ALBERT

Description: The influence of land cover change in the Asian monsoon region on present-day and mid-Holocene climate Biogeosciences, 8, 1499-1519, 2011 Author(s): A. Dallmeyer and M. Claussen Using the general circulation model ECHAM5/JSBACH, we investigate the biogeophysical effect of large-scale afforestation and deforestation in the Asian monsoon domain on present-day and mid-Holocene climate. We demonstrate that the applied land cover change does not only modify the local climate but also change the climate in North Africa and the Middle East via teleconnections. Deforestation in the Asian monsoon domain enhances the rainfall in North Africa. In parts of the Sahara summer precipitation is more than doubled. In contrast, afforestation strongly decreases summer rainfall in the Middle East and even leads to the cessation of the rainfall-activity in some parts of this region. Regarding the local climate, deforestation results in a reduction of precipitation and a cooler climate as grass mostly has a higher albedo than forests. However, in the core region of the Asian monsoon the decrease in evaporative cooling in the monsoon season overcompensates this signal and results in a net warming. Afforestation has mainly the opposite effect, although the pattern of change is less clear. It leads to more precipitation in most parts of the Asian monsoon domain and a warmer climate except for the southern regions where a stronger evaporation decreases near-surface temperatures in the monsoon season. When prescribing mid-Holocene insolation, the pattern of local precipitation change differs. Afforestation particularly increases monsoon rainfall in the region along the Yellow River which was the settlement area of major prehistoric cultures. In this region, the effect of land cover change on precipitation is half as large as the orbitally-induced precipitation change. Thus, our model results reveal that mid- to late-Holocene land cover change could strongly have contributed to the decreasing Asian monsoon precipitation during the Holocene known from reconstructions.

Description: Origin and fate of the secondary nitrite maximum in the Arabian Sea Biogeosciences, 8, 1565-1577, 2011 Author(s): P. Lam, M. M. Jensen, A. Kock, K. A. Lettmann, Y. Plancherel, G. Lavik, H. W. Bange, and M. M. M. Kuypers The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO 2 − ) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO 2 − accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO 2 − -producing and consuming processes, and examining the relationship between the overall NO 2 − balance and active N-loss in the Arabian Sea. Based on a combination of 15 N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO 2 − accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO 3 − ) reduction and to a certain extent ammonia oxidation. Meanwhile, NO 2 − consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH 4 + ) were hardly detectable in this region, though some loss to NO 2 − oxidation was predicted from modeled NO 3 − changes. No significant correlation was found between NO 2 − and N-loss rates ( p 〉0.05). This discrepancy between NO 2 − accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO 2 − reduction to N 2 O, N 2 and NH 4 + , and indirectly for the remineralized NH 4 + required by anammox. Altogether, our data do not support the long-held view that NO 2 − accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO 2 − accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.

Description: Carbon dioxide fluxes over an ancient broadleaved deciduous woodland in southern England Biogeosciences, 8, 1595-1613, 2011 Author(s): M. V. Thomas, Y. Malhi, K. M. Fenn, J. B. Fisher, M. D. Morecroft, C. R. Lloyd, M. E. Taylor, and D. D. McNeil We present results from a study of canopy-atmosphere fluxes of carbon dioxide from 2007 to 2009 above a site in Wytham Woods, an ancient temperate broadleaved deciduous forest in southern England. Gap-filled net ecosystem exchange (NEE) data were partitioned into gross primary productivity (GPP) and ecosystem respiration ( R e ) and analysed on daily, monthly and annual timescales. Over the continuous 24 month study period annual GPP was estimated to be 21.1 Mg C ha −1 yr −1 and R e to be 19.8 Mg C ha −1 yr −1 ; net ecosystem productivity (NEP) was 1.2 Mg C ha −1 yr −1 . These estimates were compared with independent bottom-up estimates derived from net primary productivity (NPP) and flux chamber measurements recorded at a plot within the flux footprint in 2008 (GPP = 26.5 ± 6.8 Mg C ha −1 yr −1 , R e = 24.8 ± 6.8 Mg C ha −1 yr −1 , biomass increment = ~1.7 Mg C ha −1 yr −1 ). Over the two years the difference in seasonal NEP was predominantly caused by changes in ecosystem respiration, whereas GPP remained similar for equivalent months in different years. Although solar radiation was the largest influence on daily values of CO 2 fluxes ( R 2 = 0.53 for the summer months for a linear regression), variation in R e appeared to be driven by temperature. Our findings suggest that this ancient woodland site is currently a substantial sink for carbon, resulting from continued growth that is probably a legacy of past management practices abandoned over 40 years ago. Our GPP and R e values are generally higher than other broadleaved temperate deciduous woodlands and may represent the influence of the UK's maritime climate, or the particular species composition of this site. The carbon sink value of Wytham Woods supports the protection and management of temperate deciduous woodlands (including those managed for conservation rather than silvicultural objectives) as a strategy to mitigate atmospheric carbon dioxide increases.

Description: Constraining global methane emissions and uptake by ecosystems Biogeosciences, 8, 1643-1665, 2011 Author(s): R. Spahni, R. Wania, L. Neef, M. van Weele, I. Pison, P. Bousquet, C. Frankenberg, P. N. Foster, F. Joos, I. C. Prentice, and P. van Velthoven Natural methane (CH 4 ) emissions from wet ecosystems are an important part of today's global CH 4 budget. Climate affects the exchange of CH 4 between ecosystems and the atmosphere by influencing CH 4 production, oxidation, and transport in the soil. The net CH 4 exchange depends on ecosystem hydrology, soil and vegetation characteristics. Here, the LPJ-WHyMe global dynamical vegetation model is used to simulate global net CH 4 emissions for different ecosystems: northern peatlands (45°–90° N), naturally inundated wetlands (60° S–45° N), rice agriculture and wet mineral soils. Mineral soils are a potential CH 4 sink, but can also be a source with the direction of the net exchange depending on soil moisture content. The geographical and seasonal distributions are evaluated against multi-dimensional atmospheric inversions for 2003–2005, using two independent four-dimensional variational assimilation systems. The atmospheric inversions are constrained by the atmospheric CH 4 observations of the SCIAMACHY satellite instrument and global surface networks. Compared to LPJ-WHyMe the inversions result in a~significant reduction in the emissions from northern peatlands and suggest that LPJ-WHyMe maximum annual emissions peak about one month late. The inversions do not put strong constraints on the division of sources between inundated wetlands and wet mineral soils in the tropics. Based on the inversion results we diagnose model parameters in LPJ-WHyMe and simulate the surface exchange of CH 4 over the period 1990–2008. Over the whole period we infer an increase of global ecosystem CH 4 emissions of +1.11 Tg CH 4 yr −1 , not considering potential additional changes in wetland extent. The increase in simulated CH 4 emissions is attributed to enhanced soil respiration resulting from the observed rise in land temperature and in atmospheric carbon dioxide that were used as input. The long-term decline of the atmospheric CH 4 growth rate from 1990 to 2006 cannot be fully explained with the simulated ecosystem emissions. However, these emissions show an increasing trend of +3.62 Tg CH 4 yr −1 over 2005–2008 which can partly explain the renewed increase in atmospheric CH 4 concentration during recent years.

Description: Corrigendum to "A culture-based calibration of benthic foraminiferal paleotemperature proxies: δ 18 O and Mg/Ca results" published in Biogeosciences, 7, 1335–1347, 2010 Biogeosciences, 8, 1521-1521, 2011 Author(s): H. L. Filipsson, J. M. Bernhard, S. A. Lincoln, and D. C. McCorkle No abstract available.

