N starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO 3 − relative to DIP, a wet deposition event resulted in a strong increase in the NO 3 − /DIP ratio from initially 〈 6 to over 150 at the end of the DUNE-2-R1 experiment suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP increased after wet dust deposition events from initially 15% to 60–70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasted responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments, changes nutrient stocks and alters the NO 3 − /DIP ratio leading to a switch in the nutrient limitation of the phytoplanktonic activity.

Description: Can current moisture responses predict soil CO 2 efflux under altered precipitation regimes? A synthesis of manipulation experiments Biogeosciences Discussions, 11, 853-899, 2014 Author(s): S. Vicca, M. Bahn, M. Estiarte, E. E. van Loon, R. Vargas, G. Alberti, P. Ambus, M. A. Arain, C. Beier, L. P. Bentley, W. Borken, N. Buchmann, S. L. Collins, G. de Dato, J. S. Dukes, C. Escolar, P. Fay, G. Guidolotti, P. J. Hanson, A. Kahmen, G. Kröel-Dulay, T. Ladreiter-Knauss, K. S. Larsen, E. Lellei-Kovacs, E. Lebrija-Trejos, F. T. Maestre, S. Marhan, M. Marshall, P. Meir, Y. Miao, J. Muhr, P. A. Niklaus, R. Ogaya, J. Peñuelas, C. Poll, L. E. Rustad, K. Savage, A. Schindlbacher, I. K. Schmidt, A. R. Smith, E. D. Sotta, V. Suseela, A. Tietema, N. van Gestel, O. van Straaten, S. Wan, U. Weber, and I. A. Janssens As a key component of the carbon cycle, soil CO 2 efflux (SCE) is being increasingly studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of current relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question to what extent these relationships remain unaltered beyond the current climatic window for which observations are available to constrain the relationships. Here, we evaluate whether current responses of SCE to fluctuations in soil temperature and soil water content can be used to predict SCE under altered rainfall patterns. Of the 58 experiments for which we gathered SCE data, 20 were discarded because either too few data were available, or inconsistencies precluded their incorporation in the analyses. The 38 remaining experiments were used to test the hypothesis that a model parameterized with data from the control plots (using soil temperature and water content as predictor variables) could adequately predict SCE measured in the manipulated treatment. Only for seven of these 38 experiments, this hypothesis was rejected. Importantly, these were the experiments with the most reliable datasets, i.e., those providing high-frequency measurements of SCE. Accordingly, regression tree analysis demonstrated that measurement frequency was crucial; our hypothesis could be rejected only for experiments with measurement intervals of less than 11 days, and was not rejected for any of the 24 experiments with larger measurement intervals. This highlights the importance of high-frequency measurements when studying effects of altered precipitation on SCE, probably because infrequent measurement schemes have insufficient capacity to detect shifts in the climate-dependencies of SCE. We strongly recommend that future experiments focus more strongly on establishing response functions across a broader range of precipitation regimes and soil moisture conditions. Such experiments should make accurate measurements of water availability, they require high-frequency SCE measurements and they should consider both instantaneous responses and the potential legacy effects of climate extremes. This is important, because we demonstrated that at least for some ecosystems, current moisture responses cannot be extrapolated to predict SCE under altered rainfall.

Description: Technical Note: Simple formulations and solutions of the dual-phase diffusive transport for biogeochemical modeling Biogeosciences Discussions, 11, 1587-1611, 2014 Author(s): J. Y. Tang and W. J. Riley Representation of gaseous diffusion in variably saturated near-surface soils is becoming more common in land biogeochemical models, yet the formulations and numerical solution algorithms applied vary widely. We present three different but equivalent formulations of the dual-phase (gaseous and aqueous) tracer diffusion transport problem that is relevant to a wide class of volatile tracers in land biogeochemical models. Of these three formulations (i.e., the gas-primary, aqueous-primary, and bulk tracer based formulations), we contend the gas-primary formulation is the most convenient for modeling tracer dynamics in biogeochemical models. We then provide finite volume approximation to the gas-primary equation and evaluate its accuracy against three analytical models: one for steady-state soil CO 2 dynamics, one for steady-state soil CO 2 dynamics, and one for transient tracer diffusion from a constant point source into two different sequentially aligned medias. All evaluations demonstrated good accuracy of the numerical approximation. We expect our result will standardize an efficient mechanistic numerical method for solving relatively simple, multi-phase, one-dimensional diffusion problems in land models.

Description: The effect of land-use change on the net exchange rates of greenhouse gases: a meta-analytical approach Biogeosciences Discussions, 11, 1053-1095, 2014 Author(s): D.-G. Kim and M. U. F. Kirschbaum One of the environmental impacts of land-use change (LUC) is a change in the net exchange of the greenhouse gases (GHGs) carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O). Here we summarize findings based on a new global database containing data sets of changes in soil organic carbon stocks and soil CH 4 and N 2 O fluxes. We combine that with estimates of biomass carbon stock changes and enteric CH 4 emissions following LUC. Data were expressed in common units by converting net CH 4 and N 2 O fluxes to CO 2 equivalents (CO 2 eq) using established global warming potentials, and carbon-stock changes were converted to annual net fluxes by averaging stock changes over 100 yr. Conversion from natural forest to cropland resulted in the greatest increase in net GHG fluxes, while conversion of cropland to secondary forest resulted in the greatest reduction in net GHG emissions. Specifically, LUC from natural forest to crop and grasslands led to net fluxes of 6.2 ± 1.6 (Mean ± 95% confidence intervals) and 4.8 ± 1.6 t CO 2 eq ha −1 yr −1 to the atmosphere, respectively. Conversely, conversion from crop and grasslands to secondary forest reduced net emissions by 6.1 ± 4.1 and 3.9 ± 1.2 t CO 2 eq ha −1 yr −1 , respectively. Land-use change impacts were generally dominated by changes in biomass carbon. A retrospective analysis indicated that LUC from natural forests to agricultural lands contributed a cumulative 1326 ± 449 Gt CO 2 eq between 1765 and 2005, which is equivalent to average emissions of 5.5 ± 1.6 Gt CO 2 eq yr −1 . This study demonstrates how specific LUCs can positively or negatively affect net GHG fluxes to the atmosphere.

Description: High temperature decreases the PIC / POC ratio and increases phosphorus requirements in Coccolithus pelagicus (Haptophyta) Biogeosciences Discussions, 11, 1021-1051, 2014 Author(s): A. C. Gerecht, L. Šupraha, B. Edvardsen, I. Probert, and J. Henderiks Rising ocean temperatures will likely increase stratification of the water column and reduce nutrient input into the photic zone. This will increase the likelihood of nutrient limitation in marine microalgae, leading to changes in the abundance and composition of phytoplankton communities, which in turn will affect global biogeochemical cycles. Calcifying algae, such as coccolithophores, influence the carbon cycle by fixing CO 2 into particulate organic carbon (POC) through photosynthesis and into particulate inorganic carbon (PIC) through calcification. As calcification produces a net release of CO 2 , the ratio of PIC / POC determines whether coccolithophores act as a source (PIC / POC 〉 1) or a sink (PIC / POC 〈 1) of atmospheric CO 2 . We studied the effect of phosphorus (P-) limitation and temperature stress on the physiology and PIC / POC ratios of two subspecies of Coccolithus pelagicus . This large and heavily calcified species (PIC / POC generally 〉 1.5) is a major contributor to calcite export from the photic zone into deep-sea reservoirs. Phosphorus limitation did not influence exponential growth rates in either subspecies, but P-limited cells had significantly lower cellular P-content. A 5 °C temperature increase did not affect exponential growth rates either, but nearly doubled cellular P-content under both high and low phosphate availability. The PIC / POC ratios did not differ between P-limited and nutrient-replete cultures, but at elevated temperature (from 10 to 15 °C) PIC / POC ratios decreased by 40–60%. Our results suggest that elevated temperature may intensify P-limitation due to a higher P-requirement to maintain growth and POC production rates, possibly reducing abundances in a warmer ocean. Under such a scenario C. pelagicus may decrease its calcification rate relative to photosynthesis, resulting in PIC / POC ratios 〈 1 and favouring CO 2 -sequestration over release. Phosphorus limitation by itself is unlikely to cause changes in the PIC / POC ratio in this species.

Description: Integrating microbial physiology and physiochemical principles in soils with the MIcrobial-MIneral Carbon Stabilization (MIMICS) model Biogeosciences Discussions, 11, 1147-1185, 2014 Author(s): W. R. Wieder, A. S. Grandy, C. M. Kallenbach, and G. B. Bonan Previous modeling efforts document divergent responses of microbial explicit soil biogeochemistry models when compared to traditional models that implicitly simulate microbial activity, particularly following environmental perturbations. However, microbial models are needed that capture current soil biogeochemical theories emphasizing the relationships between litter quality, functional differences in microbial physiology, and the physical protection of microbial byproducts in forming stable soil organic matter (SOM). To address these limitations we introduce the MIcrobial-MIneral Carbon Stabilization (MIMICS) model. In MIMICS, the turnover of litter and SOM pools are governed by temperature sensitive Michaelis–Menten kinetics and the activity of two physiologically distinct microbial functional types. The production of microbial residues through microbial turnover provides inputs to SOM pools that are considered physically or chemically protected. Soil clay content determines the physical protection of SOM in different soil environments. MIMICS adequately simulates the mean rate of leaf litter decomposition observed at a temperate and boreal forest sites, and captures observed effects of litter quality on decomposition rates. Initial results from MIMICS suggest that soil C storage can be maximized in sandy soils with low-quality litter inputs, whereas high-quality litter inputs may maximize SOM accumulation in finely textured soils that physically stabilize microbial products. Assumptions in MIMICS about the degree to which microbial functional types differ in the production, turnover, and stabilization of microbial residues provides a~mechanism by which microbial communities may influence SOM dynamics in mineral soils. Although further analyses are needed to validate model results, MIMICS allows us to begin exploring theoretical interactions between substrate quality, microbial community abundance, and the formation of stable SOM.

Description: Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds Biogeosciences Discussions, 11, 1277-1308, 2014 Author(s): A.-M. Vafeiadou, P. Materatski, H. Adão, M. De Troch, and T. Moens This study examines the resource use and trophic position of nematodes and harpacticoid copepods at the genus/species level in an estuarine food web in Zostera noltii beds and in adjacent bare sediments, using the natural abundance of stable carbon and nitrogen isotopes. Microphytobenthos is among the main resources of most taxa, but seagrass-associated resources (i.e. seagrass detritus and epiphytes) also contribute to meiobenthos nutrition, with seagrass detritus being available also in deeper sediments and in unvegetated patches close to seagrass beds. A predominant dependence on chemoautotrophic bacteria was demonstrated for the nematode genus Terschellingia and the copepod family Cletodidae. A predatory feeding mode is illustrated for Paracomesoma and other Comesomatidae, which were previously considered first-level consumers (deposit feeders) according to their buccal morphology. The considerable variation found in both resource use and trophic level among nematode genera from the same feeding type, and even among congeneric nematode species, shows that interpretation of nematode feeding ecology based purely on mouth morphology should be avoided.

Description: Seasonal contribution of terrestrial organic matter and biological oxygen demand to the Baltic Sea from three contrasting river catchments Biogeosciences Discussions, 11, 1355-1382, 2014 Author(s): H. E. Reader, C. A. Stedmon, and E. S. Kritzberg To examine the potential influence of terrestrially derived DOM on the Baltic Sea, a year-long study of dissolved organic matter (DOM) was performed in three river catchments in Sweden. One catchment drains into the Bothnian Sea, while two southern catchments drain into the Baltic Proper. Dissolved organic carbon (DOC) concentrations were positively correlated with discharge from forested catchments over the year and while the overall concentrations of DOC were several times higher in the southern two catchments, annual loading of DOC was on the same order for all three catchments, due to differences in discharge. Biological oxygen demand (BOD) was used as a proxy for the lability of carbon in the system. The range of BOD values was similar for all three catchments, however, the ratio of BOD to DOC (an indication of the labile fraction) in Ume älv was four times higher than in the southern two catchments. Total annual BOD loading to the Baltic Sea was twice as high in the northern catchment. Lower winter temperatures and preservation of organic matter in the northern catchment combined with an intense spring flood help to explain the higher concentrations of labile carbon in the northern catchment. Lower lability of DOM as well as higher colour in the southern catchments suggest that wetlands (i.e. peat bogs) may be the dominant source of DOM in these catchments, particularly in periods of low flow. With climate change expected to increase precipitation events and temperatures across the region, the supply and quality of DOM delivered to the Baltic Sea can also be expected to change. Our results indicate that DOM supply will be more stable throughout the year, and potentially have a lower bioavailability.

Description: Responses of nitrous oxide emissions to nitrogen and phosphorus additions in two tropical plantations with N-fixing vs. non-N-fixing tree species Biogeosciences Discussions, 11, 1413-1442, 2014 Author(s): W. Zhang, X. Zhu, Y. Luo, R. Rafique, H. Chen, J. Huang, and J. Mo Leguminous tree plantations at phosphorus (P) limited sites may result in higher rates of nitrous oxide (N 2 O) emissions, however, the effects of nitrogen (N) and P applications on soil N 2 O emissions from plantations with N-fixing vs. non-N-fixing tree species has rarely been studied in the field. We conducted an experimental manipulation of N and P additions in two tropical plantations with Acacia auriculiformis ( AA ) and Eucalyptus urophylla ( EU ) tree species in South China. The objective was to determine the effects of N- or P-addition alone, as well as NP application together on soil N 2 O emissions from tropical plantations with N-fixing vs. non-N-fixing tree species. We found that the average N 2 O emission from control was greater in AA (2.26 ± 0.06 kg N 2 O-N ha −1 yr −1 ) than in EU plantation (1.87 ± 0.05 kg N 2 O-N ha −1 yr −1 ). For the AA plantation, N-addition stimulated the N 2 O emission from soil while P-addition did not. Applications of N with P together significantly decreased N 2 O emission compared to N-addition alone, especially in high level treatment plots (decreased by 18%). In the EU plantation, N 2 O emissions significantly decreased in P-addition plots compared with the controls, however, N- and NP-additions did not. The differing response of N 2 O emissions to N- or P-addition was attributed to the higher initial soil N status in the AA than that of the EU plantation, due to symbiotic N fixation in the former. Our results suggest that atmospheric N deposition potentially stimulates N 2 O emissions from leguminous tree plantations in the tropics, whereas P fertilization has the potential to mitigate N deposition-induced N 2 O emissions from such plantations.

Description: An ensemble approach to simulate CO 2 emissions from natural fires Biogeosciences Discussions, 11, 1443-1478, 2014 Author(s): A. V. Eliseev, I. I. Mokhov, and A. V. Chernokulsky This paper presents ensemble simulations with the global climate model developed at the A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS CM). These simulations were forced by historical reconstruction of external forcings for 850–2005 AD and by the Representative Concentration Pathways (RCP) scenarios till year 2300. Different ensemble members were constructed by varying the governing parameters of the IAP RAS CM module to simulate natural fires. These members are constrained by the GFED–3.1 observational data set and further subjected to Bayesian averaging. This approach allows to select only changes in fire characteristics which are robust within the constrained ensemble. In our simulations, the present-day (1998–2011 AD) global area burnt due to natural fires is (2.1 ± 0.4) × 10 6 km 2 yr −1 (ensemble means and intra-ensemble standard deviations are presented), and the respective CO 2 emissions in the atmosphere are (1.4 ± 0.2) PgC yr −1 . The latter value is in agreement with the corresponding observational estimates. Regionally, the model underestimates CO 2 emissions in the tropics; in the extra-tropics, it underestimates these emissions in north-east Eurasia and overestimates them in Europe. In the 21st century, the ensemble mean global burnt area is increased by 13% (28%, 36%, 51%) under scenario RCP 2.6 (RCP 4.5, RCP 6.0, RCP 8.5). The corresponding global emissions increase is 14% (29%, 37%, 42%). In the 22nd–23rd centuries, under the mitigation scenario RCP 2.6 the ensemble mean global burnt area and respective CO 2 emissions slightly decrease, both by 5% relative to their values in year 2100. Under other RCP scenarios, these variables continue to increase. Under scenario RCP 8.5 (RCP 6.0, RCP 4.5) the ensemble mean burnt area in year 2300 is higher by 83% (44%, 15%) than its value in year 2100, and the ensemble mean CO 2 emissions are correspondingly higher by 31% (19%, 9%). All changes of natural fire characteristics in the 21st–23rd centuries are associated mostly with the corresponding changes in boreal regions of Eurasia and North America. However, under the RCP 8.5 scenario, increase of the burnt area and CO 2 emissions in boreal regions during the 22nd–23rd centuries are accompanied by the respective decreases in the tropics and subtropics.

