ALBERT

Description: Looking beyond stratification: a model-based analysis of the biological drivers of oxygen depletion in the North Sea Biogeosciences Discussions, 12, 12543-12610, 2015 Author(s): F. Große, N. Greenwood, M. Kreus, H. J. Lenhart, D. Machoczek, J. Pätsch, L. A. Salt, and H. Thomas The problem of low oxygen conditions, often referred to as hypoxia, occurs regularly in the North Sea, a temperate European shelf sea. Stratification represents a major process regulating the seasonal dynamics of bottom oxygen. However, lowest oxygen conditions in the North Sea do not occur in the regions of strongest stratification. This suggests that stratification is an important prerequisite for hypoxia, but that the complex interaction between hydrodynamics and the biological processes drives its development. In this study we use the ecosystem model HAMSOM-ECOHAM5 to provide a general characteristic of the different North Sea oxygen regimes, and to quantify the impact of the different physical and biological factors driving the oxygen dynamics below the thermocline and in the bottom layer. We show that the North Sea can be subdivided into three different regimes in terms of oxygen dynamics: (1) a highly productive, non-stratified coastal regime, (2) a productive, seasonally stratified regime with a small sub-thermocline volume, and (3) a productive, seasonally stratified regime with a large sub-thermocline volume, with regime 2 being highly susceptible to hypoxic conditions. Our analysis of the different processes driving the oxygen development reveals that inter-annual variations in the oxygen conditions are caused by variations in primary production, while spatial differences can be attributed to differences in stratification and water depth. In addition, we show that benthic bacteria represent the main oxygen consumers in the bottom layer, consistently accounting for more than 50 % of the overall consumption. By providing these valuable insights, we show that ecosystem models can be a useful tool for the interpretation of observations and the estimation of the impact of anthropogenic drivers on the North Sea oxygen conditions.

Description: Optimizing the impact of temperature on bio-hydrogen production from food waste and its derivatives under no pH control using statistical modelling Biogeosciences Discussions, 12, 12823-12850, 2015 Author(s): A. Sattar, C. Arslan, C. Ji, S. Sattar, K. Yousaf, and S. Hashim The effect of temperature on bio-hydrogen production by co-digestion of sewerage sludge with food waste and its two derivatives, i.e. noodle waste and rice waste, was investigated by statistical modelling. Experimental results showed that increasing temperature from mesophilic (37 °C) to thermophilic (55 °C) was an effective mean for increasing bio-hydrogen production from food waste and noodle waste, but it caused a negative impact on bio-hydrogen production from rice waste. The maximum cumulative bio-hydrogen production of 650 mL was obtained from noodle waste under mesophilic temperature condition. Most of the production was observed during 48 h of incubation that continued till 72 h of incubation, and a decline in pH during this interval was 4.3 and 4.4 from a starting value of 7 under mesophilic and thermophilic conditions, respectively. Most of glucose consumption was also observed during 72 h of incubation and the maximum consumption was observed during the first 24 h, which was the same duration where the maximum pH drop occurred. The maximum hydrogen yields of 82.47 mL VS −1 , 131.38 mL COD −1 , and 44.90 mL glucose −1 were obtained from mesophilic food waste, thermophilic noodle waste and mesophilic rice waste respectively. The production of volatile fatty acids increased with an increase in time and temperature from food waste and noodle waste reactors whereas it decreased with temperature in rice waste reactors. The statistical modelling returned good results with high values of coefficient of determination ( R 2 ) for each waste type when it was opted for the study of cumulative hydrogen production, glucose consumption and volatile fatty acid production. The 3-D response surface plots developed by the statistical models helped a lot in developing better understanding of the impact of temperature and incubation time.

Description: Stable isotope study of a new chondrichthyan fauna (Kimmeridgian, Porrentruy, Swiss Jura): an unusual freshwater-influenced isotopic composition for the hybodont shark Asteracanthus Biogeosciences Discussions, 12, 12899-12921, 2015 Author(s): L. Leuzinger, L. Kocsis, J.-P. Billon-Bruyat, S. Spezzaferri, and T. Vennemann Chondrichthyan teeth (sharks, rays and chimaeras) are mineralised in isotopic equilibrium with the surrounding water, and parameters such as water temperature and salinity can be inferred from the oxygen isotopic composition (δ 18 O p ) of their bioapatite. We analysed a new chondrichthyan assemblage, as well as teeth from bony fish (Pycnodontiformes). All specimens are from Kimmeridgian coastal marine deposits of the Swiss Jura (vicinity of Porrentruy, Ajoie district, NW Switzerland). While the overall faunal composition and the isotopic composition of bony fish are consistent with marine conditions, unusually low δ 18 O p values were measured for the hybodont shark Asteracanthus . These values are also lower compared to previously published data from older European Jurassic localities. Additional analyses on material from Solothurn (Kimmeridgian, NW Switzerland) also have comparable, low- 18 O isotopic compositions for Asteracanthus . The data are hence interpreted to represent a so far unique, freshwater-influenced isotopic composition for this shark that is classically considered as a marine genus. While reproduction in freshwater or brackish realms is established for other hybodonts, a similar behaviour for Asteracanthus is proposed here. Regular excursions into lower salinity waters can be linked to the age of the deposits and correspond to an ecological adaptation, most likely driven by the Kimmeridgian transgression and by the competition of the primitive shark Asteracanthus with the rapidly diversifying neoselachians (modern sharks).

Description: Climate change impacts on net primary production (NPP) and export production (EP) regulated by increasing stratification and phytoplankton community structure in CMIP5 models Biogeosciences Discussions, 12, 12851-12897, 2015 Author(s): W. Fu, J. Randerson, and J. K. Moore We examine climate change impacts on net primary production (NPP) and export production (sinking particulate flux; EP) with simulations from nine Earth System Models (ESMs) performed in the framework of the fifth Coupled Model Inter-comparison Project (CMIP5). Global NPP and EP are reduced considerably by the end of the century for the intense warming scenario of Representative Concentration Pathway (RCP) 8.5. Relative to the 1990s, global NPP in the 2090s is reduced by 2.3–16 % and EP by 7–18 %. The models with the largest increases in stratification (and largest relative reductions in NPP and EP) also show the largest positive biases in stratification for the contemporary period, suggesting some potential overestimation of climate impacts on NPP and EP. All of the CMIP5 models show an increase in stratification in response to surface ocean warming and freshening that is accompanied by decreases in NPP, EP, and surface macronutrient concentrations. There is considerable variability across models in the absolute magnitude of these fluxes, surface nutrient concentrations, and their perturbations by climate change, indicating large model uncertainties. The negative response of NPP and EP to stratification increases reflects a bottom-up control, as nutrient flux to the euphotic zone declines. Models with dynamic phytoplankton community structure show larger declines in EP than in NPP. This is driven by phytoplankton community composition shifts, with a reduced percentage of NPP by large phytoplankton under RCP 8.5, as smaller phytoplankton are favored under the increasing nutrient stress. Thus, projections of the NPP response to climate change in the CMIP5 models are critically dependent on the simulated phytoplankton community structure, the efficiency of the biological pump, and the resulting (highly variable) levels of regenerated production. Community composition is represented relatively simply in the CMIP5 models, and should be expanded to better capture the spatial patterns and the changes in export efficiency that are necessary for predicting climate impacts on NPP.

Description: Technical Note: A simple calculation algorithm to separate high-resolution CH 4 flux measurements into ebullition and diffusion-derived components Biogeosciences Discussions, 12, 12923-12945, 2015 Author(s): M. Hoffmann, M. Schulz-Hanke, J. Garcia Alba, N. Jurisch, U. Hagemann, T. Sachs, M. Sommer, and J. Augustin Processes driving the production, transformation and transport of methane (CH 4 ) in wetland ecosystems are highly complex. Thus, serious challenges are constitutes in terms of the mechanistic process understanding, the identification of potential environmental drivers and the calculation of reliable CH 4 emission estimates. We present a simple calculation algorithm to separate open-water CH 4 fluxes measured with automatic chambers into diffusion- and ebullition-derived components, which helps facilitating the identification of underlying dynamics and potential environmental drivers. Flux separation is based on ebullition related sudden concentration changes during single measurements. A variable ebullition filter is applied, using the lower and upper quartile and the interquartile range (IQR). Automation of data processing is achieved by using an established R-script, adjusted for the purpose of CH 4 flux calculation. The algorithm was tested using flux measurement data (July to September 2013) from a former fen grassland site, converted into a shallow lake as a result of rewetting ebullition and diffusion contributed 46 and 55 %, respectively, to total CH 4 emissions, which is comparable to those previously reported by literature. Moreover, the separation algorithm revealed a concealed shift in the diurnal trend of diffusive fluxes throughout the measurement period.

Description: Nitrogen export from a boreal stream network following forest harvesting: seasonal nitrate removal and conservative export of organic forms Biogeosciences Discussions, 12, 12061-12089, 2015 Author(s): J. Schelker, R. Sponseller, E. Ring, L. Högbom, S. Löfgren, and H. Laudon Boreal streams are under pressure from large scale disturbance by forestry. Recent scenarios predict an increase in forest production in Scandinavia to meet market demands and to mitigate higher anthropogenic CO 2 emissions. Increased fertilization and shorter forest rotations are anticipated which will likely enhance the pressure on boreal streams in the near future. Among the major environmental impacts of forest harvesting is the increased mobilization of inorganic nitrogen (N), primarily as nitrate (NO 3 - ) into surface waters. But whereas NO 3 - inputs to first-order streams have been previously described, their downstream fate and impact is not well understood. We evaluated the downstream fate of N inputs in a boreal landscape that has been altered by forest harvests over a 10 year period to estimate the effects of multiple clear-cuts on aquatic N export in a boreal stream network. Small streams showed substantial leaching of NO 3 - in response to harvests with concentrations increasing by ~ 15 fold. NO 3 - concentrations at two sampling stations further downstream in the network were strongly seasonal and increased significantly in response to harvesting at the medium size, but not at the larger stream. Nitrate removal efficiency, E r , calculated as the percentage of "forestry derived" NO 3 - that was retained within the landscape using a mass balance model was highest during the snow melt season followed by the growing season, but declined continuously throughout the dormant season. In contrast, export of organic N from the landscape indicated little removal and was essentially conservative. Overall, net removal of NO 3 - between 2008 and 2011 accounted for ~ 70 % of the total NO 3 - mass exported from harvested patches distributed across the landscape. These results highlight the capacity and limitation of N-limited terrestrial and aquatic ecosystems to buffer inorganic N mobilization that arises from multiple clear-cuts within meso-scale boreal watersheds.

Description: Hydroxy fatty acids in fresh snow samples from northern Japan: long-range atmospheric transport of Gram-negative bacteria by Asian winter monsoon Biogeosciences Discussions, 12, 13375-13397, 2015 Author(s): P. Tyagi, S. Yamamoto, and K. Kawamura Hydroxy fatty acids (FAs) in fresh snow from Sapporo, one of the heaviest snowfall regions in the world, have been studied to ascertain the airborne bacterial endotoxin concentrations and their biomass. The presence of β-hydroxy FAs (C 9 –C 28 ), constituents of Gram-negative bacteria (GNB), suggests long-range transport of soil microbes. Likewise, the occurrence of α- and ω-hydroxy FAs (C 9 –C 30 and C 9 –C 28 , respectively) in snow reveals their contribution from epicuticular waxes and soil microorganisms. Estimated endotoxin and GNB mass can aid in assessing their possible impacts on the diversity and functioning of aquatic and terrestrial ecosystems, as well as lethal effects on pedestrians through dispersal of microbes. Air mass back trajectories together with hydroxy FAs unveil their sources from Siberia, Russian Far East and North China by the Asian monsoon. This study highlights the role of fresh snow that reduces the human health risk of GNB and endotoxin by scavenging from the air.

Description: Nitrogen cycling in the subsurface biosphere: nitrate isotopes in porewaters underlying the oligotrophic North Atlantic Biogeosciences Discussions, 12, 13545-13591, 2015 Author(s): S. D. Wankel, C. Buchwald, W. Ziebis, C. B. Wenk, and M. F. Lehmann Nitrogen (N) is a key component of fundamental biomolecules. Hence, the cycling and availability of N is a central factor governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite limited organic matter delivery from overlying water, the extent and modes of nitrogen transformations have not been widely investigated. Here we use the N and oxygen (O) isotopic composition of porewater nitrate (NO 3 − ) from a site in the oligotrophic North Atlantic (IODP) to determine the extent and magnitude of microbial nitrate production (via nitrification) and consumption (via denitrification). We find that NO 3 − accumulates far above bottom seawater concentrations (∼ 21 μM) throughout the sediment column (up to ∼ 50 μM) down to the oceanic basement as deep as 90 mbsf, reflecting the predominance of aerobic nitrification/remineralization within the deep marine sediments. Large changes in the δ 15 N and δ 18 O of nitrate, however, reveal variable influence of nitrate respiration across the three sites. We use an inverse porewater diffusion–reaction model, constrained by the N and O isotope systematics of nitrification and denitrification and the porewater NO 3 − isotopic composition, to estimate rates of nitrification and denitrification throughout the sediment column. Results indicate variability of reaction rates across and within the three boreholes that are generally consistent with the differential distribution of dissolved oxygen at this site, though not necessarily with the canonical view of how redox thresholds separate nitrate regeneration from dissimilative consumption spatially. That is, we provide isotope evidence for expanded zones of co-ocurring nitrification and denitrification. The isotope biogeochemical modeling also yielded estimates for the δ 15 N and δ 18 O of newly produced nitrate (δ 15 N NTR and δ 18 O NTR ), as well as the isotope effect for denitrification ( 15 ϵ DNF ), parameters with high relevance to global ocean models of N cycling. Estimated values of δ 15 N NTR were generally lower than previously reported δ 15 N values for sinking PON in this region. We suggest that these values can be related to sedimentary N-fixation and remineralization of the newly fixed organic N. Values of δ 18 O NTR generally ranged between −2.8 and 0.0 ‰, consistent with recent estimates based on lab cultures of nitrifying bacteria. Notably, some δ 18 O NTR values were elevated, suggesting incorporation of 18 O-enriched dissolved oxygen during nitrification, and possibly indicating a tight coupling of NH 4 + and NO 2 − oxidation in this metabolically sluggish environment. Our findings indicate that the production of organic matter by in situ autotrophy (e.g., nitrification, nitrogen fixation) supply a large fraction of the biomass and organic substrate for heterotrophy in these sediments, supplementing the small organic matter pool derived from the overlying euphotic zone. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.

Description: Spring bloom onset in the Nordic Seas Biogeosciences Discussions, 12, 13631-13673, 2015 Author(s): A. Mignot, R. Ferrari, and K. A. Mork The North Atlantic spring bloom is a massive annual growth event of marine phytoplankton, tiny free-floating algae that form the base of the ocean's food web and generates a large fraction of the global primary production of organic matter. The conditions that trigger the onset of the spring bloom in the Nordic Seas, at the northern edge of the North Atlantic, are studied using in-situ data from five bio-optical floats released above the Arctic Circle. It is often assumed that spring blooms start as soon as phytoplankton cells daily irradiance is sufficiently abundant that division rates exceed losses. The bio-optical float data instead suggest the tantalizing hypothesis that Nordic Seas blooms start when the photoperiod, the number of daily light hours experienced by phytoplankton, exceeds a critical value, independently of division rates. This bloom behavior may be explained by realizing that photosynthesis is impossible during polar nights and phytoplankton enters in a dormant stage in winter, only to be awaken by a photoperiodic trigger. While the first accumulation of biomass recorded by the bio-optical floats is consistent with the photoperiod hypothesis, it is possible that some biomass accumulation started before the critical photoperiod but at levels too low to be detected by the fluorometers. Thus more precise observations are needed to test the photoperiod hypothesis.

Description: Unusual biogenic calcite structures in two shallow lakes, James Ross Island, Antarctica Biogeosciences Discussions, 12, 13593-13629, 2015 Author(s): J. Elster, L. Nedbalová, R. Vodrážka, K. Láska, J. Haloda, and J. Komárek The floors of two shallow endorheic lakes, located on volcanic surfaces on James Ross Island, are covered with calcareous organosedimentary structures. Their biological and chemical composition, lake water characteristics, and seasonal variability of the thermal regime are introduced. The lakes are frozen down to the bottom eight-nine months per year and their water chemistry is characterized by low conductivity and neutral to slightly alkaline pH. The photosynthetic microbial mat is composed of filamentous cyanobacteria and microalgae that are considered to be Antarctic endemic species. The mucilaginous black biofilm is covered by green spots formed by a green microalga and the macroscopic structures are packed together with fine material. Thin sections consist of rock substrate, soft biofilm, calcite spicules and mineral grains originating from different sources. The morphology of the spicules is typical of calcium carbonate monocrystals having a layered structure and worn surface, which reflect growth and degradation processes. The spicules chemical composition and structure correspond to pure calcite. Lakes age, altitude, morphometry, geomorphological and hydrological stability, including low sedimentation rates, together with thermal regime predispose the existence of this community. We hypothesize that the precipitation of calcite is connected with the photosynthetic activity of the green microalgae that were not recorded in any other lake in the region. This study has shown that the unique community producing biogenic calcite spicules is quite different to any yet described.

Description: Characterizing Leaf Area Index (LAI) and Vertical Foliage Profile (VFP) over the United States Biogeosciences Discussions, 12, 13675-13710, 2015 Author(s): H. Tang, S. Ganguly, G. Zhang, M. A. Hofton, R. F. Nelson, and R. Dubayah Leaf area index (LAI) and vertical foliage profile (VFP) are among the important canopy structural variables. Recent advances in lidar remote sensing technology have demonstrated the capability of accurately mapping LAI and VFP over large areas. The primary objective of this study was to derive and validate a LAI and VFP product over the contiguous United States using spaceborne waveform lidar data. This product was derived at the footprint level from the Geoscience Laser Altimeter System (GLAS) using a biophysical model. We validated GLAS derived LAI and VFP across major forest biomes using airborne waveform lidar. The comparison results showed that GLAS retrievals of total LAI were generally accurate with little bias ( r 2 = 0.67, bias = −0.13, RMSE = 0.75). The derivations of GLAS retrievals of VFP within layers was not as accurate overall ( r 2 = 0.36, bias = −0.04, RMSE = 0.26), and these varied as a function of height, increasing from understory to overstory −0 to 5 m layer: r 2 = 0.04, bias = 0.09, RMSE = 0.31; 10 to 15 m layer: r 2 = 0.53, bias = −0.08, RMSE = 0.22; and 15 to 20 m layer: r 2 = 0.66, bias =−0.05, RMSE = 0.20. Significant relationships were also found between GLAS LAI products and different environmental factors, in particular elevation and annual precipitation. In summary, our results provide a unique insight into vertical canopy structure distribution across North American ecosystems. This data set is a first step towards a baseline of canopy structure needed for evaluating climate and land use induced forest changes at continental scale in the future and should help deepen our understanding of the role of vertical canopy structure on terrestrial ecosystem processes across varying scales.