Description: Underestimated effects of low temperature during early growing season on carbon sequestration of a subtropical coniferous plantation Biogeosciences, 8, 1667-1678, 2011 Author(s): W.-J. Zhang, H.-M. Wang, F.-T. Yang, Y.-H. Yi, X.-F. Wen, X.-M. Sun, G.-R. Yu, Y.-D. Wang, and J.-C. Ning The impact of air temperature in early growing season on the carbon sequestration of a subtropical coniferous plantation was discussed through analyzing the eddy flux observations at Qianyanzhou (QYZ) site in southern China from 2003 to 2008. This site experienced two cold early growing seasons (with temperature anomalies of 2–5 °C) in 2005 and 2008, and a severe summer drought in 2003. Results indicated that the low air temperature from January to March was the major factor controlling the inter-annual variations in net carbon uptake at this site, rather than the previously thought summer drought. The accumulative air temperature from January to February showed high correlation ( R 2 =0.970, p

Description: Mega fire emissions in Siberia: potential supply of bioavailable iron from forests to the ocean Biogeosciences, 8, 1679-1697, 2011 Author(s): A. Ito Significant amounts of carbon and nutrients are released to the atmosphere due to large fires in forests. Characterization of the spatial distribution and temporal variation of the intense fire emissions is crucial for assessing the atmospheric loadings of trace gases and aerosols. This paper discusses issues of the representation of forest fires in the estimation of emissions and the application to an atmospheric chemistry transport model (CTM). The potential contribution of forest fires to the deposition of bioavailable iron (Fe) into the ocean is highlighted, with a focus on mega fires in eastern Siberia. Satellite products of burned area, active fire, and land cover are used to estimate biomass burning emissions in conjunction with a biogeochemical model. Satellite-derived plume height from MISR is used for the injection height of boreal forest fire emissions. This methodology is applied to quantify fire emission rates in each three-dimensional grid location in the high latitude Northern Hemisphere (〉30° N latitude) over a 5-yr period from 2001 to 2005. There is large interannual variation in forest burned area during 2001–2005 (13–49 × 10 3 km 2 yr −1 ) which results in a corresponding variation in the annual emissions of carbon monoxide (CO) (14–81 Tg CO y −1 ). Satellite observations of CO column from MOPITT are used to evaluate the model performance in simulating the spatial distribution and temporal variation of the fire emissions. The model results for CO enhancements due to eastern Siberian fires are in good agreement with MOPITT observations. These validation results suggest that the model using emission rates estimated in this work is able to describe the interannual changes in CO due to intense forest fires. Bioavailable iron is derived from atmospheric processing of relatively insoluble iron from desert sources by anthropogenic pollutants (mainly sulfuric acid formed from oxidation of SO 2 ) and from direct emissions of soluble iron from combustion sources. Emission scenarios for IPCC AR5 report (Intergovernmental Panel on Climate Change; Fifth Assessment Report) suggest that anthropogenic SO 2 emissions are suppressed in the future to improve air quality. In future warmer and drier climate, severe fire years such as 2003 may become more frequent in boreal regions. The fire emission rates estimated in this study are applied to the aerosol chemistry transport model to examine the relative importance of biomass burning sources of soluble iron compared to those from dust sources. The model reveals that extreme fire events contribute to a significant deposition of soluble iron (20–40 %) to downwind regions over the western North Pacific Ocean, compared to the dust sources with no atmospheric processing by acidic species. These results suggest that the supply of nutrients from large forest fires plays a role as a negative biosphere-climate feedback with regards to the ocean fertilization.

Description: The effect of resource history on the functioning of soil microbial communities is maintained across time Biogeosciences, 8, 1477-1486, 2011 Author(s): A. D. Keiser, M. S. Strickland, N. Fierer, and M. A. Bradford Historical resource conditions appear to influence microbial community function. With time, historical influences might diminish as populations respond to the contemporary environment. Alternatively, they may persist given factors such as contrasting genetic potentials for adaptation to a new environment. Using experimental microcosms, we test competing hypotheses that function of distinct soil microbial communities in common environments (H1 a ) converge or (H1 b ) remain dissimilar over time. Using a 6 × 2 (soil community inoculum × litter environment) full-factorial design, we compare decomposition rates in experimental microcosms containing grass or hardwood litter environments. After 100 days, communities that develop are inoculated into fresh litters and decomposition followed for another 100 days. We repeat this for a third, 100-day period. In each successive, 100-day period, we find higher decomposition rates (i.e. functioning) suggesting communities function better when they have an experimental history of the contemporary environment. Despite these functional gains, differences in decomposition rates among initially distinct communities persist, supporting the hypothesis that dissimilarity is maintained across time. In contrast to function, community composition is more similar following a common, experimental history. We also find that "specialization" on one experimental environment incurs a cost, with loss of function in the alternate environment. For example, experimental history of a grass-litter environment reduced decomposition when communities were inoculated into a hardwood-litter environment. Our work demonstrates experimentally that despite expectations of fast growth rates, physiological flexibility and rapid evolution, initial functional differences between microbial communities are maintained across time. These findings question whether microbial dynamics can be omitted from models of ecosystem processes if we are to predict reliably global change effects on biogeochemical cycles.

Description: Experimental fossilisation of viruses from extremophilic Archaea Biogeosciences, 8, 1465-1475, 2011 Author(s): F. Orange, A. Chabin, A. Gorlas, S. Lucas-Staat, C. Geslin, M. Le Romancer, D. Prangishvili, P. Forterre, and F. Westall The role of viruses at different stages of the origin of life has recently been reconsidered. It appears that viruses may have accompanied the earliest forms of life, allowing the transition from an RNA to a DNA world and possibly being involved in the shaping of tree of life in the three domains that we know presently. In addition, a large variety of viruses has been recently identified in extreme environments, hosted by extremophilic microorganisms, in ecosystems considered as analogues to those of the early Earth. Traces of life on the early Earth were preserved by the precipitation of silica on the organic structures. We present the results of the first experimental fossilisation by silica of viruses from extremophilic Archaea (SIRV2 – Sulfolobus islandicus rod-shaped virus 2, TPV1 – Thermococcus prieurii virus 1, and PAV1 – Pyrococcus abyssi virus 1). Our results confirm that viruses can be fossilised, with silica precipitating on the different viral structures (proteins, envelope) over several months in a manner similar to that of other experimentally and naturally fossilised microorganisms. This study thus suggests that viral remains or traces could be preserved in the rock record although their identification may be challenging due to the small size of the viral particles.

Description: Soil organic matter dynamics in a North America tallgrass prairie after 9 yr of experimental warming Biogeosciences, 8, 1487-1498, 2011 Author(s): X. Cheng, Y. Luo, X. Xu, R. Sherry, and Q. Zhang The influence of global warming on soil organic matter (SOM) dynamics in terrestrial ecosystems remains unclear. In this study, we combined soil fractionation with isotope analyses to examine SOM dynamics after nine years of experimental warming in a North America tallgrass prairie. Soil samples from the control plots and the warmed plots were separated into four aggregate sizes (〉2000 μm, 250–2000 μm, 53–250 μm, and

Description: How to link soil C pools with CO 2 fluxes? Biogeosciences, 8, 1523-1537, 2011 Author(s): Y. Kuzyakov Despite the importance of carbon (C) pools and CO 2 fluxes in terrestrial ecosystems and especially in soils, as well as many attempts to assign fluxes to specific pools, this challenge remains unsolved. Interestingly, scientists investigating pools are not closely linked with scientists studying fluxes. This review therefore focused on experimental approaches enabling soil C pools to be linked with CO 2 flux from the soil. The background, advantages and shortcomings of uncoupled approaches (measuring only pools or fluxes) and of coupled approaches (measuring both pools and fluxes) were evaluated and their prerequisites – steady state of pools and isotopic steady state – described. The uncoupled approaches include: (i) monitoring the decrease of C pools in long-term fallow bare soil lacking C input over decades, (ii) analyzing components of CO 2 efflux dynamics by incubating soil without new C input over months or years, and (iii) analyzing turnover rates of C pools based on their 13 C and 14 C isotopic signature. The uncoupled approaches are applicable for non-steady state conditions only and have limited explanatory power. The more advantageous coupled approaches partition simultaneously pools and fluxes based on one of three types of changes in the isotopic signature of input C compared to soil C: (i) abrupt permanent, (ii) gradual permanent, and (iii) abrupt temporary impacts. I show how the maximal sensitivity of the approaches depends on the differences in the isotopic signature of pools with fast and slow turnover rates. The promising coupled approaches include: (a) δ 13 C of C pools and CO 2 efflux from soil after C 3 /C 4 vegetation changes or in FACE experiments (both corresponding to continuous labeling), (b) addition of 13 C or 14 C labeled organics (corresponding to pulse labeling), and (c) bomb- 14 C. I show that physical separation of soil C pools is not a prerequisite to estimate pool size or to link pools with fluxes. Based on simple simulation of C aging in soil after the input, the discordance of MRT of C in pools and of C released in CO 2 was demonstrated. This discordance of MRT between pools and fluxes shows that the use of MRT of pools alone underestimates the fluxes at least for two times. The future challenges include combining two or more promising approaches to elucidate more than two C sources for CO 2 fluxes, and linking scientific communities investigating the pools with those investigating the fluxes.