Description: Perspectives of the microbial carbon pump with special references to microbial respiration and ecological efficiency Biogeosciences Discussions, 11, 1479-1533, 2014 Author(s): H. Dang and N. Jiao Although respiration consumes fixed carbon and produce CO 2 , it provides energy for essential biological processes of an ecosystem, including the microbial carbon pump (MCP). In MCP-driving biotransformation of labile DOC to recalcitrant DOC (RDOC), microbial respiration provides the metabolic energy for environmental organic substrate sensing, cellular enzyme syntheses and catalytic processes such as uptake, secretion, modification, fixation and storage of carbon compounds. The MCP efficiency of a heterotrophic microorganism is thus related to its energy production efficiency and hence to its respiration efficiency. Anaerobically respiring microbes usually have lower energy production efficiency and lower energy-dependent carbon transformation efficiency, and consequently lower MCP efficiency at per cell level. This effect is masked by the phenomena that anoxic environments often store more organic matter. Here we point out that organic carbon preservation and RDOC production is different in mechanisms, and anaerobically respiring ecosystems could also have lower MCP ecological efficiency. Typical cases can be found in large river estuarine ecosystems. Due to strong terrigenous input of nutrients and organic matter, estuarine ecosystems usually experience intense heterotrophic respiration processes that rapidly consume dissolved oxygen, potentially producing hypoxic and anoxic zones in the water column. The lowered availability of dissolved oxygen and the excessive supply of nutrients such as nitrate from river input prompt enhanced anaerobic respiration processes. Thus, some nutrients may be consumed by anaerobically respiring heterotrophic microorganisms, instead of being utilized by phytoplankton for carbon fixation and primary production. In this situation, the ecological functioning of the estuarine ecosystem is altered and the ecological efficiency is lowered, as less carbon is fixed and less energy is produced. Ultimately this would have negatively impacts on the ecological functioning and efficiency of the MCP which depends on both organic carbon and energy supply.

Description: Annual CO 2 budget and seasonal CO 2 exchange signals at a High Arctic permafrost site on Spitsbergen, Svalbard archipelago Biogeosciences Discussions, 11, 1535-1559, 2014 Author(s): J. Lüers, S. Westermann, K. Piel, and J. Boike The annual variability of CO 2 exchange in most ecosystems is primarily driven by the activities of plants and soil microorganisms. However, little is known about the carbon balance and its controlling factors outside the growing season in arctic regions dominated by soil freeze/thaw-processes, long-lasting snow cover, and several months of darkness. This study presents a complete annual cycle of the CO 2 net ecosystem exchange (NEE) dynamics for a High Arctic tundra area on the west coast of Svalbard based on eddy-covariance flux measurements. The annual cumulative CO 2 budget is close to zero grams carbon per square meter per year, but shows a very strong seasonal variability. Four major CO 2 exchange seasons have been identified. (1) During summer (ground snow-free), the CO 2 exchange occurs mainly as a result of biological activity, with a predominance of strong CO 2 assimilation by the ecosystem. (2) The autumn (ground snow-free or partly snow-covered) is dominated by CO 2 respiration as a result of biological activity. (3) In winter and spring (ground snow-covered), low but persistent CO 2 release occur, overlain by considerable CO 2 exchange events in both directions associated with changes of air masses and air and atmospheric CO 2 pressure. (4) The snow melt season (pattern of snow-free and snow-covered areas), where both, meteorological and biological forcing, resulting in a visible carbon uptake by the high arctic ecosystem. Data related to this article are archived under: http://doi.pangaea.de/10.1594/PANGAEA.809507 .

Description: Spatio-temporal variability of soil respiration in a spruce-dominated headwater catchment in western Germany Biogeosciences Discussions, 11, 691-728, 2014 Author(s): A. Y. Bossa and B. Diekkrüger CO 2 production and transport from forest floors is an important component of the carbon cycle and is closely related to the global atmosphere CO 2 concentration. If we are to understand the feedback between soil processes and atmospheric CO 2 , we need to know more about the spatio-temporal variability of this soil respiration under different environmental conditions. In this study, long-term measurements were conducted in a spruce-dominated forest ecosystem in western Germany. Multivariate analysis-based similarities between different measurements sites led to the detection of site clusters along two CO 2 emission axes: (1) mainly controlled by soil temperature and moisture condition, and (2) mainly controlled by root biomass and the forest floor litter. The combined effects of soil temperature and soil moisture were used as a time-dependent rating factor affecting the optimal CO 2 production and transport at cluster level. High/moderate/weak time-dependent rating factors were associated with the different clusters. The process-based most distant clusters were identified using specified pattern characteristics: the reaction rates in the soil layers, the activation energy for bio-chemical reactions, the water sorption and desorption constant, the root biomass factor, the litter layer factor and the organic matter factor. A HYDRUS-1D model system was inversely used to compute soil hydraulic parameters from soil moisture measurements. Heat transport parameters were adjusted based on observed soil temperatures. The results were used to adjust CO 2 production and transport characteristics such as the molecular diffusion coefficient of carbon dioxide in air and water and the CO 2 production by soil microorganisms and plant roots under optimal conditions for each cluster. Although the uncertainty associated with the HYDRUS-1-D simulations is higher, the results were consistent with both the multivariate clustering and the time-dependent rating of site production/transport. Finally, four clusters with significantly different environmental conditions (i.e., permanent high soil moisture condition, accumulated litter amount, high variability in soil moisture content, dominant temperature-dependence) were found relevant in explaining the spatio-temporal variability of CO 2 efflux and providing reference specific characteristic values for the investigated area.

Description: Magnetic quantification of Fe and S bound as magnetosomal greigite in laminated sapropels in deeper basins of the Baltic Sea Biogeosciences Discussions, 11, 729-752, 2014 Author(s): M. Reinholdsson and I. Snowball Magnetotactic bacteria (MTB) biomineralize magnetite and/or greigite for navigation purposes and it have been suggested that their magnetosomes make a significant contribution to the burial of Fe (and S and O) in sedimentary environments. To test this hypothesis and improve our understanding of MTBs impact on the rate of burial of these two elements we have quantified the abundance of Fe and S bound as greigite magnetofossils in laminated Baltic Sea sapropels, which were formed during periods of hypoxia and anoxia, using mineral magnetic measurements. Fluxes of Fe and S in the form of preserved greigite magnetofossils were calculated for three sedimentary sequences. The magnetosomal Fe (and S) fluxes range between 0.19 and 1.46 × 10 −6 g cm −2 yr −1 (0.15 and 1.12 × 10 −6 g cm −2 yr −1 ), and varied in time and space. The contribution of magnetosomal Fe to total Fe fluxes is relatively low, 〈 0.2%, although its contribution can be important in other stratified waters that suffer from hypoxia/anoxia. We show that the magnetosomal fluxes of Fe in the Baltic Sea are, however, similar to fluxes of Fe derived from mineral magnetic studies of magnetite magnetosomes in organic rich, varved freshwater lake sediments in Sweden.

Description: Controls on pH in surface waters of northwestern European shelf seas Biogeosciences Discussions, 11, 943-974, 2014 Author(s): V. M. C. Rérolle, M. Ribas-Ribas, V. Kitidis, I. Brown, D. C. E. Bakker, G. A. Lee, T. Shi, M. C. Mowlem, and E. P. Achterberg We present here a high resolution surface water pH dataset obtained in the Northwest European shelf seas in summer 2011. This is the first time that pH has been measured at such a high spatial resolution (10 measurements h –1 ) in this region. The aim of our paper is to investigate the carbonate chemistry dynamics of the surface water using pH and ancillary data. The main processes controlling the pH distribution along the ship's transect, and their relative importance, were determined using a statistical approach. The study highlights the impact of biological activity, temperature and riverine inputs on the carbonate chemistry dynamics of the shelf seas surface water. For this summer cruise, the biological activity formed the main control of the pH distribution along the cruise transect. Variations in chlorophyll and nutrients explained 29% of the pH variance along the full transect and as much as 68% in the northern part of the transect. In contrast, the temperature distribution explained ca. 50% of the pH variation in the Skagerrak region. Riverine inputs were evidenced by high dissolved organic carbon (DOC) levels in the Strait of Moyle (northern Irish Sea) and the southern North Sea with consequent remineralisation processes and a reduction in pH. The DOC distribution described 15% of the pH variance along the full transect. This study highlights the high spatial variability of the surface water pH in shelf seawaters where a range of processes simultaneously impacts the carbonate chemistry.

Description: Characterisation of NO production and consumption: new insights by an improved laboratory dynamic chamber technique Biogeosciences Discussions, 11, 1187-1275, 2014 Author(s): T. Behrendt, P. R. Veres, F. Ashuri, G. Song, M. Flanz, B. Mamtimin, M. Bruse, J. Williams, and F. X. Meixner Biogenic NO x emissions from natural and anthropogenically influenced soils are currently estimated to amount to 9 Tg a −1 , hence a significant fraction of global NO x emissions (45 Tg a −1 ). During the last three decades, a large number of field measurements have been performed to quantify biogenic NO emissions. To study biogenic NO emissions as a function of soil moisture, soil temperature, and soil nutrients, several laboratory approaches have been developed to estimate local/regional NO emissions by suitable up-scaling. This study presents an improved and automated laboratory dynamic chamber system (consisting of six individual soil chambers) for investigation and quantification of all quantities necessary to characterize biogenic NO release from soil (i.e., net NO release rate, NO production and consumption rate, and respective Q 10 values). In contrast to former versions of the laboratory dynamic chamber system, the four experiments for complete characterization can now be performed on a single soil sample, whereas former studies had to be performed on four sub-samples. This study discovered that the sub-sample variability biased former measurements of net NO release rates tremendously. Furthermore, it was also shown that the previously reported variation of optimum soil moisture (i.e., where a maximum net NO release rate occurs) between individual sub-samples is most likely a methodical artefact of former versions of the laboratory dynamic chamber system. A comprehensive and detailed methodical concept description of the improved laboratory dynamic chamber system is provided. Response of all quantities (necessary to characterize net NO release) to soil temperature and NO mixing ratio of the flushing air-stream are determined by automatic monitoring of these variables during one single drying-out experiment with one single soil sample only. The method requires precise measurements of NO mixing ratio at the inlet and outlet of each soil chamber; finally, four pairs of inlet/outlet NO mixing ratios are sufficient to derive all necessary quantities. Soil samples from drylands exhibit particularly low NO production, but even lower NO consumption rates. However, with the improved laboratory dynamic chamber system those low levels can be quantified, as well as corresponding NO compensation point mixing ratios and respective Q 10 values. It could be shown, that the NO compensation point mixing ratio seems to be generally independent of gravimetric soil moisture content, but, particularly for dryland soils, strongly dependent on soil temperature. New facilities have been included into the improved system (e.g. for investigation of net release rates of other trace gases, namely CO 2 and VOCs). First results are shown for net release rates of acetone (C 3 H 6 O), acetaldehyde (C 2 H 4 O) and CO 2 . This new system is thus able to simultaneously investigate potential mechanistic links between NO, multitudinous VOC and CO 2 .

Description: Degradation changes stable carbon isotope depth profiles in palsa peatlands Biogeosciences Discussions, 11, 1383-1412, 2014 Author(s): J. P. Krüger, J. Leifeld, and C. Alewell Palsa peatlands are a significant carbon pool in the global carbon cycle and are projected to change by global warming due to accelerated permafrost thaw. Our aim was to use stable carbon isotopes as indicators of palsa degradation. Depth profiles of stable carbon isotopes generally reflect organic matter dynamics in soils with an increase of δ 13 C values during aerobic decomposition and stable or decreasing δ 13 C values with depth during anaerobic decomposition. Stable carbon isotope depth profiles of undisturbed and degraded sites of hummocks as well as hollows at three palsa peatlands in northern Sweden were used to investigate the degradation processes. The depth patterns of stable isotopes clearly differ between intact and degraded hummocks at all sites. Erosion and cryoturbation at the degraded sites significantly changes the stable carbon isotope depth profiles. At the intact hummocks the uplifting of peat material by permafrost is indicated by a turning in the δ 13 C depth trend and this assessment is supported by a change in the C / N ratios. For hollows isotope patterns were less clear, but some hollows and degraded hollows in the palsa peatlands show differences in their stable carbon isotope depth profiles indicating enhanced degradation rates. We conclude that the degradation of palsa peatlands by accelerated permafrost thawing could be identified with stable carbon isotope depth profiles. At intact hummocks δ 13 C depth patterns display the uplifting of peat material by a change in peat decomposition processes.

Description: Vapor pressure deficit controls on fire ignition and fire spread in boreal forest ecosystems Biogeosciences Discussions, 11, 1309-1353, 2014 Author(s): F. Sedano and J. T. Randerson Climate-driven changes in the fire regime within boreal forest ecosystems are likely to have important effects on carbon cycling and species composition. In the context of improving fire management options and developing more realistic scenarios of future change, it is important to understand how meteorology regulates different fire processes, including ignition, daily fire spread rates, and cumulative annual burned area. Here we combined MODIS active fires (MCD14ML), MODIS imagery (MOD13A1) and ancillary historic fire perimeter information to produce a dataset of daily fire spread maps of Alaska for the period 2002–2011. This approach provided a spatial and temporally continuous representation of fire progression and a precise identification of ignition and extinction locations and dates for each wildfire. The fire-spread maps were analyzed together with daily vapor pressure deficit (VPD) observations from the North American Regional Reanalysis (NARR) and lightning strikes from the Alaska Lightning Detection Network (ALDN). We found a significant relationship between daily VPD and probability that a lightning strike would develop into a fire ignition. In the first 5 days after ignition, above average VPD increased the probability that fires would grow to large or very large sizes. Strong relationships also were identified between VPD and burned area at several levels of temporal and spatial aggregation. As a consequence of regional coherence in meteorology, ignition, daily fire spread rates, and fire extinction events were often synchronized across different fires in interior Alaska. At a regional scale, the sum of positive VPD anomalies during the fire season was positively correlated with annual burned area during the NARR era (1979–2011; R 2 = 0.45). Some of the largest fires we mapped had slow initial growth, indicating opportunities may exist for suppression efforts to adaptively manage these forests for climate change. The results of our spatiotemporal analysis provide new information about temporal and spatial dynamics of wildfires and have implications for modeling the terrestrial carbon cycle.

Description: The influence of the geo-morphological and sedimentological settings on the distribution of epibenthic assemblages on a flat topped hill on the over-deepened shelf of the Western Weddell Sea Biogeosciences Discussions, 11, 1631-1672, 2014 Author(s): B. Dorschel, J. Gutt, D. Piepenburg, M. Schröder, and J.-E. Arndt Epibenthos communities play an important role in the marine ecosystems of the Weddell Sea. Information on the factors controlling their structure and distribution are, however, still rare. Especially the interactions between environmental factors and biotic assemblages are not fully understood. Nachtigaller Hill, a newly discovered seabed structure on the over-deepened shelf of the Northwest Weddell Sea (Southern Ocean), offers a unique site to study these interactions in a high-latitude Antarctic setting. Based on high-resolution bathymetry, geo-referenced biological data, the effect of the terrain and related environmental parameters on the epibenthos was assessed. At Nachtigaller Hill, both geo-morphological and biological data showed complex distribution patterns, reflecting local processes such as iceberg scouring and locally amplified bottom currents. This variability is also generally reflected in the variable epibenthos distribution patterns although statistical analyses did not show strong correlations between the selected environmental parameters and species abundances. By analysing the interactions between environmental and biological patterns, this study provides crucial information towards a better understanding of the factors and processes that drive epibenthos communities on the shelves of the Weddell Sea and probably also on other Antarctic shelves.

Description: Future climate variability impacts on potential erosion and soil organic carbon in European croplands Biogeosciences Discussions, 11, 1561-1585, 2014 Author(s): M. van der Velde, J. Balkovič, C. Beer, N. Khabarov, M. Kuhnert, M. Obersteiner, R. Skalský, W. Xiong, and P. Smith We investigate the impact of future climate variability on the potential vulnerability of soils to erosion and the consequences for soil organic carbon (SOC) in European croplands. Soil erosion is an important carbon flux not characterized in Earth System Models. We use a~European implementation of EPIC, driven by reference climate data (CNTRL), and climate data with reduced variability (REDVAR). Whether erosion regimes will change across European cropland depends on the spatial conjunction of expected changes in climate variability and physiographic conditions conducive to erosion. We isolated the effect of erosion by performing simulations with and without erosion. Median CNTRL and REDVAR erosion rates equalled 14.4 and 9.1 ton ha −1 , and 19.1 and 9.7, for 1981–2010 and 2071–2100, respectively. The total amount of carbon lost from European cropland due to erosion was estimated at 769 Tg C for 1981–2010 (from a total storage of 6197 Tg C without erosion) under CNTRL climate. Climate trend impacts reduce the European cropland SOC stock by 578 Tg C without – and by 683 Tg C with erosion, from 1981 to 2100. Climate variability compounds these impacts and decreases the stock by an estimated 170 Tg without erosion and by 314 Tg C with erosion, by the end of the century. Future climate variability and erosion will thus compound impacts on SOC stocks arising from gradual climate change alone.