Description: Application of the 15 N-Gas Flux method for measuring in situ N 2 and N 2 O fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique Biogeosciences Discussions, 12, 12653-12689, 2015 Author(s): F. Sgouridis, S. Ullah, and A. Stott Soil denitrification is considered the most un-constrained process in the global N cycle due to uncertain in situ N 2 flux measurements, particularly in natural and semi-natural terrestrial ecosystems. 15 N tracer approaches can provide in situ measurements of both N 2 and N 2 O simultaneously, but their use has been limited to fertilised agro-ecosystems due to the need for large 15 N additions in order to detect 15 N 2 production against the high atmospheric N 2 . For 15 N-N 2 analyses, we have used an "in house" laboratory designed and manufactured N 2 preparation instrument which can be interfaced to any commercial continuous flow isotope ratio mass spectrometer (CF-IRMS). The N 2 prep unit has gas purification steps, a copper based reduction furnace, and allows the analysis of small gas injection volumes (4 μL) for 15 N-N 2 analysis. For the analysis of N 2 O, an automated Tracegas Pre-concentrator (Isoprime Ltd) coupled to an IRMS was used to measure the 15 N-N 2 O (4 mL gas injection volume). Consequently, the coefficient of variation for the determination of isotope ratios for N 2 in air and in standard N 2 O (0.5 ppm) was better than 0.5 %. The 15 N Gas-Flux method was adapted for application in natural and semi-natural land use types (peatlands, forests and grasslands) by lowering the 15 N tracer application rate to 0.04–0.5 kg 15 N ha −1 . For our chamber design (volume / surface = 8:1) and a 20 h incubation period, the minimum detectable flux rates were 4 μg N m −2 h −1 and 0.2 ng N m −2 h −1 for the N 2 and N 2 O fluxes respectively. The N 2 flux ranged between 2.4 and 416.6 μg N m −2 h −1 , and the grassland soils showed on average 3 and 14 times higher denitrification rates than the woodland and organic soils respectively. The N 2 O flux was on average 20 to 200 times lower than the N 2 flux, while the denitrification product ratio (N 2 O/N 2 + N 2 O) was low, ranging between 0.03 and 13 %. Total denitrification rates measured by the acetylene inhibition technique under the same field conditions correlated ( r = 0.58) with the denitrification rates measured under the 15 N Gas-Flux method but were underestimated by a factor of 4 and this was attributed to the incomplete inhibition of N 2 O reduction to N 2 under relatively high soil moisture content. The results show that the 15 N Gas-Flux method can be used for quantifying N 2 and N 2 O production rates in natural terrestrial ecosystems, thus significantly improving our ability to constrain ecosystem N budgets.

Description: Natural variability in the surface ocean carbonate ion concentration Biogeosciences Discussions, 12, 13123-13157, 2015 Author(s): N. S. Lovenduski, M. C. Long, and K. Lindsay We investigate variability in the surface ocean carbonate ion concentration ([CO 3 2− ]) on the basis of a long control simulation with a fully-coupled Earth System Model. The simulation is run with a prescribed, pre-industrial atmospheric CO 2 concentration for 1000 years, permitting investigation of natural [CO 3 2− ] variability on interannual to multi-decadal timescales. We find high interannual variability in surface [CO 3 2− ] in the tropical Pacific and at the boundaries between the subtropical and subpolar gyres in the Northern Hemisphere, and relatively low interannual variability in the centers of the subtropical gyres and in the Southern Ocean. Statistical analysis of modeled [CO 3 2− ] variance and autocorrelation suggests that significant anthropogenic trends in the saturation state of aragonite (Ω aragonite ) are already or nearly detectable at the sustained, open-ocean timeseries sites, whereas several decades of observations are required to detect anthropogenic trends in Ω aragonite in the tropical Pacific, North Pacific, and North Atlantic. The detection timescale for anthropogenic trends in pH is shorter than that for Ω aragonite , due to smaller noise-to-signal ratios and lower autocorrelation in pH. In the tropical Pacific, the leading mode of surface [CO 3 2− ] variability is primarily driven by variations in the vertical advection of dissolved inorganic carbon (DIC) in association with El Niño–Southern Oscillation. In the North Pacific, surface [CO 3 2− ] variability is caused by circulation-driven variations in surface DIC and strongly correlated with the Pacific Decadal Oscillation, with peak spectral power at 20–30 year periods. North Atlantic [CO 3 2− ] variability is also driven by variations in surface DIC, and exhibits weak correlations with both the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation. As the scientific community seeks to detect the anthropogenic influence on ocean carbonate chemistry, these results will aid the interpretation of trends calculated from spatially- and temporally-sparse observations.

Description: Spatial and temporal trends in summertime climate and water quality indicators in the coastal embayments of Buzzards Bay, Massachusetts Biogeosciences Discussions, 12, 13159-13192, 2015 Author(s): J. E. Rheuban, S. C. Williamson, J. E. Costa, D. M. Glover, R. W. Jakuba, D. C. McCorkle, C. Neill, T. Williams, and S. C. Doney Degradation of coastal ecosystems by eutrophication is largely defined by nitrogen loading from land via surface and groundwater flows. However, indicators of water quality are highly variable due to a myriad of other drivers, including temperature and precipitation. To evaluate these drivers, we examined spatial and temporal trends in a 22 year record of summer water quality data from 122 stations in 17 embayments within Buzzards Bay, MA (USA), collected through a citizen science monitoring program managed by Buzzards Bay Coalition. To identify spatial patterns across Buzzards Bay's embayments, we used a principle component and factor analysis and found that rotated factor loadings indicated little correlation between inorganic nutrients and organic matter and chlorophyll a (Chl a ) concentration. Factor scores showed that embayment geomorphology in addition to nutrient loading was a strong driver of water quality, where embayments with surface water inputs showed larger biological impacts than embayments dominated by groundwater influx. A linear regression analysis of annual summertime water quality indicators over time revealed that from 1992 to 2013, most embayments (15 of 17) exhibited an increase in temperature (mean rate of 0.082 ± 0.025 (SD) °C yr −1 ) and Chl a (mean rate of 0.0171 ± 0.0088 log 10 (Chl a ; mg m −3 ) yr −1 , equivalent to a 4.0 % increase per year). However, only 7 embayments exhibited an increase in total nitrogen (TN) concentration (mean rate 0.32 ± 0.47 (SD) μM yr −1 ). Average summertime log 10 (TN) and log 10 (Chl a ) were correlated with an indication that yield of Chl a per unit total nitrogen increased with time suggesting the estuarine response to TN may have changed because of other stressors such as warming, altered precipitation patterns, or changing light levels. These findings affirm that nitrogen loading and physical aspects of embayments are essential in explaining observed ecosystem response. However, climate-related stressors may also need to be considered by managers because increased temperature and precipitation may worsen water quality and partially offset benefits achieved by reducing nitrogen loading.

Description: Carbon dynamics in highly heterotrophic subarctic thaw ponds Biogeosciences Discussions, 12, 11707-11749, 2015 Author(s): T. Roiha, I. Laurion, and M. Rautio Global warming has accelerated the formation of permafrost thaw ponds in several subarctic and arctic regions. These ponds are net heterotrophic as evidenced by their greenhouse gas (GHG) supersaturation levels (CO 2 and CH 4 ), and generally receive large terrestrial carbon inputs from the thawing and eroding permafrost. We measured seasonal and vertical variations in the concentration and type of dissolved organic matter (DOM) in five subarctic thaw (thermokarst) ponds in northern Quebec, and explored how environmental gradients influenced heterotrophic and phototrophic biomass and productivity. Late winter DOM had low aromaticity indicating reduced inputs of terrestrial carbon, while the high concentration of dissolved organic carbon (DOC) suggests that some production of non-chromophoric dissolved compounds by the microbial food web took place under the ice cover. Summer DOM had a strong terrestrial signature, but was also characterized with significant inputs of algal-derived carbon, especially at the pond surface. During late winter, bacterial production was low (maximum of 0.8 mg C m −3 d −1 ) and was largely based on free-living bacterioplankton (58 %). Bacterial production in summer was high (up to 58 mg C m −3 d −1 ), dominated by particle-attached bacteria (67 %), and strongly correlated to the amount of terrestrial carbon. Primary production was restricted to summer surface waters due to strong light limitation deeper in the water column or in winter. The phototrophic biomass was equal to the heterotrophic biomass, but as the algae were mostly composed of mixotrophic species, most probably they used bacteria rather than solar energy in such shaded ponds. According to the δ 13 C analyses, non-algal carbon supported 51 % of winter and 37 % of summer biomass of the phantom midge larvae, Chaoborus sp., that are at the top of the trophic chain. Our results point to a strong heterotrophic energy pathway in these thaw pond ecosystems, where bacterioplankton dominates the production of new carbon in both summer and winter.

Description: Phototrophic pigment diversity and picophytoplankton abundance in permafrost thaw lakes Biogeosciences Discussions, 12, 12121-12156, 2015 Author(s): A. Przytulska, J. Comte, S. Crevecoeur, C. Lovejoy, I. Laurion, and W. F. Vincent Permafrost thaw lakes (thermokarst lakes) are widely distributed across the northern landscape, and are known to be biogeochemically active sites that emit large amounts of carbon to the atmosphere as CH 4 and CO 2 . However, the abundance and composition of the photosynthetic communities that consume CO 2 have been little explored in this ecosystem type. In order to identify the major groups of phototrophic organisms and their controlling variables, we sampled 12 permafrost thaw lakes along a permafrost degradation gradient in northern Québec, Canada. Additional samples were taken from 5 rock-basin reference lakes in the region to determine if the thaw waters differed in limnological properties and phototrophs. Phytoplankton community structure was determined by high performance liquid chromatography analysis of their photoprotective and photosynthetic pigments, and autotrophic picoplankton concentrations were assessed by flow cytometry. One of the black colored lakes located in a andscape of rapidly degrading palsas (permafrost mounds) was selected for high-throughput 18S rRNA sequencing to help interpret the pigment and cytometry data. The results showed that the limnological properties of the thaw lakes differed significantly from the reference lakes, and were more highly stratified. However, both waterbody types contained similarly diverse phytoplankton groups, with dominance of the pigment assemblages by fucoxanthin-containing taxa, as well as chlorophytes, cryptophytes and cyanobacteria. Chlorophyll a concentrations (Chl a ) were correlated with total phosphorus (TP), and both were significantly higher in the thaw lakes (overall means of 3.3 μg Chl a L −1 and 34 μg TP L −1 ) relative to the reference lakes (2.0 μg Chl a L −1 and 8.2 μg TP L −1 ). Stepwise multiple regression of Chl a against the other algal pigments showed that it was largely a function of lutein, fucoxanthin and peridinin ( R 2 = 0.78). The bottom waters of two of the thaw lakes also contained high concentrations of bacteriochlorophyll d , showing the presence of green photosynthetic sulphur bacteria. The molecular analyses indicated a relatively minor contribution of diatoms, while chrysophytes, dinoflagellates and chlorophytes were well represented; the heterotrophic eukaryote fraction was dominated by numerous ciliate taxa, and also included Heliozoa, Rhizaria, chytrids and flagellates. Autotrophic picoplankton occurred in cell concentrations up to 8.8 × 10 5 mL −1 (picocyanobacteria) and 4.6 × 10 5 mL −1 (picoeukaryotes). Both groups of picophytoplankton were positively correlated with total phytoplankton abundance, as measured by Chl a ; picocyanobacteria were inversely correlated with dissolved organic carbon, while picoeukaryotes were correlated with conductivity. Despite their net heterotrophic character, subarctic thaw lakes are rich habitats for diverse phototrophic communities.

Description: Skeletal mineralogy of coral recruits under high temperature and p CO 2 Biogeosciences Discussions, 12, 12485-12500, 2015 Author(s): T. Foster and P. L. Clode Aragonite, which is the polymorph of CaCO 3 precipitated by modern corals during skeletal formation, has a higher solubility than the more stable polymorph calcite. This higher solubility leaves animals that produce aragonitic skeletons more vulnerable to anthropogenic ocean acidification. It is therefore, important to determine whether scleractinian corals have the plasticity to adapt and produce calcite in their skeletons in response to changing environmental conditions. Both high p CO 2 and lower Mg / Ca ratios in seawater are thought to have driven changes in the skeletal mineralogy of major marine calcifiers in the past ∼540 myr. Experimentally reduced Mg / Ca ratios in ambient seawater have been shown to induce some calcite precipitation in both adult and newly settled modern corals, however, the impact of high p CO 2 on the mineralogy of recruits is unknown. Here we determined the skeletal mineralogy of one-month old Acropora spicifera coral recruits grown under high temperature (+3 °C) and p CO 2 (∼900 μatm) conditions, using X-ray diffraction and Raman spectroscopy. We found that newly settled coral recruits produced entirely aragonitic skeletons regardless of the treatment. Our results show that elevated p CO 2 alone is unlikely to drive changes in the skeletal mineralogy of young corals. Not having an ability to switch from aragonite to calcite precipitation may leave corals and ultimately coral reef ecosystems more susceptible to predicted ocean acidification. An important area for prospective research would be to investigate the combined impact of high p CO 2 and reduced Mg / Ca ratio on coral skeletal mineralogy.

Description: The impact of sedimentary alkalinity release on the water column CO 2 system in the North Sea Biogeosciences Discussions, 12, 12395-12453, 2015 Author(s): H. Brenner, U. Braeckman, M. Le Guitton, and F. J. R. Meysman Recently, it has been proposed that alkalinity release from sediments can play an important role in the carbonate dynamics on continental shelves, lowering the p CO 2 of seawater and hence increasing the CO 2 uptake from the atmosphere. To test this hypothesis, sedimentary alkalinity generation was quantified within permeable and muddy sediments across the North Sea during two cruises in September 2011 (basin-wide) and June 2012 (Dutch coastal zone). Benthic fluxes of alkalinity ( A T ) and dissolved inorganic carbon (DIC) were determined using shipboard closed sediment incubations. These results show that sediments can be an important source for alkalinity, particularly in the shallow southern North Sea, where high A T and DIC fluxes were recorded in near shore sediments of the Belgian, Dutch and German coastal zone. In contrast, fluxes of A T and DIC are substantially lower in the deeper, seasonally stratified, northern part of the North Sea. Overall, our results show that sedimentary alkalinity generation should be considered an important factor in the CO 2 dynamics of shallow coastal systems.

Description: Contribution of Marine Group II Euryarchaeota to cyclopentyl tetraethers in the Pearl River estuary and coastal South China Sea: impact on the TEX 86 paleothermometer Biogeosciences Discussions, 12, 12455-12484, 2015 Author(s): J. X. Wang, C. L. Zhang, W. Xie, Y. G. Zhang, and P. Wang TEX 86 (TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms) has been widely applied to reconstruct (paleo-) sea surface temperature (SST). While Marine Group I (MG I) Thaumarchaeota have been commonly believed to be the source for GDGTs, Marine Group II (MG II Euryarchaeota ) have recently been suggested to contribute significantly to the GDGT pool in the ocean. However, little is known how the MG II Euryarchaeota -derived GDGTs may influence TEX 86 in marine sediment record. In this study, we characterize MG II Euryarchaeota -produced GDGTs and assess the likely effect of these tetraether lipids on TEX 86 . Analyses of core lipid (CL-) and intact polar lipid (IPL-) based GDGTs, 454 sequencing and quantitative polymerase chain reaction (qPCR) targeting MG II Euryarchaeota were performed on suspended particulate matter (SPM) and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (seawater). The results showed that the community composition varied along the salinity gradient with MG II Euryarchaeota as the second dominant group in the mixing water and seawater. qPCR data indicated that the abundance of MG II Euryarchaeota in the mixing water was three to four orders of magnitude higher than the river water and seawater. Significant linear correlations were observed between the gene abundance ratio of MG II Euryarchaeota vs. total archaea and the relative abundance of GDGTs-1, -2, -3, or -4 as well as the ring index based on these compounds, which collectively suggest that MG II Euryarchaeota may actively produce GDGTs in the water column. These results also show strong evidence that MG II Euryarchaeota synthesizing GDGTs with 1–4 cyclopentane moieties may bias TEX 86 in the water column and sediments. This study highlights that valid interpretation of TEX 86 in sediment record, particularly in coastal oceans, needs to consider the contribution from MG II Euryarchaeota .

Description: Fate of peat-derived carbon and associated CO 2 and CO emissions from two Southeast Asian estuaries Biogeosciences Discussions, 12, 8299-8340, 2015 Author(s): D. Müller, T. Warneke, T. Rixen, M. Müller, A. Mujahid, H. W. Bange, and J. Notholt Coastal peatlands in Southeast Asia release large amounts of organic carbon to rivers, which transport it further to the adjacent estuaries. However, little is known about the fate of this terrestrial material in the coastal ocean. Although Southeast Asia is, by area, considered a hotspot of estuarine CO 2 emissions, studies in this region are very scarce. We measured dissolved and particulate organic carbon, carbon dioxide (CO 2 ) partial pressure and carbon monoxide (CO) concentrations in two tropical estuaries in Sarawak, Malaysia, whose coastal area is covered by peatlands. We surveyed the estuaries of the rivers Lupar and Saribas during the wet and dry season, respectively. The spatial distribution and the carbon-to-nitrogen ratios of dissolved organic matter (DOM) suggest that peat-draining rivers convey terrestrial organic carbon to the estuaries. We found evidence that a large fraction of this carbon is respired. The median p CO 2 in the estuaries ranged between 618 and 5064 μatm with little seasonal variation. CO 2 fluxes were determined with a floating chamber and estimated to amount to 14–272 mol m −2 yr −1 , which is high compared to other studies from tropical and subtropical sites. In contrast, CO concentrations and fluxes were relatively moderate (0.3–1.4 nmol L −1 and 0.8–1.9 mmol m −2 yr −1 ) if compared to published data for oceanic or upwelling systems. We attributed this to the large amounts of suspended matter (4–5004 mg L −1 ), limiting the light penetration depth. However, the diurnal variation of CO suggests that it is photochemically produced, implying that photodegradation might play a role for the removal of DOM from the estuary as well. We concluded that unlike smaller peat-draining tributaries, which tend to transport most carbon downstream, estuaries in this region function as an efficient filter for organic carbon and release large amounts of CO 2 to the atmosphere. The Lupar and Saribas mid-estuaries release 0.4 ± 0.2 Tg C yr −1 , which corresponds to approximately 80% of the emissions from the aquatic systems in these two catchments.