Description: Sources of short-lived bromocarbons in the Iberian upwelling system Biogeosciences, 8, 1551-1564, 2011 Author(s): S. Raimund, B. Quack, Y. Bozec, M. Vernet, V. Rossi, V. Garçon, Y. Morel, and P. Morin Seawater concentrations of the four brominated trace gases, dibromomethane (CH 2 Br 2 ), bromodichloromethane (CHBrCl 2 ), dibromochloromethane (CHBr 2 Cl) and bromoform (CHBr 3 ) were measured at different depths of the water column in the Iberian upwelling off Portugal during summer 2007. Statistical analysis of the data set revealed three distinct clusters, caused by different sea surface temperature. Bromocarbon concentrations were elevated in recently upwelled and aged upwelled waters (mean values of 30 pmol l −1 for CHBr 3 ), while concentrations in the open ocean were significantly lower (7.4 pmol l −1 for CHBr 3 ). Comparison with other productive marine areas revealed that the Iberian upwelling had higher halocarbon concentrations than the Mauritanian upwelling. However, the concentrations off the Iberian Peninsula were still much lower than those of coastal macroalgal-influenced waters or those of Polar regions dominated by cold water adapted diatoms. Correlations with biological variables and marker pigments indicated that phytoplankton was a source of bromocarbon in the open ocean. By contrast, in upwelled water masses along the coast, halocarbons showed weaker correlations to marker pigments but were significantly influenced by the tidal frequency. Our results indicate a strong intertidal coastal source of bromocarbon and transport by surface currents of these enriched waters towards the upwelling region.

Description: A systematic approach for comparing modeled biospheric carbon fluxes across regional scales Biogeosciences, 8, 1579-1593, 2011 Author(s): D. N. Huntzinger, S. M. Gourdji, K. L. Mueller, and A. M. Michalak Given the large differences between biospheric model estimates of regional carbon exchange, there is a need to understand and reconcile the predicted spatial variability of fluxes across models. This paper presents a set of quantitative tools that can be applied to systematically compare flux estimates despite the inherent differences in model formulation. The presented methods include variogram analysis, variable selection, and geostatistical regression. These methods are evaluated in terms of their ability to assess and identify differences in spatial variability in flux estimates across North America among a small subset of models, as well as differences in the environmental drivers that best explain the spatial variability of predicted fluxes. The examined models are the Simple Biosphere (SiB 3.0), Carnegie Ames Stanford Approach (CASA), and CASA coupled with the Global Fire Emissions Database (CASA GFEDv2), and the analyses are performed on model-predicted net ecosystem exchange, gross primary production, and ecosystem respiration. Variogram analysis reveals consistent seasonal differences in spatial variability among modeled fluxes at a 1° × 1° spatial resolution. However, significant differences are observed in the overall magnitude of the carbon flux spatial variability across models, in both net ecosystem exchange and component fluxes. Results of the variable selection and geostatistical regression analyses suggest fundamental differences between the models in terms of the factors that explain the spatial variability of predicted flux. For example, carbon flux is more strongly correlated with percent land cover in CASA GFEDv2 than in SiB or CASA. Some of the differences in spatial patterns of estimated flux can be linked back to differences in model formulation, and would have been difficult to identify simply by comparing net fluxes between models. Overall, the systematic approach presented here provides a set of tools for comparing predicted grid-scale fluxes across models, a task that has historically been difficult unless standardized forcing data were prescribed, or a detailed sensitivity analysis performed.

Description: Effect of ocean acidification on otolith development in larvae of a tropical marine fish Biogeosciences, 8, 1631-1641, 2011 Author(s): P. L. Munday, V. Hernaman, D. L. Dixson, and S. R. Thorrold Calcification in many invertebrate species is predicted to decline due to ocean acidification. The potential effects of elevated CO 2 and reduced carbonate saturation state on other species, such as fish, are less well understood. Fish otoliths (earbones) are composed of aragonite, and thus, might be susceptible to either the reduced availability of carbonate ions in seawater at low pH, or to changes in extracellular concentrations of bicarbonate and carbonate ions caused by acid-base regulation in fish exposed to high p CO 2 . We reared larvae of the clownfish Amphiprion percula from hatching to settlement at three pH NBS and p CO 2 levels (control: ~pH 8.15 and 404 μatm CO 2 ; intermediate: pH 7.8 and 1050 μatm CO 2 ; extreme: pH 7.6 and 1721 μatm CO 2 ) to test the possible effects of ocean acidification on otolith development. There was no effect of the intermediate treatment (pH 7.8 and 1050 μatm CO 2 ) on otolith size, shape, symmetry between left and right otoliths, or otolith elemental chemistry, compared with controls. However, in the more extreme treatment (pH 7.6 and 1721 μatm CO 2 ) otolith area and maximum length were larger than controls, although no other traits were significantly affected. Our results support the hypothesis that pH regulation in the otolith endolymph can lead to increased precipitation of CaCO 3 in otoliths of larval fish exposed to elevated CO 2 , as proposed by an earlier study, however, our results also show that sensitivity varies considerably among species. Importantly, our results suggest that otolith development in clownfishes is robust to even the more pessimistic changes in ocean chemistry predicted to occur by 2100.

Description: Organic sediment formed during inundation of a degraded fen grassland emits large fluxes of CH 4 and CO 2 Biogeosciences, 8, 1539-1550, 2011 Author(s): M. Hahn-Schöfl, D. Zak, M. Minke, J. Gelbrecht, J. Augustin, and A. Freibauer Peatland restoration by inundation of drained areas can alter local greenhouse gas emissions as CO 2 and CH 4 . Factors that can influence these emissions include the quality and amount of substrates available for anaerobic degradation processes and the sources and availability of electron acceptors. In order to learn about possible sources of high CO 2 and CH 4 . emissions from a rewetted degraded fen grassland, we performed incubation experiments that tested the effects of fresh plant litter in the flooded peats on pore water chemistry and CO 2 and CH 4 . production and emission. The position in the soil profile of the pre-existing drained peat substrate affected initial rates of anaerobic CO 2 production subsequent to flooding, with the uppermost peat layer producing the greatest specific rates of CO 2 evolution. CH 4 production rates depended on the availability of electron acceptors and was significant only when sulfate concentrations were reduced in the pore waters. Very high specific rates of both CO 2 (maximum of 412 mg C d −1 kg −1 C) and CH 4 production (788 mg C d −1 kg −1 C) were observed in a new sediment layer that accumulated over the 2.5 years since the site was flooded. This new sediment layer was characterized by overall low C content, but represented a mixture of sand and relatively easily decomposable plant litter from reed canary grass killed by flooding. Samples that excluded this new sediment layer but included intact roots remaining from flooded grasses had specific rates of CO 2 (max. 28 mg C d −1 kg −1 C) and CH 4 (max. 34 mg C d −1 kg −1 C) production that were 10–20 times lower than for the new sediment layer and were comparable to those of a newly flooded upper peat layer. Lowest rates of anaerobic CO 2 and CH 4 production (range of 4–8 mg C d −1 kg −1 C and