Description: Fluctuations of sulfate, S-bearing amino acids and magnesium in a giant clam shell Biogeosciences Discussions, 11, 1613-1629, 2014 Author(s): T. Yoshimura, Y. Tamenori, H. Kawahata, and A. Suzuki We used micro-X-ray fluorescence combined with X-ray photoabsorption spectroscopy to investigate speciation-specific sulfur profiles in the inner shell layer of a giant clam ( Hippopus hippopus ). The sulfate, S-bearing amino acids, and total sulfur profiles indicated that inorganic sulfate was the dominant component in the shell of this bivalve. Sulfur profiles in the inner shell layer showed clear annual fluctuations that varied by more than one order of magnitude, from 〈 50 to 1420 ppm, and sulfate and total sulfur maxima became higher with age, whereas no ontogenetic trend was noticeable in the profile of S-bearing amino acids. A changes in the carbonate ion concentration in the calcifying fluid would suggest that an ontogenetic increase in the relative activity of sulfate ions to carbonate ions in the calcifying fluid affects sulfate concentrations in the shells. These results suggest that trace sulfur profiles in the shell of the giant clam may reflect both cyclic shell growth related to environmental factors such as insolation and temperature and ontogenetic changes of the calcifying fluid chemistry mediated by physiological processes. The observed S profile implies a clear change in calcifying fluid chemistry towards less alkaline condition with age. Magnesium fluctuations suggested that Mg was incorporated into the shells at high growth rates during warm seasons. The spectrum of Mg K -edge XANES and comparison of Mg and S-bearing amino acids profiles indicated that a pronounced effect of the organic fraction or disordered phases were observed in aragonitic shell of H. hippopus rather than regulated substitution into the aragonite crystal lattice.

Description: CO 2 and CH 4 in sea ice from a subarctic fjord Biogeosciences Discussions, 11, 4047-4083, 2014 Author(s): O. Crabeck, B. Delille, D. N. Thomas, N. X. Geilfus, S. Rysgaard, and J. L. Tison We present CH 4 concentration [CH 4 ] and the partial pressure of CO 2 ( p CO 2 ) in bulk sea ice from subarctic, land-fast sea ice in the Kapisillit fjord, Greenland. The bulk ice [CH 4 ] ranged from 1.8 to 12.1 nmol L −1 , which corresponds to a partial pressure range of 3 to 28 ppmv. This is markedly higher than the average atmospheric methane content of 1.9 ppmv. Most of the trapped methane within the sea ice was evidently contained inside bubbles, and only a minor portion was dissolved in the brine. The bulk ice p CO 2 ranged from 60 to 330 ppmv showing that sea ice at temperatures above −4 °C is under-saturated compared to the atmosphere (390 ppmv). Our study adds to the few existing studies of CH 4 and CO 2 in sea ice and concludes that sub-arctic sea can be a sink for atmospheric CO 2 , while being a net source of CH 4 . Processes related to the freezing and melting of sea ice represents large unknowns to the exchange of CO 2 but also CH 4 . It is therefore imperative to assess the consequences of these unknowns through further field campaigns and targeted research under other sea ice conditions at both hemispheres.

Description: Estimating net anthropogenic nitrogen inputs (NANI) in the Lake Dianchi Basin of China Biogeosciences Discussions, 11, 4123-4150, 2014 Author(s): W. Gao, R. W. Howarth, B. Hong, D. P. Swaney, and H. C. Guo Net anthropogenic nitrogen inputs (NANI) with components of atmospheric N deposition, synthetic N fertilizer, agricultural N fixation and N in net food and feed imports from 15 catchments in Lake Dianchi Basin were determined over an 11 year period (2000–2010). The 15 catchments range in size from 44 km 2 to 316 km 2 with an average of 175 km 2 . To reduce uncertainty from scale change methodology, results from data extracting by area-weighting and land use-weighting methods were compared. Results show that methodology for extrapolating data from county scale to watersheds has a great influence on NANI computation for catchments in the Lake Dianchi Basin, and estimates of NANI between two methods have an average difference of 30% on catchments basis while a smaller difference (15%) was observed on the whole Lake Dianchi Basin basis. The riverine N export has stronger linear relationship with NANI computed by land use-weighting method, which we believe is more reliable. Overall, nitrogen inputs assessed by the NANI approach for the Lake Dianchi Basin are 9900 kg N km −2 yr −1 , ranging from 6600 to 28 000 kg N km −2 yr −1 among the 15 catchments. Synthetic N fertilizer is the largest component of NANI in most subwatersheds. On average, riverine flux of nitrogen in catchments of the Lake Dianchi Basin averages 83% of NANI, far higher than generally observed in North America and Europe. Saturated N sinks and limited capacity for denitrification in rivers may be responsible for this high percent of riverine N export. A negative intercept observed in the linear relationship between NANI and riverine N export suggests the influence of pollution control measures on N flux in small watershed. The NANI methodology should be applicable in small watersheds when sufficiently detailed data are available to estimate its components.

Description: Lena River Delta formation during the Holocene Biogeosciences Discussions, 11, 4085-4122, 2014 Author(s): D. Bolshiyanov, A. Makarov, and L. Savelieva The Lena River Delta, the largest delta of the Arctic Ocean, differs from other deltas because it consists mainly of organomineral sediments, commonly called peat, that contain a huge organic carbon reservoir. The analysis of Delta sediment radiocarbon ages showed that they could not have formed as peat during floodplain bogging, but accumulated when Laptev Sea water level was high and green mosses and sedges grew and were deposited on the surface of flooded marshes. The Lena River Delta formed as organomineral masses and layered sediments accumulated during transgressive phases when sea level rose. In regressive phases, the islands composed of these sediments and other, more ancient islands were eroded. Each new sea transgression led to further accumulation of layered sediments. As a result of alternating transgressive and regressive phases the first alluvial-marine terrace formed, consisting of geological bodies of different ages. Determining the formation age of different areas of the first terrace and other marine terraces on the coast allowed the periods of increasing (8–6 Ka, 4.5–4 Ka, 2.5–1.5 Ka, 0.4–0.2 Ka) and decreasing (5 Ka, 3 Ka, 0.5 Ka) Laptev Sea levels to be distinguished in the Lena Delta area.

Description: A red tide alga grown under ocean acidification up-regulates its tolerance to lower pH by increasing its photophysiological functions Biogeosciences Discussions, 11, 6303-6328, 2014 Author(s): S.-W. Chen, J. Beardall, and K.-S. Gao Phaeocystis globosa , a red tide alga, often forms blooms in or adjacent to coastal waters and experiences changes of pH and seawater carbonate chemistry caused by either diel/periodic fluctuation in biological activity, human activity or, in the longer term, ocean acidification due to atmospheric CO 2 rise. We examined the photosynthetic physiology of this species while growing it under different pH levels induced by CO 2 enrichment and investigated its acclimation to carbonate chemistry changes under different light levels. Short-term exposure to reduced pH nbs (7.70) decreased the alga's photosynthesis and light use efficiency. However, acclimation to the reduced pH level for 1–19 generations led to recovered photosynthetic activity, being equivalent to that of cells grown under pH 8.07 (control), though such acclimation required a different time span (number of generations) under different light regimes. The low-pH grown cells increased their contents of chlorophyll and carotenoids with prolonged acclimation to the acidification, with increased photosynthetic quantum yield and decreased non-photochemical quenching. The specific growth rate of the low-pH grown cells also increased to emulate that grown under the ambient pH level. This study clearly shows that Phaeocystis globosa is able to acclimate to seawater acidification by increasing its energy capture and decreasing its non-photochemcial energy loss.

Description: The low temperature hyperalkaline hydrothermal system of the Prony bay (New Caledonia) Biogeosciences Discussions, 11, 6221-6267, 2014 Author(s): C. Monnin, V. Chavagnac, C. Boulart, B. Ménez, M. Gérard, E. Gérard, M. Quéméneur, G. Erauso, A. Postec, L. Guentas-Dombrowski, C. Payri, and B. Pelletier The terrestrial hyperalkaline springs of the Prony bay (southern lagoon, New Caledonia) have been known since the XIXth century, but a recent high resolution bathymetric survey of the seafloor has revealed the existence of numerous submarine structures similar to the well-known Aiguille de Prony, which are also the location of high pH fluid discharge into the lagoon. During the HYDROPRONY cruise (28 October to 13 November 2011) samples of waters, gases and concretions have been collected by scuba divers at underwater vents. Four of these sampling sites are located in the Prony bay at depths up to 50 m. One (Bain des Japonais spring) is also in the Prony Bay but uncovered at low tide and another (Rivière des Kaoris spring) is on land slightly above the seawater level at high tide. We report the chemical composition (Na, K, Ca, Mg, Cl, SO 4 , Dissolved Inorganic Carbon, SiO 2 (aq)) of 45 water samples collected at 6 sites of high pH water discharge, as well as the composition of gases. Temperatures reach 37 °C at the Bain des Japonais and 32 °C at the spring of the Kaoris. Gas bubbling was observed only at these two springs. The emitted gases contain between 12 and 30% of hydrogen in volume of dry gas, 6 to 14% of methane, and 56 to 72% of nitrogen, with trace amounts of carbon dioxide, ethane and propane. pH values and salinities of all the 45 collected water samples range from the seawater values (8.2 and 35 g L −1 ) to hyperalkaline freshwaters of the Ca-OH type (pH 11 and salinities as low as 0.3 g L −1 ) showing that the collected samples are always a mixture of a hyperalkaline fluid of meteoric origin and ambient seawater. Cl-normalized concentrations of dissolved major elements first show that the Bain des Japonais is distinct from the other sites. Waters collected at this site are three component mixtures involving the high pH fluid, the lagoon seawater and the river water from the nearby Rivière du Carénage. The chemical compositions of the hyperalkaline end members (at pH 11) are not significantly different from one site to the other although the sites are several km away from each other and are located on different ultramafic substrata. The very low salinity of the hyperalkaline end members shows that seawater does not percolate through the ultramafic formation. Mixing of the hyperalkaline hydrothermal end member with local seawater produces large ranges and very sharp gradients of pH, salinity and dissolved element concentrations. There is a major change in the composition of the water samples at a pH around 10, which delimitates the marine environment from the hyperalkaline environment. The redox potential evolves toward negative values at high pH indicative of the reducing conditions due to bubbling of the H 2 -rich gas. The calculation of the mineral saturation states carried out for the Na-K-Ca-Mg-Cl-SO 4 -DIC-SiO 2 -H2O system shows that this change is due to the onset of brucite formation. While the saturation state of the Ca-carbonates over the whole pH range is typical of that found in a normal marine environment, Mg- and Mg-Ca-carbonates (magnesite, hydromagnesite, huntite, dolomite) exhibit very large supersaturations with maximum values at pH around 10, very well marked for the Bain des Japonais, emphasizing the role of water mixing in mineral formation. The discharge of high pH waters of meteoric origin into the lagoon marine environment makes the hydrothermal system of the Prony bay unique compared to other low temperature serpentinizing environments such as Oman (fully continental) or Lost City (fully marine).

Description: Disparities between Phaeocystis in situ and optically-derived carbon biomass and growth rates: potential effect on remote-sensing primary production estimates Biogeosciences Discussions, 11, 6119-6149, 2014 Author(s): L. Peperzak, H. J. van der Woerd, and K. R. Timmermans The oceans play a pivotal role in the global carbon cycle. Unfortunately, the daily production of organic carbon, the product of phytoplankton standing stock and growth rate cannot be measured globally by discrete oceanographic methods. Instead, optical proxies from Earth-orbiting satellites must be used. To test the accuracy of optically-derived proxies of phytoplankton physiology and growth rate, standard ex situ data from the wax and wane of a Phaeocystis bloom in laboratory mesocosms were compared with hyperspectral reflectance data. Chlorophyll biomass could be estimated accurately from reflectance using specific chlorophyll absorption algorithms. However, the conversion of chlorophyll (Chl) to carbon (C) was obscured by the observed increase in C : Chl under nutrient-limited growth. C : Chl was inversely correlated ( r 2 = 0.88) with Photosystem II quantum efficiency (Fv/Fm), the in situ fluorometric oceanographic proxy for growth rate. In addition, the optical proxy for growth rate, the quantum efficiency of fluorescence ϕ was linearly correlated to Fv/Fm ( r 2 = 0.84), but not – as by definition – by using total phytoplankton absorption, because during nutrient-limited growth the concentrations of non-fluorescent light-absorbing pigments increased. As a consequence, none of the three proxies (C : Chl, Fv/Fm, φ) was correlated to carbon or cellular phytoplankton growth rates. Therefore, it is concluded that although satellite derived estimates of chlorophyll biomass may be accurate, physiologically-induced non-linear shifts in growth rate proxies may obscure accurate phytoplankton growth rates and hence global carbon production estimates.

Description: Quantification of iron-rich volcanogenic dust emissions and deposition over ocean from Icelandic dust sources Biogeosciences Discussions, 11, 5941-5967, 2014 Author(s): O. Arnalds, H. Olafsson, and P. Dagsson-Waldhauserova Iceland has extremely active dust sources that result in large scale emissions and deposition on land and sea. The dust has volcanogenic origin of basaltic composition with about 10 % Fe content. We used two independent methods to quantify dust emission from Iceland and dust deposition on sea. Firstly, aerial extent (map) of deposition on land was extended to ocean areas around Iceland. Secondly, survey of number of dust events over the past decades and calculations of emissions and sea deposition for the dust storms were made. The results show total emissions range from 30.5 (dust event based calculation) to 40.1 million tons (map calculation), which places Iceland among the most active dust sources on Earth. Ocean deposition ranges between 5.5 (dust events calculations) and 13.8 million tons (map calculation). Calculated iron deposition from Icelandic dust ranges between 0.56 to 1.4 million tons, which are distributed over wide areas (〉 370 000 km 2 ) and consist of fine reactive volcanic materials. The paper provides the first quantitative estimate of total dust emissions and oceanic deposition from Iceland. Iron is a limiting nutrient for primary production in the oceans around Iceland and the dust is likely to affect Fe levels in Icelandic ocean waters.

Description: Trimethylamine emissions in animal husbandry Biogeosciences Discussions, 11, 6519-6553, 2014 Author(s): J. Sintermann, S. Schallhardt, M. Kajos, M. Jocher, A. Bracher, A. Münger, D. Johnson, A. Neftel, and T. Ruuskanen Degradation of plant material by animals is an important transformation pathway in the nitrogen (N) cycle. During the involved processes, volatile reduced alkaline nitrogen compounds, mainly ammonia (NH 3 ) and aliphatic amines such as trimethylamine (TMA), are formed. Today, animal husbandry is estimated to constitute a main source of aliphatic amines into the atmosphere with TMA being the main emitted compound. Here, we show how the interaction between faeces and urine in animal production systems provides the primary source for agricultural TMA emissions. Excreted urine contains large quantities of urea and TMA-N-oxide, which are transformed into NH 3 and TMA, respectively, via enzymatic processes provided by microbes present in faeces. TMA emissions from areas polluted with urine-faeces mixture are on average in the order of 10 to 50 nmol m −2 s −1 . Released amines promote secondary aerosol particle formation in the agricultural emission plume. The atmospheric lifetime of TMA, which was estimated to be in the order of 30 to 1000 s, is determined by the condensation on aerosol particles.

Description: Accumulation of nitrogen and organic matter during primary succession of Leymus arenarius dunes on the volcanic island Surtsey, Iceland Biogeosciences Discussions, 11, 6591-6613, 2014 Author(s): G. Stefansdottir, A. L. Aradottir, and B. D. Sigurdsson The volcanic island of Surtsey has been a natural laboratory where the primary succession of flora and fauna has been monitored, since it emerged from the N-Atlantic Ocean in 1963. We quantified the accumulation rates of nitrogen (N) and soil organic matter (SOM) in a 37 year long chronosequence of Leymus arenarius dunes in order to illuminate the spatiotemporal patterns in their build-up in primary succession. The Leymus dune area, volume and height grew exponentially over time. Aboveground plant biomass, cover or number of shoots per unit area did not change significantly with time, but root biomass accumulated with time, giving a root-shoot ratio of 19. The dunes accumulated on average 6.6 kg N ha −1 year −1 , which was 3.5 times more than is received annually by atmospheric deposition. The extensive root system of Leymus seems to effectively retain and accumulate large part of the annual N deposition, not only deposition directly on the dunes but also from the adjacent unvegetated areas. SOM per unit area increased exponentially with dune age, but the accumulation of roots, aboveground biomass and SOM was more strongly linked to soil N than time: 1 g m −2 increase in soil N led on the average to 6 kg C m −2 increase in biomass and SOM. The Leymus dunes, where most of the N has been accumulated, will therefore probably act as hot-spots for further primary succession of flora and fauna on the tephra sands of Surtsey.