Description: Carbonate saturation state of surface waters in the Ross Sea and Southern Ocean: controls and implications for the onset of aragonite undersaturation Biogeosciences Discussions, 12, 8429-8465, 2015 Author(s): H. B. DeJong, R. B. Dunbar, D. A. Mucciarone, and D. A. Koweek Predicting when surface waters of the Ross Sea and Southern Ocean will become undersaturated with respect to biogenic carbonate minerals is challenging in part due to the lack of baseline high resolution carbon system data. Here we present ~ 1700 surface total alkalinity measurements from the Ross Sea and along a transect between the Ross Sea and southern Chile from the austral autumn (February–March 2013). We calculate the saturation state of aragonite (Ω Ar ) and calcite (Ω Ca ) using measured total alkalinity and p CO 2 . In the Ross Sea and south of the Polar Front, variability in carbonate saturation state (Ω) is mainly driven by algal photosynthesis. Freshwater dilution and calcification have minimal influence on Ω variability. We estimate an early spring surface water Ω Ar value of ~ 1.2 for the Ross Sea using a total alkalinity–salinity relationship and historical p CO 2 measurements. Our results suggest that the Ross Sea is not likely to become undersaturated with respect to aragonite until the year 2070.

Description: Biodegradability of dissolved organic carbon in permafrost soils and waterways: a meta-analysis Biogeosciences Discussions, 12, 8353-8393, 2015 Author(s): J. E. Vonk, S. E. Tank, P. J. Mann, R. G. M. Spencer, C. C. Treat, R. G. Striegl, B. W. Abbott, and K. P. Wickland As Arctic regions warm, the large organic carbon pool stored in permafrost becomes increasingly vulnerable to thaw and decomposition. The transfer of newly mobilized carbon to the atmosphere and its potential influence upon climate change will largely depend on the reactivity and subsequent fate of carbon delivered to aquatic ecosystems. Dissolved organic carbon (DOC) is a key regulator of aquatic metabolism and its biodegradability will determine the extent and rate of carbon release from aquatic ecosystems to the atmosphere. Knowledge of the mechanistic controls on DOC biodegradability is however currently poor due to a scarcity of long-term data sets, limited spatial coverage of available data, and methodological diversity. Here, we performed parallel biodegradable DOC (BDOC) experiments at six Arctic sites (16 experiments) using a standardized incubation protocol to examine the effect of methodological differences used as common practice in the literature. We further synthesized results from 14 aquatic and soil leachate BDOC studies from across the circum–arctic permafrost region to examine pan-Arctic trends in BDOC. An increasing extent of permafrost across the landscape resulted in higher BDOC losses in both soil and aquatic systems. We hypothesize that the unique composition of permafrost-derived DOC combined with limited prior microbial processing due to low soil temperature and relatively shorter flow path lengths and transport times, resulted in higher overall terrestrial and freshwater BDOC loss. Additionally, we found that the fraction of BDOC decreased moving down the fluvial network in continuous permafrost regions, i.e. from streams to large rivers, suggesting that highly biodegradable DOC is lost in headwater streams. We also observed a seasonal (January–December) decrease in BDOC losses in large streams and rivers, but no apparent change in smaller streams and soil leachates. We attribute this seasonal change to a combination of factors including shifts in carbon source, changing DOC residence time related to increasing thaw-depth, increasing water temperatures later in the summer, as well as decreasing hydrologic connectivity between soils and surface water as the seasons progress. Our results suggest that future, climate warming-induced shifts of continuous permafrost into discontinuous permafrost regions could affect the degradation potential of thaw-released DOC as well as its variability throughout the Arctic summer. We lastly present a recommended standardized BDOC protocol to facilitate the comparison of future work and improve our knowledge of processing and transport of DOC in a changing Arctic.

Description: Ideas and perspectives: use of tree-ring width as an indicator of tree growth Biogeosciences Discussions, 12, 8341-8352, 2015 Author(s): R. A. Hember, W. A. Kurz, and J. M. Metsaranta By taking core samples, dendroecological studies can reconstruct radial growth over the lifespan of a tree, providing a valuable way to estimate the sensitivity of tree productivity to environmental change. With increasing prevalence of such studies in global change science, it is worth cautioning that the incremental growth rate of a sub-dimension of a tree organ, such as annual ring width ( w ), does not respond to extrinsic perturbations with the same relative magnitude as the primary production of that organ. For example, if an extrinsic force causes a two-fold increase in the absolute growth rate of stemwood biomass (AGR), it should only theoretically translate into a 1.3-fold increase in w , or a 1.7-fold increase in basal area increment (BAI), when a 2:1 ratio in resource allocation to lateral and apical meristems is assumed. Expressing the magnitude of a response in relative terms does not, therefore, provide a valid means of comparing estimates of relative growth derived from measurement of different dimensional traits of the tree. From our perspective, enough conformity to facilitate comparison of environmental sensitivity across studies of tree growth is warranted so we emphasize the benefit of dimension analysis to transform measurements of w and BAI into the AGR. Although conversion to AGR introduces an error from the use of allometric equations, the approach is widely accepted in mainstream ecology and global change science at least partially because it avoids discrepancies in response magnitude owing to differences in dimension. Studies of organ elongation have historically provided invaluable information, yet it must be recognized that they systematically underestimate the response magnitude of primary production, and confound comparisons of growth sensitivity between many dendroecological studies that focus on w and studies of primary production.

Description: Spatial variability of diploptene δ 13 C values in thermokarst lakes: the potential to analyse the complexity of lacustrine methane cycling Biogeosciences Discussions, 12, 12157-12189, 2015 Author(s): K. L. Davies, R. D. Pancost, M. E. Edwards, K. M. Walter Anthony, P. G. Langdon, and L. Chaves Torres Cryospheric changes in northern high latitudes are linked to significant greenhouse gas flux to the atmosphere, including methane release that originates from organic matter decomposition in thermokarst lakes. The connections between methane production in sediments, transport pathways and oxidation are not well understood and this has implications for any attempts to reconstruct methane production from sedimentary archives. We assessed methane oxidation as represented by methane oxidising bacteria across the surface sediments of two interior Alaska thermokarst lakes in relation to methane emissions via ebullition (bubbling). The bacterial biomarker diploptene was present and had low δ 13 C values (lower than −38 ‰) in all sediments analysed, suggesting methane oxidation was widespread. The most δ 13 C-depleted diploptene was found in the area of highest methane ebullition emissions in Ace Lake (δ 13 C diplotene values between −68.2 and −50.1 ‰), suggesting a positive link between methane production, oxidation, and emission in this area. In contrast, significantly less depleted diploptene δ 13 C values (between −42.9 and −38.8 ‰) were found in the area of highest methane ebullition emissions in Smith Lake. Lower δ 13 C values of diploptene were found in the central area of Smith Lake (between −56.8 and −46.9 ‰), where methane ebullition rates are low but methane diffusion appears high. Using δ 13 C-diplotene as a proxy for methane oxidation activity, we suggest the observed differences in methane oxidation levels among sites within the two lakes could be linked to differences in source area of methane production (e.g. age and type of organic carbon) and bathymetry as it relates to varying oxycline depths and changing pressure gradients. As a result, methane oxidation is highly lake-dependent. The diploptene δ 13 C values also highlight strong within-lake variability, implying that single-value, down-core records of hopanoid isotopic signatures are not secure indicators of changing methane flux at the whole-lake scale.

Description: Metagenomic analyses of the late Pleistocene permafrost – additional tools for reconstruction of environmental conditions Biogeosciences Discussions, 12, 12091-12119, 2015 Author(s): E. Rivkina, L. Petrovskaya, T. Vishnivetskaya, K. Krivushin, L. Shmakova, M. Tutukina, A. Meyers, and F. Kondrashov A comparative analysis of the metagenomes from two 30 000 year-old permafrost samples, one of lake-alluvial origin and the other from late Pleistocene Ice Complex sediments, revealed significant differences within microbial communities. The late Pleistocene Ice Complex sediments (which have been characterized by the absence of methane with lower values of redox-potential and Fe 2+ content) showed both a low abundance of methanogenic archaea and enzymes from the carbon, nitrogen and sulfur cycles. The metagenomic and geochemical analyses described in the paper provide evidence that the formation of the late Pleistocene Ice Complex sediments likely took place under much more aerobic conditions than lake-alluvial sediments.

Description: Nonlinear thermal and moisture dynamics of high Arctic wetland polygons following permafrost disturbance Biogeosciences Discussions, 12, 11797-11831, 2015 Author(s): E. Godin, D. Fortier, and E. Lévesque Low-centre polygonal terrain developing within gentle sloping surfaces and lowlands in the high Arctic have a potential to retain snowmelt water in their bowl-shaped centre and as such are considered high latitude wetlands. Such wetlands in the continuous permafrost regions have an important ecological role in an otherwise generally arid region. In the valley of the glacier C-79 on Bylot Island (Nunavut, Canada), thermal erosion gullies are rapidly eroding the permafrost along ice wedges affecting the integrity of the polygons by breaching and collapsing the surrounding rims. While intact polygons were characterized by a relative homogeneity (topography, snow cover, maximum active layer thaw depth, ground moisture content, vegetation cover), eroded polygons had a non-linear response for the same elements following their perturbation. The heterogeneous nature of disturbed terrains impacts active layer thickness, ground ice aggradation in the upper portion of permafrost, soil moisture and vegetation dynamics, carbon storage and terrestrial green-house gas emissions.

Description: Controls on microalgal community structures in cryoconite holes upon high Arctic glaciers, Svalbard Biogeosciences Discussions, 12, 11751-11795, 2015 Author(s): T. R. Vonnahme, M. Devetter, J. D. Žárský, M. Šabacká, and J. Elster Glaciers are known to harbor surprisingly complex ecosystems. On their surface, distinct cylindrical holes filled with meltwater and sediments are considered as hot spots for microbial life. The present paper addresses possible biological interactions within the community of prokaryotic cyanobacteria and eukaryotic microalgae (microalgae) and relations to their potential grazers, additional to their environmental controls. Svalbard glaciers with substantial allochthonous input of material from local sources reveal high microalgal densities. Small valley glaciers with high sediment coverages and high impact of birds show high biomasses and support a high biological diversity. Invertebrate grazer densities do not show any significant negative correlation with microalgal abundances, but a positive correlation with eukaryotic microalgae. Most microalgae found in this study form large colonies ( 〈 10 cells, or 〉 25 μm), which may protect them against invertebrate grazing. This finding rather indicates grazing as a positive control on eukaryotic microalgae by nutrient recycling. Density differences between the eukaryotic microalgae and prokaryotic cyanobacteria and their high distinction in RDA and PCA analyses indicate that these two groups are in strong contrast. Eukaryotic microalgae occurred mainly in unstable cryoconite holes with high sediment loads, high N : P ratios, and a high impact of bird guano, as a proxy for nutrients. In these environments autochthonous nitrogen fixation appears to be negligible. Selective wind transport of Oscillatoriales via soil and dust particles is proposed to explain their dominance in cryoconites further away from the glacier margins. We propose that, for the studied glaciers, nutrient levels related to recycling of limiting nutrients is the main factor driving variation in the community structure of microalgae and grazers.

Description: Thermo-erosion gullies boost the transition from wet to mesic vegetation Biogeosciences Discussions, 12, 12191-12228, 2015 Author(s): N. Perreault, E. Lévesque, D. Fortier, and L. J. Lamarque Continuous permafrost zones with well-developed polygonal ice-wedge networks are particularly vulnerable to climate change. Thermo-mechanical erosion can initiate the development of gullies that lead to substantial drainage of adjacent wet habitats. How vegetation responds to this particular disturbance is currently unknown but has the potential to strongly disrupt function and structure of Arctic ecosystems. Focusing on three major gullies of Bylot Island, Nunavut, we aimed at estimating the effects of thermo-erosion processes in shaping plant community changes. Over two years, we explored the influence of environmental factors on plant species richness, abundance and biomass studying 197 polygons that covered the whole transition from intact wet to disturbed and mesic habitats. While gullying decreased soil moisture by 40 % and thaw front depth by 10 cm in breached polygons, we observed a gradual vegetation shift within five to ten years with mesic habitat plant species such as Arctagrostis latifolia and Salix arctica replacing wet habitat dominant Carex aquatilis and Dupontia fisheri . This transition was accompanied by a five time decrease in graminoid above-ground biomass in mesic sites. Our results illustrate that wetlands are highly vulnerable to thermo-erosion processes that may rapidly promote the decrease of food availability for herbivores and reduce methane emissions of Arctic ecosystems.

Description: Effects of dust additions on phytoplankton growth and DMS production in high CO 2 northeast Pacific HNLC waters Biogeosciences Discussions, 12, 12281-12319, 2015 Author(s): J. Mélançon, M. Levasseur, M. Lizotte, M. Scarratt, J.-É. Tremblay, P. Tortell, G.-P. Yang, G.-Y. Shi, H.-W. Gao, D. M. Semeniuk, M. Robert, M. Arychuk, K. Johnson, N. Sutherland, M. Davelaar, N. Nemcek, A. Peña, and W. Richardson Ocean acidification (OA) is likely to have an effect on the fertilizing potential of desert dust in high-nutrient, low-chlorophyll oceanic regions, either by modifying Fe speciation and bioavailability, or by altering phytoplankton Fe requirements and acquisition. To address this issue, short incubations (4 days) of northeast subarctic Pacific waters enriched with either FeSO 4 or dust, and set at pH 8.0 (in situ) and 7.8 were conducted in August 2010. We assessed the impact of a decrease in pH on dissolved Fe concentration, phytoplankton biomass, taxonomy and productivity, and the production of dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP). Chlorophyll a (chl a ) remained unchanged in the controls and doubled in both the FeSO 4 -enriched and dust-enriched incubations, confirming the Fe-limited status of the plankton assemblage during the experiment. In the acidified treatments, a significant reduction (by 16–38 %) of the final concentration of chl a was measured compared to their non-acidified counterparts, and a 15 % reduction in particulate organic carbon (POC) concentration was measured in the dust-enriched acidified treatment compared to the dust-enriched non-acidified treatment. FeSO 4 and dust additions had a fertilizing effect mainly on diatoms and cyanobacteria. Lowering the pH affected mostly the haptophytes, but pelagophyte concentrations were also reduced in some acidified treatments. Acidification did not significantly alter DMSP and DMS concentrations. These results show that dust deposition events in a low-pH iron-limited Northeast subarctic Pacific are likely to stimulate phytoplankton growth to a lesser extent than in today's ocean during the few days following fertilization and point to a low initial sensitivity of the DMSP and DMS dynamics to OA.

Description: Projected climate change impacts on North Sea and Baltic Sea: CMIP3 and CMIP5 model based scenarios Biogeosciences Discussions, 12, 12229-12279, 2015 Author(s): D. Pushpadas, C. Schrum, and U. Daewel Climate change impacts on the marine biogeochemistry and lower trophic level dynamics in the North Sea and Baltic Sea have been assessed using regional downscaling in a number of recent studies. However, most of these where only forced by physical conditions from Global Climate Models (GCMs) and regional downscaling considering the climate change impact on oceanic nutrient conditions from Global Earth System Models (ESMs) are rare and so far solely based on CMIP3-generation climate models. The few studies published show a large range in projected future primary production and hydrodynamic condition. With the addition of CMIP5 models and scenarios, the demand to explore the uncertainty in regional climate change projections increased. Moreover, the question arises how projections based on CMIP5-generation models compare to earlier projections and multi-model ensembles comprising both AR4 and AR5 generation forcing models. Here, we investigated the potential future climate change impacts to the North Sea and the Baltic Sea ecosystem using a coherent regional downscaling strategy based on the regional coupled bio-physical model ECOSMO. ECOSMO was forced by output from different ESMs from both CMIP3 and CMIP5 models. Multi-model ensembles using CMIP3/A1B and CMIP5/RCP4.5 scenarios are examined, where the selected CMIP5 models are the successors of the chosen CMIP3 models. Comparing projected changes with the present day reference condition, all these simulations predicted an increase in Sea Surface Temperature (SST) in both North Sea and Baltic Sea, reduction in sea ice in the Baltic, decrease in primary production in the North Sea and an increase in primary production in the Baltic Sea. Despite these largely consistent results on the direction of the projected changes, our results revealed a broad range in the amplitude of projected climate change impacts. Our study strengthens the claim that the choice of the ESM is a major factor for regional climate projections. The change in oceanic nutrient input appeared to be the major driver for the projected changes in North Sea primary production. Assessing the spread in ensemble groups, we found that there is for the North Sea a significant reduction in the spread of projected changes among CMIP5 forced model simulations compared to those forced by CMIP3 ESMs, except for salinity. The latter was due to an unexpected salinification observed in one of the CMIP5 model while all other models exhibit freshening in the future. However, for the Baltic Sea substantial differences in inter-model variability in projected climate change impact to primary production is lacking.

Description: Dissolved organic carbon lability and stable isotope shifts during microbial decomposition in a tropical river system Biogeosciences Discussions, 12, 12761-12782, 2015 Author(s): N. Geeraert, F. O. Omengo, G. Govers, and S. Bouillon A significant amount of carbon is transported to the ocean as dissolved organic carbon (DOC) in rivers. During transport, it can be transformed through microbial consumption and photochemical oxidation. In dark incubation experiments with water from the Tana River, Kenya, we examined the consumption of DOC through microbial decomposition and the associated change in its carbon stable isotope composition (δ 13 C). In 15 of the 18 incubations, DOC concentrations decreased significantly by 10 to 60 %, with most of the decomposition taking place within the first 24–48 h. After 8 days, the remaining DOC was up to 3 ‰ more depleted in 13 C compared with the initial pool, and the change in δ 13 C correlated strongly with the fraction of DOC remaining. We propose that the shift in δ 13 C is consistent with greater microbial lability of DOC originating from herbaceous C 4 vegetation than DOC derived from woody C 3 vegetation in the semi-arid lower Tana. The findings complement earlier data that riverine C sources do not necessarily reflect their proportion in the catchment: besides spatial distribution, also processing within the river can further influence the riverine δ 13 C.