Description: Controls over aboveground forest carbon density on Barro Colorado Island, Panama Biogeosciences, 8, 1615-1629, 2011 Author(s): J. Mascaro, G. P. Asner, H. C. Muller-Landau, M. van Breugel, J. Hall, and K. Dahlin Despite the importance of tropical forests to the global carbon cycle, ecological controls over landscape-level variation in live aboveground carbon density (ACD) in tropical forests are poorly understood. Here, we conducted a spatially comprehensive analysis of ACD variation for a continental tropical forest – Barro Colorado Island, Panama (BCI) – and tested site factors that may control such variation. We mapped ACD over 1256 ha of BCI using airborne Light Detection and Ranging (LiDAR), which was well-correlated with ground-based measurements of ACD in Panamanian forests of various ages ( r 2 = 0.84, RMSE = 17 Mg C ha −1 , P 〈 0.0001). We used multiple regression to examine controls over LiDAR-derived ACD, including slope angle, forest age, bedrock, and soil texture. Collectively, these variables explained 14 % of the variation in ACD at 30-m resolution, and explained 33 % at 100-m resolution. At all resolutions, slope (linked to underlying bedrock variation) was the strongest driving factor; standing carbon stocks were generally higher on steeper slopes. This result suggests that physiography may be more important in controlling ACD variation in Neotropical forests than currently thought. Although BCI has been largely undisturbed by humans for a century, past land-use over approximately half of the island still influences ACD variation, with younger forests (80–130 years old) averaging ~15 % less carbon storage than old-growth forests (〉400 years old). If other regions of relatively old tropical secondary forests also store less carbon aboveground than primary forests, the effects on the global carbon cycle could be substantial and difficult to detect with traditional satellite monitoring.

Description: Beyond the Fe-P-redox connection: preferential regeneration of phosphorus from organic matter as a key control on Baltic Sea nutrient cycles Biogeosciences, 8, 1699-1720, 2011 Author(s): T. Jilbert, C. P. Slomp, B. G. Gustafsson, and W. Boer Patterns of regeneration and burial of phosphorus (P) in the Baltic Sea are strongly dependent on redox conditions. Redox varies spatially along water depth gradients and temporally in response to the seasonal cycle and multidecadal hydrographic variability. Alongside the well-documented link between iron oxyhydroxide dissolution and release of P from Baltic Sea sediments, we show that preferential remineralization of P with respect to carbon (C) and nitrogen (N) during degradation of organic matter plays a key role in determining the surplus of bioavailable P in the water column. Preferential remineralization of P takes place both in the water column and upper sediments and its rate is shown to be redox-dependent, increasing as reducing conditions become more severe at greater water-depth in the deep basins. Existing Redfield-based biogeochemical models of the Baltic may therefore underestimate the imbalance between N and P availability for primary production, and hence the vulnerability of the Baltic to sustained eutrophication via the fixation of atmospheric N. However, burial of organic P is also shown to increase during multidecadal intervals of expanded hypoxia, due to higher net burial rates of organic matter around the margins of the deep basins. Such intervals may be characterized by basin-scale acceleration of all fluxes within the P cycle, including productivity, regeneration and burial, sustained by the relative accessibility of the water column P pool beneath a shallow halocline.

Description: How accurately can soil organic carbon stocks and stock changes be quantified by soil inventories? Biogeosciences, 8, 1193-1212, 2011 Author(s): M. Schrumpf, E. D. Schulze, K. Kaiser, and J. Schumacher Precise determination of changes in organic carbon (OC) stocks is prerequisite to understand the role of soils in the global cycling of carbon and to verify changes in stocks due to management. A large dataset was collected to form base to repeated soil inventories at 12 CarboEurope sites under different climate and land-use, and with different soil types. Concentration of OC, bulk density (BD), and fine earth fraction were determined to 60 cm depth at 100 sampling points per site. We investigated (1) time needed to detect changes in soil OC, assuming future re-sampling of 100 cores; (2) the contribution of different sources of uncertainties to OC stocks; (3) the effect of OC stock calculation on mass rather than volume base for change detection; and (4) the potential use of pedotransfer functions (PTF) for estimating BD in repeated inventories. The period of time needed for soil OC stocks to change strongly enough to be detectable depends on the spatial variability of soil properties, the depth increment considered, and the rate of change. Cropland sites, having small spatial variability, had lower minimum detectable differences (MDD) with 100 sampling points (105 ± 28 gC m −2 for the upper 10 cm of the soil) than grassland and forest sites (206 ± 64 and 246 ± 64 gC m −2 for 0–10 cm, respectively). Expected general trends in soil OC indicate that changes could be detectable after 2–15 yr with 100 samples if changes occurred in the upper 10 cm of stone-poor soils. Error propagation analyses showed that in undisturbed soils with low stone contents, OC concentrations contributed most to OC stock variability while BD and fine earth fraction were more important in upper soil layers of croplands and in stone rich soils. Though the calculation of OC stocks based on equivalent soil masses slightly decreases the chance to detect changes with time at most sites except for the croplands, it is still recommended to account for changing bulk densities with time. Application of PTF for the estimation of bulk densities caused considerable underestimation of total variances of OC stocks if the error associated with the PTF was not accounted for, which rarely is done in soil inventories. Direct measurement of all relevant parameters approximately every 10 yr is recommended for repeated soil OC inventories.

Description: Quantifying methane emissions from rice paddies in Northeast China by integrating remote sensing mapping with a biogeochemical model Biogeosciences, 8, 1225-1235, 2011 Author(s): Y. Zhang, Y. Y. Wang, S. L. Su, and C. S. Li The Sanjiang Plain located in Northeastern China is one of the major rice producing regions in the country. However, differing from the majority rice regions in Southern China, the Sanjinag Plain possesses a much cooler climate. Could the rice paddies in this domain be an important source of global methane? To answer this question, methane (CH 4 ) emissions from the region were calculated by integrating remote sensing mapping with a process-based biogeochemistry model, Denitrification and Decomposition or DNDC. To quantify regional CH 4 emissions from the plain, the model was first tested against a two-year dataset of CH 4 fluxes measured at a typical rice field within the domain. A sensitivity test was conducted to find out the most sensitive factors affecting CH 4 emissions in the region. Based on the understanding gained from the validation and sensitivity tests, a geographic information system (GIS) database was constructed to hold the spatially differentiated input information to drive DNDC for its regional simulations. The GIS database included a rice map derived from the Landsat TM images acquired in 2006, which provided crucial information about the spatial distribution of the rice fields within the domain of 10.93 million ha. The modeled results showed that the total 1.44 million ha of rice paddies in the plain emitted 0.48–0.58 Tg CH 4 -C in 2006 with spatially differentiated annual emission rates ranging between 38.6–943.9 kg CH 4 -C ha −1 , which are comparable with that observed in Southern China. The modeled data indicated that the high SOC contents, long crop season and high rice biomass enhanced CH 4 production in the cool paddies. The modeled results proved that the northern wetland agroecosystems could make important contributions to global greenhouse gas inventory.

Description: Soil carbon stock increases in the organic layer of boreal middle-aged stands Biogeosciences, 8, 1279-1289, 2011 Author(s): M. Häkkinen, J. Heikkinen, and R. Mäkipää Changes in the soil carbon stock can potentially have a large influence on global carbon balance between terrestrial ecosystems and atmosphere. Since carbon sequestration of forest soils is influenced by human activities, reporting of the soil carbon pool is a compulsory part of the national greenhouse gas (GHG) inventories. Various soil carbon models are applied in GHG inventories, however, the verification of model-based estimates is lacking. In general, the soil carbon models predict accumulation of soil carbon in the middle-aged stands, which is in good agreement with chronosequence studies and flux measurements of eddy sites, but they have not been widely tested with repeated measurements of permanent plots. The objective of this study was to evaluate soil carbon changes in the organic layer of boreal middle-aged forest stands. Soil carbon changes on re-measured sites were analyzed by using soil survey data that was based on composite samples as a first measurement and by taking into account spatial variation on the basis of the second measurement. By utilizing earlier soil surveys, a long sampling interval, which helps detection of slow changes, could be readily available. The range of measured change in the soil organic layer varied from −260 to 1260 g m −2 over the study period of 16–19 years and 23 ± 2 g m −2 per year, on average. The increase was significant in 6 out of the 38 plots from which data were available. Although the soil carbon change was difficult to detect at the plot scale, the overall increase measured across the middle-aged stands agrees with predictions of the commonly applied soil models. Further verification of the soil models is needed with larger datasets that cover wider geographical area and represent all age classes, especially young stands with potentially large soil carbon source.