Description: Soil CO 2 efflux from mountainous windthrow areas: dynamics over 12 years post-disturbance Biogeosciences Discussions, 11, 6383-6417, 2014 Author(s): M. Mayer, B. Matthews, A. Schindlbacher, and K. Katzensteiner Windthrow driven changes in carbon (C) allocation and soil microclimate can affect soil carbon dioxide (CO 2 ) efflux ( F soil ) of forest ecosystems. Although F soil is the dominant C flux following stand-replacing disturbance, the effects of catastrophic windthrow on F soil are still poorly understood. We measured F soil at a montane mixed forest site and at a subalpine spruce forest site from 2009 until 2012. Both sites consisted of undisturbed forest stands and two adjacent windthrow areas which differed in time since disturbance. The combination of chronosequence and direct time-series approaches enabled us to investigate F soil dynamics over 12 years post-disturbance. In the initial phase after disturbance (1–6 years), F soil rates did not differ significantly from those of the undisturbed stands, but in the later phase (9–12 years after disturbance) F soil rates were significantly higher than corresponding undisturbed stand values. The higher F soil rates in the later phase post-disturbance are likely explained by a dense vegetation cover and correspondingly higher autotrophic respiration rates. Soil temperature increased significantly following windthrow (by 2.9–4.8 °C) especially in the initial phase post-disturbance when vegetation cover was sparse. A significant part (20–36%) of F soil from the windthrow areas was thus attributed to disturbance induced changes in soil temperature. According to our estimates, ~500 to 700 g C m −2 yr −1 are released via F soil from south-facing forest sites in the Austrian Calcareous Alps in the initial 6 years after windthrow. With high game pressure suppressing primary production in these areas, post-disturbance loss of ecosystem C to the atmosphere is likely to be substantial unless management is proactive in regenerating such sites. An increase in the frequency of forest disturbance by windthrow could therefore decrease soil C stocks and positively feedback on rising atmospheric CO 2 concentrations.

Description: Assessing effects of permafrost thaw on C fluxes based on a multi-year modeling across a permafrost thaw gradient at Stordalen, Sweden Biogeosciences Discussions, 11, 3963-3999, 2014 Author(s): J. Deng, C. Li, S. Frolking, Y. Zhang, K. Bäckstrand, and P. Crill Northern peatlands in permafrost regions contain large amount of organic carbon (C) in the soil. Climate warming and associated permafrost degradation are expected to have significant impacts on the C balance of these ecosystems, but the magnitude is uncertain. We incorporated a permafrost model, Northern Ecosystem Soil Temperature (NEST), into a biogeochemical model, DeNitrification-DeComposition (DNDC), to model C dynamics in high-latitude peatland ecosystems. The enhanced model was applied to assess effects of permafrost thaw on C fluxes of a sub-arctic peatland at Stordalen, Sweden. DNDC simulated soil freeze/thaw dynamics, net ecosystem exchange of CO 2 (NEE), and CH 4 fluxes across three typical land cover types, which represent different stages in the process of ongoing permafrost thaw at Stordalen. Model results were compared with multi-year field measurements and the validation indicates that DNDC was able to simulate observed differences in soil thaw, NEE, and CH 4 fluxes across the three land cover types at Stordalen. Consistent with the results from field studies, the modeled C fluxes across the permafrost thaw gradient demonstrate that permafrost thaw and the associated changes in soil hydrology and vegetation increase net uptake of C from the atmosphere, but also increase the radiative forcing impacts on climate due to increased CH 4 emissions. This study indicates the potential of utilizing biogeochemical models, such as DNDC, to predict soil thermal regime in permafrost areas and to investigate impacts of permafrost thaw on ecosystem C fluxes after incorporating a permafrost component into the model framework.

Description: Constraint of soil moisture on CO 2 efflux from tundra lichen, moss, and tussock in Council, Alaska using a hierarchical Bayesian model Biogeosciences Discussions, 11, 5903-5939, 2014 Author(s): Y. Kim, K. Nishina, N. Chae, S. Park, Y. Yoon, and B. Lee The tundra ecosystem is quite vulnerable to drastic climate change in the Arctic, and the quantification of carbon dynamics is of significant importance in response to thawing permafrost, changes in the snow-covered period and snow and shrub community extent, and the decline of sea ice in the Arctic. Here, CO 2 efflux measurements using a manual chamber system within a 40 m × 40 m (5 m interval; 81 total points) plot were conducted in dominant tundra vegetation on the Seward Peninsula of Alaska, during the growing seasons of 2011 and 2012, for the assessment of the driving parameters of CO 2 efflux. We applied a hierarchical Bayesian (HB) model – which is a function of soil temperature, soil moisture, vegetation type and thaw depth – to quantify the effect of environmental parameters on CO 2 efflux, and to estimate growing season CO 2 emission. Our results showed that average CO 2 efflux in 2011 is 1.4-fold higher than in 2012, resulting from the distinct difference in soil moisture between the two years. Tussock-dominated CO 2 efflux is 1.4 to 2.3 times higher than those measured in lichen and moss communities, reflecting tussock as a significant CO 2 source in the Arctic, with wide area distribution on a circumpolar scale. CO 2 efflux followed soil temperature nearly exponentially from both the observed data and the posterior medians of the HB model. This reveals soil temperature as the most important parameter in regulating CO 2 efflux, rather than soil moisture and thaw depth. Obvious changes in soil moisture during the growing seasons of 2011 and 2012 resulted in an explicit difference in CO 2 efflux – 742 and 539 g CO 2 m −2 period −1 in 2011 and 2012, respectively, suggesting that the 2012 CO 2 emission rate was constrained by 27% (95% credible interval: 17–36%) compared to 2011, due to higher soil moisture from severe rain. Estimated growing season CO 2 emission rate ranged from 0.86 Mg CO 2 period −1 in 2012 to 1.2 Mg CO 2 period −1 in 2011 within a 40 m × 40 m plot, corresponding to 86% and 80% of the annual CO 2 emission rates within the Alaska western tundra ecosystem. Therefore, the HB model can be readily applied to observed CO 2 efflux, as it demands only four environmental parameters and can also be effective for quantitatively assessing the driving parameters of CO 2 efflux.

Description: Contrasting responses of terrestrial ecosystem production to hot temperature extreme regimes between grassland and forest Biogeosciences Discussions, 11, 5997-6017, 2014 Author(s): Y. Zhang, M. Voigt, and H. Liu Observational data during the past several decades show faster increase of hot temperature extremes over land than changes in mean temperature. Towards more extreme temperature is expected to affect terrestrial ecosystem function. However, the ecological impacts of hot extremes on vegetation production remain uncertain across biomes in natural climatic conditions. In this study, we investigated the effects of hot temperature extremes on aboveground net primary production (ANPP) by combining MODIS EVI dataset and in situ climatic records during 2000 to 2009 from 12 long-term experimental sites across biomes and climates. Our results showed that higher mean annual maximum temperatures ( T max ) greatly reduced grassland production, and yet enhanced forest production after removing the effects of precipitation. Relative decreases in ANPP were 16% for arid grassland and 7% for mesic grassland, and the increase were 5% for forest. We also observed a significant positive relationship between interannual ANPP and T max for forest biome ( R 2 = 0.79, P 〈 0.001). This line of evidence suggests that hot temperature extreme leads to contrasting ecosystem-level response of vegetation production to warming climate between grassland and forest. Given that many terrestrial ecosystem models use average daily temperature as input, predictions of ecosystem production should consider these contrasting responses to more hot temperature extreme regimes associated with climate change.

Description: The temperature sensitivity of organic matter decay in tidal marshes Biogeosciences Discussions, 11, 6019-6037, 2014 Author(s): M. L. Kirwan, G. R. Guntenspergen, and J. A. Langley Approximately half of marine carbon sequestration takes place in coastal wetlands, including tidal marshes, where ecosystems accumulate organic matter to build soil elevation and survive sea level rise. The long-term viability of marshes, and their carbon pools, depends in part on how the balance between productivity and decay responds to climate change. Here, we report the sensitivity of soil organic matter decay in tidal marshes to seasonal and latitudinal variations in temperature measured over a 3 year period. We find a moderate increase in decay rate at warmer temperatures (3–6% °C −1 , Q 10 = 1.3–1.5). Despite the profound differences between microbial metabolism in wetlands and uplands, our results indicate a strong conservation of temperature sensitivity. Moreover, simple comparisons with organic matter production suggest that elevated atmospheric CO 2 and warmer temperatures will accelerate carbon accumulation in marsh soils, and enhance their ability to survive sea level rise.

Description: Bio-geographic classification of the Caspian Sea Biogeosciences Discussions, 11, 4409-4450, 2014 Author(s): F. Fendereski, M. Vogt, M. R. Payne, Z. Lachkar, N. Gruber, A. Salmanmahiny, and S. A. Hosseini Like other inland seas, the Caspian Sea (CS) has been influenced by climate change and anthropogenic disturbance during recent decades, yet the scientific understanding of this water body remains poor. In this study, an eco-geographical classification of the CS based on physical information derived from space and in-situ data is developed and tested against a set of biological observations. We used a two-step classification procedure, consisting of (i) a data reduction with self-organizing maps (SOMs) and (ii) a synthesis of the most relevant features into a reduced number of marine ecoregions using the Hierarchical Agglomerative Clustering (HAC) method. From an initial set of 12 potential physical variables, 6 independent variables were selected for the classification algorithm, i.e., sea surface temperature (SST), bathymetry, sea ice, seasonal variation of sea surface salinity (DSSS), total suspended matter (TSM) and its seasonal variation (DTSM). The classification results reveal a robust separation between the northern and the middle/southern basins as well as a separation of the shallow near-shore waters from those off-shore. The observed patterns in ecoregions can be attributed to differences in climate and geochemical factors such as distance from river, water depth and currents. A comparison of the annual and monthly mean Chl a concentrations between the different ecoregions shows significant differences (Kruskal–Wallis rank test, P 〈 0.05). In particular, we found differences in phytoplankton phenology, with differences in the date of bloom initiation, its duration and amplitude between ecoregions. A first qualitative evaluation of differences in community composition based on recorded presence-absence patterns of 27 different species of plankton, fish and benthic invertebrate also confirms the relevance of the ecoregions as proxies for habitats with common biological characteristics.

Description: Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling Biogeosciences Discussions, 11, 5969-5995, 2014 Author(s): C. C. van Heerwaarden and A. J. Teuling This study investigates the difference in land–atmosphere interactions between grassland and forest during typical heat wave conditions in order to understand the controversial results of Teuling et al. (2010) (T10, hereafter), who have found the systematic occurrence of higher sensible heat fluxes over forest than over grassland during heat wave conditions. With a simple, but accurate coupled land–atmosphere model, we are able to reproduce the findings of T10 for both normal summer and heat wave conditions, and to carefully explore the sensitivity of the coupled land–atmosphere system to changes in incoming radiation and early-morning temperature. Our results emphasize the importance of fast processes during the onset of heat waves, since we are able to explain the results of T10 without having to take into account changes in soil moisture. In order to disentangle the contribution of differences in several static and dynamic properties between forest and grassland, we have performed an experiment in which new land use types are created that are equal to grassland, but with one of its properties replaced by that of forest. From these, we conclude that the closure of stomata in the presence of dry air is by far the most important process in creating the different behavior of grassland and forest during the onset of a heat wave. However, we conclude that for a full explanation of the results of T10 also the other properties (albedo, roughness and the ratio of minimum stomatal resistance to leaf-area index) play an important, but indirect role; their influences mainly consist of strengthening the feedback that leads to the closure of the stomata by providing more energy that can be converted into sensible heat. The model experiment also confirms that, in line with the larger sensible heat flux, higher atmospheric temperatures occur over forest.

Description: Carbon cycling and phytoplankton responses within highly-replicated shipboard carbonate chemistry manipulation experiments conducted around Northwest European Shelf Seas Biogeosciences Discussions, 11, 3489-3534, 2014 Author(s): S. Richier, E. P. Achterberg, C. Dumousseaud, A. J. Poulton, D. J. Suggett, T. Tyrrell, M. V. Zubkov, and C. M. Moore The ongoing oceanic uptake of anthropogenic carbon dioxide (CO 2 ) is significantly altering the carbonate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidification will potentially impact marine ecosystems and their associated biogeochemical cycles in the future; however, results amongst studies, particularly when performed on natural communities, are highly variable, which in part likely reflects inconsistencies in experimental approach. To investigate the potential for identification of more generic responses and greater experimentally reproducibility, we devised and implemented a series of highly replicated ( n = 8), short term (2–4 days) multi-level (≥ 4 conditions) carbonate chemistry/nutrient manipulation experiments on a range of natural microbial communities sampled in Northwest European shelf seas. Carbonate chemistry manipulations and resulting biological responses were found to be highly reproducible within individual experiments and to a lesser extent between geographically different experiments. Statistically robust reproducible physiological responses of phytoplankton to increasing p CO 2 , characterized by a suppression of net growth for small sized cells ( 〈 10 μm), were observed in the majority of the experiments, irrespective of nutrient status. Remaining between-experiment variability was potentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cycling within the experiments revealed the expected increased sensitivity of carbonate chemistry to biological processes at higher p CO 2 and hence lower buffer capacity. The results thus emphasize how biological-chemical feedbacks may be altered in the future ocean.

Description: Bacterial production and transformation of dissolved neutral sugars and amino acids in seawater Biogeosciences Discussions, 11, 6151-6184, 2014 Author(s): L. Jørgensen, O. Lechtenfeld, R. Benner, M. Middelboe, and C. A. Stedmon Dissolved organic matter (DOM) in the ocean consists of a heterogeneous mixture of molecules, most of which are of unknown origin. Neutral sugars and amino acids are among the few recognizable biomolecules in DOM, and the molecular composition of these biomolecules is shaped primarily by biological production and degradation processes. This study provides insight into the bioavailability of biomolecules as well as the chemical composition of DOM produced by bacteria. The molecular compositions of neutral sugars and amino acids were investigated in DOM produced by bacteria and in DOM remaining after long-term bacterial degradation. Results from bioassay incubations (32 days) with natural and artificial seawater, indicate that the molecular compositions following bacterial degradation are not strongly influenced by the initial substrate or bacterial community. The molecular composition of neutral sugars released by bacteria was characterized by a high glucose content (47 mol%) and heterogeneous contributions from other neutral sugars (3–14 mol%). DOM remaining after bacterial degradation was characterized by a high galactose content (33 mol%), followed by glucose (22 mol%) and the remaining neutral sugars (7–11 mol%). The ratio of D-amino acids to L-amino acids increased during the experiments as a response to bacterial degradation, and after 32 days the D/L ratios of aspartic acid, glutamic acid, serine and alanine reached around 0.79, 0.32, 0.30 and 0.51 in all treatments, respectively. The striking similarity in neutral sugar and amino acid compositions between natural and artificial seawater samples, suggests that the microbial carbon pump also applies for neutral sugars and amino acids and that bacterially-produced biomolecules persist for long periods in the ocean.

Description: Atmospheric inversion of the surface carbon flux with consideration of the spatial distributions of US crop production and consumption Biogeosciences Discussions, 11, 6069-6117, 2014 Author(s): J. M. Chen, J. W. Fung, G. Mo, F. Deng, and T. O. West In order to improve quantification of the spatial distribution of carbon sinks and sources in the conterminous USA, we conduct a~nested global atmospheric inversion with consideration of the spatial information of crop production and consumption. Spatially distributed county-level cropland net primary productivity, harvested biomass, soil carbon change, and human and livestock consumption data over the conterminous USA are used for this purpose. Time-dependent Bayesian synthesis inversions are conducted based on CO 2 observations at 210 stations to infer CO 2 fluxes globally at monthly time steps with a nested focus on 30 regions in North America. Prior land surface carbon fluxes are first generated using a biospheric model, and the inversions are constrained using prior fluxes with and without adjustments for crop production and consumption over the 2002–2007 period. After these adjustments, the inverted regional carbon sink in the US Midwest increases from 0.25 ± 0.03 Pg C yr −1 to 0.42 ± 0.13 Pg C yr −1 , whereas the large sink in the US Southeast forest region is weakened from 0.41 ± 0.12 Pg C yr −1 to 0.29 ± 0.12 Pg C yr −1 . These adjustments also reduce the inverted sink in the West region from 0.066 ± 0.04 Pg C yr −1 to 0.040 ± 0.02 Pg C yr −1 because of high crop consumption and respiration by humans and livestock. The general pattern of sink increase in crop production areas and sink decreases (or source increases) in crop consumption areas highlights the importance of considering the lateral carbon transfer in crop products in atmospheric inverse modeling, which provides an atmospheric perspective of the overall carbon balance of a region.