Description: Ocean dynamic processes causing spatially heterogeneous distribution of sedimentary caesium-137 massively released from the Fukushima Dai-ichi Nuclear Power Plant Biogeosciences Discussions, 12, 12713-12759, 2015 Author(s): H. Higashi, Y. Morino, N. Furuichi, and T. Ohara Massive amounts of anthropogenic radiocaesium 137 Cs that was released into the environment by the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011 are widely known to have extensively migrated to Pacific oceanic sediment off of east Japan. Several recent reports have stated that the sedimentary 137 Cs is now stable with a remarkably heterogeneous distribution. The present study elucidates ocean dynamic processes causing this heterogeneous sedimentary 137 Cs distribution in and around the shelf off Fukushima and adjacent prefectures. We performed a numerical simulation of oceanic 137 Cs behaviour for about 10 months after the accident, using a comprehensive dynamic model involving advection–diffusion transport in seawater, adsorption and desorption to and from particulate matter, sedimentation and suspension on and from the bottom, and vertical diffusion transport in the sediment. A notable simulated result was that the sedimentary 137 Cs significantly accumulated in a swath just offshore of the shelf break (along the 50–100 m isobath) as in recent observations, although the seabed in the entire simulation domain was assumed to have ideal properties such as identical bulk density, uniform porosity, and aggregation of particles with a single grain diameter. This result indicated that the heterogeneous sedimentary 137 Cs distribution was not necessarily a result of the spatial distribution of 137 Cs sediment adsorptivity. The present simulation suggests that the shape of the swath is mainly associated with spatiotemporal variation between bottom shear stress in the shallow shelf ( 〈 50 m depths) and that offshore of the shelf break. In a large part of the shallow shelf, the simulation indicated that strong bottom friction suspending particulate matter from the seabed frequently occurred via a periodic spring tide about every 2 weeks and via occasional strong wind. The sedimentary 137 Cs thereby could hardly stay on the surface of the seabed with the result that the simulated sediment-surface 137 Cs activity tended to decrease steadily for a long term after the initial 137 Cs migration. By contrast, in the offshore region, neither the spring tide nor the strong wind caused bottom disturbance. Hence, the particulate matter incorporated with 137 Cs, which was horizontally transported from the adjacent shallow shelf, readily settled and remained on the surface of the sediment just offshore of the shelf break. The present simulation also clearly demonstrated that the bottom disturbance influenced the sedimentary 137 Cs distributions not only horizontally but also vertically. In particular, within a part of the near-shore off the nuclear power plant, the simulation indicated that large amounts of the sedimentary 137 Cs were present in both upper and deeper sediments. As a result, total sedimentary 137 Cs in the entire simulation domain (1.4 x 10 5 km 2 ) at the end of 2011 was 3.2 x 10 15 Bq, more than 10 times that in previous estimates using samples of upper sediments.

Description: Soil N 2 O and NO emissions from land use and land-use change in the tropics and subtropics: a meta-analysis Biogeosciences Discussions, 12, 12783-12821, 2015 Author(s): J. van Lent, K. Hergoualc'h, and L. V. Verchot Deforestation and forest degradation in the tropics may substantially alter soil N-oxide emissions. It is particularly relevant to accurately quantify those changes to properly account for them in a REDD+ climate change mitigation scheme that provides financial incentives to reduce the emissions. With this study we provide updated land use (LU)-based emission rates (103 studies, 387 N 2 O and 111 NO case studies), determine the trend and magnitude of flux changes with land-use change (LUC) using a meta-analysis approach (43 studies, 132 N 2 O and 37 NO cases) and evaluate biophysical drivers of N 2 O and NO emissions and emission changes for the tropics. The average N 2 O and NO emissions in intact upland tropical forest amounted to 2.0 ± 0.2 ( n = 88) and 1.7 ± 0.5 ( n = 36) kg N ha −1 yr −1 , respectively. In agricultural soils annual N 2 O emissions were exponentially related to N fertilization rates and average water-filled pore space (WFPS) whereas in non-agricultural sites a Gaussian response to WFPS fit better the observed NO and N 2 O emissions. The sum of soil N 2 O and NO fluxes and the ratio of N 2 O to NO increased exponentially and significantly with increasing nitrogen availability (expressed as NO 3 − /[NO 3 − +NH 4 + ]) and WFPS, respectively; following the conceptual Hole-In-the-Pipe model. Nitrous and nitric oxide fluxes did not overall increase significantly as a result of LUC (Hedges's d of 0.11 ± 0.11 and 0.16 ± 0.19, respectively), however individual LUC trajectories or practices did. Nitrous oxide fluxes increased significantly after intact upland forest conversion to croplands (Hedges's d = 0.78 ± 0.24) and NO increased significantly following the conversion of low forest cover (secondary forest younger than 30 years, woodlands, shrublands) (Hedges's d of 0.44 ± 0.13). Forest conversion to fertilized systems significantly and highly raised both N 2 O and NO emission rates (Hedges's d of 1.03 ± 0.23 and 0.52 ± 0.09, respectively). Changes in nitrogen availability and WFPS were the main factors explaining changes in N 2 O emissions following LUC, therefore it is important that experimental designs monitor their spatio-temporal variation. Gaps in the literature on N oxide fluxes included geographical gaps (Africa, Oceania) and LU gaps (degraded forest, wetland (notably peat) forest, oil palm plantation and soy cultivation).

Description: Phytoplankton calcification as an effective mechanism to prevent cellular calcium poisoning Biogeosciences Discussions, 12, 12691-12712, 2015 Author(s): M. N. Müller, J. Barcelos e Ramos, K. G. Schulz, U. Riebesell, J. Kaźmierczak, F. Gallo, L. Mackinder, Y. Li, P. N. Nesterenko, T. W. Trull, and G. M. Hallegraeff Marine phytoplankton has developed the remarkable ability to tightly regulate the concentration of free calcium ions in the intracellular cytosol at a level of ~ 0.1 μmol L −1 in the presence of seawater Ca 2+ concentrations of 10 mmol L −1 . The low cytosolic calcium ion concentration is of utmost importance for proper cell signalling function. While the regulatory mechanisms responsible for the tight control of intracellular Ca 2+ concentration are not completely understood, phytoplankton taxonomic groups appear to have evolved different strategies, which may affect their ability to cope with changes in seawater Ca 2+ concentrations in their environment on geological time scales. For example, the Cretaceous (145 to 66 Ma ago), an era known for the high abundance of coccolithophores and the production of enormous calcium carbonate deposits, exhibited seawater calcium concentrations up to four times present-day levels. We show that calcifying coccolithophore species ( Emiliania huxleyi , Gephyrocapsa oceanica and Coccolithus braarudii ) are able to maintain their relative fitness (in terms of growth rate and photosynthesis) at simulated Cretaceous seawater calcium concentrations, whereas these rates are severely reduced under these conditions in some non-calcareous phytoplankton species ( Chaetoceros sp., Ceratoneis closterium and Heterosigma akashiwo ). Most notably, this also applies to a non-calcifying strain of E. huxleyi which displays a calcium-sensitivity similar to the non-calcareous species. We hypothesize that the process of calcification in coccolithophores provides an efficient mechanism to prevent cellular calcium poisoning and thereby offered a potential key evolutionary advantage, responsible for the proliferation of coccolithophores during times of high seawater calcium concentrations.

Description: EUROSPEC: at the interface between remote sensing and ecosystem CO 2 flux measurements in Europe Biogeosciences Discussions, 12, 13069-13121, 2015 Author(s): A. Porcar-Castell, A. Mac Arthur, M. Rossini, L. Eklundh, J. Pacheco-Labrador, K. Anderson, M. Balzarolo, M. P. Martín, H. Jin, E. Tomelleri, S. Cerasoli, K. Sakowska, A. Hueni, T. Julitta, C. J. Nichol, and L. Vescovo Resolving the spatial and temporal dynamics of gross primary productivity (GPP) of terrestrial ecosystems across different scales remains a challenge. Remote sensing is regarded as the solution to upscale point observations conducted at the ecosystem level, using the eddy covariance (EC) technique, to the landscape and global levels. In addition to traditional vegetation indices, the photochemical reflectance index (PRI) and the emission of solar-induced chlorophyll fluorescence (SIF), now measurable from space, provide a new range of opportunities to monitor the global carbon cycle using remote sensing. However, the scale mismatch between EC observations and the much coarser satellite-derived data complicates the integration of the two sources of data. The solution is to establish a network of in situ spectral measurements that can act as bridge between EC measurements and remote sensing data. In situ spectral measurements have been already conducted for many years at EC sites, but using variable instrumentation, setups, and measurement standards. In Europe in particular, in situ spectral measurements remain highly heterogeneous. The goal of EUROSPEC Cost Action ES0930 was to promote the development of common measuring protocols and new instruments towards establishing best practices and standardization of in situ spectral measurements. In this review we describe the background and main tradeoffs of in situ spectral measurements, review the main results of EUROSPEC Cost Action, and discuss the future challenges and opportunities of in situ spectral measurements for improved estimation of local and global carbon cycle.

Description: Improved end-member characterization of modern organic matter pools in the Ohrid Basin (Albania, Macedonia) and evaluation of new palaeoenvironmental proxies Biogeosciences Discussions, 12, 12975-13039, 2015 Author(s): J. Holtvoeth, D. Rushworth, A. Imeri, M. Cara, H. Vogel, T. Wagner, and G. A. Wolff We present elemental, lipid biomarker and compound-specific isotope (δ 13 C, δ 2 H) data for soils and leaf litter collected in the catchment of Lake Ohrid (Albania, Macedonia), as well as macrophytes, particulate organic matter and sediments from the lake itself. Lake Ohrid provides an outstanding archive of continental environmental change of at least 1.2 M years and the purpose of our study is to ground truth organic geochemical proxies that we developed in order to study past changes in the terrestrial biome. We show that soils dominate the lipid signal of the lake sediments rather than the vegetation or aquatic biomass, while compound-specific isotopes (δ 13 C, δ 2 H) determined for n-alkanoic acids confirm a dominant terrestrial source of organic matter to the lake. There is a strong imprint of suberin monomers on the composition of total lipid extracts and chain-length distributions of n-alkanoic acids, n-alcohols, ω-hydroxy acids and α,ω-dicarboxylic acids. Our end-member survey identifies that ratios of mid-chain length suberin-derived to long-chain length cuticular-derived alkyl compounds as well as their average chain length distributions can be used as new molecular proxies of organic matter sources to the lake. We tested these for the 8.2 ka event, a pronounced and widespread Holocene climate fluctuation. In SE Europe climate became drier and cooler in response to the event, as is clearly recognizable in the carbonate and organic carbon records of Lake Ohrid sediments. Our new proxies indicate biome modification in response to hydrological changes, identifying two phases of increased soil OM supply, first from topsoils and then from mineral soils. Our study demonstrates that geochemical fingerprinting of terrestrial OM should focus on the main lipid sources, rather than the living biomass. Both can exhibit climate-controlled variability, but are generally not identical.

Description: The root economics spectrum: divergence of absorptive root strategies with root diameter Biogeosciences Discussions, 12, 13041-13067, 2015 Author(s): D. Kong, J. Wang, P. Kardol, H. Wu, H. Zeng, X. Deng, and Y. Deng Plant roots usually vary along a dominant ecological axis, the root economics spectrum (RES), depicting a tradeoff between resource acquisition and conservation. For absorptive roots, which are mainly responsible for resource acquisition, we hypothesized that root strategies as predicted from the RES shift with increasing root diameter. To test this hypothesis, we used seven contrasting plant species for which we separated absorptive roots into two categories: thin roots ( 〈 247 μm diameter) and thick roots. For each category, we analyzed a~range of root traits closely related to resource acquisition and conservation, including root tissue density, carbon (C) and nitrogen (N) fractions as well as root anatomical traits. The results showed that trait relationships for thin absorptive roots followed the expectations from the RES while no clear trait relationships were found in support of the RES for thick absorptive roots. Our results suggest divergence of absorptive root strategies in relation to root diameter, which runs against a single economics spectrum for absorptive roots.

Description: Millennial changes in North Atlantic oxygen concentrations Biogeosciences Discussions, 12, 12947-12973, 2015 Author(s): B. A. A. Hoogakker, D. J. R. Thornalley, and S. Barker Glacial–interglacial changes in bottom water oxygen concentrations [O 2 ] in the deep Northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O 2 ] changes in relation to millennial climate oscillations in the North Atlantic ocean over the last glacial cycle, using bottom water [O 2 ] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015), and (2) ODP 1055 from the intermediate northwest Atlantic. Deep northeast Atlantic core MD95-2042 shows decreased bottom water [O 2 ] during millennial scale cool events, with lowest bottom water [O 2 ] of 170, 144, and 166 ± 17 μmol kg −1 during Heinrich ice rafting events H6, H4 and H1. Importantly, at intermediate core ODP 1055 bottom water [O 2 ] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with lowest values of 179 and 194 μmol kg −1 recorded during millennial cool events C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O 2 ] across the North Atlantic below ~1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O 2 ] during North Atlantic Heinrich events and earlier cold events at the deep site can be linked to water mass changes in relation to ocean circulation changes, and possibly productivity changes. At the intermediate depth site a strong North Atlantic Intermediate Water cell precludes water mass changes as a cause for decreased bottom water [O 2 ]. Instead we propose that the lower bottom [O 2 ] there can be linked to productivity changes through increased export of organic material from the surface ocean.

Description: The carbon cycle in Mexico: past, present and future of C stocks and fluxes Biogeosciences Discussions, 12, 12501-12541, 2015 Author(s): G. Murray-Tortarolo, P. Friedlingstein, S. Sitch, V. J. Jaramillo, F. Murguía-Flores, A. Anav, Y. Liu, A. Arneth, A. Arvanitis, A. Harper, A. Jain, E. Kato, C. Koven, B. Poulter, B. D. Stocker, A. Wiltshire, S. Zaehle, and N. Zeng We modelled the carbon (C) cycle in Mexico with a process-based approach. We used different available products (satellite data, field measurements, models and flux towers) to estimate C stocks and fluxes in the country at three different time frames: present (defined as the period 2000–2005), the past century (1901–2000) and the remainder of this century (2010–2100). Our estimate of the gross primary productivity (GPP) for the country was 2137 ± 1023 Tg C yr −1 and a total C stock of 34 506 ± 7483 Tg C, with 20 347 ± 4622 Pg C in vegetation and 14 159 ± 3861 in the soil. Contrary to other current estimates for recent decades, our results showed that Mexico was a C sink over the period 1990–2009 (+31 Tg C yr −1 ) and that C accumulation over the last century amounted to 1210 ± 1040 Tg C. We attributed this sink to the CO 2 fertilization effect on GPP, which led to an increase of 3408 ± 1060 Tg C, while both climate and land use reduced the country C stocks by −458 ± 1001 and −1740 ± 878 Tg C, respectively. Under different future scenarios the C sink will likely continue over 21st century, with decreasing C uptake as the climate forcing becomes more extreme. Our work provides valuable insights on relevant driving processes of the C-cycle such as the role of drought in marginal lands (e.g. grasslands and shrublands) and the impact of climate change on the mean residence time of C in tropical ecosystems.

Description: Influence of timing of sea ice retreat on phytoplankton size during marginal ice zone bloom period in the Chukchi and Bering shelves Biogeosciences Discussions, 12, 12611-12651, 2015 Author(s): A. Fujiwara, T. Hirawake, K. Suzuki, L. Eisner, I. Imai, S. Nishino, T. Kikuchi, and S. I. Saitoh Timing of sea ice retreat (TSR) as well as cell size of primary producers (i.e., phytoplankton) plays crucial roles in seasonally ice-covered marine ecosystem. Thus, it is important to monitor the temporal and spatial distribution of phytoplankton community size structure. Prior to this study, an ocean color algorithm has been developed to derive phytoplankton size index F L , which is defined as the ratio of chlorophyll a derived from the cells larger than 5 μm to the total chl a using satellite remote sensing for the Chukchi and Bering shelves. Using this method, we analyzed pixel-by-pixel relationships between F L during marginal ice zone (MIZ) bloom period and TSR over a period of 1998–2013. The influence of TSR on sea surface temperature (SST) and changes in ocean heat content (ΔOHC) during the MIZ bloom period were also investigated. A significant negative relationship between F L and TSR was widely found in the shelf region during MIZ bloom season. On the other hand, we found a significant positive (negative) relationship between SST (ΔOHC) and TSR. That is, earlier sea-ice retreat was associated with a dominance of larger phytoplankton during a colder and weakly stratified MIZ bloom season, suggesting that duration of nitrate supply, which is important for large-sized phytoplankton growth in this region (i.e., diatoms), can change according to TSR. In addition, under-ice phytoplankton blooms are likely to occur in years with late ice retreat, because sufficient light for phytoplankton growth can pass through the ice and penetrate into the water columns due to an increase in solar radiation toward the summer solstice. Moreover, we found not only the length of ice-free season but also annual median of F L positively correlated with annual net primary production (APP). Thus, both phytoplankton community composition and growing season are important for APP in the study area. Our findings showed quantitative relationship between the inter-annual variability of F L , TSR and APP suggesting satellite remote sensing of phytoplankton community size structure is suitable to document the impact of recent rapid sea ice loss on ecosystem of the study region.

Description: Southwestern Tropical Atlantic coral growth response to atmospheric circulation changes induced by ozone depletion in Antarctica Biogeosciences Discussions, 12, 13193-13213, 2015 Author(s): H. Evangelista, I. Wainer, A. Sifeddine, T. Corrège, R. C. Cordeiro, S. Lamounier, D. Godiva, C.-C. Shen, F. Le Cornec, B. Turcq, C. E. Lazareth, and C.-Y. Hu Climate changes induced by stratospheric ozone depletion over Antarctica have been recognized as an important consequence of the recently observed Southern Hemisphere atmospheric circulation. Here we present evidences that the Brazilian coast (Southwestern Atlantic) may have been impacted from both winds and sea surface temperature changes derived from this process. Skeleton analysis of massive coral species living in shallow waters off Brazil are very sensitive to air–sea interactions, and seem to record this impact. Growth rates of Brazilian corals show a trend reversal that fits the ozone depletion evolution, confirming that ozone impacts are far reaching and potentially affect coastal ecosystems in tropical environments.

Description: Contrasting pH buffering patterns in neutral-alkaline soils along a 3600 km transect in northern China Biogeosciences Discussions, 12, 13215-13240, 2015 Author(s): W. Luo, P. N. Nelson, M.-H. Li, J. Cai, Y. Zhang, Y. Zhang, Y. Shan, R. Wang, X. Han, and Y. Jiang Soil pH buffering capacity (pHBC) plays a crucial role in predicting acidification rates, yet its large-scale patterns and controls are poorly understood, especially for neutral-alkaline soils. Here, we evaluated the spatial patterns and drivers of pHBC along a 3600 km long transect (1900 km sub-transect with carbonate containing soils and 1700 km sub-transect with non-carbonate containing soils) across northern China. Soil pHBC was greater in the carbonate containing soils than in the non-carbonate containing soils. Acid addition decreased soil pH in the non-carbonate containing soils more markedly than in the carbonate containing soils. Within the carbonate soil sub-transect, soil pHBC was positively correlated with cation exchange capacity (CEC), carbonate content and exchangeable sodium (Na) concentration, but negatively correlated with initial pH and clay content, and not correlated with soil organic carbon (SOC) content. Within the non-carbonate sub-transect, soil pHBC was positively related to initial pH, clay content, CEC and exchangeable Na concentration, but not related to SOC content. Carbonate content was the primary determinant of pHBC in the carbonate containing soils and CEC was the main determinant of buffering capacity in the non-carbonate containing soils. Soil pHBC was positively related to aridity index and carbonate content across the carbonate containing soil sub-transect. Our results indicated that mechanisms controlling pHBC differ among neutral-alkaline soils of northern China, especially between carbonate and non-carbonate containing soils, leading to different rates, risks, and impacts of acidification. This understanding should be incorporated into the acidification risk assessment and landscape management in a changing world.