Description: Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment Biogeosciences, 8, 1333-1350, 2011 Author(s): U. Gamnitzer, A. B. Moyes, D. R. Bowling, and H. Schnyder The carbon isotopic composition (δ 13 C) of CO 2 efflux (δ 13 C efflux ) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δ 13 C Rs ). However, soils contain a large CO 2 pool in air-filled pores. This pool receives CO 2 from belowground respiration and exchanges CO 2 with the atmosphere (via diffusion and advection) and the soil liquid phase (via dissolution). Natural or artificial modification of δ 13 C of atmospheric CO 2 (δ 13 C atm ) or δ 13 C Rs causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of δ 13 C efflux from δ 13 C Rs (termed "disequilibrium effect"). Here, we use a soil CO 2 transport model and data from a 13 CO 2 / 12 CO 2 tracer experiment to quantify the disequilibrium between δ 13 C efflux and δ 13 C Rs in ecosystem respiration. The model accounted for diffusion of CO 2 in soil air, advection of soil air, dissolution of CO 2 in soil water, and belowground and aboveground respiration of both 12 CO 2 and 13 CO 2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δ 13 C atm of −46.9 ‰ during daytime for 2 weeks. Nighttime δ 13 C efflux from the ecosystem was estimated with three independent methods: a laboratory-based cuvette system, in-situ steady-state open chambers, and in-situ closed chambers. Earlier work has shown that the δ 13 C efflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δ 13 C Rs . Conversely, the δ 13 C efflux measured using the closed chamber technique differed from these by −11.2 ‰. Most of this disequilibrium effect (9.5 ‰) was predicted by the CO 2 transport model. Isotopic disequilibria in the soil-chamber system were introduced by changing δ 13 C atm in the chamber headspace at the onset of the measurements. When dissolution was excluded, the simulated disequilibrium effect was only 3.6 ‰. Dissolution delayed the isotopic equilibration between soil CO 2 and the atmosphere, as the storage capacity for labelled CO 2 in water-filled soil pores was 18 times that of soil air. These mechanisms are potentially relevant for many studies of δ 13 C Rs in soils and ecosystems, including FACE experiments and chamber studies in natural conditions. Isotopic disequilibria in the soil-atmosphere system may result from temporal variation in δ 13 C Rs or diurnal changes in the mole fraction and δ 13 C of atmospheric CO 2 . Dissolution effects are most important under alkaline conditions.

Description: Spatial and temporal resolution of carbon flux estimates for 1983–2002 Biogeosciences, 8, 1309-1331, 2011 Author(s): L. M. P. Bruhwiler, A. M. Michalak, and P. P. Tans We discuss the spatial and temporal resolution of monthly carbon flux estimates for the period 1983–2002 using a fixed-lag Kalman Smoother technique with a global chemical transport model, and the GLOBALVIEW data product. The observational network has expanded substantially over this period, and the flux estimates are better constrained provided by observations for the 1990's in comparison to the 1980's. The estimated uncertainties also decrease as observational coverage expands. In this study, we use the Globalview data product for a network that changes every 5 yr, rather than using a small number of continually-operating sites (fewer observational constraints) or a large number of sites, some of which may consist almost entirely of extrapolated data. We show that the discontinuities resulting from network changes reflect uncertainty due to a sparse and variable network. This uncertainty effectively limits the resolution of trends in carbon fluxes, and is a potentially significant source of noise in assimilation systems that allow changes in observation distribution between assimilation time steps. The ability of the inversion to distinguish, or resolve, carbon fluxes at various spatial scales is examined using a diagnostic known as the resolution kernel. We find that the global partition between land and ocean fluxes is well-resolved even for the very sparse network of the 1980's, although prior information makes a significant contribution to the resolution. The ability to distinguish zonal average fluxes has improved significantly since the 1980's, especially for the tropics, where the zonal ocean and land biosphere fluxes can be distinguished. Care must be taken when interpreting zonal average fluxes, however, since the lack of air samples for some regions in a zone may result in a large influence from prior flux estimates for these regions. We show that many of the TransCom 3 source regions are distinguishable throughout the period over which estimates are produced. Examples are Boreal and Temperate North America. The resolution of fluxes from Europe and Australia has greatly improved since the 1990's. Other regions, notably Tropical South America and the Equatorial Atlantic remain practically unresolved. Comparisons of the average seasonal cycle of the estimated carbon fluxes with the seasonal cycle of the prior flux estimates reveals a large adjustment of the summertime uptake of carbon for Boreal Eurasia, and an earlier onset of springtime uptake for Temperate North America. In addition, significantly larger seasonal cycles are obtained for some ocean regions, such as the Northern Ocean, North Pacific, North Atlantic and Western Equatorial Pacific, regions that appear to be well-resolved by the inversion.

Description: Longitudinal variability of the biogeochemical role of Mediterranean aerosols in the Mediterranean Sea Biogeosciences, 8, 1067-1080, 2011 Author(s): E. Ternon, C. Guieu, C. Ridame, S. L'Helguen, and P. Catala The Mediterranean Sea is a semi-enclosed basin characterized by a strong thermal stratification during summer during which the atmosphere is the main source of new nutrients to the nutrient-depleted surface layer. From aerosol sampling and microcosm experiments performed during the TransMed BOUM cruise (June–July 2008) we showed that: (i) the Mediterranean atmosphere composition (Al, Fe, P) was homogeneous over ~28° of longitude and was a mixture with a constant proportion of anthropogenic contribution and a variable but modest contribution of crustal aerosols. This quite stable composition over a one month period and a long transect (~2500 km) allowed to define the Mediterranean atmospheric "background" that characterizes the summer season in the absence of major Saharan event and forest fires, (ii) primary production significantly increased at all tested stations after aerosols addition collected on-board and after Saharan dust analog addition, indicating that both additions relieved on-going (co)-limitations. Although both additions significantly increased the N 2 fixation rates at the western station, diazotrophic activity remained very low (~0.2 nmol N L −1 d −1 ), (iii) due to the presence of anthropogenic particles, the probable higher solubility of nutrients associated with mixed aerosols (crustal + anthropogenic contribution), conferred a higher fertilizing potential to on-board collected aerosol as compared to Saharan dust analog. Finally, those experiments showed that atmospheric inputs from a mixed atmospheric event ("summer rain" type) or from a high-intensity Saharan event would induce comparable response by the biota in the stratified Mediterranean SML, during summer.

Description: Height-diameter allometry of tropical forest trees Biogeosciences, 8, 1081-1106, 2011 Author(s): T. R. Feldpausch, L. Banin, O. L. Phillips, T. R. Baker, S. L. Lewis, C. A. Quesada, K. Affum-Baffoe, E. J. M. M. Arets, N. J. Berry, M. Bird, E. S. Brondizio, P. de Camargo, J. Chave, G. Djagbletey, T. F. Domingues, M. Drescher, P. M. Fearnside, M. B. França, N. M. Fyllas, G. Lopez-Gonzalez, A. Hladik, N. Higuchi, M. O. Hunter, Y. Iida, K. A. Salim, A. R. Kassim, M. Keller, J. Kemp, D. A. King, J. C. Lovett, B. S. Marimon, B. H. Marimon-Junior, E. Lenza, A. R. Marshall, D. J. Metcalfe, E. T. A. Mitchard, E. F. Moran, B. W. Nelson, R. Nilus, E. M. Nogueira, M. Palace, S. Patiño, K. S.-H. Peh, M. T. Raventos, J. M. Reitsma, G. Saiz, F. Schrodt, B. Sonké, H. E. Taedoumg, S. Tan, L. White, H. Wöll, and J. Lloyd Tropical tree height-diameter ( H:D ) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were: 1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap). 2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A ). 3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass. Annual precipitation coefficient of variation ( P V ), dry season length ( S D ), and mean annual air temperature ( T A ) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D . After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D . Pantropical and continental-level models provided less robust estimates of H , especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account.