Description: Biogeochemical variability in the equatorial Indian Ocean during the monsoon transition Biogeosciences Discussions, 11, 6185-6219, 2014 Author(s): P. G. Strutton, V. J. Coles, R. R. Hood, R. J. Matear, M. J. McPhaden, and H. E. Phillips In this paper we examine time-series measurements of near-surface chlorophyll concentration from a mooring that was deployed at 80.5° E on the equator in the Indian Ocean in 2010. These data reveal at least six striking spikes in chlorophyll in October through December, with approximately 2 week periodicity, that coincide with the development of the fall Wyrtki jets during the transition between the summer and winter monsoons. Concurrent meteorological and in situ physical measurements from the mooring reveal that the chlorophyll pulses are associated with intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as occurs in the Atlantic and Pacific Oceans, since eastward winds that force Wyrtki jet intensification should drive downwelling. The chlorophyll spikes could be explained by two alternative mechanisms: (1) turbulent entrainment of nutrients and/or chlorophyll from across the base of the mixed layer by wind stirring or Wyrtki jet-induced shear instability; or (2) enhanced horizontal advection of high chlorophyll concentrations into the convergent equatorial zone. The first mechanism is supported by the phasing and amplitude of the relationship between wind stress and chlorophyll, which suggests that the chlorophyll spikes are the result of turbulent entrainment driven by synoptic zonal wind events. The second mechanism is supported by satellite chlorophyll observations that reveal a clear connection between the increased chlorophyll concentrations at the mooring location and larger-scale topographic wake effects from the Chagos–Lacadive Ridge upstream. The biweekly periodicity of the chlorophyll spikes appears to be related to the presence of mixed Rossby-gravity waves, also known as Yanai waves, which can be seen throughout the time-series as a biweekly periodicity in the meridional velocities with upward phase propagation. Consistent with hypothesis 2, eastward flows over the Chagos–Lacadive Ridge generate high chlorophyll concentrations to the north of the equator and periodic southward advection in the meridional flows associated with Yanai waves produces the chlorophyll spikes that are observed in the mooring record. Yanai waves may also contribute to vertical shear across the base of the mixed layer that could help support entrainment. The OFAM3 eddy-resolving model suggests that both of our proposed mechanisms may be important. Climatological satellite chlorophyll data show that the elevated chlorophyll concentrations in this region are consistently observed year after year and so are reflective of recurring large-scale wind and circulation-induced productivity enhancement in the central equatorial Indian Ocean.

Description: Reduction of ferrihydrite with adsorbed and coprecipitated organic matter: microbial reduction by Geobacter bremensis vs. abiotic reduction by Na-dithionite Biogeosciences Discussions, 11, 6039-6067, 2014 Author(s): K. Eusterhues, A. Hädrich, J. Neidhardt, K. Küsel, T. F. Keller, K. D. Jandt, and K. U. Totsche Ferrihydrite (Fh) is a widespread poorly crystalline Fe oxide which becomes easily coated by natural organic matter (OM) in the environment. This mineral-bound OM entirely changes the mineral surface properties and therefore the reactivity of the original mineral. Here, we investigated the reactivity of 2-line Fh, Fh with adsorbed OM and Fh coprecipitated with OM towards microbial and abiotic reduction of Fe(III). As a surrogate for dissolved soil OM we used a water extract of a Podzol forest floor. Fh-OM associations with different OM-loadings were reduced either by Geobacter bremensis or abiotically by Na-dithionite. Both types of experiments showed decreasing initial Fe reduction rates and decreasing degrees of reduction with increasing amounts of mineral-bound OM. At similar OM-loadings, coprecipitated Fhs were more reactive than Fhs with adsorbed OM. The difference can be explained by the smaller crystal size and poor crystallinity of such coprecipitates. At small OM loadings this led to even faster Fe reduction rates than found for pure Fh. The amount of mineral-bound OM also affected the formation of secondary minerals: goethite was only found after reduction of OM-free Fh and siderite was only detected when Fhs with relatively low amounts of mineral-bound OM were reduced. We conclude that direct contact of G. bremensis to the Fe oxide mineral surface was inhibited when blocked by OM. Consequently, mineral-bound OM shall be taken into account besides Fe(II) accumulation as a further widespread mechanism to slow down reductive dissolution.

Description: A model–data intercomparison of simulated runoff in the contiguous United States: results from the North America Carbon Regional and Continental Interim-Synthesis Biogeosciences Discussions, 11, 1801-1826, 2014 Author(s): C. R. Schwalm, D. N. Huntinzger, R. B. Cook, Y. Wei, I. T. Baker, R. P. Neilson, B. Poulter, P. Caldwell, G. Sun, H. Q. Tian, and N. Zeng Significant changes in the water cycle are expected under current global environmental change. Robust assessment of these changes at global scales is confounded by shortcomings in the observed record. Modeled assessments yield conflicting results which are linked to differences in model structure and simulation protocol. Here we compare simulated runoff from six terrestrial biosphere models (TBMs), five reanalysis products, and one gridded surface station product with observations from a network of stream gauges in the contiguous United States (CONUS) from 2001 to 2005. We evaluate the consistency of simulated runoff with stream gauge data at the CONUS and water resource region scale, as well as examining similarity across TBMs and reanalysis products at the grid cell scale. Mean runoff across all simulated products and regions varies widely (range: 71–356 mm yr -1 ) relative to observed continental-scale runoff (209 mm yr -1 ). Across all 12 products only two are within 10% of the observed value and only four exhibit Nash–Sutcliffe efficiency values in excess of 0.8. Region-level mismatch exhibits a weak pattern of overestimation in western and underestimation in eastern regions; although two products are systematically biased across all regions. In contrast, bias in a temporal sense, within region by water year, is highly consistent. Although gridded composite TBM and reanalysis runoff show some regional similarities for 2001–2005 with CONUS means, individual product values are highly variable. To further constrain simulated runoff and to link model-observation mismatch to model structural characteristics would require watershed-level simulation studies coupled with river routing schemes, standardized forcing data, and explicit consideration of water cycle management.

Description: Insights into oxygen transport and net community production in sea ice from oxygen, nitrogen and argon concentrations Biogeosciences Discussions, 11, 2045-2081, 2014 Author(s): J. Zhou, B. Delille, F. Brabant, and J.-L. Tison We present the evolution of O 2 standing stocks, saturation levels and concentrations in landfast sea ice, collected in Barrow (Alaska), from February to June 2009. The comparison of the standing stocks and saturation levels of O 2 against those of N 2 and Ar suggests that the dynamic of O 2 in sea ice strongly depends on physical processes (gas incorporation and subsequent transport). We then discuss on the use of O 2 / Ar and O 2 / N 2 to correct for the physical contribution and to determine the biological contribution (NCP) to O 2 supersaturations. We conclude that O 2 / Ar suits better than O 2 / N 2 , because O 2 / N 2 is more sensitive due to the relative abundance of O 2 , N 2 and Ar, and less biased when gas bubble formation and gas diffusion are maximized. We further estimate the NCP in the impermeable layers during ice growth and in the permeable layers during ice decay. Our results indicate that NCP contributed to a~release of carbon to the atmosphere in the upper ice layers, but to an uptake of carbon at sea ice bottom. Overall, seawater (rather than the atmosphere) may be the main supplier of carbon for sea ice microorganisms.

Description: An experimental study on the effects of nutrient enrichment on organic carbon storage in western Pacific oligotrophic gyre Biogeosciences Discussions, 11, 2973-2991, 2014 Author(s): J. Liu, N. Jiao, and K. Tang Carbon sequestration in the ocean is of great concern with respect to the mitigation of global warming. How to hold the fixed organic carbon in the presence of tremendous heterotrophic microorganisms in marine environments is the central issue. We have previously hypothesized that excessive nutrients would ultimately decrease the storage of organic carbon in marine environments. To test it out, a series of in situ nutrient enrichment incubation experiments were conducted at a site (17.59° N, 127.00° E) within the Western Pacific oligotrophic gyre. Five treatments were employed: glucose or algal exudation organic material (EOM) and nitrate and phosphate were added alone or in combination to approximate final concentrations of 10 μmol C kg −1 , 1 μmol N kg −1 and 0.11 μmol P kg −1 respectively. The results showed that the dissolved organic carbon (DOC) consumption rates and bacterial community specific growth rates were enhanced by inorganic nutrients enrichment treatments during the initial 48 h incubation. At the end of 14 days incubation, about 1/3 (average 3.29 μmol C kg −1 ) more organic carbon was respired from the glucose enriched incubation with addition of inorganic nutrients compared to that without addition of inorganic nutrients. In the case no essential nutrients were available, even glucose could not be efficiently used by bacteria and thus remained in the environment. These results suggest that repletion of inorganic nutrients has negative impacts on carbon preservation, presumably due to elevated nutrient-stimulated bacterial metabolism and respiration, which is meaningful for potential coastal water management and worth for further studies.

Description: Molecular insights into the microbial formation of marine dissolved organic matter: recalcitrant or labile? Biogeosciences Discussions, 11, 3065-3111, 2014 Author(s): B. P. Koch, G. Kattner, M. Witt, and U. Passow The degradation of marine dissolved organic matter (DOM) is an important control variable in the global carbon cycle and dependent on the DOM composition. For our understanding of the kinetics of organic matter cycling in the ocean, it is therefore crucial to achieve a mechanistic and molecular understanding of its transformation processes. A long-term microbial experiment was performed to follow the production of non-labile DOM by marine bacteria. Two different glucose concentrations and dissolved algal exudates were used as substrates. We monitored the bacterial abundance, concentrations of dissolved and particulate organic carbon (DOC, POC), nutrients, amino acids, and transparent exopolymer particles (TEP) for two years. Ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) allowed the molecular characterization of extracted DOM after 70 days and after ∼2 years of incubation. Although glucose was quickly degraded, a DOC background was generated in glucose incubations. Only 20% of the organic carbon from algal exudate was degraded within the 2 years of incubation. TEP, which are released by micro-organisms, were produced during glucose degradation but decreased within less than three weeks back to half of the maximum concentration and were below detection in all treatments after 2 years. The molecular analysis demonstrated that DOM generated during glucose degradation differed appreciably from DOM produced during the degradation of the algal exudates. Our results led to several conclusions: (i) Higher substrate levels result in a higher level of non-labile DOC which is an important prerequisite for carbon sequestration in the ocean; (ii) TEP are generated by bacteria but are also degraded rapidly, thus limiting their potential contribution to carbon sequestration; (iii) The molecular signatures of DOM derived from algal exudates or glucose after 70 days of incubation differed strongly from refractory DOM. After 2 years, however, the molecular patterns of DOM in glucose incubations were more similar to deep ocean DOM whereas the degraded exudate was still different.

Description: Physical controls on CH 4 emissions from a newly flooded subtropical freshwater hydroelectric reservoir: Nam Theun 2 Biogeosciences Discussions, 11, 3271-3317, 2014 Author(s): C. Deshmukh, D. Serça, C. Delon, R. Tardif, M. Demarty, C. Jarnot, Y. Meyerfeld, V. Chanudet, P. Guédant, W. Rode, S. Descloux, and F. Guérin In the present study, we measured CH 4 ebullition and diffusion with funnels and floating chambers in the footprint of an eddy-covariance system measuring CH 4 emissions at high frequency (30 mn) in the Nam Theun 2 Reservoir, a recently impounded (in 2008) subtropical hydroelectric reservoir located in Lao PDR, southeast Asia. The EC fluxes were very consistent with the sum of the two terms measured independently (diffusive fluxes + ebullition = EC fluxes), indicating that the EC system picked-up both diffusive fluxes and ebullition from the reservoir. The EC system permitted to evidence a diurnal bimodal pattern of CH 4 emissions anti-correlated with atmospheric pressure. During daytime, a large atmospheric pressure drop triggers CH 4 ebullition (up to 100 mmol m –2 d –1 ) whereas at night, a more moderate peak of CH 4 emission was recorded. As a consequence, fluxes during daytime were twice higher than during nighttime. A total of 4811 measurements of CH 4 ebullition with submerged funnels at a weekly/fortnightly frequency were performed. The data set covers a water depth ranging from 0.4 to 16 m, and all types of flooded ecosystems. This dataset allowed to determine that ebullition depends mostly on water level change among many other variables tested. On average, ebullition was 8.5 ± 10.5 mmol m –2 d –1 (10–90 percentile range: 0.03–21.5 mmol m –2 d –1 ) and ranged from 0–201.7 mmol m –2 d –1 . An artificial neural network model could explain up to 45% of variability of ebullition using total static pressure (sum of hydrostatic and atmospheric pressure), variations in the water level and atmospheric pressure, and bottom temperature as inputs. This model allowed extrapolation of CH 4 ebullition at the reservoir scale and performing gap-filling over four years. Our results clearly showed a very high seasonality: 50% of the yearly CH 4 ebullition occurs within four months of the warm dry season. Overall, ebullition contributed 60–80% of total emissions from the surface of the reservoir (disregarding downstream emissions) suggesting that ebullition is a major pathway in young hydroelectric reservoirs in the tropics.

Description: Global cropland monthly Gross Primary Production in the year 2000 Biogeosciences Discussions, 11, 3465-3488, 2014 Author(s): T. Chen, G. R. van der Werf, N. Gobron, E. J. Moors, and A. J. Dolman Croplands cover about 12% of the ice-free terrestrial land surface. Compared with natural ecosystems, croplands have distinct characteristics due to anthropogenic influences. Their global gross primary production (GPP) is not well constrained and estimates vary between 8.2 and 14.2 Pg C yr −1 . We quantified global cropland GPP using a light use efficiency (LUE) model, employing satellite observations and survey data of crop types and distribution. A novel step in our analysis was to assign a maximum light use efficiency estimate (ϵ* GPP ) to each of the 26 different crop types, instead of taking a uniform value as done in the past. These ϵ* GPP values were calculated based on flux tower CO 2 exchange measurements and a literature survey of field studies, and ranged from 1.20 g CMJ −1 to 2.96 g CMJ −1 . Global cropland GPP was estimated to be 11.05 Pg C yr −1 in the year 2000. Maize contributed most to this (1.55 Pg C yr −1 ), and the continent of Asia contributed most with 38.9% of global cropland GPP. In the continental United States, annual cropland GPP (1.28 Pg C yr −1 ) was close to values reported previously (1.24 Pg C yr −1 ) constrained by harvest records, but our estimates of ϵ* GPP values were much higher. Our results are sensitive to satellite information and survey data on crop type and extent, but provide a consistent and data-driven approach to generate a look-up table of ϵ* GPP for the 26 crop types for potential use in other vegetation models.

Description: Impacts of sea ice on the marine iron cycle and phytoplankton productivity Biogeosciences Discussions, 11, 2383-2418, 2014 Author(s): S. Wang, D. Bailey, K. Lindsay, K. Moore, and M. Holland Iron is a key nutrient for phytoplankton growth in the surface ocean. At high latitudes, the iron cycle is closely related to sea ice. In recent decades, Arctic sea ice cover has been declining rapidly and Antarctic sea ice has exhibited large regional trends. A significant reduction of sea ice in both hemispheres is projected in future climate scenarios. To study impacts of sea ice on the iron cycle, iron sequestration in ice is incorporated to the Biogeochemical Elemental Cycling (BEC) model. Sea ice acts as a reservoir of iron during winter and releases iron to the surface ocean in spring and summer. Simulated iron concentrations in sea ice generally agree with observations, in regions where iron concentrations are lower. The maximum iron concentrations simulated in the Arctic sea ice and the Antarctic sea ice are 192 nM and 134 nM, respectively. These values are much lower than observed, which is likely due to missing biological processes in sea ice. The largest iron source to sea ice is suspended sediments, contributing fluxes of iron of 2.2 × 10 8 mol Fe month −1 to the Arctic and 4.1 × 10 6 mol Fe month −1 to the Southern Ocean during summer. As a result of the iron flux from ice, iron concentrations increase significantly in the Arctic. Iron released from melting ice increases phytoplankton production in spring and summer and shifts phytoplankton community composition in the Southern Ocean. Simulation results for the period of 1998 to 2007 indicate that a reduction of sea ice in the Southern Ocean will have a negative influence on phytoplankton production. Iron transport by sea ice appears to be an important process bringing iron to the central Arctic. Impacts of iron fluxes from ice to ocean on marine ecosystems are negligible in the current Arctic Ocean, as iron is not typically the growth-limiting nutrient. However, it may become a more important factor in the future, particularly in the central Arctic, as iron concentrations will decrease with declining sea ice cover and transport.

Description: A stand-alone tree demography and landscape structure module for Earth system models: integration with global forest data Biogeosciences Discussions, 11, 2343-2382, 2014 Author(s): V. Haverd, B. Smith, L. P. Nieradzik, and P. R. Briggs Poorly constrained rates of biomass turnover are a key limitation of Earth system models (ESM). In light of this, we recently proposed a new approach encoded in a model called Populations-Order-Physiology (POP), for the simulation of woody ecosystem stand dynamics, demography and disturbance-mediated heterogeneity. POP is suitable for continental to global applications and designed for coupling to the terrestrial ecosystem component of any ESM. POP bridges the gap between first generation Dynamic Vegetation Models (DVMs) with simple large-area parameterisations of woody biomass (typically used in current ESMs) and complex second generation DVMs, that explicitly simulate demographic processes and landscape heterogeneity of forests. The key simplification in the POP approach, compared with second-generation DVMs, is to compute physiological processes such as assimilation at grid-scale (with CABLE or a similar land surface model), but to partition the grid-scale biomass increment among age classes defined at sub grid-scale, each subject to its own dynamics. POP was successfully demonstrated along a savanna transect in northern Australia, replicating the effects of strong rainfall and fire disturbance gradients on observed stand productivity and structure. Here, we extend the application of POP to a range of forest types around the globe, employing paired observations of stem biomass and density from forest inventory data to calibrate model parameters governing stand demography and biomass evolution. The calibrated POP model is then coupled to the CABLE land surface model and the combined model (CABLE-POP) is evaluated against leaf-stem allometry observations from forest stands ranging in age from 3 to 200 yr. Results indicate that simulated biomass pools conform well with observed allometry. We conclude that POP represents a preferable alternative to large-area parameterisations of woody biomass turnover, typically used in current ESMs.