Description: Smallholder African farms in western Kenya have limited greenhouse gas fluxes Biogeosciences Discussions, 12, 15301-15336, 2015 Author(s): D. E. Pelster, M. C. Rufino, T. Rosenstock, J. Mango, G. Saiz, E. Diaz-Pines, G. Baldi, and K. Butterbach-Bahl Few field studies examine greenhouse gas (GHG) emissions from African agricultural systems resulting in high uncertainty for national inventories. We provide here the most comprehensive study in Africa to date, examining annual CO 2 , CH 4 and N 2 O emissions from 59 plots, across different vegetation types, field types and land classes in western Kenya. The study area consists of a lowland area (approximately 1200 m a.s.l.) rising approximately 600 m to a highland plateau. Cumulative annual fluxes ranged from 2.8 to 15.0 Mg CO 2 -C ha −1 , −6.0 to 2.4 kg CH 4 -C ha −1 and −0.1 to 1.8 kg N 2 O-N ha −1 . Management intensity of the plots did not result in differences in annual fluxes for the GHGs measured ( P = 0.46, 0.67 and 0.14 for CO 2 , N 2 O and CH 4 respectively). The similar emissions were likely related to low fertilizer input rates (≤ 20 kg ha −1 ). Grazing plots had the highest CO 2 fluxes ( P = 0.005); treed plots were a larger CH 4 sink than grazing plots ( P = 0.05); while N 2 O emissions were similar across vegetation types ( P = 0.59). This case study is likely representative for low fertilizer input, smallholder systems across sub-Saharan Africa, providing critical data for estimating regional or continental GHG inventories. Low crop yields, likely due to low inputs, resulted in high (up to 67 g N 2 O-N kg −1 aboveground N uptake) yield-scaled emissions. Improving crop production through intensification of agricultural production (i.e. water and nutrient management) may be an important tool to mitigate the impact of African agriculture on climate change.

Description: Combining two complementary micrometeorological methods to measure CH 4 and N 2 O fluxes over pasture Biogeosciences Discussions, 12, 15245-15299, 2015 Author(s): J. Laubach, M. Barthel, A. Fraser, J. E. Hunt, and D. W. T. Griffith New Zealand's largest industrial sector is pastoral agriculture, giving rise to a large fraction of the country's emissions of methane (CH 4 ) and nitrous oxide (N 2 O). We designed a system to continuously measure CH 4 and N 2 O fluxes at the field scale on two adjacent pastures that differed with respect to management. At the core of this system was a closed-cell Fourier-transform infrared spectrometer (FTIR), measuring the mole fractions of CH 4 , N 2 O and carbon dioxide (CO 2 ) at two heights at each site. In parallel, CO 2 fluxes were measured using eddy-covariance instrumentation. We applied two different micrometeorological ratio methods to infer the CH 4 and N 2 O fluxes from their respective mole fractions and the CO 2 fluxes. The first is a variant of the flux-gradient method, where it is assumed that the turbulent diffusivities of CH 4 and N 2 O equal that of CO 2 . This method was reliable when the CO 2 mole-fraction difference between heights was at least 4 times greater than the FTIR's resolution of differences. For the second method, the temporal increases of mole fractions in the stable nocturnal boundary layer, which are correlated for concurrently-emitted gases, are used to infer the unknown fluxes of CH 4 and N 2 O from the known flux of CO 2 . This method was sensitive to "contamination" from trace gas sources other than the pasture of interest and therefore required careful filtering. With both methods combined, estimates of mean daily CH 4 and N 2 O fluxes were obtained for 60 % of days at one site and 77 % at the other. Both methods indicated both sites as net sources of CH 4 and N 2 O. Mean emission rates for one year at the unfertilised, winter-grazed site were 8.2 (± 0.91) nmol CH 4 m −2 s −1 and 0.40 (± 0.018) nmol N 2 O m −2 s −1 . During the same year, mean emission rates at the irrigated, fertilised and rotationally-grazed site were 7.0 (± 0.89) nmol CH 4 m −2 s −1 and 0.57 (± 0.019) nmol N 2 O m −2 s −1 . At this site, the N 2 O emissions amounted to 1.19 (± 0.15) % of the nitrogen inputs from animal excreta and fertiliser application.

Description: Increasing P-stress and viral infection impact lipid remodeling of the picophytoplankter Micromonas pusilla Biogeosciences Discussions, 12, 15583-15610, 2015 Author(s): D. S. Maat, N. J. Bale, E. C. Hopmans, J. S. Sinninghe Damsté, S. Schouten, and C. P. D. Brussaard The intact polar lipid (IPL) composition of phytoplankton is plastic and dependent on environmental factors. Previous studies have shown that phytoplankton under phosphorus (P)-stress substitute phosphatidylglycerols (PGs) with sulphoquinovosyldiacylglycerols (SQDGs) and digalactosyldiacylglycerols (DGDGs). However, these studies focused merely on P-depletion, while phytoplankton in the natural environment often experience P-limitation whereby the degree of limitation depends on the supply rate of the limiting nutrient. Here we demonstrate a linear increase in SQDG : PG and DGDG : PG ratios with increasing cellular P-stress in the picophotoeukaryote Micromonas pusilla , obtained by P-replete, P-limited (chemostat) and P-starved (no supply of P) culturing conditions. These ratios were not affected by the degree of the P-limiting conditions itself (i.e. 0.97 and 0.32 μ max chemostats), suggesting there is a minimum requirement of PGs for the maintenance of cell growth. Viral infection reduced the increase in SQDG : PG and DGDG : PG ratios in P-starved cells, but the extent did depend on the growth rate of the cultures before infection. The membrane of M. pusilla virus MpV itself was lacking some IPLs compared to the host as, e.g. no monogalactosyldiacylglycerols could be detected. Growth of the phytoplankton cultures under enhanced CO 2 concentration did not affect the lipid remodeling results. The present study provides new insights into how the P-related trophic state of an ecosystem as well as viral infection can affect phytoplankton IPL composition, and therefore influence food web dynamics and biogeochemical cycling.

Description: No-tillage lessens soil CO 2 emissions the most under arid and sandy soil conditions: results from a meta-analysis Biogeosciences Discussions, 12, 15495-15535, 2015 Author(s): K. Abdalla, P. Chivenge, P. Ciais, and V. Chaplot The management of agroecosystems plays a crucial role in the global carbon cycle with soil tillage leading to known organic carbon redistributions within soils and changes in soil CO 2 emissions. Yet, discrepancies exist on the impact of tillage on soil CO 2 emissions and on the main soil and environmental controls. A meta-analysis was conducted using 46 peer-reviewed publications totaling 174 paired observations comparing CO 2 emissions over entire seasons or years from tilled and untilled soils across different climates, crop types and soil conditions with the objective of quantifying tillage impact on CO 2 emissions and assessing the main controls. On average, tilled soils emitted 21 % more CO 2 than untilled soils, which corresponded to a significant difference at P 〈 0.05. The difference increased to 29 % in sandy soils from arid climates with low soil organic carbon content (SOC C 〈 1 %) and low soil moisture, but tillage had no impact on CO 2 fluxes in clayey soils with high background SOC C (〉 3 %). Finally, nitrogen fertilization and crop residue management had little effect on the CO 2 responses of soils to no-tillage. These results suggest no-tillage is an effective mitigation measure of carbon dioxide losses from dry land soils. They emphasize the importance of including information on soil factors such as texture, aggregate stability and organic carbon content in global models of the carbon cycle.

Description: Predicting biomass of hyperdiverse and structurally complex Central Amazon forests – a virtual approach using extensive field data Biogeosciences Discussions, 12, 15537-15581, 2015 Author(s): D. Magnabosco Marra, N. Higuchi, S. E. Trumbore, G. H. P. M. Ribeiro, J. dos Santos, V. M. C. Carneiro, A. J. N. Lima, J. Q. Chambers, R. I. Negrón-Juárez, F. Holzwarth, B. Reu, and C. Wirth Old-growth forests are subject to substantial changes in structure and species composition due to the intensification of human activities, gradual climate change and extreme weather events. Trees store ca. 90 % of the total AGB above-ground biomass in tropical forests and AGB estimation models are crucial for forest management and conservation. In the Central Amazon, predicting AGB at large spatial-scales is a challenging task due to the heterogeneity of successional stages, high tree species diversity and inherent variations in allometry and architecture. We parameterized generic AGB estimation models applicable across species and a wide range of structural and compositional variation related to species sorting into height layers as well as frequent natural disturbances. We used 727 trees from 101 genera and at least 135 species harvested in a contiguous forest near Manaus, Brazil. Sampling from this dataset we assembled six scenarios designed to span existing gradients in floristic composition and size distribution in order to select models that best predict AGB at the landscape-level across successional gradients. We found that good individual tree model fits do not necessarily translate into good predictions of AGB at the landscape level. When predicting AGB (dry mass) over scenarios using our different models and an available pantropical model, we observed systematic biases ranging from −31 % (pantropical) to +39 %, with RMSE root-mean-square error values of up to 130 Mg ha −1 (pantropical). Our first and second best models had both low mean biases (0.8 and 3.9 %, respectively) and RMSE (9.4 and 18.6 Mg ha −1 ) when applied over scenarios. Predicting biomass correctly at the landscape-level in complex tropical forests, especially allowing good performance at the margins of data availability for model parametrization, requires the inclusion of predictors related to species architecture. The model of interest should comprise the floristic composition and size-distribution variability of the target forest, implying that even generic global or pantropical biomass estimation models can lead to strong biases. Reliable biomass assessments for the Amazon basin still depend on the collection of destructive allometry data at the local/regional scale and forest inventories including species-specific attributes, which are often unavailable or estimated imprecisely in most regions.

Description: Carbon dynamics and changing winter conditions: a review of current understanding and future research directions Biogeosciences Discussions, 12, 15763-15808, 2015 Author(s): M. Haei and H. Laudon Despite the important role of winters for northern ecosystems, it remains the least understood of all the seasons. Here, we summarize existing empirical studies on winter climate and carbon dynamics and highlight some important future research directions. The existing studies include field-scale snow-cover manipulation experiments representing extreme soil climate conditions, laboratory soil incubations studying the influential factors, and time-series of climate and carbon data showing long-term natural variations and existing trends. Most of the field and laboratory experiments indicate an increased soil organic carbon loss due to soil frost. Long-term data demonstrate temporal changes in winter CO 2 efflux and its important contribution to the annual fluxes. A number of research priorities to improve our understanding of winter conditions include (i) ecosystem processes in the fall-winter and winter-spring shoulder seasons, (ii) extreme events, (iii) partitioning into organic- and inorganic carbon, (iv) carry-over effects of winter and growing season on each other, (v) long-term cumulative impacts, and (vi) improved winter process modelling. These areas of research would enable an improved understanding of the role of the snow covered period for carbon cycling, and provide a basis for more realistic models that include winter processes.

Description: Vanishing coccolith vital effects with alleviated CO 2 limitation Biogeosciences Discussions, 12, 15835-15866, 2015 Author(s): M. Hermoso, I. Z. X. Chan, H. L. O. McClelland, A. M. C. Heureux, and R. E. M. Rickaby By recreating a range of geologically relevant concentrations of dissolved inorganic carbon (DIC) in the laboratory, we demonstrate that the magnitude of the vital effects in both carbon and oxygen isotopes of coccolith calcite of multiple species relates to ambient DIC concentration. Under high DIC levels, all the examined coccoliths lacked any offset from inorganic calcite, whereas in low (present-day) DIC concentrations, these vital effects and interspecies differences become substantial. These laboratory observations support the recent hypothesis from field observations that the appearance of interspecific vital effect in coccolithophores coincides with the long-term Neogene decline of atmospheric CO 2 concentrations. The present study brings further valuable constraints on coccolith isotopic compositions by demonstrating the threshold for the absence of vital effects under high DIC regimes. From a mechanistic viewpoint, we show that the vital effect is determined by physiology; growth rate, cell size and relative rates of photosynthesis and calcification, and a modulation of these parameters with ambient carbon availability. This study provides palaeoceanographers with a biogeochemical framework that can be utilised to further develop the use of calcareous nannofossils in palaeoceanography to derive sea surface temperature and p CO 2 levels.

Description: Ocean acidification modulates expression of genes and physiological performance of a marine diatom Biogeosciences Discussions, 12, 15809-15833, 2015 Author(s): Y. Li, S. Zhuang, Y. Wu, H. Ren, F. Cheng, X. Lin, K. Wang, J. Beardall, and K. Gao Ocean Acidification (OA) is known to affect various aspects of the physiological performance of diatoms, but there is little information on the underlining molecular mechanisms involved. Here, we show that in the model diatom Phaeodactylum tricornutum expression of the genes related to light harvesting, carbon acquisition and carboxylation, nitrite assimilation and ATP synthesis are modulated by OA. Growth and photosynthetic carbon fixation were enhanced by elevated CO 2 (1000 μatm) under both constant indoor and fluctuating outdoor light regimes. The genetic expression of nitrite reductase (NiR) was up-regulated by OA regardless of light levels and/or regimes. The transcriptional expression of fucoxanthin chlorophyll a/c protein (lhcf type (FCP)) and mitochondrial ATP synthase (mtATP synthase) genes were also enhanced by OA, but only under high light intensity. OA treatment decreased the expression of β-carbonic anhydrase (β-CA) along with down-regulation of CO 2 concentrating mechanisms (CCMs). Additionally, the genes for these proteins (NiR, FCP, mtATP synthase, β-CA) showed diel expressions either under constant indoor light or fluctuating sunlight. Thus, OA enhanced photosynthetic and growth rates by stimulating nitrogen assimilation and indirectly by down-regulating the energy-costly inorganic carbon acquisition process.

Description: Comparing solubility algorithms of greenhouse gases in Earth-System modelling Biogeosciences Discussions, 12, 15925-15945, 2015 Author(s): V. M. N. C. S. Vieira, E. Sahlée, P. Jurus, E. Clementi, H. Pettersson, and M. Mateus Accurate solubility estimates are fundamental for (i) Earth-System models forecasting the climate change taking into consideration the atmosphere–ocean balances and trades of greenhouse gases, and (ii) using field data to calibrate and validate the algorithms simulating those trades. We found important differences between the formulation generally accepted and a recently proposed alternative relying on a different chemistry background. First, we tested with field data from the Baltic Sea, which also enabled finding differences between using water temperatures measured at 0.5 or 4 m depths. Then, we used data simulated by atmospheric (Meteodata application of WRF) and oceanographic (WW3-NEMO) models of the European Coastal Ocean and Mediterranean to compare the use of the two solubility algorithms in Earth-System modelling. The mismatches between both formulations lead to a difference of millions of tons of CO 2 , and hundreds of tons of CH 4 and N 2 O, dissolved in the first meter below the sea surface of the whole modelled region.

Description: Seasonal variations in metallic mercury (Hg 0 ) vapor exchange over biannual wheat – corn rotation cropland in the North China Plain Biogeosciences Discussions, 12, 16105-16158, 2015 Author(s): J. Sommar, W. Zhu, L. Shang, C.-J. Lin, and X. B. Feng Air-surface gas exchange of Hg 0 was measured in five approximately bi-weekly campaigns (in total 87 days) over a wheat-corn rotation cropland located in the North China Plain using the relaxed eddy accumulation (REA) technique. The campaigns were separated over duration of a full year period (201–2013) aiming to capture the flux pattern over essential growing stages of the planting system with a low homogeneous topsoil Hg content (~ 45 ng g −1 ). Contrasting pollution regimes influenced air masses at the site and corresponding Hg 0 concentration means (3.3 in late summer to 6.2 ng m −3 in winter) were unanimously above the typical hemispheric background of 1.5–1.7 ng m −3 during the campaigns. Extreme values in bi-directional net Hg 0 exchange were primarily observed during episodes of peaking Hg 0 concentrations. In tandem with under-canopy chamber measurements, the above-canopy REA measurements provided evidence for a balance between Hg 0 ground emissions and uptake of Hg 0 by the developed canopies. During the wheat growing season covering ~ 2/3 of the year at the site, net field-scale Hg 0 emission was prevailing for periods of active plant growth until canopy senescence (mean flux: 20.0 ng m −3 ) disclosing the dominance of Hg 0 soil efflux during warmer seasons. In the final vegetative stage of corn and wheat, ground and above-canopy Hg 0 flux displayed inversed daytime courses with a near mid-day maximum (emission) and minimum (deposition), respectively. In contrast to wheat, Hg 0 uptake of the corn canopy at this stage offset ground Hg 0 emissions with additional removal of Hg 0 from the atmosphere. Differential uptake of Hg 0 between wheat (C 3 species) and corn (C 4 species) foliage is discernible from estimated Hg 0 flux (per leaf area) and Hg content in mature cereal leaves being a factor of 〉 3 higher for wheat (at ~ 120 ng g −1 dry weight). Furthermore, this study shows that intermittent flood irrigation of the air-dry field induced a short pulse of Hg 0 emission due to displacement of Hg 0 present in the surface soil horizon. A more lingering effect of flood irrigation is however suppressed Hg 0 soil emissions, which for wet soil (~ 30 %-vol) beneath the corn canopy was on an average a factor of ~ 3 lower than that for drier soil ( 〈 10 %-vol) within wheat stands. Extrapolation of the campaign Hg 0 flux data (mean: 7.1 ng m −2 h −1 ) to the whole year suggests the wheat-corn rotation cropland a net source of atmospheric Hg 0 . The observed magnitude of annual wet deposition flux (~ 8.8 μg Hg m −2 ) accounted for a minor fraction of soil Hg 0 evasion flux prevailing over the majority of year. Therefore, we suggest that dry deposition of other forms of airborne Hg constitutes the dominant pathway of Hg input to this local ecosystem and that these deposited forms would be gradually transformed and re-emitted as Hg 0 rather than being sequestered here. In addition, after crop harvesting, the practice of burning agricultural residue with considerable Hg content rather than straw return management yields seasonally substantial atmospheric Hg 0 emissions from croplands in the NCP region.

Description: Gastropod diversification and community structuring processes in ancient Lake Ohrid: a metacommunity speciation perspective Biogeosciences Discussions, 12, 16081-16103, 2015 Author(s): T. Hauffe, C. Albrecht, and T. Wilke The Balkan Lake Ohrid is the oldest and most speciose freshwater lacustrine system in Europe. However, it remains unclear whether the diversification of its endemic taxa is mainly driven by neutral processes, environmental factors, or species interactions. This calls for a holistic perspective involving both evolutionary processes and ecological dynamics. Such a unifying framework – the metacommunity speciation model – considers how community assembly affects diversification and vice versa by assessing the relative contribution of the three main community assembly processes, dispersal limitation, environmental filtering, and species interaction. The current study therefore used the species-rich model taxon Gastropoda to assess how extant communities in Lake Ohrid are structured by performing process based metacommunity analyses. Specifically, the study aimed at (i) identifying the relative importance of the three community assembly processes and (ii) to test whether the importance of these individual processes changes gradually with lake depth or whether they are distinctively related to eco-zones. Based on specific simulation steps for each of the three processes, it could be demonstrated that dispersal limitation had the strongest influence on gastropod community structures in Lake Ohrid. However, it was not the exclusive assembly process but acted together with the other two processes – environmental filtering, and species interaction. In fact, the relative importance of the three community assembly processes varied both with lake depth and eco-zones, though the processes were better predicted by the latter. The study thus corroborated the high importance of dispersal limitation for both maintaining species richness in Lake Ohrid (through its impact on community structure) and generating endemic biodiversity (via its influence on diversification processes). However, according to the metacommunity speciation model, the inferred importance of environmental filtering and biotic interaction also suggests a small but significant influence of ecological speciation. These findings contribute to the main goal of the SCOPSCO initiative – inferring the drivers of biotic evolution – and might provide an integrative perspective on biological and limnological dynamics in ancient Lake Ohrid.