Description: Eutrophication and warming effects on long-term variation of zooplankton in Lake Biwa Biogeosciences, 8, 1383-1399, 2011 Author(s): C. H. Hsieh, Y. Sakai, S. Ban, K. Ishikawa, T. Ishikawa, S. Ichise, N. Yamamura, and M. Kumagai We compiled and analyzed long-term (1961–2005) zooplankton community data in response to environmental variations in Lake Biwa. Environmental data indicate that Lake Biwa had experienced eutrophication (according to the total phosphorus concentration) in the late 1960s and recovered to a normal trophic status around 1985, and then has exhibited warming since 1990. Total zooplankton abundance showed a significant correlation with total phytoplankton biomass. Following a classic pattern, the cladoceran/calanoid and cyclopoid/calanoid abundance ratio was related positively to eutrophication. The zooplankton community exhibited a significant response to the boom and bust of phytoplankton biomass as a consequence of eutrophication-reoligotriphication and warming. Moreover, our analyses suggest that the Lake Biwa ecosystem exhibited a hierarchical response across trophic levels; that is, higher trophic levels may show a more delayed response or no response to eutrophication than lower ones. We tested the hypothesis that the phytoplankton community can better explain the variation of the zooplankton community than bulk environmental variables, considering that the phytoplankton community may directly affect the zooplankton succession through predator-prey interactions. Using a variance partition approach, however, we did not find strong evidence to support this hypothesis. We further aggregated zooplankton according to their feeding types (herbivorous, carnivorous, omnivorous, and parasitic) and taxonomic groups, and analyzed the aggregated data. While the pattern remains similar, the results are less clear comparing the results based on finely resolved data. Our research suggests that zooplankton can be bio-indicators of environmental changes; however, the efficacy depends on data resolution.

Description: Carbonate system in the water masses of the Southeast Atlantic sector of the Southern Ocean during February and March 2008 Biogeosciences, 8, 1401-1413, 2011 Author(s): M. González-Dávila, J. M. Santana-Casiano, R. A. Fine, J. Happell, B. Delille, and S. Speich Carbonate system variables were measured in the South Atlantic sector of the Southern Ocean along a transect from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC) from February to March 2008. Eddies detached from the retroflection of the Agulhas Current increased the gradients observed along the fronts. Minima in the fugacity of CO 2 , f CO 2 , and maxima in pH on either side of the frontal zone were observed, noting that within the frontal zone f CO 2 reached maximum values and pH was at a minimum. Vertical distributions of water masses were described by their carbonate system properties and their relationship to CFC concentrations. Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) offered pH T,25 values of 7.56 and 7.61, respectively. The UCDW also had higher concentrations of CFC-12 (〉0.2 pmol kg −1 ) as compared to deeper waters, revealing that UCDW was mixed with recently ventilated waters. Calcite and aragonite saturation states (Ω) were also affected by the presence of these two water masses with high carbonate concentrations. The aragonite saturation horizon was observed at 1000 m in the subtropical area and north of the Subantarctic Front. At the position of the Polar Front, and under the influence of UCDW and LCDW, the aragonite saturation horizon deepened from 800 m to 1500 m at 50.37° S, and reached 700 m south of 57.5° S. High latitudes proved to be the most sensitive areas to predicted anthropogenic carbon increase. Buffer coefficients related to changes in [CO 2 ], [H + ] and Ω with changes in dissolved inorganic carbon ( C T ) and total alkalinity ( A T ) offered minima values in the Antarctic Intermediate Water and UCDW layers. These coefficients suggest that a small increase in C T will sharply decrease the status of pH and carbonate saturation. Here we present data that suggest that south of 55° S, surface water will be under-saturated with respect to aragonite within the next few decades.

Description: Assessment of the importance of dissimilatory nitrate reduction to ammonium for the terrestrial nitrogen cycle Biogeosciences, 8, 1779-1791, 2011 Author(s): T. Rütting, P. Boeckx, C. Müller, and L. Klemedtsson The nitrogen (N) cycle contains two different processes of dissimilatory nitrate (NO 3 − ) reduction, denitrification and dissimilatory NO 3 − reduction to ammonium (DNRA). While there is general agreement that the denitrification process takes place in many soils, the occurrence and importance of DNRA is generally not considered. Two approaches have been used to investigate DNRA in soil, (1) microbiological techniques to identify soil microorganisms capable of DNRA and (2) 15 N tracing to elucidate the occurrence of DNRA and to quantify gross DNRA rates. There is evidence that many soil bacteria and fungi have the ability to perform DNRA. Redox status and C/NO 3 − ratio have been identified as the most important factors regulating DNRA in soil. 15 N tracing studies have shown that gross DNRA rates can be a significant or even a dominant NO 3 − consumption process in some ecosystems. Moreover, a link between heterotrophic nitrification and DNRA provides an alternative pathway of ammonium (NH 4 + ) production to mineralisation. Numerical 15 N tracing models are particularly useful when investigating DNRA in the context of other N cycling processes. The results of correlation and regression analyses show that highest gross DNRA rates can be expected in soils with high organic matter content in humid regions, while its relative importance is higher in temperate climates. With this review we summarise the importance and current knowledge of this often overlooked NO 3 − consumption process within the terrestrial N cycle. We strongly encourage considering DNRA as a relevant process in future soil N cycling investigations.

Description: Stable carbon isotopes as indicators for environmental change in palsa peats Biogeosciences, 8, 1769-1778, 2011 Author(s): C. Alewell, R. Giesler, J. Klaminder, J. Leifeld, and M. Rollog Palsa peats are unique northern ecosystems formed under an arctic climate and characterized by a high biodiversity and sensitive ecology. The stability of the palsas are seriously threatened by climate warming which will change the permafrost dynamic and induce a degradation of the mires. We used stable carbon isotope depth profiles in two palsa mires of Northern Sweden to track environmental change during the formation of the mires. Soils dominated by aerobic degradation can be expected to have a clear increase of carbon isotopes (δ 13 C) with depth, due to preferential release of 12 C during aerobic mineralization. In soils with suppressed degradation due to anoxic conditions, stable carbon isotope depth profiles are either more or less uniform indicating no or very low degradation or depth profiles turn to lighter values due to an enrichment of recalcitrant organic substances during anaerobic mineralisation which are depleted in 13 C. The isotope depth profile of the peat in the water saturated depressions (hollows) at the yet undisturbed mire Storflaket indicated very low to no degradation but increased rates of anaerobic degradation at the Stordalen site. The latter might be induced by degradation of the permafrost cores in the uplifted areas (hummocks) and subsequent breaking and submerging of the hummock peat into the hollows due to climate warming. Carbon isotope depth profiles of hummocks indicated a turn from aerobic mineralisation to anaerobic degradation at a peat depth between 4 and 25 cm. The age of these turning points was 14 C dated between 150 and 670 yr and could thus not be caused by anthropogenically induced climate change. We found the uplifting of the hummocks due to permafrost heave the most likely explanation for our findings. We thus concluded that differences in carbon isotope profiles of the hollows might point to the disturbance of the mires due to climate warming or due to differences in hydrology. The characteristic profiles of the hummocks are indicators for micro-geomorphic change during permafrost up heaving.