Description: Evaluating remote sensing of deciduous forest phenology at multiple spatial scales using PhenoCam imagery Biogeosciences Discussions, 11, 2305-2342, 2014 Author(s): S. T. Klosterman, K. Hufkens, J. M. Gray, E. Melaas, O. Sonnentag, I. Lavine, L. Mitchell, R. Norman, M. A. Friedl, and A. D. Richardson Plant phenology regulates ecosystem services at local and global scales and is a sensitive indicator of global change. Estimates of phenophase transition dates, such as the start of spring or end of autumn, can be derived from sensor-based time series data at the near-surface and remote scales, but must be interpreted in terms of biologically relevant events. We use the PhenoCam archive of digital repeat photography to implement a consistent protocol for visual assessment of canopy phenology at 13 temperate deciduous forest sites throughout eastern North America, as well as to perform digital image analysis for time series-based estimates of phenology dates. We then compare these near-surface results to remote sensing metrics of phenology at the landscape scale, derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) sensors. We present a new type of curve fit, using a generalized sigmoid, to estimate phenology dates. We quantify the statistical uncertainty of phenophase transition dates estimated using this method and show that the generalized sigmoid results in less statistical uncertainty than other curve-fitting methods. Additionally, we find that dates derived from analysis of high-frequency PhenoCam imagery have smaller uncertainties than remote sensing metrics of phenology, and that dates derived from the remotely-sensed enhanced vegetation index (EVI) have smaller uncertainty than those derived from the normalized difference vegetation index (NDVI). Near-surface time series estimates for the start of spring are found to closely match visual assessment of leaf out, as well as remote sensing-derived estimates of the start of spring. However late spring and autumn phenology exhibit larger differences between near-surface and remote scales. Differences in late spring phenology between near-surface and remote scales are found to correlate with a landscape metric of deciduous forest cover. These results quantify the effect of landscape heterogeneity when aggregating to the coarser spatial scales of remote sensing, and demonstrate the importance of accurate curve fitting and vegetation index selection when analyzing and interpreting phenology time series.

Description: Temperature-dependence of planktonic metabolism in the Subtropical North Atlantic Ocean Biogeosciences Discussions, 11, 3241-3269, 2014 Author(s): L. S. García-Corral, E. Barber, A. Regaudie-de-Gioux, S. Sal, J. Holding, S. Agustí, N. Navarro, P. Serret, P. Mozetič, and C. M. Duarte The temperature dependence of planktonic metabolism in the Subtropical North Atlantic Ocean was assessed on the basis of measurements of gross primary production (GPP), community respiration (CR) and net community production (NCP), as well as experimental assessments of the response of CR to temperature manipulations. Metabolic rates were measured at 68 stations along three consecutive longitudinal transects completed during the Malaspina 2010 Expedition, in three different seasons. Temperatures gradients were observed in depth and at basin and seasonal scale. The results showed seasonal variability in the metabolic rates, being the highest rates observed during the spring transect. The overall mean integrated GPP/CR ratio was of 1.39 ± 0.27 decreasing from winter to summer and the NCP for the Subtropical North Atlantic Ocean during this cruises, was net autotrophy (NCP 〉 0) in about two-thirds of the total sampled communities (68.2%). Here, we reported the activation energies describing the temperature-dependence of planktonic community metabolism, which generally was higher for CR than for GPP in the Subtropical North Atlantic Ocean, as the metabolic theory of ecology predicts. Also, we performed an assessment of the activation energies describing the responses to in situ temperature at field ( E aCR = 1.64 ± 0.36 eV) and those derive experimentally by temperature manipulations ( E aCR = 1.45 ± 0.6 eV), which showed a great consistency.

Description: Improved estimates show large circumpolar stocks of permafrost carbon while quantifying substantial uncertainty ranges and identifying remaining data gaps Biogeosciences Discussions, 11, 4771-4822, 2014 Author(s): G. Hugelius, J. Strauss, S. Zubrzycki, J. W. Harden, E. A. G. Schuur, C. L. Ping, L. Schirrmeister, G. Grosse, G. J. Michaelson, C. D. Koven, J. A. O'Donnell, B. Elberling, U. Mishra, P. Camill, Z. Yu, J. Palmtag, and P. Kuhry Soils and other unconsolidated deposits in the northern circumpolar permafrost region store large amounts of soil organic carbon (SOC). This SOC is potentially vulnerable to remobilization following soil warming and permafrost thaw, but stock estimates are poorly constrained and quantitative error estimates were lacking. This study presents revised estimates of the permafrost SOC pool, including quantitative uncertainty estimates, in the 0–3 m depth range in soils as well as for deeper sediments (〉3 m) in deltaic deposits of major rivers and in the Yedoma region of Siberia and Alaska. The revised estimates are based on significantly larger databases compared to previous studies. Compared to previous studies, the number of individual sites/pedons has increased by a factor ×8–11 for soils in the 1–3 m depth range,, a factor ×8 for deltaic alluvium and a factor ×5 for Yedoma region deposits. Upscaled based on regional soil maps, estimated permafrost region SOC stocks are 217 ± 15 and 472 ± 34 Pg for the 0–0.3 m and 0–1 m soil depths, respectively (±95% confidence intervals). Depending on the regional subdivision used to upscale 1–3 m soils (following physiography or continents), estimated 0–3 m SOC storage is 1034 ± 183 Pg or 1104 ± 133 Pg. Of this, 34 ± 16 Pg C is stored in thin soils of the High Arctic. Based on generalised calculations, storage of SOC in deep deltaic alluvium (〉3 m to ≤60 m depth) of major Arctic rivers is estimated to 91 ± 39 Pg (of which 69 ± 34 Pg is in permafrost). In the Yedoma region, estimated 〉3 m SOC stocks are 178 +140/−146 Pg, of which 74 +54/−57 Pg is stored in intact, frozen Yedoma (late Pleistocene ice- and organic-rich silty sediments) with the remainder in refrozen thermokarst deposits (±16/84th percentiles of bootstrapped estimates). A total estimated mean storage for the permafrost region of ca. 1300–1370 Pg with an uncertainty range of 930–1690 Pg encompasses the combined revised estimates. Of this, ≤819–836 Pg is perennially frozen. While some components of the revised SOC stocks are similar in magnitude to those previously reported for this region, there are also substantial differences in individual components. There is evidence of remaining regional data-gaps. Estimates remain particularly poorly constrained for soils in the High Arctic region and physiographic regions with thin sedimentary overburden (mountains, highlands and plateaus) as well as for 〉3 m depth deposits in deltas and the Yedoma region.

Description: Bryophyte colonization history of the virgin volcanic island Surtsey, Iceland Biogeosciences Discussions, 11, 4823-4856, 2014 Author(s): G. V. Ingimundardóttir, H. Weibull, and N. Cronberg The island Surtsey was formed in a volcanic eruption south of Iceland in 1963–1967 and has since then been protected and monitored by scientists. The first two moss species were found on Surtsey as early as 1967 and several new bryophyte species were discovered every year until 1973 when regular sampling ended. Systematic bryophyte inventories in a grid of 100 m × 100 m quadrats were made in 1971 and 1972. The number of observed species almost doubled between years with 36 species found in 1971 and 72 species in 1972. Here we report results from an inventory in 2008, when every other of the grid's quadrats were searched for bryophytes. Despite lower sampling intensity than in 1972, distributional expansion and contraction of earlier colonists was revealed as well as presence of new colonists. A total of 38 species were discovered, 15 of those were not encountered in 1972 and eight had never been reported from Surtsey before ( Bryum elegans , Ceratodon heterophyllus , Didymodon rigidulus , Eurhynchium praelongum , Schistidium confertum , S. papillosum , Tortula hoppeana and T. muralis ). Habitat loss due to erosion and reduced thermal activity in combination with successional vegetation changes are likely to have played a significant role in the decline of some bryophyte species which were abundant in 1972 ( Leptobryum pyriforme , Schistidium apocarpum coll., Funaria hygrometrica , Philonotis spp., Pohlia spp, Schistidium strictum , Sanionia uncinata ) while others have continued to thrive and expand (e.g. Schistidium maritimum, Racomitrium lanuginosum, R. ericoides, R. fasciculare and Bryum argenteum ). Some species (especially Bryum spp.) benefit from the formation of new habitats, such as grassland within a gull colony, which was established in 1984. Several newcomers are rarely producing sporophytes on Iceland and unlikely to have dispersed by airborne spores. They are more likely to have been introduced to Surtsey by seagulls in the form of vegetative fragments or dispersal agents ( Bryum elegans, Didymodon rigidulus, Eurhynchium praelongum, Ceratodon heterophyllus and Ulota phyllantha ). The establishment of the gull colony also means that leakage of nutrients from the nesting area is, at least locally, downplaying the relative importance of nitrogen fixation by cyanobacteria growing in bryophyte shoots.

Description: Rapid formation of large aggregates during the spring bloom of Kerguelen Island: observations and model comparisons Biogeosciences Discussions, 11, 4949-4993, 2014 Author(s): M.-P. Jouandet, G. A. Jackson, F. Carlotti, M. Picheral, L. Stemmann, and S. Blain We recorded vertical profiles of particle size distributions (PSD, sizes ranging from 0.052 to several mm in equivalent spherical diameter) in the natural iron-fertilized bloom southeast of Kerguelen Island (Southern Ocean) from pre-bloom to early bloom stage. PSD were measured by the Underwater Vision Profiler during the Kerguelen Ocean and Plateau Compared Study cruise 2 (KEOPS 2, October–November 2011). The total particle numerical abundance was more than 4 fold higher during the early bloom phase compared to pre-bloom conditions as a result of the 2-weeks bloom development. We witnessed the rapid formation of large particles and their accumulation at the base of the mixed layer within a two days period, as indicated by changes in total particle volume ( V T ) and particle size distribution. The V T profiles suggest sinking of particles from the mixed layer to 200 m, but little export deeper than 200 m during the observation period. The results of a one dimensional particles dynamic model support coagulation as the mechanism responsible for the rapid aggregate formation and the development of the V T subsurface maxima. Comparison with KEOPS1, which investigated the same area during late summer, and previous iron fertilization experiments highlights physical aggregation as the primary mechanism for large particulate production during the earlier phase of iron fertilized bloom and its export from the surface mixed layer.

Description: Chemical fate and settling of mineral dust in surface seawater after atmospheric deposition observed from dust seeding experiments in large mesocosms Biogeosciences Discussions, 11, 4909-4947, 2014 Author(s): K. Desboeufs, N. Leblond, T. Wagener, E. B. Nguyen, and C. Guieu We report here the elemental composition of sinking particles in sediment traps and in the water column following 4 artificial mineral dust seedings (representing a flux of 10 g m −2 ) in mesocosms, simulating dry or wet dust deposition into oligotrophic marine waters. These data were used to examine the rates and mechanisms of total mass, particulate organic carbon (POC) and elemental (Al, Ba, Ca, Co, Cu, Fe, K, Li, Mg, Mn, Mo, N, Nd, P, S, Sr and Ti) transfer from the surface to the sediment traps after dust deposition. The dust additions were carried out with fresh or artificially aged dust (i.e. enriched in nitrate and sulfate by mimicking cloud processing) for various biogeochemical conditions, enabling us to test the effect of these parameters on the chemical evolution and settling of dust after deposition. Whatever the type of seeding (using fresh dust to simulate dry deposition or artificially aged dust to simulate wet deposition), the dust was predominant in the particulate phase in the sediment traps at the bottom of mesocosms and within the water column during each experiment. 15% of initial dust mass was dissolved in the water column in the first 24 h after seeding. For artificially aged dust, this released fraction was mainly nitrate, sulfate and calcium and hence represented a significant source of new N for the marine biota. Except for Ca, S and N, the elemental composition of dust particles was constant during their settling, showing the relevance of using interelemental ratios, such as Ti/Al or Ba/Al as proxy of lithogenic fluxes or of productivity. After 7 days, between 30 and 68% of added dust was still in suspension in the mesocosms depending on the experiment. This difference in the dust settling was directly associated to a difference in POC export, since POC fluxes were highly correlated to dust lithogenic fluxes signifying a ballast effect of dust. The highest fraction of remaining dust in the mesocosm at the end of the experiment was found when the lowest chl a increase was observed, and inversely. This suggests a high interaction between a fertilizing effect of dust, a ballast effect, and POC fluxes. Our data emphasize a typical ratio Lithogenic/POC fluxes around 30 which could be used as reference to estimate the POC export triggered by wet dust deposition event. The elemental fluxes associated to the dust settling presented in this paper constitute also an original database on the export of atmospheric metals in a case of dry or wet dust deposition event.

Description: Remotely sensed land-surface energy fluxes at sub-field scale in heterogeneous agricultural landscape and coniferous plantation Biogeosciences Discussions, 11, 4857-4908, 2014 Author(s): R. Guzinski, H. Nieto, R. Jensen, and G. Mendiguren In this study we evaluate a methodology for disaggregating land surface energy fluxes estimated with the Dual Time Difference (DTD) model which uses the day and night polar orbiting satellites observations of Land Surface Temperature (LST) as a remotely sensed input. The DTD model is run with MODIS input data at a spatial resolution of around 1 km while the disaggregation uses Landsat observations of LST to produce fluxes at a nominal spatial resolution of 30 m. The higher resolution modeled fluxes can be directly compared against eddy-covariance based flux tower measurements to ensure more accurate model validation and also provide a better visualization of fluxes' spatial patterns in heterogeneous areas allowing for development of, for example, more efficient irrigation practices. The disaggregation technique is evaluated in an area covered by the Danish Hydrological Observatory (HOBE), in the west of the Jutland peninsula, and the modeled fluxes are compared against measurements from two flux towers: first one in a heterogeneous agricultural landscape and second one in a homogeneous conifer plantation. The results indicate that the disaggregated fluxes have greatly improved accuracy as compared to high resolution fluxes derived directly with Landsat data without the disaggregation. At the agricultural site the disaggregated fluxes display negligible bias and almost perfect correlation ( r 〉 0.90) with Eddy Covariance based measurements, while at the plantation site the results are encouraging but not ideal. In addition we introduce a modification to the DTD model by replacing the "parallel" configuration of the resistances to sensible heat exchange by the "series" configuration. The later takes into account the in-canopy air temperature and substantially improves the accuracy of the DTD model.

Description: Biophysical controls on net ecosystem CO 2 exchange over a semiarid shrubland in northwest China Biogeosciences Discussions, 11, 5089-5122, 2014 Author(s): X. Jia, T. S. Zha, B. Wu, Y. Q. Zhang, J. N. Gong, S. G. Qin, G. P. Chen, S. Kellomäki, and H. Peltola The carbon (C) cycling in semiarid and arid areas remains largely unexplored, despite the wide distribution of drylands globally. Rehabilitation practices have been carried out in many desertified areas, but information on the C sequestration potential of recovering vegetation is still largely lacking. Using the eddy-covariance technique, we measured the net ecosystem CO 2 exchange (NEE) over a recovering shrub ecosystem in northwest China throughout 2012 in order to (1) quantify NEE and its components, (2) examine the dependence of C fluxes on biophysical factors at multiple timescales. The annual budget showed a gross ecosystem productivity (GEP) of 456 ± 8 g C m −2 yr −1 and an ecosystem respiration ( R e ) of 379 ± 3 g C m −2 yr −1 , resulting in a net C sink of 77 ± 7 g C m −2 yr −1 . The maximum daily NEE, GEP and R e were −4.7, 6.8 and 3.3 g C m −2 day −1 , respectively. Both the maximum C assimilation rate (i.e., at optimum light intensity) and the quantum yield varied strongly over the growing season, being higher in summer and lower in spring and autumn. At the half-hourly scale, water stress exerted a major control over daytime NEE, and interacted with heat stress and photoinhibition in constraining C fixation by the vegetation. Low soil moisture also reduced the temperature sensitivity of R e ( Q 10 ). At the synoptic scale, rain events triggered immediate pulses of C release from the ecosystem, followed by peaks of CO 2 uptake 1–2 days later. Over the entire growing season, leaf area index accounted for 45 and 65% of the seasonal variation in NEE and GEP, respectively. There was a linear dependence of daily R e on GEP, with a slope of 0.34. These results highlight the role of abiotic stresses and their alleviation in regulating C cycling in the face of an increasing frequency and intensity of extreme climatic events.