Description: Technical Note: Semi-rigid chambers for methane gas flux measurements on tree-stems Biogeosciences Discussions, 12, 16019-16048, 2015 Author(s): A. Siegenthaler, B. Welch, S. R. Pangala, M. Peacock, and V. Gauci There is increasing interest in the measurement of methane (CH 4 ) emissions from tree stems in a wide range of ecosystems so as to determine how they contribute to the total ecosystem flux. To date, tree CH 4 fluxes are commonly measured using rigid closed chambers (static or dynamic), which often pose challenges as these are bulky and limit measurement of CH 4 fluxes to only a very narrow range of tree stem sizes and shapes. To overcome these challenges we aimed to design, describe and test new semi-rigid stem-flux chambers (or sleeves). We compared semi-rigid chamber's gas permeability to CH 4 against the traditional rigid chamber approach, in the laboratory and in the field, with continuous flow or syringe injections. We found that the semi-rigid chambers performed well, and had numerous benefits including reduced gas permeability and optimal stem gas exchange surface to total chamber volume ratio ( S c / V tot ) allowing better headspace mixing, especially when connected in a dynamic mode to a continuous flow gas analyser. Semi-rigid sleeves can easily be constructed and transported in multiple sizes, are extremely light, cheap to build and fast to deploy. This makes them ideal for use in remote ecosystems where access logistics are complicated.

Description: Differential resilience of ancient sister lakes Ohrid and Prespa to environmental disturbances during the Late Pleistocene Biogeosciences Discussions, 12, 16049-16079, 2015 Author(s): E. Jovanovska, A. Cvetkoska, T. Hauffe, Z. Levkov, B. Wagner, R. Sulpizio, A. Francke, C. Albrecht, and T. Wilke Ancient lakes, like lakes Ohrid and Prespa on the Balkan Peninsula, have become model systems for studying the link between geological and biotic evolution. Recently the scientific deep drilling program "Scientific Collaboration on Past Speciation Conditions in Lake Ohrid" (SCOPSCO) has been launched to better understand the environmental, climatic and limnological evolution of the lake. It revealed that Lake Ohrid experienced a number of environmental disturbances during its ca. 2.0 million year long history. They comprise disturbances that lasted over longer periods of times ("press events") such as Heinrich events as well as sudden and short disturbances ("pulse events") like the deposition of volcanic ashes. The latter include one of the most severe volcanic episodes during the Late Pleistocene, the eruption of the Campanian Ignimbrite (known as Y-5 marine tephra layer) from the Campi Flegrei caldera, dated at 39.6 ± 0.1 ka ago. The event is recorded by the deposition of a ca. 15 cm thick Y-5 tephra layer in sediment cores of lakes Ohrid (DEEP-5045-1) and Prespa (Co1204). This pulse event is overlain by the Heinrich event 4 (H4), 40.0–38.0 ka ago. In the current paper, diatoms were used as proxies to compare the responses of these lakes to the Y-5 (pulse) and the H4 (press) disturbances. Based on stratigraphically constrained incremental sum of squares cluster (CONISS) and unconstrained Partitioning Around Medoids (PAM) analyses, we found only little evidence that the diatom community compositions in either lake responded to the H4 event. However, the Y-5 influx caused clear and rapid diatom community changes. After the initial response, community composition in Lake Ohrid and, to a lesser extent, in Lake Prespa slowly returned to their quasi pre-disturbance state. Moreover, there is no evidence for disturbance-related extinction events. The combined evidence from these findings suggests that lakes Ohrid and Prespa likely did not experience regime shifts. It is therefore concluded that both lakes show resilience to environmental disturbance. However, it seems that Lake Ohrid is more resilient than Lake Prespa as the recovery of diatom communities is more pronounced and as its estimated recovery time is only ca. 1400 years vs. ca. 3600 years in Lake Prespa. The reasons for the differential responses remain largely unknown, but differences in geology, lake age, limnology, and intrinsic parameters of the diatom proxies may play a role. Given the relative robust results obtained, this study provides important new insights into the response of lakes to (multiple) environmental disturbances. Moreover, it contributes to one of the major goals of the SCOPSCO project – to evaluate the influence of major geological events onto the evolution of endemic taxa in Lake Ohrid.

Description: Technical Note: Sampling and processing of mesocosm sediment trap material for quantitative biogeochemical analysis Biogeosciences Discussions, 12, 18693-18722, 2015 Author(s): T. Boxhammer, L. T. Bach, J. Czerny, and U. Riebesell Sediment traps are the most common tool to investigate vertical particle flux in the marine realm. However, the spatial decoupling between particle formation and collection often handicaps reconciliation of these two processes even within the euphotic zone. Pelagic mesocosms have the advantage of being closed systems and are therefore ideally suited to study how processes in natural plankton communities influence particle formation and settling in the ocean's surface. We therefore developed a protocol for efficient sample recovery and processing of quantitatively collected pelagic mesocosm sediment trap samples. Sedimented material was recovered by pumping it under gentle vacuum through a silicon tube to the sea surface. The particulate matter of these samples was subsequently concentrated by passive settling, centrifugation or flocculation with ferric chloride and we discuss the advantages of each approach. After concentration, samples were freeze-dried and ground with an easy to adapt procedure using standard lab equipment. Grain size of the finely ground samples ranges from fine to coarse silt (2–63 μm), which guarantees homogeneity for representative subsampling, a widespread problem in sediment trap research. Subsamples of the ground material were perfectly suitable for a variety of biogeochemical measurements and even at very low particle fluxes we were able to get a detailed insight on various parameters characterizing the sinking particles. The methods and recommendations described here are a key improvement for sediment trap applications in mesocosms, as they facilitate processing of large amounts of samples and allow for high-quality biogeochemical flux data.

Description: Distribution of Arctic and Pacific copepods and their habitat in the northern Bering Sea and Chukchi Sea Biogeosciences Discussions, 12, 18661-18691, 2015 Author(s): H. Sasaki, K. Matsuno, A. Fujiwara, M. Onuka, A. Yamaguchi, H. Ueno, Y. Watanuki, and T. Kikuchi The advection of warm Pacific water and the reduction of sea-ice extent in the western Arctic Ocean may influence the abundance and distribution of copepods, i.e., a key component in food webs. To understand the factors affecting abundance of copepods in the northern Bering Sea and Chukchi Sea, we constructed habitat models explaining the spatial patterns of the large and small Arctic copepods and the Pacific copepods, separately, using generalized additive models. Copepods were sampled by NORPAC net. Vertical profiles of density, temperature and salinity in the seawater were measured using CTD, and concentration of chlorophyll a in seawater was measured with a fluorometer. The timing of sea-ice retreat was determined using the satellite image. To quantify the structure of water masses, the magnitude of pycnocline and averaged density, temperature and salinity in upper and bottom layers were scored along three axes using principal component analysis (PCA). The structures of water masses indexed by the scores of PCAs were selected as explanatory variables in the best models. Large Arctic copepods were abundant in the water mass with high salinity water in bottom layer or with cold/low salinity water in upper layer and cold/high salinity water in bottom layer, and small Arctic copepods were abundant in the water mass with warm/saline water in upper layer and cold/high salinity water in bottom layers, while Pacific copepods were abundant in the water mass with warm/saline in upper layer and cold/high salinity water in bottom layer. All copepod groups were abundant in areas with deeper depth. Although chlorophyll a in upper and bottom layers were selected as explanatory variables in the best models, apparent trends were not observed. All copepod groups were abundant where the sea-ice retreated at earlier timing. Our study might indicate potential positive effects of the reduction of sea-ice extent on the distribution of all groups of copepods in the Arctic Ocean.

Description: The interaction between nitrogen and phosphorous is a strong predictor of intra-plant variation in nitrogen isotope composition in a desert species Biogeosciences Discussions, 12, 18769-18794, 2015 Author(s): J. Zhang, L. Gu, J. Zhang, R. Wu, F. Wang, G. Lin, B. Wu, Q. Lu, and P. Meng Understanding intra-plant variations in δ 15 N, which can be large, is essential for fully utilizing the role of δ 15 N as an integrator of the terrestrial nitrogen cycle. Studying such variations can yield insights into nitrogen metabolisms by the plant as a whole or by specific plant organs. However, systematical evaluation of intra-plant variations in δ 15 N and their relationship with organ nutrient contents is rare. We excavated whole plant architectures of Nitraria tangutorum Bobrov, a C 3 species that has an exceptional capability of fixing sands and building sand dunes, in two deserts in northwestern China. We systematically and simultaneously measured nitrogen isotope ratios and nitrogen and phosphorous contents of different parts of the excavated plants. We found that intra-plant variations in δ 15 N of N. tangutorum were positively correlated with corresponding organ nitrogen (N) and phosphorous (P) contents. However, it was the N × P interaction, not N and P individually or their linear combination, that was the strongest predictor of intra-plant δ 15 N. We hypothesized that this strong positive intra-plant δ 15 N–N/P relationship was caused by fractionating gaseous N losses (volatilization) from plants and that the volatilization depended on the interaction between organ N and P contents. We also showed that root δ 15 N increased with depth into soil. This pattern was similar to profiles of soil δ 15 N reported by previous studies in different ecosystems although the exact relationship between root and soil profiles in δ 15 N was not clear. Studies of intra-plant variations in δ 15 N in different species, ecosystems, and climates and measurements of plant nitrogen volatilization and associated isotope fractionation are needed in order to determine the exact mechanisms responsible for the significant patterns first reported in this study.

Description: Changing seasonality of the Baltic Sea Biogeosciences Discussions, 12, 18855-18882, 2015 Author(s): M. Kahru, R. Elmgren, and O. P. Savchuk Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystems. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17–35 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes such as earlier start of the summer season and prolongation of the productive season in multiple variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m −2 of surface incoming shortwave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 °C has almost doubled (from 29 days in 1982 to 56 days in 2014), the period with Ked490 over 0.4 m −1 has increased from about 60 days in 1998 to 240 days in 2013, i.e. quadrupled. The period with satellite-detected CHL of at least 3 mg m −3 has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now switched to the summer cyanobacteria bloom in July.

Description: Blooms of cyanobacteria in a temperate Australian lagoon system post and prior to European settlement Biogeosciences Discussions, 12, 18829-18853, 2015 Author(s): P. L. M. Cook, M. Jennings, D. P. Holland, J. Beardall, C. Briles, A. Zawadzki, P. Doan, K. Mills, and P. Gell Blooms of noxious N 2 fixing cyanobacteria such as Nodularia spumigena are a recurring problem in some estuaries. Here we report the results of a palaeoecological study on a temperate Australian lagoon system (The Gippsland Lakes) where we used stable isotopes and pigment biomarkers in dated cores as proxies for eutrophication and blooms of cyanobacteria. Pigment proxies show a clear signal, with an increase in cyanobacterial pigments (echinenone, canthaxanthin and zeaxanthin) in the period coinciding with recent blooms. Another excursion in these proxies was observed prior to the opening of an artificial entrance to the lakes in 1889, which markedly increased the salinity of the Gippsland Lakes. A coincident increase in the sediment organic carbon content in the period prior to the opening of the artificial entrance suggests the bottom waters of the lakes were increasingly stratified and hypoxic, which would have led to an increase in the recycling of phosphorus. After the opening of the artificial entrance there was a ~ 60 year period with low values for the cyanobacterial proxies as well as a low sediment organic carbon content suggesting a period of low bloom activity associated with the increased salinity of the lakes. During the 1940s, the current period of re-eutrophication commenced as indicated by a steadily increasing sediment organic carbon content and cyanobacterial pigments. We suggest increasing nitrogen inputs from the catchment led to the return of hypoxia and increased phosphorus release from the sediment, which drove the re-emergence of cyanobacterial blooms.

Description: Marine regime shifts in ocean biogeochemical models: a case study in the Gulf of Alaska Biogeosciences Discussions, 12, 14003-14048, 2015 Author(s): C. Beaulieu, H. Cole, S. Henson, A. Yool, T. R. Anderson, L. de Mora, E. T. Buitenhuis, M. Butenschön, I. J. Totterdell, and J. I. Allen Regime shifts have been reported in many marine ecosystems, and are often expressed as an abrupt change occurring in multiple physical and biological components of the system. In the Gulf of Alaska, a regime shift in the late 1970s was observed, indicated by an abrupt increase in sea surface temperature and major shifts in the catch of many fish species. This late 1970s regime shift in the Gulf of Alaska was followed by another shift in the late 1980s, not as pervasive as the 1977 shift, but which nevertheless did not return to the prior state. A thorough understanding of the extent and mechanisms leading to such regime shifts is challenged by data paucity in time and space. We investigate the ability of a suite of ocean biogeochemistry models of varying complexity to simulate regime shifts in the Gulf of Alaska by examining the presence of abrupt changes in time series of physical variables (sea surface temperature and mixed layer depth), nutrients and biological variables (chlorophyll, primary productivity and plankton biomass) using change-point analysis. Our study demonstrates that ocean biogeochemical models are capable of simulating the late 1970s shift, indicating an abrupt increase in sea surface temperature forcing followed by an abrupt decrease in nutrients and biological productivity. This predicted shift is consistent among all the models, although some of them exhibit an abrupt transition (i.e. a significant shift from one year to the next), whereas others simulate a smoother transition. Some models further suggest that the late 1980s shift was constrained by changes in mixed layer depth. Our study demonstrates that ocean biogeochemical can successfully simulate regime shifts in the Gulf of Alaska region, thereby providing better understanding of how changes in physical conditions are propagated from lower to upper trophic levels through bottom-up controls.

Description: Methane dynamics in warming tundra of Northeast European Russia Biogeosciences Discussions, 12, 13931-13965, 2015 Author(s): M. E. Marushchak, T. Friborg, C. Biasi, M. Herbst, T. Johansson, I. Kiepe, M. Liimatainen, S. E. Lind, P. J. Martikainen, T. Virtanen, H. Soegaard, and N. J. Shurpali Methane (CH 4 ) fluxes were investigated in a subarctic Russian tundra site in a multi-approach study combining plot scale data, ecosystem scale eddy covariance (EC) measurements and fine resolution land cover classification scheme for regional upscaling. The flux data as measured by the two independent techniques resulted in a seasonal (May–October 2008) cumulative CH 4 emission of 2.4 (EC) and 3.7 g CH 4 m −2 (manual chambers) for the source area representative of the footprint of the EC instruments. Upon upscaling for the entire study region of 98.6 km 2 , the chamber measured flux data yielded a regional flux estimate of 6.7 g CH 4 m −2 yr −1 . Our upscaling efforts accounted for the large spatial variability in the distribution of the various land cover types (LCTs) predominant at our study site. In particular, wetlands with emissions ranging from 34 to 53 g CH 4 m −2 yr −1 were the most dominant CH 4 emitting surfaces. Emissions from thermokarst lakes were an order of magnitude lower, while the rest of the landscape (mineral tundra) was a weak sink for atmospheric methane. Vascular plant cover was a key factor in explaining the spatial variability of CH 4 emissions among wetland types, as indicated by the positive correlation of emissions with the leaf area index (LAI). As elucidated through a stable isotope analysis, the plant transport was the dominant CH 4 release pathway that discriminates against heavier δ 13 C-CH 4 . The methane released from wetlands was lighter than that in the surface porewater and δ 13 C in the emitted CH 4 correlated with the vascular plant cover (LAI) implying that the plant-mediated CH 4 release dominates. A mean value of δ 13 C obtained here for the emitted CH 4 , −68.2 ± 2.0 ‰, is within the range of values from other wetlands, thus reinforcing the use of inverse modeling tools to better constrain the CH 4 budget. Based on the IPCC A1B emission scenario, a temperature increase of 7 °C has been predicted for the tundra region of European Russia by the end of the 21st Century. A regional warming of this magnitude will have profound effects on the permafrost distribution leading to considerable changes in the regional landscape with a potential for an increase in the areal extent of methane emitting wet surfaces. We estimate that a projected increase in air temperature of 7 °C with a mere 10 % associated increase in the combined areal coverage of willow stands, fens and lakes in the region will lead to a 51 % higher amount of CH 4 being emitted into the atmosphere by the end of this century.

Description: Uncertainty analysis of gross primary production partitioned from net ecosystem exchange measurements Biogeosciences Discussions, 12, 13967-14002, 2015 Author(s): R. Raj, N. A. S. Hamm, C. van der Tol, and A. Stein Gross primary production (GPP), separated from flux tower measurements of net ecosystem exchange (NEE) of CO 2 , is used increasingly to validate process-based simulators and remote sensing-derived estimates of simulated GPP at various time steps. Proper validation should include the uncertainty associated with this separation at different time steps. This can be achieved by using a Bayesian framework. In this study, we estimated the uncertainty in GPP at half hourly time steps. We used a non-rectangular hyperbola (NRH) model to separate GPP from flux tower measurements of NEE at the Speulderbos forest site, The Netherlands. The NRH model included the variables that influence GPP, in particular radiation, and temperature. In addition, the NRH model provided a robust empirical relationship between radiation and GPP by including the degree of curvature of the light response curve. Parameters of the NRH model were fitted to the measured NEE data for every 10-day period during the growing season (April to October) in 2009. Adopting a Bayesian approach, we defined the prior distribution of each NRH parameter. Markov chain Monte Carlo (MCMC) simulation was used to update the prior distribution of each NRH parameter. This allowed us to estimate the uncertainty in the separated GPP at half-hourly time steps. This yielded the posterior distribution of GPP at each half hour and allowed the quantification of uncertainty. The time series of posterior distributions thus obtained allowed us to estimate the uncertainty at daily time steps. We compared the informative with non-informative prior distributions of the NRH parameters. The results showed that both choices of prior produced similar posterior distributions GPP. This will provide relevant and important information for the validation of process-based simulators in the future. Furthermore, the obtained posterior distributions of NEE and the NRH parameters are of interest for a range of applications.