Description: Contribution of riverine nutrients to the silicon biogeochemistry of the global ocean – a model study Biogeosciences, 8, 551-564, 2011 Author(s): C. Y. Bernard, H. H. Dürr, C. Heinze, J. Segschneider, and E. Maier-Reimer Continental shelf seas are known to support a large fraction of the global primary production. Yet, they are mostly ignored or neglected in global biogeochemical models. A number of processes that control the transfer of dissolved nutrients from rivers to the open ocean remain poorly understood. This applies in particular to dissolved silica which drives the growth of diatoms that form a large part of the phytoplankton biomass and are thus an important contributor to export production of carbon. Here, the representation of the biogeochemical cycling along continents is improved by coupling a high resolution database of riverine fluxes of nutrients to the global biogeochemical ocean general circulation model HAMOCC5-OM. Focusing on silicon (Si), but including the whole suite of nutrients – carbon (C), nitrogen (N) and phosphorus (P) in various forms – inputs are implemented in the model at coastal coupling points using the COSCAT global database of 156 mega-river-ensemble catchments from Meybeck et al. (2006). The catchments connect to the ocean through coastal segments according to three sets of criteria: natural limits, continental shelf topography, and geophysical dynamics. According to the model the largest effects on nutrient concentrations occur in hot spots such as the Amazon plume, the Arctic – with high nutrient inputs in relation to its total volume, and areas that encounter the largest increase in human activity, e.g., Southern Asia.

Description: Large-scale shifts in phytoplankton groups in the Equatorial Pacific during ENSO cycles Biogeosciences, 8, 539-550, 2011 Author(s): I. Masotti, C. Moulin, S. Alvain, L. Bopp, A. Tagliabue, and D. Antoine The El Niño Southern Oscillation (ENSO) drives important changes in the marine productivity of the Equatorial Pacific, in particular during major El Niño/La Niña transitions. Changes in environmental conditions associated with these climatic events also likely impact phytoplankton composition. In this work, the distribution of four major phytoplankton groups (nanoeucaryotes, Prochlorococcus, Synechococcus, and diatoms) was examined between 1996 and 2007 by applying the PHYSAT algorithm to the ocean color data archive from the Ocean Color and Temperature Sensor (OCTS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Coincident with the decrease in chlorophyll concentrations, a large-scale shift in the phytoplankton composition of the Equatorial Pacific, that was characterized by a decrease in Synechococcus and an increase in nanoeucaryote dominance, was observed during the early stages of both the strong El Niño of 1997 and the moderate El Niño of 2006. A significant increase in diatoms dominance was observed in the Equatorial Pacific during the 1998 La Niña and was associated with elevated marine productivity. An analysis of the environmental variables using a coupled physical-biogeochemical model (NEMO-PISCES) suggests that the Synechococcus dominance decrease during the two El Niño events was associated with an abrupt decline in nutrient availability (−0.9 to −2.5 μM NO 3 month −1 ). Alternatively, increased nutrient availability (3 μM NO 3 month −1 ) during the 1998 La Niña resulted in Equatorial Pacific dominance diatom increase. Despite these phytoplankton community shifts, the mean composition is restored after a few months, which suggests resilience in community structure.

Description: Carbon monoxide apparent quantum yields and photoproduction in the Tyne estuary Biogeosciences, 8, 703-713, 2011 Author(s): A. Stubbins, C. S. Law, G. Uher, and R. C. Upstill-Goddard Carbon monoxide (CO) apparent quantum yields (AQYs) are reported for a suite of riverine, estuarine and sea water samples, spanning a range of coloured dissolved organic matter (CDOM) sources, diagenetic histories, and concentrations (absorption coefficients). CO AQYs were highest for high CDOM riverine samples and almost an order of magnitude lower for low CDOM coastal seawater samples. CO AQYs were between 47 and 80% lower at the mouth of the estuary than at its head. Whereas, a conservative mixing model predicted only 8 to 14% decreases in CO AQYs between the head and mouth of the estuary, indicating that a highly photoreactive pool of terrestrial CDOM is lost during estuarine transit. The CDOM absorption coefficient ( a ) at 412 nm was identified as a good proxy for CO AQYs (linear regression r 2 〉 0.8; n = 12) at all CO AQY wavelengths studied (285, 295, 305, 325, 345, 365, and 423 nm) and across environments (high CDOM river, low CDOM river, estuary and coastal sea). These regressions are presented as empirical proxies suitable for the remote sensing of CO AQYs in natural waters, including open ocean water, and were used to estimate CO AQY spectra and CO photoproduction in the Tyne estuary based upon annually averaged estuarine CDOM absorption data. A minimum estimate of annual CO production was determined assuming that only light absorbed by CDOM leads to the formation of CO and a maximum limit was estimated assuming that all light entering the water column is absorbed by CO producing photoreactants (i.e. that particles are also photoreactive). In this way, annual CO photoproduction in the Tyne was estimated to be between 0.99 and 3.57 metric tons of carbon per year, or 0.004 to 0.014% of riverine dissolved organic carbon (DOC) inputs to the estuary. Extrapolation of CO photoproduction rates to estimate total DOC photomineralisation indicate that less than 1% of DOC inputs are removed via photochemical processes during transit through the Tyne estuary.

Description: Anaerobic oxidation of methane: an underappreciated aspect of methane cycling in peatland ecosystems? Biogeosciences, 8, 779-793, 2011 Author(s): K. A. Smemo and J. B. Yavitt Despite a large body of literature on microbial anaerobic oxidation of methane (AOM) in marine sediments and saline waters and its importance to the global methane (CH 4 ) cycle, until recently little work has addressed the potential occurrence and importance of AOM in non-marine systems. This is particularly true for peatlands, which represent both a massive sink for atmospheric CO 2 and a significant source of atmospheric CH 4 . Our knowledge of this process in peatlands is inherently limited by the methods used to study CH 4 dynamics in soil and sediment and the assumption that there are no anaerobic sinks for CH 4 in these systems. Studies suggest that AOM is CH 4 -limited and difficult to detect in potential CH 4 production assays against a background of CH 4 production. In situ rates also might be elusive due to background rates of aerobic CH 4 oxidation and the difficulty in separating net and gross process rates. Conclusive evidence for the electron acceptor in this process has not been presented. Nitrate and sulfate are both plausible and favorable electron acceptors, as seen in other systems, but there exist theoretical issues related to the availability of these ions in peatlands and only circumstantial evidence suggests that these pathways are important. Iron cycling is important in many wetland systems, but recent evidence does not support the notion of CH 4 oxidation via dissimilatory Fe(III) reduction or a CH 4 oxidizing archaea in consortium with an Fe(III) reducer. Calculations based on published rates demonstrate that AOM might be a significant and underappreciated constraint on the global CH 4 cycle, although much about the process is unknown, in vitro rates may not relate well to in situ rates, and projections based on those rates are fraught with uncertainty. We suggest electron transfer mechanisms, C flow and pathways, and quantifying in situ peatland AOM rates as the highest priority topics for future research.