Description: Disentangling residence time and temperature sensitivity of microbial decomposition in a global soil carbon model Biogeosciences Discussions, 11, 4995-5021, 2014 Author(s): J.-F. Exbrayat, A. J. Pitman, and G. Abramowitz Recent studies have identified the first-order parameterization of microbial decomposition as a major source of uncertainty in simulations and projections of the terrestrial carbon balance. Here, we use a reduced complexity model representative of the current state-of-the-art parameterization of soil organic carbon decomposition. We undertake a systematic sensitivity analysis to disentangle the effect of the time-invariant baseline residence time ( k ) and the sensitvity of microbial decomposition to temperature ( Q 10 ) on soil carbon dynamics at regional and global scales. Our simulations produce a range in total soil carbon at equilibrium of ~ 592 to 2745 Pg C which is similar to the ~ 561 to 2938 Pg C range in pre-industrial soil carbon in models used in the fifth phase of the Coupled Model Intercomparison Project. This range depends primarily on the value of k , although the impact of Q 10 is not trivial at regional scales. As climate changes through the historical period, and into the future, k is primarily responsible for the magnitude of the response in soil carbon, whereas Q 10 determines whether the soil remains a sink, or becomes a source in the future mostly by its effect on mid-latitudes carbon balance. If we restrict our simulations to those simulating total soil carbon stocks consistent with observations of current stocks, the projected range in total soil carbon change is reduced by 42% for the historical simulations and 45% for the future projections. However, while this observation-based selection dismisses outliers it does not increase confidence in the future sign of the soil carbon feedback. We conclude that despite this result, future estimates of soil carbon, and how soil carbon responds to climate change should be constrained by available observational data sets.

Description: Estimating the carbon dynamics of South Korean forests from 1954 to 2012 Biogeosciences Discussions, 11, 5023-5052, 2014 Author(s): J. Lee, T. K. Yoon, S. Han, S. Kim, M. J. Yi, G. S. Park, C. Kim, R. Kim, and Y. Son Forests play an important role in the global carbon (C) cycle, and the South Korean forests also contribute to this global C cycle. While the South Korean forest ecosystem was almost completely destroyed by exploitation and the Korean War, it has successfully recovered because of national-scale reforestation programs since 1973. There have been several studies on the estimation of C stocks and balances in the South Korean forests over the past decades. However, a retrospective long-term study including biomass and dead organic matter (DOM) C and validating DOM C is still insufficient. Accordingly, we estimated the C stocks and balances of both biomass and DOM C during 1954–2012 using a~process-based model, the Korean Forest Soil Carbon model, and the 5th Korean National Forest Inventory (NFI) report. Validation processes were also conducted based on the 5th NFI and statistical data. Simulation results showed that the biomass C stocks increased from 36.4 to 440.4 Tg C and sequestered C at a rate of 7.0 Tg C yr −1 during 1954–2012. The DOM C stocks increased from 386.0 to 463.1 Tg C and sequestered C at a rate of 1.3 Tg C yr −1 during the same period. The estimates of biomass and DOM C stocks agreed well with observed C stock data. The annual net biome production (NBP) during 1954–2012 was 141.3 g C m −2 yr −1 , which increased from −8.8 to 436.6 g C m −2 yr −1 in 1955 and 2012, respectively. Compared to forests in other countries and global forests, the annual C sink rate of South Korean forests was much lower, but the NBP was much higher. Our results could provide the forest C dynamics in South Korean forests before and after the onset of reforestation programs.

Description: Mangrove habitats provide refuge from climate change for reef-building corals Biogeosciences Discussions, 11, 5053-5088, 2014 Author(s): K. K. Yates, C. S. Rogers, J. J. Herlan, G. R. Brooks, N. A. Smiley, and R. A. Larson Risk analyses indicate that more than 90% of the world's reefs will be threatened by climate change and local anthropogenic impacts by the year 2030 under "business as usual" climate scenarios. Increasing temperatures and solar radiation cause coral bleaching that has resulted in extensive coral mortality. Increasing carbon dioxide reduces seawater pH, slows coral growth, and may cause loss of reef structure. Management strategies include establishment of marine protected areas with environmental conditions that promote reef resiliency. However, few resilient reefs have been identified, and resiliency factors are poorly defined. Here we characterize the first natural, non-reef, coral refuge from thermal stress and ocean acidification and identify resiliency factors for mangrove–coral habitats. We measured diurnal and seasonal variations in temperature, salinity, photosynthetically active radiation (PAR), and seawater chemistry; characterized substrate parameters; and examined water circulation patterns in mangrove communities where scleractinian corals are growing attached to and under mangrove prop roots in Hurricane Hole, St. John, US Virgin Islands. Additionally, we inventoried the coral species and quantified incidences of coral bleaching, mortality and recovery for two major reef-building corals, Colpophyllia natans and Diploria labyrinthiformis , growing in mangrove shaded and exposed (unshaded) areas. At least 33 species of scleractinian corals were growing in association with mangroves. Corals were thriving in low-light (more than 70% attenuation of incident PAR) from mangrove shading and at higher temperatures than nearby reef tract corals. A higher percentage of C. natans colonies was living shaded by mangroves, and no shaded colonies bleached. Fewer D. labyrinthiformis colonies were shaded by mangroves, however more unshaded colonies bleached. A combination of substrate and habitat heterogeniety, proximity of different habitat types, hydrographic conditions, and biological influences on seawater chemistry generate chemical conditions that buffer against ocean acidification. This previously undocumented refuge for corals provides evidence for adaptation of coastal organisms and ecosystem transition due to recent climate change. Identifying and protecting other natural, non-reef coral refuges is critical for sustaining corals and other reef species into the future.

Description: Methanotrophic activity and bacterial diversity in volcanic-geothermal soils at Pantelleria island (Italy) Biogeosciences Discussions, 11, 5147-5178, 2014 Author(s): A. L. Gagliano, W. D'Alessandro, M. Tagliavia, F. Parello, and P. Quatrini Volcanic and geothermal systems emit endogenous gases by widespread degassing from soils, including CH 4 , a greenhouse gas twenty-five times as potent as CO 2 . Recently, it has been demonstrated that volcanic/geothermal soils are source of methane, but also sites of methanotrophic activity. Methanotrophs are able to consume 10–40 Tg of CH 4 a −1 and to trap more than 50% of the methane degassing through the soils. We report on methane microbial oxidation in the geothermally most active site of Pantelleria island (Italy), Favara Grande, whose total methane emission was previously estimated in about 2.5 t a −1 . Laboratory incubation experiments with three top-soil samples from Favara Grande indicated methane consumption values up to 950 ng g −1 dry soil h −1 . One of the three sites, FAV2, where the highest oxidation rate was detected, was further analysed on a vertical soil profile and the maximum methane consumption was measured in the top-soil layer but values 〉 100 ng g −1 h −1 were maintained up to a depth of 15 cm. The highest consumption rate was measured at 37 °C, but a still recognizable consumption at 80 °C (〉 20 ng g −1 h −1 ) was recorded. In order to estimate the bacterial diversity, total soil DNA was extracted from Favara Grande and analysed using a Temporal Temperature Gradient gel Electrophoresis (TTGE) analysis of the amplified bacterial 16S rRNA gene. The three soil samples were probed by PCR using standard proteobacterial primers and newly designed verrucomicrobial primers targeting the unique methane monooxygenase gene pmoA ; the presence of methanotrophs was detected in sites FAV2 and FAV3, but not in FAV1, where harsher chemical-physical conditions and negligible methane oxidation were detected. The pmoA gene libraries from the most active site FAV2 pointed out a high diversity of gammaproteobacterial methanotrophs distantly related to Methylococcus/Methylothermus genera and the presence of the newly discovered acido-thermophilic methanotrophs Verrucomicrobia. Alphaproteobacteria of the genus Methylocystis were isolated from enrichment cultures, under a methane containing atmosphere at 37 °C. The isolates grow at pH 3.5–8 and temperatures of 18–45 °C, and show a methane oxidation rate of ~ 450 μmol mol −1 h −1 . Soils from Favara Grande showed the largest diversity of methanotrophic bacteria until now detected in a geothermal soil. While methanotrophic Verrucomicrobia are reported to dominate highly acidic geothermal sites, our results suggest that slightly acidic soils, in high enthalpy geothermal systems, host a more diverse group of both culturable and uncultivated methanotrophs.

Description: Distribution of ammonia oxidizers in relation to vegetation characteristics in the Qilian Mountains, northwestern China Biogeosciences Discussions, 11, 5123-5146, 2014 Author(s): X. S. Tai, W. L. Mao, G. X. Liu, T. Chen, W. Zhang, X. K. Wu, H. Z. Long, B. G. Zhang, and T. P. Gao Nitrogen is the major limiting nutrient in cold environments, and its availability is strongly dependent on nitrification. However, microbial communities driving this process remain largely uncharacterized in alpine meadow soils in northwestern China, namely those catalyzing the rate-limiting step of ammonia oxidation. In this study, ammonia-oxidizing communities in alpine meadow soils were characterized by real-time PCR and clone sequencing by targeting on amoA genes, which putatively encode ammonia monooxygenase subunit A. The results demonstrated that ammonia-oxidizing archaea (AOA) outnumbered ammonia-oxidizing bacteria (AOB) in the alpine meadow soils. Most of the AOA phylotypes detected in the study region fell within typical Group I.1b of Thaumarchaeota. Interestingly, a new ammonia-oxidizing archaeal group named " Kobresia meadow soil group" was found. Phylogenetic analysis of AOB communities exhibited a dominance of Nitrosospira -like sequences affiliated to beta-Proteobacteria. Compared with other alpine environments, Qilian Mountains had a great phylogenetic diversity of ammonia oxidizers. Principal Component Analysis (PCA) analysis showed that distinct AOA/AOB phylotype groups were attributed to different meadow types, reflecting an overall distribution of ammonia-oxidizing communities associated with meadow types. Redundancy Analysis (RDA) analysis showed that Axis 1 (90.9%) together with Axis 2 (9.1%) explained all the variables while Axis 1 exhibited a significant explanatory power. So that vegetation coverage mostly correlated to Axis 1 was the most powerful environmental factor in the study region. Characteristics of ammonia-oxidizing communities showed a close association with vegetation coverage.

Description: Flexible C : N ratio enhances metabolism of large phytoplankton when resource supply is intermittent Biogeosciences Discussions, 11, 5179-5214, 2014 Author(s): D. Talmy, J. Blackford, N. J. Hardman-Mountford, L. Polimene, M. J. Follows, and R. J. Geider Phytoplankton cell size influences particle sinking rate, food web interactions and biogeographical distributions. We present a model in which the uptake, storage and assimilation of nitrogen and carbon are explicitly resolved in different sized phytoplankton cells. In the model, metabolism and cellular C : N ratio are influenced by accumulation of carbon polymers such as carbohydrate and lipid, which is greatest when cells are nutrient starved, or exposed to high light. Allometric relations and empirical datasets are used to constrain the range of possible C : N, and indicate larger cells can accumulate significantly more carbon storage compounds than smaller cells. When forced with extended periods of darkness combined with brief exposure to saturating irradiance, the model predicts organisms large enough to accumulate significant carbon reserves may on average synthesize protein and other functional apparatus up to five times faster than smaller organisms. The advantage of storage in terms of average daily protein synthesis rate is greatest when modeled organisms were previously nutrient starved, and carbon storage reservoirs saturated. Small organisms may therefore be at a disadvantage in terms of average daily growth rate in environments that involve prolonged periods of darkness and intermittent nutrient limitation. We suggest this mechanism is a significant constraint on phytoplankton C : N variability and cell size distribution in different oceanic regimes.

Description: Trace elements in shells of common gastropods in the near vicinity of a natural CO 2 vent: no evidence of pH-dependent contamination Biogeosciences Discussions, 11, 5215-5237, 2014 Author(s): J. B. McClintock, C. D. Amsler, M. O. Amsler, A. Duquette, R. A. Angus, J. M. Hall-Spencer, and M. Milazzo There is concern that the use of natural volcanic CO 2 vents as analogs for studies of the impacts of ocean acidification on marine organisms are biased due to physiochemical influences other than seawater pH alone. One issue that has been raised is whether potentially harmful trace elements in sediments that are rendered more soluble and labile in low pH environments are made more bioavailable, and sequestered in the local flora and fauna at harmful levels. In order to evaluate this hypothesis, we analyzed the concentrations of trace elements in shells (an established proxy for tissues) of four species of gastropods (two limpets, a topshell and a whelk) collected from three sites in Levante Bay, Vulcano Island. Each sampling site increased in distance from the primary CO 2 vent and thus represented low, moderate, and ambient seawater pH conditions. Concentrations of As, Cd, Co, Cr, Hg, Mo, Ni, Pb, and V measured in shells using ICP-OES were below detection thresholds for all four gastropod species at all three sites. However, there were measurable concentrations of Sr, Mn, and U in the shells of the limpets Patella caerulea , P. rustica , and the snail Osilinus turbinatus , and similarly, Sr, Mn, U, and also Zn in the shells of the whelk Hexaplex trunculus . Levels of these elements were within the ranges measured in gastropod shells in non-polluted environments, and with the exception of U in the shells of P. caerulea , where the concentration was significantly lower at the collecting site closest to the vent (low pH site), there were no site-specific spatial differences in concentrations for any of the trace elements in shells. Thus trace element enhancement in sediments in low-pH environments was not reflected in greater bioaccumulations of potentially harmful elements in the shells of common gastropods.

Description: CO 2 and nutrient-driven changes across multiple levels of organization in Zostera noltii ecosystems Biogeosciences Discussions, 11, 5239-5274, 2014 Author(s): B. Martínez-Crego, I. Olivé, and R. Santos Increasing evidence emphasizes that the effects of human impacts on ecosystems must be investigated using designs that incorporate the responses across levels of biological organization as well as the effects of multiple stressors. Here we implemented a mesocosm experiment to investigate how the effects of CO 2 enrichment and its interaction with eutrophication, scale-up from changes in primary producers at the individual- (biochemistry) or population-level (production, reproduction, and/or abundance) to higher levels of community (macroalgae abundance, herbivory, and global metabolism) and ecosystem organization (detritus release and carbon sink capacity). The responses of Zostera noltii seagrass meadows growing in low- and high- nutrient field conditions were compared. In both meadows, the effect of elevated CO 2 levels was mediated by epiphyte proliferation (mostly the cyanobacterium Microcoleus spp.), but not through changes in plant biochemistry or population-level traits. In the low-nutrient meadow, epiphyte proliferation suppressed the CO 2 benefits on Z. noltii leaf production and led to increased detritus and decreased organic matter in sediment. Faster and stronger responses to nutrients than to CO 2 were observed. Nutrient addition enhanced the nutritional quality of Z. noltii (high N, low C : N and phenolics) and the loss of leaves and shoots, while promoted the proliferation of pennate diatoms and purple bacteria. These changes led to a reduced sediment organic matter, but had no significant effects on herbivory nor on community metabolism. Interestingly, the interaction with CO 2 attenuated eutrophication effects. In the high-nutrient meadow, a striking shoot decline caused by amphipod overgrazing was observed, with no response to CO 2 and nutrient additions. Our results reveal that under future scenarios of CO 2 , the responses of seagrass ecosystems will be complex, being mediated by epiphyte proliferation rather than by effects on plant biochemistry. The multi-level responses of the system to nutrients overwhelmed those to CO 2 enrichment, but the interaction between stressors reduced the effects of eutrophication. Both, CO 2 and nutrient enrichments can reduce the carbon sink capacity of seagrass meadows.

Description: Comparing the influence of net and gross anthropogenic land use and land cover changes on the carbon cycle in the MPI-ESM Biogeosciences Discussions, 11, 5443-5469, 2014 Author(s): S. Wilkenskjeld, S. Kloster, J. Pongratz, T. Raddatz, and C. Reick Global vegetation models traditionally treat anthropogenic land use and land cover changes (LULCC) only as the changes in vegetation cover seen from one year to the next (net transitions). This approach ignores sub-grid-scale processes such as shifting cultivation which do not affect the net vegetation distribution but which have an impact on the carbon budget. The simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) all describe LULCC using the Harmonized Land-Use Protocol. Though this protocol describes such sub-grid-scale processes (gross transitions), some of the CMIP5 models still use the traditional approach. Using JSBACH/CBALANCE – the land carbon component of the Max Planck Institute Earth System Model (MPI-ESM), this study demonstrates how this potentially leads to a severe underestimation of the carbon emissions from LULCC. Using net transitions lowers the average land use emissions from 1.44 to 0.90 Pg C yr –1 (38%) during the historical period (1850–2005) – a total lowering by 85 Pg C. The difference between the methods is smaller in the RCP-scenarios (2006–2100) but in RCP2.6 and RCP8.5 still cumulates to 30–40 Pg C (on average 0.3–0.4 Pg C yr –1 or 13–25%). In RCP4.5 essentially no difference between the methods is found. Results from models using net transitions are furthermore found to be sensitive to model resolution.

Description: Impacts of physical data assimilation on the Global Ocean Carbonate System Biogeosciences Discussions, 11, 5399-5441, 2014 Author(s): L. Visinelli, S. Masina, M. Vichi, and A. Storto Prognostic simulations of ocean carbon distribution are largely dependent on an adequate representation of physical dynamics. In this work we show that the assimilation of temperature and salinity in a coupled ocean-biogeochemical model significantly improves the reconstruction of the carbonate system variables over the last two decades. For this purpose, we use the NEMO ocean global circulation model, coupled to the Biogeochemical Flux Model (BFM) in the global PELAGOS configuration. The assimilation of temperature and salinity is included into the coupled ocean-biogeochemical model by using a variational assimilation method. The use of ocean physics data assimilation improves the simulation of alkalinity and dissolved organic carbon against the control run as assessed by comparing with independent time series and gridded datasets. At the global scale, the effects of the assimilation of physical variables in the simulation of p CO 2 improves the seasonal cycle in all basins, getting closer to the SOCAT estimates. Biases in the partial pressure of CO 2 with respect to data that are evident in the control run are reduced once the physical data assimilation is used. The root mean squared errors in the p CO 2 are reduced by up to 30% depending on the ocean basin considered. In addition, we quantify the relative contribution of biological carbon uptake on surface p CO 2 by performing another simulation in which biology is neglected in the assimilated run.