Description: Data-based estimates of the ocean carbon sink variability – first results of the Surface Ocean p CO 2 Mapping intercomparison (SOCOM) Biogeosciences Discussions, 12, 14049-14104, 2015 Author(s): C. Rödenbeck, D. C. E. Bakker, N. Gruber, Y. Iida, A. R. Jacobson, S. Jones, P. Landschützer, N. Metzl, S. Nakaoka, A. Olsen, G.-H. Park, P. Peylin, K. B. Rodgers, T. P. Sasse, U. Schuster, J. D. Shutler, V. Valsala, R. Wanninkhof, and J. Zeng Using measurements of the surface-ocean CO 2 partial pressure ( p CO 2 ) and 14 different p CO 2 mapping methods recently collated by the Surface Ocean p CO 2 Mapping intercomparison (SOCOM) initiative, variations in regional and global sea–air CO 2 fluxes have been investigated. Though the available mapping methods use widely different approaches, we find relatively consistent estimates of regional p CO 2 seasonality, in line with previous estimates. In terms of interannual variability (IAV), all mapping methods estimate the largest variations to occur in the Eastern equatorial Pacific. Despite considerable spead in the detailed variations, mapping methods with closer match to the data also tend to be more consistent with each other. Encouragingly, this includes mapping methods belonging to complementary types – taking variability either directly from the p CO 2 data or indirectly from driver data via regression. From a weighted ensemble average, we find an IAV amplitude of the global sea–air CO 2 flux of 0.31 PgC yr −1 (standard deviation over 1992–2009), which is larger than simulated by biogeochemical process models. On a decadal perspective, the global CO 2 uptake is estimated to have gradually increased since about 2000, with little decadal change prior to 2000. The weighted mean total ocean CO 2 sink estimated by the SOCOM ensemble is consistent within uncertainties with estimates from ocean-interior carbon data or atmospheric oxygen trends.

Description: Contributions of dynamic environmental signals during life-cycle transitions to early life-history traits in lodgepole pine ( Pinus contorta Dougl.) Biogeosciences Discussions, 12, 14105-14138, 2015 Author(s): Y. Liu, T. Wang, and Y. A. El-Kassaby Environmental signals are important triggers in the life-cycle transitions and play a crucial role in the life-history evolution. Yet, very little is known about the leading ecological factors contributing to the variations of life-history traits in perennial plants. This paper explores both the causes and consequences for the evolution of life-history traits (i.e., seed dormancy and size) in lodgepole pine ( Pinus contorta Dougl.) across British Columbia (B.C.), Canada. We selected 83 logepole pine populations covering 22 ecosystem zones of B.C. and through their geographic coordinate, 197 climatic variables were generated accordingly for the reference (1961–1990) and future (2041–2070) periods. We found that dynamic climatic variables rather than constant geographic variables are the true environmental driving forces in seed dormancy and size variations and thus provide reliable predictors in response to global climate change. Evapotranspiration and precipitation in the plant-to-seed chronology are the most critical climate variables for seed dormancy and size variations, respectively. Hence, we predicted that levels of seed dormancy in lodgepole pine would increase across large tracts of B.C. in 2050s. Winter-chilling is able to increase the magnitude of life-history plasticity and lower the bet-hedge strategy in the seed-to-plant transition; however, winter-chilling is likely to be insufficient in the north of 49° N in 2050s, which may delay germination while unfavourable conditions during dry summers may result in adverse consequences in the survival of seedlings owing to extended germination span.

Description: A novel paleo-bleaching proxy using boron isotopes and high-resolution laser ablation to reconstruct coral bleaching events Biogeosciences Discussions, 12, 8131-8155, 2015 Author(s): G. Dishon, J. Fisch, I. Horn, K. Kaczmarek, J. Bijma, D. F. Gruber, O. Nir, Y. Popovich, and D. Tchernov Coral reefs occupy only ~0.1% of the oceans habitat, but are the most biologically diverse marine ecosystem. In recent decades, coral reefs have experienced significant global declines due to a variety of causes, one of the major being widespread coral bleaching events. During bleaching the coral expels its symbiotic algae losing its main source of nutrition generally obtained through photosynthesis. While recent coral bleaching events have been extensively investigated, there is no scientific data on historical coral bleaching prior to 1979. In this study, we employ high-resolution femtosecond Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) to demonstrate a distinct biologically-induced decline of boron (B) isotopic composition (δ 11 B) as a result of coral bleaching. These findings and methodology offer a new use for a previously developed isotopic proxy to reconstruct paleo-coral bleaching events. Based on a literature review of published δ 11 B data and our recorded "vital effect" of coral bleaching on the δ 11 B signal, we also describe at least two possible coral bleaching events since the Last Glacial Maximum. The implementation of this bleaching proxy holds the potential of identifying occurrences of coral bleaching throughout the geological record. A deeper temporal view of coral bleaching will enable scientists to determine if it occurred in the past during times of environmental change and what outcome it may have had on coral population structure.

Description: Nitrogen cycling in shallow low oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes Biogeosciences Discussions, 12, 7257-7299, 2015 Author(s): H. Hu, A. Bourbonnais, J. Larkum, H. W. Bange, and M. A. Altabet O 2 minimum zones (OMZ) of the world's oceans are important locations for microbial dissimilatory NO 3 - reduction and subsequent loss of combined nitrogen (N) to biogenic N 2 gas. This is particularly so when the OMZ is coupled to a region of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N isotope ratios (and O in the case of NO 3 - and NO 2 - ) can be used as natural tracers of OMZ N-cycling because of distinct kinetic isotope effects associated with microbially-mediated N-cycle transformations. Here we present NO 2 - and NO 3 - stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface O 2 was generally depleted below about 30 m depth with O 2 less than 10 μM, while NO 2 - concentrations were high, ranging from 6 to 10 μM and NO 3 - was in places strongly depleted to near 0 μM. We observed for the first time, a positive linear relationship between NO 2 - δ 15 N and δ 18 O at our coastal stations, analogous to that of NO 3 - N and O isotopes during assimilatory and dissimilatory reduction. This relationship is likely the result of rapid NO 2 - turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO 2 - facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ 15 N of [NO 3 - ], DIN, or biogenic N 2 , values for ε are generally much lower than previously reported, reaching as low as 6.5‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.

Description: Characterization of active and total fungal communities in the atmosphere over the Amazon rainforest Biogeosciences Discussions, 12, 7177-7207, 2015 Author(s): A. M. Womack, P. E. Artaxo, F. Y. Ishida, R. C. Mueller, S. R. Saleska, K. T. Wiedemann, B. J. M. Bohannan, and J. L. Green Fungi are ubiquitous in the atmosphere and may play an important role in atmospheric processes. We investigated the composition and diversity of fungal communities over the Amazon rainforest canopy and compared these communities to fungal communities found in terrestrial environments. We characterized the total fungal community and the metabolically active portion of the community using high-throughout DNA and RNA sequencing and compared these data to predictions generated by a mass-balance model. We found that the total community was primarily comprised of fungi from the phylum Basidiomycota. In contrast, the active community was primarily composed of members of the phylum Ascomycota and included a high relative abundance of lichen fungi, which were not detected in the total community. The relative abundance of Basidiomycota and Ascomycota in the total and active communities was consistent with our model predictions, suggesting that this result was driven by the relative size and number of spores produced by these groups. When compared to other environments, fungal communities in the atmosphere were most similar to communities found in tropical soils and leaf surfaces, suggesting that inputs of fungi to the atmosphere are from local, rather than distant, sources. Our results demonstrate that there are significant differences in the composition of the total and active fungal communities in the atmosphere, and that lichen fungi, which have been shown to be efficient ice nucleators, may be abundant members of active atmospheric fungal communities over the forest canopy.

Description: Effects of nitrate and phosphate supply on chromophoric and fluorescent dissolved organic matter in the Eastern Tropical North Atlantic: a mesocosm study Biogeosciences Discussions, 12, 7209-7255, 2015 Author(s): A. N. Loginova, C. Borchard, J. Meyer, H. Hauss, R. Kiko, and A. Engel The Eastern Tropical North Atlantic (ETNA) is an open ocean region with little input of terrestrial dissolved organic matter (DOM), suggesting that pelagic production has to be the main source of DOM. Inorganic nitrogen (DIN) and phosphorus (DIP) concentrations affect pelagic production, leading to DOM modifications. The quantitative and qualitative changes in DOM are often estimated by its optical properties. Colored DOM (CDOM) is often used to estimate dissolved organic carbon (DOC) concentrations by applied techniques, e.g. through remote sensing, whereas DOM properties, such as molecular weight, can be estimated from the slopes of the CDOM absorption spectra ( S ). Fluorescence properties of CDOM (FDOM) allow discriminating between different structural CDOM properties. The investigation of distribution and cycling of CDOM and FDOM was recognized to be important for understanding of physical and biogeochemical processes, influencing DOM. However, little information is available about effects of nutrient variability on CDOM and FDOM dynamics. Here we present results from two mesocosm experiments conducted with a natural plankton community of the ETNA, where effects of DIP ("Varied P") and DIN ("Varied N") supply on optical properties of DOM were studied. CDOM accumulated proportionally to phytoplankton biomass during the experiments. S decreased over time indicating accumulation of high molecular weight DOM. In Varied N, an additional CDOM portion, as a result of bacterial DOM reworking, was determined. It increased the CDOM fraction in DOC proportionally to the supplied DIN. The humic-like FDOM component (Comp.1) was derived by bacteria proportionally to DIN supply. The bound-to-protein amino acid-like FDOM component (Comp.2) was released irrespectively to phytoplankton biomass, but depending on DIP and DIN concentrations, as a part of an overflow mechanism. Under high DIN supply, Comp.2 was removed by bacterial reworking processes, leading to an accumulation of humic-like Comp.1. No influence of nutrient availability on amino acid-like FDOM component in peptide form (Comp.3) was observed. Comp.3 potentially acted as an intermediate product during formation or degradation Comp.2. Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and "quality" of optically active DOM and, therefore, might bias results of the applied techniques for an estimation of DOC concentrations in open ocean regions.

Description: Greenhouse gas balance of cropland conversion to bioenergy poplar short rotation coppice Biogeosciences Discussions, 12, 8035-8084, 2015 Author(s): S. Sabbatini, N. Arriga, T. Bertolini, S. Castaldi, T. Chiti, C. Consalvo, S. Njakou Djomo, B. Gioli, G. Matteucci, and D. Papale The production of bioenergy in Europe is one of the strategies conceived to reduce greenhouse gas (GHG) emissions. The suitability of the land use change from a cropland (REF site) to a short rotation coppice plantation of hybrid poplar (SRC site) was investigated by comparing the GHG budgets of these two systems over 24 months in Viterbo, Italy. Eddy covariance measurements were carried out to quantify the net ecosystem exchange of CO 2 ( F CO 2 ), whereas chambers were used to measure N 2 O and CH 4 emissions from soil. Soil organic carbon (SOC) of an older poplar plantation was used to estimate via a regression the SOC loss due to SRC establishment. Emissions from tractors and from production and transport of agricultural inputs ( F MAN ) were modelled and GHG emission offset due to fossil fuel substitution was credited to the SRC site considering the C intensity of natural gas. Emissions due to the use of the biomass ( F EXP ) were also considered. The suitability was finally assessed comparing the GHG budgets of the two sites. F CO 2 was the higher flux in the SRC site (−3512 ± 224 g CO 2 eq m −2 in two years), while in the REF site it was −1838 ± 107 g CO 2 m −2 in two years. F EXP was equal to 1858 ± 240 g CO 2 m −2 in 24 months in the REF site, thus basically compensating F CO 2 , while it was 1118 ± 521 g CO 2 eq m −2 in 24 months in the SRC site. This latter could offset −379.7 ± 175.1 g CO 2 eq m −2 from fossil fuel displacement. Soil CH 4 and N 2 O fluxes were negligible. F MAN weighed 2 and 4% in the GHG budgets of SRC and REF sites respectively, while the SOC loss weighed 455 ± 524 g CO 2 m −2 in two years. Overall, the REF site was close to neutrality in a GHG perspective (156 ± 264 g CO 2 eq m −2 ), while the SRC site was a net sink of −2202 ± 792 g CO 2 eq m −2 . In conclusion the experiment led to a positive evaluation of the conversion of cropland to bioenergy SRC from a GHG viewpoint.

Description: Interpreting canopy development and physiology using the EUROPhen camera network at flux sites Biogeosciences Discussions, 12, 7979-8034, 2015 Author(s): L. Wingate, J. Ogée, E. Cremonese, G. Filippa, T. Mizunuma, M. Migliavacca, C. Moisy, M. Wilkinson, C. Moureaux, G. Wohlfahrt, A. Hammerle, L. Hörtnagl, C. Gimeno, A. Porcar-Castell, M. Galvagno, T. Nakaji, J. Morison, O. Kolle, A. Knohl, W. Kutsch, P. Kolari, E. Nikinmaa, A. Ibrom, B. Gielen, W. Eugster, M. Balzarolo, D. Papale, K. Klumpp, B. Köstner, T. Grünwald, R. Joffre, J.-M. Ourcival, M. Hellstrom, A. Lindroth, G. Charles, B. Longdoz, B. Genty, J. Levula, B. Heinesch, M. Sprintsin, D. Yakir, T. Manise, D. Guyon, H. Ahrends, A. Plaza-Aguilar, J. H. Guan, and J. Grace Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO 2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time-series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations we found that these phenological events could be detected adequately (RMSE 〈 8 and 11 days for leaf out and leaf fall respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring "green hump" observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO 2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO 2 in the future.

Description: Sources and Transformations of Anthropogenic Nitrogen along an Urban River-Estuarine Continuum Michael J. Pennino, Sujay S. Kaushal, Sudhir Murthy, Joel Blomquist, Jeff Cornwell, and Lora Harris Biogeosciences Discuss., doi:10.5194/bg-2016-264,2016 Manuscript under review for BG (discussion: open, 0 comments) The results of this manuscript report the analysis of the fate and transport of wastewater and anthropogenic nitrogen along the Potomac River Estuary, from Washington D.C. to the Chesapeake Bay. In conjunction with a mass balance approach, nitrate isotopes were used to estimate fluxes and trace the sources and transformations N along the estuary. This study shows that estuaries have a large capacity to transform N inputs, but with large seasonal variability due to hydrological extremes.

Description: Contrasting radiation and soil heat fluxes in Arctic shrub and wet sedge tundra Inge Juszak, Werner Eugster, Monique M. P. D. Heijmans, and Gabriela Schaepman-Strub Biogeosciences, 13, 4049-4064, doi:10.1594/PANGAEA.860561, 2016 Changes in Arctic vegetation composition and structure feed back to climate and permafrost. Using field observations at a Siberian tundra site, we find that dwarf shrubs absorb more solar radiation than wet sedges and thus amplify surface warming, especially during snow melt. On the other hand, permafrost thaw was enhanced below sedges as a consequence of high soil moisture. Standing dead sedge leaves affected the radiation budget strongly and deserve more scientific attention.

Description: Diversity and mineral substrate preference in endolithic microbial communities from marine intertidal outcrops (Isla de Mona, Puerto Rico) Estelle Couradeau, Daniel Roush, Brandon Scott Guida, and Ferran Garcia-Pichel Biogeosciences Discuss., doi:10.5194/bg-2016-254,2016 Manuscript under review for BG (discussion: open, 0 comments) Endolithic (inside rock) microbial communities are dominated by cyanobacteria, among which the true boring cyanobacteria actively perforate the mineral and play a significant role in the erosion of coastal outcrops that may increase with ocean acidification. We interrogated the microbial communities associated with various intertidal substrates of Mona Island (PR) and brought the first evidence that there exists a mineralogical substrate preference among the pioneers true boring cyanobacteria.

Description: Anaerobic methane oxidation in an East African great lake (Lake Kivu) Fleur A. E. Roland, François Darchambeau, Cédric Morana, Sean A. Crowe, Bo Thamdrup, and Alberto V. Borges Biogeosciences Discuss., doi:10.5194/bg-2016-300,2016 Manuscript under review for BG (discussion: open, 0 comments) We studied methane consumption in a tropical Great Lake (Lake Kivu, East Africa). Lake Kivu has huge methane concentrations in its deep anoxic waters, but is a very poor emitter of methane to the atmosphere, which suppose a strong methane consumption in the water column. During this study, we put in evidence high aerobic and anaerobic consumption rates, whose relative importance varied with the season (higher aerobic rates in dry season, when the oxic compartment is wider).

Description: Coastal-ocean uptake of anthropogenic carbon Timothée Bourgeois, James C. Orr, Laure Resplandy, Jens Terhaar, Christian Ethé, Marion Gehlen, and Laurent Bopp Biogeosciences, 13, 4167-4185, doi:10.5194/bg-13-4167-2016, 2016 The global coastal ocean took up 0.1 Pg C yr −1 of anthropogenic carbon during 1993–2012 based on new biogeochemical simulations with an eddying 3-D global model. That is about half of the most recent estimate, an extrapolation based on surface areas. It should not be confused with the continental shelf pump, perhaps 10 times larger, which includes natural as well as anthropogenic carbon. Coastal uptake of anthropogenic carbon is limited by its offshore transport.

Description: Anthropogenically induced environmental changes in the northeastern Adriatic Sea in the last 400 years (Panzano Bay, Gulf of Trieste) Jelena Vidović, Rafał Nawrot, Ivo Gallmetzer, Alexandra Haselmair, Adam Tomašových, Michael Stachowitsch, Vlasta Ćosović, and Martin Zuschin Biogeosciences Discuss., doi:10.5194/bg-2016-273,2016 Manuscript under review for BG (discussion: open, 0 comments) Shallow and sheltered marine embayments in urbanized areas are prone to the accumulation of pollutants, but little is known about the historical baselines of such marine ecosystems. Here we study foraminiferal assemblages, geochemical proxies and sedimentological data from 1.6 m long sediment cores to uncover ~ 400 years of anthropogenic pressure from mining, port and industrial zones in the Gulf of Trieste, Italy. From 1600 to 1900 AD, element concentrations and foraminiferal assemblages point to negligible effects of agricultural activities. The only significant anthropogenic activity during this period is mercury mining in the hinterlands of the gulf, releasing high amounts of mercury into the bay and significantly exceeding today's Italian sediment quality guidelines (SQG) and the standards on the effects of trace elements to benthic organisms (ERL and ERM). Nonetheless, the fluctuations in the concentrations of mercury do not correlate with changes in the composition and diversity of foraminiferal assemblages due to its nonbioavailability. Intensified agricultural and maricultural activities in the first half of the 20th century caused slight nutrient enrichment and a minor increase in foraminiferal diversity. Intensified port and industrial activities in the second half of 20th century increased the normalised trace element concentrations and persistent organic pollutants (PAH, PCB) in the topmost part of the core, with solely Ni exceeding Italian SQG, ERL and ERM. This increase caused only minor changes in the foraminiferal community because foraminifera in Panzano Bay have a long history of adaptation to naturally elevated trace element concentrations. Our study underlines the importance of using an integrated, multidisciplinary approach in reconstructing the history of environmental and anthropogenic changes in marine systems. Given the prolonged human impacts in coastal areas like the Gulf of Trieste, such long term baseline data are crucial for interpreting the present state of marine ecosystems.

Description: Mechanisms of Trichodesmium demise within the New Caledonian lagoon during the VAHINE mesocosm experiment Dina Spungin, Ulrike Pfreundt, Hugo Berthelot, Sophie Bonnet, Dina AlRoumi, Frank Natale, Wolfgang R. Hess, Kay D. Bidle, and Ilana Berman-Frank Biogeosciences, 13, 4187-4203, doi:10.5194/bg-13-4187-2016, 2016 The marine cyanobacterium Trichodesmium spp. forms massive blooms important to carbon and nitrogen cycling in the oceans that often collapse abruptly. We investigated a Trichodesmium bloom in the lagoon waters of New Caledonia to specifically elucidate the cellular processes mediating the bloom decline. We demonstrate physiological, biochemical, and genetic evidence for nutrient and oxidative stress that induced a genetically controlled programmed cell death (PCD) pathway leading to bloom demise.

Description: Spring phytoplankton communities of the Labrador Sea (2005–2014): pigment signatures, photophysiology and elemental ratios Glaucia M. Fragoso, Alex J. Poulton, Igor M. Yashayaev, Erica J. H. Head, and Duncan A. Purdie Biogeosciences Discuss., doi:10.5194/bg-2016-295,2016 Manuscript under review for BG (discussion: open, 0 comments) This research describes a detailed analysis of current distributions of spring phytoplankton communities in the Labrador Sea based on ten years of observations. Phytoplankton community composition varied mainly according to the contrasting hydrographical zones of the Labrador Sea. The taxonomic distinctions of these communities influenced the photosynthetic and biochemical signatures of near surface waters, which may have a profound impact on the carbon cycle in high latitude seas.

Description: Sedimentary response to sea ice and atmospheric variability over the instrumental period off Adélie Land, East Antarctica Philippine Campagne, Xavier Crosta, Sabine Schmidt, Marie Noëlle Houssais, Olivier Ther, and Guillaume Massé Biogeosciences, 13, 4205-4218, doi:10.5194/bg-13-4205-2016, 2016 Diatoms and biomarkers have been recently used for palaeoclimate reconstructions in the Southern Ocean. Few sediment-based ecological studies have investigated their relationships with environmental conditions. Here, we compare high-resolution sedimentary records with meteorological data to study relationships between our proxies and recent atmospheric and sea surface changes. Our results indicate that coupled wind pattern and sea surface variability act as the proximal forcing at that scale.

Description: Observing and modelling phytoplankton community structure in the North Sea: can ERSEM-type models simulate biodiversity? David A. Ford, Johan van der Molen, Kieran Hyder, John Bacon, Rosa Barciela, Veronique Creach, Robert McEwan, Piet Ruardij, and Rodney Forster Biogeosciences Discuss., doi:10.5194/bg-2016-304,2016 Manuscript under review for BG (discussion: open, 0 comments) This study presents a novel set of in situ observations of phytoplankton community structure for the North Sea. These observations were used to validate two physical-biogeochemical ocean model simulations, each of which used different variants of the widely-used European Regional Seas Ecosystem Model (ERSEM). It was found that the ability of the models to reproduce the observed biodiversity was strongly dependent on the details of the biogeochemical model formulations and parameterisations used.

Description: Evaluation of 4 years of continuous δ 13 C(CO 2 ) data using a moving Keeling plot method Sanam Noreen Vardag, Samuel Hammer, and Ingeborg Levin Biogeosciences, 13, 4237-4251, doi:10.5194/bg-13-4237-2016, 2016 Using a synthetic dataset, we show how to best determine the mean source signature, δ S , at high temporal resolution using continuous CO 2 and δ 13 C(CO 2 ) data. We apply this method to measured data from Heidelberg and find a distinct seasonal cycle of δ S . Disentangling this record into its source components requires the isotopic end members of CO 2 from the biosphere and those from the fuel mix. They can be estimated from the δ S record, but only when their relative share is close to 100 %.

Description: Long-term drainage reduces CO 2 uptake and increases CO 2 emission on a Siberian floodplain due to shifts in vegetation community and soil thermal characteristics Min Jung Kwon, Martin Heimann, Olaf Kolle, Kristina A. Luus, Edward A. G. Schuur, Nikita Zimov, Sergey A. Zimov, and Mathias Göckede Biogeosciences, 13, 4219-4235, doi:10.5194/bg-13-4219-2016, 2016 A decade-long drainage on an Arctic floodplain has altered dominant plant species and soil temperature regimes. Consequently, CO 2 exchange rates between the atmosphere and the terrestrial ecosystem were modified: CO 2 uptake rates by the terrestrial ecosystem decreased and CO 2 emission rates to the atmosphere increased. Ongoing global warming may thaw ice-rich permafrost and make some regions drier in the Arctic, and this will reduce carbon accumulation in the terrestrial ecosystem.

Description: Physiological responses of coastal and oceanic diatoms to diurnal fluctuations in seawater carbonate chemistry under two CO 2 concentrations Futian Li, Yaping Wu, David A. Hutchins, Feixue Fu, and Kunshan Gao Biogeosciences Discuss., doi:10.5194/bg-2016-281,2016 Manuscript under review for BG (discussion: open, 0 comments) Ongoing ocean acidification is being superimposed on the natural carbonate buffer system to influence the physiology of phytoplankton. Here, we show that coastal and oceanic diatoms respond differentially to diurnal fluctuating carbonate chemistry in current and ocean acidification scenarios. We propose that the ability to acclimate to dynamic carbonate chemistry may act as one determinant of the spatial distribution of diatom species.

Description: Hydrogen dynamics in soil organic matter as determined by 13 C and 2 H labeling experiments Alexia Paul, Christine Hatté, Lucie Pastor, Yves Thiry, Françoise Siclet, and Jérôme Balesdent Biogeosciences Discuss., doi:10.5194/bg-2016-317,2016 Manuscript under review for BG (discussion: open, 0 comments) The terrestrial environment has been affected by tritium contamination. There is a need to assess the residence time and the dynamics of organic hydrogen in soils organic matter in order to predict the fate of tritium. In the present study we traced carbon and hydrogen from plant derived molecule or from water in different soil types. We showed that water is the main donor of hydrogen in soil and it is dependent on carbon biosynthesis and on soil type.

Description: Ocean acidification over the next three centuries using a simple global climate carbon-cycle model: projections and sensitivities Corinne A. Hartin, Benjamin Bond-Lamberty, Pralit Patel, and Anupriya Mundra Biogeosciences, 13, 4329-4342, doi:10.5194/bg-13-4329-2016, 2016 Continued oceanic uptake of anthropogenic CO 2 is projected to significantly alter the chemistry of the upper oceans over the next three centuries, with potentially serious consequences for marine ecosystems. Relatively few models have the capability to make projections of ocean acidification, limiting our ability to assess the impacts and probabilities of ocean changes. In this study we examine the ability of Hector v1.1, a reduced-form global model, to project changes in the upper ocean carbonate system over the next three centuries, and quantify the model's sensitivity to parametric inputs. Hector is run under prescribed emission pathways from the Representative Concentration Pathways (RCPs) and compared to both observations and a suite of Coupled Model Intercomparison (CMIP5) model outputs. Current observations confirm that ocean acidification is already taking place, and CMIP5 models project significant changes occurring to 2300. Hector is consistent with the observational record within both the high- (〉 55°) and low-latitude oceans (〈 55°). The model projects low-latitude surface ocean pH to decrease from preindustrial levels of 8.17 to 7.77 in 2100, and to 7.50 in 2300; aragonite saturation levels (Ω Ar ) decrease from 4.1 units to 2.2 in 2100 and 1.4 in 2300 under RCP 8.5. These magnitudes and trends of ocean acidification within Hector are largely consistent with the CMIP5 model outputs, although we identify some small biases within Hector's carbonate system. Of the parameters tested, changes in [H + ] are most sensitive to parameters that directly affect atmospheric CO 2 concentrations – Q 10 (terrestrial respiration temperature response) as well as changes in ocean circulation, while changes in Ω Ar saturation levels are sensitive to changes in ocean salinity and Q 10 . We conclude that Hector is a robust tool well suited for rapid ocean acidification projections and sensitivity analyses, and it is capable of emulating both current observations and large-scale climate models under multiple emission pathways.

Description: Decadal and long-term boreal soil carbon and nitrogen sequestration rates across a variety of ecosystems Kristen L. Manies, Jennifer W. Harden, Christopher C. Fuller, and Merritt R. Turetsky Biogeosciences, 13, 4315-4327, doi:10.5194/bg-13-4315-2016, 2016 Boreal soils are important to the global C cycle. We need to understand what controls how C accumulates and is lost from this soil. To help we examined C & N accumulation rates for five boreal ecosystems. Most ecosystems were similar. But the rich fen had higher long-term C & N accumulation rates, likely due to differences in nutrient cycling & because it burns less. Therefore, shifts among ecosystems will not change regional C & N dynamics much, unless there is a shift to or from a rich fen.

Description: Exploring the distance between nitrogen and phosphorus limitation in mesotrophic surface waters using a sensitive bioassay Enis Hrustic, Risto Lignell, Ulf Riebesell, and Tron Frede Thingstad Biogeosciences Discuss., doi:10.5194/bg-2016-313,2016 Manuscript under review for BG (discussion: open, 0 comments) In the stratified season, phytoplankton in the ocean's top layer are limited by mineral nutrients, normally nitrogen, phosphorous or iron. It is important to know, not only which element is limiting, but also the surplus of the secondary limiting element, referred to as e.g. N* when N is the secondary limiting element. Determination of *N by chemical methods in surface waters is not straight forward. We here show how one instead can "ask the organisms".

Description: The importance of an estuarine salinity gradient on soil organic carbon stocks of tidal marshes Marijn Van de Broek, Stijn Temmerman, Roel Merckx, and Gerard Govers Biogeosciences Discuss., doi:10.5194/bg-2016-285,2016 Manuscript under review for BG (discussion: open, 0 comments) The results of this study on the OC stocks of tidal marshes show that variations in OC stocks of tidal marshes along estuaries are important and should be taken into account in order to make of accurate estimates of the total amount of OC stored in these ecosystems. Moreover, our results clearly show that most studies underestimate the variation in OC stocks along estuaries due to a shallow sampling depth, neglecting the variation in OC decomposition after burial along estuaries.

Description: Ecological response to collapse of the biological pump following the mass extinction at the Cretaceous-Paleogene boundary Johan Vellekoop, Lineke Woelders, Sanem Açikalin, Jan Smit, Bas van de Schootbrugge, Ismail Ö. Yilmaz, Henk Brinkhuis, and Robert P. Speijer Biogeosciences Discuss., doi:10.5194/bg-2016-275,2016 Manuscript under review for BG (discussion: open, 0 comments) The Cretaceous-Paleogene boundary, ~ 66 Ma, is characterized by a mass-extinction. We studied groups of both surface-dwelling and bottom-dwelling organisms to unravel the oceanographic consequences of these extinctions. Our integrated records indicate that a reduction of the transport of organic matter to the sea floor resulted in enhanced recycling of nutrients in the upper water column and decreased food supply at the sea floor, in the first tens of thousands of years after the extinctions.

Description: The long-solved problem of the best-fit straight line: Application to isotopic mixing lines Richard Wehr and Scott R . Saleska Biogeosciences Discuss., doi:10.5194/bg-2016-315,2016 Manuscript under review for BG (discussion: open, 0 comments) In 1969, Derek York published a highly general solution to the common problem of how to fit a straight line to points measured with error in both x and y . Unfortunately York’s solution is almost unknown outside the geophysical literature, and new studies wrestle with the problem each year. We introduce York’s solution and demonstrate it using an example from biogeochemistry: the isotopic mixing line. By Monte Carlo simulation, we show that York’s solution is superior to all popular fit methods.

Description: Patterns of carbon processing at the seafloor: the role of faunal and microbial communities in moderating carbon flows Clare Woulds, Steven Bouillon, Gregory L. Cowie, Emily Drake, Jack J. Middelburg, and Ursula Witte Biogeosciences, 13, 4343-4357, doi:10.5194/bg-13-4343-2016, 2016 Estuarine sediments are important locations for carbon cycling and burial. We used tracer experiments to investigate how site conditions affect the way in which seafloor biological communities cycle carbon. We showed that while total respiration rates are primarily determined by temperature, total carbon processing by the biological community is strongly related to its biomass. Further, we saw a distinct pattern of carbon cycling in sandy sediment, in which uptake by bacteria dominates.

Description: Temporal changes in photoreactivity of dissolved organic carbon and implications for aquatic carbon fluxes from peatlands Amy E. Pickard, Kate V. Heal, Andrew R. McLeod, and Kerry J. Dinsmore Biogeosciences Discuss., doi:10.5194/bg-2016-296,2016 Manuscript under review for BG (discussion: open, 0 comments) Peatland catchments export significant volumes of photoreactive carbon to aquatic systems, particularly headwater streams. Delivery of photoreactive material is subject to seasonal variation, and is also influenced by the timing and magnitude of rainfall events. We suggest that photoprocessing of peatland derived carbon may contribute to carbon dioxide emissions from aquatic systems, although considerable uncertainty remains as to how much material is processed 'in situ' within these systems.

Description: Species-specific temporal variation in photosynthesis as a moderator of peatland carbon sequestration Aino Korrensalo, Tomáš Hájek, Pavel Alekseychik, Janne Rinne, Timo Vesala, Lauri Mehtätalo, Ivan Mammarella, and Eeva-Stiina Tuittila Biogeosciences Discuss., doi:10.5194/bg-2016-265,2016 Manuscript under review for BG (discussion: open, 0 comments) Photosynthetic parameters of peatland plant species were measured over one growing season in an ombrotrophic bog. Based on these measurements, ecosystem-level photosynthesis was calculated for the whole growing season and compared with an estimate derived from micrometeorological measurements. These two estimates corresponded well. Species with low areal cover at the site but high photosynthetic efficiency appeared to potentially important for the ecosystem-level carbon balance.

Description: The effect of warm summer 2012 on seasonal and annual methane dynamics in adjacent small lakes on the ice-free margin of Greenland Sarah B. Cadieux, Jeffrey R. White, and Lisa M. Pratt Biogeosciences Discuss., doi:10.5194/bg-2016-293,2016 Manuscript under review for BG (discussion: open, 0 comments) In thermally stratified lakes, the greatest annual methane emissions typically occur during thermal overturn events. In July of 2012, Greenland experienced significant warming that resulted in substantial melting of the Greenland Ice Sheet and enhanced runoff events. This unusual climate phenomenon provided an opportunity to examine the effects of short-term natural heating on lake thermal structure and methane dynamics and compare these observations with those from the following year when temperatures were normal. Here, we focus on methane concentrations within the water column of 5 adjacent small lakes on the ice-free margin of Southwest Greenland under open-water and ice-covered conditions from 2012–2014. Enhanced warming of the epilimnion in the lakes under open-water conditions in 2012 led to strong thermal stability and the development of anoxic hypolimnions in each of the lakes. As a result, during open-water conditions, mean dissolved methane concentrations in the water column were significantly ( p  

Description: Quantifying the Cenozoic marine diatom deposition history: links to the C and Si cycles Johan Renaudie Biogeosciences Discuss., doi:10.5194/bg-2016-290,2016 Manuscript under review for BG (discussion: open, 0 comments) Marine planktonic diatoms are, today, among the world's main primary producers as well as the main organic carbon exporter to the deep-sea despite the fact that they were a very minor component of the plankton at the beginning of the Cenozoic. They are also the main silica exporter to the deep-sea, thus balancing global chemical weathering. This study reviews their global Cenozoic depositional pattern in order to understand the modality and the context of their rise to dominance, but also to understand how diatom evolution affected the Cenozoic functioning of the ocean's biological pump. After two short-lived major abundance peaks in the late Eocene and the late Oligocene, diatom abundance in sediments shifted in the mid-Miocene to globally higher values which have largely persisted to the modern day. These quantitative findings provide support for the hypothesis according to which diatoms, through their ecological role in the ocean's biological carbon pump, have contributed to the Cenozoic changes in atmospheric carbon dioxide pressure and consequently to changes in the global climate state. Additionally, correlations between diatom abundance peaks and shifts in seawater strontium and osmium isotopic composition hint at a strong control of the silicate weathering on diatom deposition.

Description: Drivers of atmospheric methane uptake by montane forest soils in the southern Peruvian Andes Sam P. Jones, Torsten Diem, Lidia P. Huaraca Quispe, Adan J. Cahuana, Dave S. Reay, Patrick Meir, and Yit Arn Teh Biogeosciences, 13, 4151-4165, doi:10.5194/bg-13-4151-2016, 2016 Tropical montane forests represent a significant portion of Andean land cover, however, soil-atmosphere methane exchange in these ecosystems is under studied. Here we report on soil methane cycling in montane forests of the southern Peruvian Andes. These soils acted as a net sink for atmospheric methane and variation in uptake across the studied forests was best explained by nitrate inhibition of oxidation and/or limitations on the inward diffusion of methane from the atmosphere into the soil.

Description: Biochar carrying hydrocarbon decomposers promotes degradation during the early stage of bioremediation P. Galitskaya, L. Akhmetzyanova, and S. Selivanovskaya Biogeosciences Discuss., doi:10.5194/bg-2016-292,2016 Manuscript under review for BG (discussion: open, 0 comments) Biochar carrying hydrocarbon degrading bacterial strains was used as a tool for remediation of oil polluted soil. It was shown, that amendment of oil polluted soil by biochar causes acceleration of hydrocarbon decomposition as compared with mixing and moistening. Inoculation of hydrocarbon degrading bacteria on biochar may be recommended as an additional tool of remediation. New generation sequencing methods were used to analyze how microbial community changes in the process of bioremediation.

Description: Ideas and Perspectives: Climate-Relevant Marine Biologically-Driven Mechanisms in Earth System Models Inga Hense, Irene Stemmler, and Sebastian Sonntag Biogeosciences Discuss., doi:10.5194/bg-2016-289,2016 Manuscript under review for BG (discussion: open, 0 comments) Marine biota drives a number of climate-relevant mechanisms not all of which are included in current Earth system models (ESMs) used for climate projections. We identify three classes of mechanisms and argue that to adequately resolve these mechanisms and to ensure links to and feedbacks with other Earth system components, ESMs need to account for five marine organism groups.

Description: CH 4 exchange at the forest floor of a forestry-drained fen: low flux rates but high temporal variation Mika Korkiakoski, Markku Koskinen, Kari Minkkinen, Paavo Ojanen, Timo Penttilä, Juuso Rainne, Tuomas Laurila, and Annalea Lohila Biogeosciences Discuss., doi:10.5194/bg-2016-239,2016 Manuscript under review for BG (discussion: open, 0 comments) We measured methane exchange rates at the forest floor of a nutrient-rich drained peatland in southern Finland. The forest floor acted mainly as a small methane sink, but emission peaks were occasionally observed during spring and rainfall events. The strength of the sink correlated best with groundwater level and soil temperatures at 20 and 30 cm depths. Even thought the exchange rates were often low, observed diurnal variations during warm and dry summer periods were relatively high.