Description: Two perspectives on the coupled carbon, water, and energy exchange in the planetary boundary layer Biogeosciences Discussions, 11, 5275-5325, 2014 Author(s): M. Combe, J. Vilà-Guerau de Arellano, H. G. Ouwersloot, C. M. J. Jacobs, and W. Peters Understanding the interactions between the land surface and the atmosphere is key to model boundary-layer meteorology and cloud formation, as well as carbon cycling and crop yield. In this study we explore these interactions in the exchange of water, heat, and CO 2 in a cropland–atmosphere system at the diurnal and local scale. We thereto couple an atmospheric mixed-layer model (MXL) to two land-surface schemes, developed from two different perspectives: while one land-surface scheme (A-g s ) simulates vegetation from an atmospheric point of view, the other (GECROS) simulates vegetation from a carbon-storage point of view. We calculate surface fluxes of heat, moisture and carbon, as well as the resulting atmospheric state and boundary-layer dynamics, over a maize field in the Netherlands, for a day on which we have a rich set of observations available. Particular emphasis is placed on understanding the role of upper atmosphere conditions like subsidence, in comparison to the role of surface forcings like soil moisture. We show that the atmospheric-oriented model (MXL-A-g s ) outperforms the carbon storage-oriented model (MXL-GECROS) on this diurnal scale. This performance strongly depends on the sensitivity of the modelled stomatal conductance to water stress, which is implemented differently in each model. This sensitivity also influences the magnitude of the surface fluxes of CO 2 , water and heat (surface control), and subsequently impacts the boundary-layer growth and entrainment fluxes (upper atmosphere control), which alter the atmospheric state. These findings suggest that observed CO 2 mole fractions in the boundary layer can reflect strong influences of both the surface and upper atmospheric conditions, and the interpretation of CO 2 mole fraction variations depends on the assumed land-surface coupling. We illustrate this with a sensitivity analysis where increased subsidence, typical for periods of drought, can induce a change of 12 ppm in atmospheric CO 2 mole fractions, solely by decreasing the boundary-layer volume. The effect of such high subsidence on the Bowen ratio is of the same magnitude as induced by the depletion of soil moisture that would typically occur during a corresponding drought event. Correctly including such two-way land-surface interactions on the diurnal scale can thus potentially improve our understanding and interpretation of observed variations in atmospheric CO 2 , as well as improve crop yield forecasts by better describing the water loss and carbon gain.

Description: Comparison of seven packages that compute ocean carbonate chemistry Biogeosciences Discussions, 11, 5327-5397, 2014 Author(s): J. C. Orr, J.-M. Epitalon, and J.-P. Gattuso To study ocean acidification and the carbon cycle, marine scientists often use two measured or modeled carbonate system variables to compute others. These carbonate chemistry calculations, based on well-known thermodynamic equilibria, are now available from seven public packages: CO2SYS, csys, seacarb, swco2, CO2calc, ODV, and mocsy. We compared results from these packages using common input data and the set of equilibrium constants recommended for best practices. All packages agree within ±0.00025 units for pH and ±0.5 μmol kg −1 for CO 3 2− , and six packages agree within ±0.2 μatm for p CO 2 in terms of zonal-mean surface values. In the remaining package (csys), the surface p CO 2 variable is up to 1.4 μatm lower than in other packages, but that is because it is mislabeled. When compared to surface f CO 2 , it differs by less than 0.2 μatm. The csys deviations in f CO 2 , pH, and CO 3 2− grow with depth but remain small. Another package (swco2) also diverges significantly but only in warm deep waters as found in the Mediterranean Sea. Discrepancies between packages derive largely from their code for the equilibrium constants. Analysis of the sensitivity of each computed variable to changes in each constant showed the expected dominance of K 1 and K 2 , while also revealing comparable sensitivity to K B , e.g., with the A T – C T input pair. Best-practice formulations for K 1 and K 2 are implemented consistently among packages, except those in csys deviate slightly at depth (e.g., 0.5% larger values at 4000 db) due to its pressure corrections made on the total instead of the seawater pH scale. With more recent formulations for K 1 and K 2 designed to cover a wider range of salinities, packages disagree more, e.g., by 8 μatm in p CO 2 , 1 μmol kg −1 in CO 3 2− , and 0.006 units in pH under typical surface conditions. These discrepancies stem from packages using different sets of coefficients for the corresponding salinity dependence of the new formulations. Although each set should be equally viable after simple conversions, we show they are fundamentally inconsistent. Despite general agreement between current packages, agreement was much worse with outdated versions, e.g., differences reached up to 2.5 μatm in p CO 2 , 1.4 μmol kg −1 in CO 3 2− , and 0.007 units in pH for surface zonal means when using the best-practice constants.

Description: Methane and carbon dioxide emissions from 40 lakes along a north–south latitudinal transect in Alaska Biogeosciences Discussions, 11, 13251-13307, 2014 Author(s): A. Sepulveda-Jauregui, K. M. Walter Anthony, K. Martinez-Cruz, S. Greene, and F. Thalasso Uncertainties in the magnitude and seasonality of various gas emission modes, particularly among different lake types, limit our ability to estimate methane (CH 4 ) and carbon dioxide (CO 2 ) emissions from northern lakes. Here we assessed the relationship between CH 4 and CO 2 emission modes in 40 lakes along a latitudinal transect in Alaska to physicochemical limnology and geographic characteristics, including permafrost soil type surrounding lakes. Emission modes included Direct Ebullition, Diffusion, Storage flux, and a newly identified Ice-Bubble Storage (IBS) flux. We found that all lakes were net sources of atmospheric CH 4 and CO 2 , but the climate warming impact of lake CH 4 emissions was two times higher than that of CO 2 . Ebullition and Diffusion were the dominant modes of CH 4 and CO 2 emissions respectively. IBS, ~ 10% of total annual CH 4 emissions, is the release to the atmosphere of seasonally ice-trapped bubbles when lake ice confining bubbles begins to melt in spring. IBS, which has not been explicitly accounted for in regional studies, increased the estimate of springtime emissions from our study lakes by 320%. Geographically, CH 4 emissions from stratified, dystrophic interior Alaska thermokarst (thaw) lakes formed in icy, organic-rich yedoma permafrost soils were 6-fold higher than from non-yedoma lakes throughout the rest of Alaska. Total CH 4 emission was correlated with concentrations of phosphate and total nitrogen in lake water, Secchi depth and lake area, with yedoma lakes having higher nutrient concentrations, shallower Secchi depth, and smaller lake areas. Our findings suggest that permafrost type plays important roles in determining CH 4 emissions from lakes by both supplying organic matter to methanogenesis directly from thawing permafrost and by enhancing nutrient availability to primary production, which can also fuel decomposition and methanogenesis.

Description: Effect of reed canary grass cultivation on greenhouse gas emission from peat soil at controlled rewetting Biogeosciences Discussions, 11, 13309-13341, 2014 Author(s): S. Karki, L. Elsgaard, and P. E. Lærke Cultivation of bioenergy crops in rewetted peatland (paludiculture) is considered as a possible land use option to mitigate greenhouse gas (GHG) emissions. However, bioenergy crops like reed canary grass (RCG) can have a complex influence on GHG fluxes. Here we determined the effect of RCG cultivation on GHG emission from peatland rewetted to various extents. Mesocosms were manipulated to three different ground water levels (GWL), i.e., 0, −10 and −20 cm below the soil surface in a controlled semi-field facility. Emissions of CO 2 (ecosystem respiration, ER), CH 4 and N 2 O from mesocosms with RCG and bare soil were measured at weekly to fortnightly intervals with static chamber techniques for a period of one year. Cultivation of RCG increased both ER and CH 4 emissions, but decreased the N 2 O emissions. The presence of RCG gave rise to 69, 75 and 85% of total ER at −20, −10 and 0 cm GWL, respectively However, this difference was due to decreased soil respiration at the rising GWL as the plant-derived CO 2 flux was similar at all three GWL. For methane, 70–95% of the total emission was due to presence of RCG, with the highest contribution at −20 cm GWL. In contrast, cultivation of RCG decreased N 2 O emission by 33–86% with the major reductions at −10 and −20 cm GWL. In terms of global warming potential, the increase in CH 4 emissions due to RCG cultivation was more than off-set by the decrease in N 2 O emissions at −10 and −20 cm GWL; at 0 cm GWL the CH 4 emissions was offset only by 23%. CO 2 emissions from ER obviously were the dominant RCG-derived GHG flux, but above-ground biomass yields, and preliminary measurements of gross photosynthetic production, show that ER could be more than balanced due to the uptake of CO 2 by RCG. Our results support that RCG cultivation could be a good land use option in terms of mitigating GHG emission from rewetted peatlands, potentially turning these ecosystems into a sink of atmospheric CO 2 .

Description: Quantifying environmental stress induced emissions of algal isoprene and monoterpenes using laboratory measurements Biogeosciences Discussions, 11, 13533-13570, 2014 Author(s): N. Meskhidze, A. Sabolis, R. Reed, and D. Kamykowski We report here production rates of isoprene and monoterpene compounds (α-pinene, β-pinene, camphene and d-limonene) from six phytoplankton monocultures as a function of irradiance and temperature. Irradiance experiments were carried out for diatom strains – Thalassiosira weissflogii and Thalassiosira pseudonana ; prymnesiophyte strains – Pleurochrysis carterae ; dinoflagellate strains – Karenia brevis and Prorocentrum minimum ; cryptophyte strains – Rhodomonas salina , while temperature experiments were carried out for diatom strains – Thalassiosira weissflogii and Thalassiosira pseudonana . Phytoplankton species, incubated in a climate-controlled room, were subject to variable light (90 to 900 μmol m −2 s −1 ) and temperature (18 to 30 °C) regimes. Compared to isoprene, monoterpene emissions were an order of magnitude lower at all light and temperature levels. Emission rates are normalized by cell count and Chlorophyll a (Chl a ) content. Diatom strains were the largest emitters, with ~2x10 17 g (cell) −1 h −1 (~35 μg (g Chl a ) −1 h −1 ) for isoprene and ~5x10 −19 g (cell) −1 h −1 (~1μg (g Chl a ) −1 ) h −1 ) for α-pinene. The contribution to the total monoterpene production was ~70% from α-pinene, ~20% for d-limonene, and

Description: Satellite observations of the small-scale cyclonic eddies in the western South China Sea Biogeosciences Discussions, 11, 13515-13532, 2014 Author(s): F. Liu, S. Tang, and C. Chen High-resolution ocean color observation offers an opportunity to investigate the oceanic small-scale processes. In this study, The Medium Resolution Imaging Spectrometer (MERIS) daily 300 m data are used to study small-scale processes in the western South China Sea. It is indicated that the cyclonic eddies with horizontal scales of the order of 10 km are frequently observed during upwelling season of each year over 2004–2009. These small-scale eddies are generated in the vicinity of the southern front of the cold tongue, and then propagate eastward with a speed of approximately 12 cm s −1 . This propagation speed is consistent with the velocity of the western boundary current. As a result, the small-scale eddies keep rotating high levels of the phytoplankton away from the coastal areas, resulting in the accumulation of phytoplankton in the interior of the eddies. The generation of the small-scale eddies may be associated with strengthening of the relative movement between the rotation speed of the anticylconic mesoscale eddies and the offshore transport. With the increases of the normalized rotation speed of the anticyclonic mesoscale eddies relative to the offshore transport, the offshore current become meander under the impacts of the anticyclonic mesoscale eddies. The meandered cold tongue and instability front may stimulate the generation of the small-scale eddies. Unidirectional uniform wind along cold tongue may also contribute to the formation of the small-scale eddies.

Description: Biostratigraphic evidence of dramatic Holocene uplift of Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific Ocean Biogeosciences Discussions, 11, 13605-13622, 2014 Author(s): P. Sepúlveda, J. P. Le Roux, L. E. Lara, G. Orozco, and V. Astudillo A study of the biostratigraphy and sedimentology of Holocene deposits on Robinson Crusoe Island (RCI) on the Juan Fernández Ridge (JFR) indicates that a~dramatic but localized uplift occurred since 8000 BP, at a rate of about 8.5 mm yr −1 . In fact, supratidal flats and sand layers with marine gastropods (mostly Nerita sp.) are now exposed ca. 70 m a.s.l., and covered by transitional dunes. The last volcanic activity on RCI occurred at ca. 0.8 Ma (active hotspot located 280 km further west) and there is no sign of a compensating bulge that explains this uplift, isobaths of the sea floor instead suggesting general subsidence. However, modeling indicates that large-scale landslides followed by isostatic rebound are a viable explanation, partially reflected in the low-resolution bathymetry of the area.

Description: Nitrous oxide emission and nitrogen use efficiency in response to nitrophosphate, N-(n-butyl) thiophosphoric triamide and dicyandiamide of a wheat cultivated soil under sub-humid monsoon conditions Biogeosciences Discussions, 11, 13571-13603, 2014 Author(s): W. X. Ding, Z. M. Chen, H. Y. Yu, J. F. Luo, G. Y. Yoo, J. Xiang, H. J. Zhang, and J. J. Yuan A field experiment was designed to study the effects of nitrogen (N) source and urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) or nitrification inhibitor dicyandiamide (DCD) on nitrous oxide (N 2 O) emission and N use efficiency (NUE) in a sandy loam soil. Six treatments including no N fertilizer (control), N fertilizer urea alone (U), urea plus NBPT (NBPT), urea plus DCD (DCD), urea plus NBPT and DCD (NBPT + DCD), and nitrate-based fertilizer nitrophosphate (NP) were designed and implemented separately during the wheat growth period. Seasonal cumulative N 2 O emissions with urea alone amounted to 0.49 ± 0.12 and were significantly ( P 〈 0.05) reduced to 0.28 ± 0.03, 0.31 ± 0.01 and 0.26 ± 0.01kg N 2 O-N ha −1 by application of DCD, NBPT and NBPT + DCD, respectively. Cumulative N 2 O emissions from NP were 0.28 ± 0.01kg N 2 O-N ha −1 . A single N 2 O flux peak was identified following basal fertilization, and DCD and/or NBPT inhibition effects mainly occurred during the peak emission period. The NP application significantly ( P 〈 0.05) increased wheat yield by 12.3% and NUE from 28.8% (urea alone) to 35.9%, while urease and/or nitrification inhibitors showed a slight increase effect. Our results clearly indicated that the application of urea as basal fertilizer, but not as supplemental fertilizer, together with DCD and NBPT is an effective practice to reduce N 2 O emissions. The application of NP instead of urea would be an optimum agricultural strategy for reducing N 2 O emissions and increasing crop yield and NUE for wheat cultivation in soils of the North China Plain.

Description: Thermal-based modeling of coupled carbon, water and energy fluxes using nominal light use efficiencies constrained by leaf chlorophyll observations Biogeosciences Discussions, 11, 14133-14171, 2014 Author(s): M. A. Schull, M. C. Anderson, R. Houborg, A. Gitelson, and W. P. Kustas Recent studies have shown that estimates of leaf chlorophyll content (Chl), defined as the combined mass of chlorophyll a and chlorophyll b per unit leaf area, can be useful for constraining estimates of canopy light-use-efficiency (LUE). Canopy LUE describes the amount of carbon assimilated by a vegetative canopy for a given amount of Absorbed Photosynthetically Active Radiation (APAR) and is a key parameter for modeling land-surface carbon fluxes. A carbon-enabled version of the remote sensing-based Two-Source Energy Balance (TSEB) model simulates coupled canopy transpiration and carbon assimilation using an analytical sub-model of canopy resistance constrained by inputs of nominal LUE (β n ), which is modulated within the model in response to varying conditions in light, humidity, ambient CO 2 concentration and temperature. Soil moisture constraints on water and carbon exchange are conveyed to the TSEB-LUE indirectly through thermal infrared measurements of land-surface temperature. We investigate the capability of using Chl estimates for capturing seasonal trends in the canopy β n from in situ measurements of Chl acquired in irrigated and rain-fed fields of soybean and maize near Mead, Nebraska. The results show that field-measured Chl is non-linearly related to β n , with variability primarily related to phenological changes during early growth and senescence. Utilizing seasonally varying β n inputs based on an empirical relationship with in-situ measured Chl resulted in improvements in carbon flux estimates from the TSEB model, while adjusting the partitioning of total water loss between plant transpiration and soil evaporation. The observed Chl-β n relationship provides a functional mechanism for integrating remotely sensed Chl into the TSEB model, with the potential for improved mapping of coupled carbon, water, and energy fluxes across vegetated landscapes.