Description: Intriguing diversity among diazotrophic picoplankton along a Mediterranean transect: a dominance of rhizobia Biogeosciences, 8, 827-840, 2011 Author(s): M. Le Moal, H. Collin, and I. C. Biegala The Mediterranean Sea is one of the most oligotrophic marine areas on earth where nitrogen fixation has formally believed to play an important role in carbon and nitrogen fluxes. Although this view is under debate, the diazotrophs responsible for this activity have still not been investigated in the open sea. In this study, we characterised the surface distribution and species richness of unicellular and filamentous diazotrophs across the Mediterranean Sea by combining microscopic counts with size fractionated in situ hybridization (TSA-FISH), and 16S rDNA and nifH genes phylogenies. These genetic analyses were possible owing to the development of a new PCR protocol adapted to scarce microorganisms that can detect as few as 1 cell ml −1 in cultures. Low concentrations of diazotrophic cyanobacteria were detected and this community was dominated at 99.9% by picoplankton hybridized to the Nitro821 probe, specific for unicellular diazotrophic cyanobacteria (UCYN). Among filamentous cyanobacteria only 0.02 filament ml −1 of Richelia were detected in the eastern basin, while small (0.7–1.5 μm) and large (2.5–3.2 μm) Nitro821-targeted cells were recovered at all stations with a mean concentration of 3.5 cell ml −1 . The affiliation of the small Nitro821-targeted cells to UCYN-A was confirmed by 16S and nifH phylogenies in the western Mediterranean Sea. In the central and the eastern Mediterranean Sea no 16S rDNA and nifH sequence from UCYN was obtained as cells concentration were close to, or below PCR detection limit. Bradyrhizobium sequences dominated nifH clone libraries from picoplanktonic size fractions. A few sequences of γ-proteobacteria were also detected in the central Mediterranean Sea. While low phosphate and iron concentrations could explain the absence of Trichodesmium sp., the factors that prevent the development of UCYN-B and C remain unknown. We also propose that the dominating picoplankters probably developed specific strategies, such as associations with protists or particles, and/or photosynthetic activity, to acquire carbon for sustaining diazotrophy.

Description: An evaluation of ocean color model estimates of marine primary productivity in coastal and pelagic regions across the globe Biogeosciences, 8, 489-503, 2011 Author(s): V. S. Saba, M. A. M. Friedrichs, D. Antoine, R. A. Armstrong, I. Asanuma, M. J. Behrenfeld, A. M. Ciotti, M. Dowell, N. Hoepffner, K. J. W. Hyde, J. Ishizaka, T. Kameda, J. Marra, F. Mélin, A. Morel, J. O'Reilly, M. Scardi, W. O. Smith Jr., T. J. Smyth, S. Tang, J. Uitz, K. Waters, and T. K. Westberry Nearly half of the earth's photosynthetically fixed carbon derives from the oceans. To determine global and region specific rates, we rely on models that estimate marine net primary productivity (NPP) thus it is essential that these models are evaluated to determine their accuracy. Here we assessed the skill of 21 ocean color models by comparing their estimates of depth-integrated NPP to 1156 in situ 14 C measurements encompassing ten marine regions including the Sargasso Sea, pelagic North Atlantic, coastal Northeast Atlantic, Black Sea, Mediterranean Sea, Arabian Sea, subtropical North Pacific, Ross Sea, West Antarctic Peninsula, and the Antarctic Polar Frontal Zone. Average model skill, as determined by root-mean square difference calculations, was lowest in the Black and Mediterranean Seas, highest in the pelagic North Atlantic and the Antarctic Polar Frontal Zone, and intermediate in the other six regions. The maximum fraction of model skill that may be attributable to uncertainties in both the input variables and in situ NPP measurements was nearly 72%. On average, the simplest depth/wavelength integrated models performed no worse than the more complex depth/wavelength resolved models. Ocean color models were not highly challenged in extreme conditions of surface chlorophyll- a and sea surface temperature, nor in high-nitrate low-chlorophyll waters. Water column depth was the primary influence on ocean color model performance such that average skill was significantly higher at depths greater than 250 m, suggesting that ocean color models are more challenged in Case-2 waters (coastal) than in Case-1 (pelagic) waters. Given that in situ chlorophyll- a data was used as input data, algorithm improvement is required to eliminate the poor performance of ocean color NPP models in Case-2 waters that are close to coastlines. Finally, ocean color chlorophyll- a algorithms are challenged by optically complex Case-2 waters, thus using satellite-derived chlorophyll- a to estimate NPP in coastal areas would likely further reduce the skill of ocean color models.

Description: Effect of permafrost thawing on organic carbon and trace element colloidal speciation in the thermokarst lakes of western Siberia Biogeosciences, 8, 565-583, 2011 Author(s): O. S. Pokrovsky, L. S. Shirokova, S. N. Kirpotin, S. Audry, J. Viers, and B. Dupré To examine the mechanisms of carbon mobilization and biodegradation during permafrost thawing and to establish a link between organic carbon (OC) and other chemical and microbiological parameters in forming thermokarst (thaw) lakes, we studied the biogeochemistry of OC and trace elements (TEs) in a chronosequence of small lakes that are being formed due to permafrost thawing in the northern part of western Siberia. Twenty lakes and small ponds of various sizes and ages were sampled for dissolved and colloidal organic carbon, metals and culturable heterotrophic bacterial cell number. We observed a sequence of ecosystems from peat thawing and palsa degradation due to permafrost subsidence in small ponds to large, km-size lakes that are subject to drainage to, finally, the khasyrey (drained lake) formation. There is a systematic evolution of both total dissolved and colloidal concentration of OC and TEs in the lake water along with the chronosequence of lake development that may be directly linked to the microbial mineralization of dissolved organic matter and the liberation of the inorganic components (Fe, Al, and TEs) from the organo-mineral colloids. In this chronosequence of lake development, we observed an apparent decrease in the relative proportion of low molecular weight

Description: Direct contribution of nitrogen deposition to nitrous oxide emissions in a temperate beech and spruce forest – a 15 N tracer study Biogeosciences, 8, 621-635, 2011 Author(s): N. Eickenscheidt, R. Brumme, and E. Veldkamp The impact of atmospheric nitrogen (N) deposition on nitrous oxide (N 2 O) emissions in forest ecosystems is still unclear. Our study assessed the direct contribution of N deposition to N 2 O emissions in temperate forests exposed to chronic high N depositions using a 15 N labelling technique. In a Norway spruce stand ( Picea abies ) and in a beech stand ( Fagus sylvatica ) at the Solling, Germany, we used a low concentrated 15 N-labelled ammonium-nitrate solution to simulate N deposition. Nitrous oxide fluxes and 15 N isotope abundances in N 2 O were measured using the closed chamber method combined with 15 N isotope analyses. Emissions of N 2 O were higher in the beech stand (2.6 ± 0.6 kg N ha −1 yr −1 ) than in the spruce stand (0.3 ± 0.1 kg N ha −1 yr −1 ). We observed a direct effect of N input on 15 N-N 2 O emissions, which lasted for less than three weeks and was mainly caused by denitrification. No further increase in 15 N enrichment of N 2 O occurred during a one-year experiment, which was probably due to immobilisation of deposited N. The annual emission factor for N 2 O from deposited N was 0.1% for the spruce stand and 0.6% for the beech stand. Standard methods used in the literature applied to the same stands grossly overestimated emission factors with values of up to 25%. Only 6–13% of the total N 2 O emissions were derived from direct N depositions. Whether the remaining emissions resulted from accumulated anthropogenic N depositions or native soil N, could not be distinguished with the applied methods. The 15 N tracer technique is a useful tool, which may improve estimates of the current contribution of N deposition to N 2 O emissions.

Description: Global spatial distribution of natural riverine silica inputs to the coastal zone Biogeosciences, 8, 597-620, 2011 Author(s): H. H. Dürr, M. Meybeck, J. Hartmann, G. G. Laruelle, and V. Roubeix Silica, SiO 2 , in dissolved (DSi) and particulate (PSi) form, is both a major product of continental weathering as well as an essential nutrient in terrestrial and aquatic systems. Here we present estimates of the spatial distribution of riverine silica fluxes under natural conditions, i.e. without human influence, to ~140 segments of the global coastal zone. Focussing on the construction of the DSi budget, natural DSi concentration is multiplied with discharge of rivers for each segment for documented basins and segments. Segments with no documentation available are estimated using clustered information based mainly on considerations of local lithology, climate, and lake retention. We approximate fluxes of particulate silica in various forms (PSi) from fluxes of suspended matter, calculated from existing models. Results have been established for silica fluxes, concentrations and yields for drainage basins of the different continents, oceans basins as well as coastal segment basins. For the continental surfaces actually draining into the oceans (exorheic regions, representing 114.7 million (M) km 2 ), 371 M t y −1 of DSi and 8835 M t y −1 of PSi are transported, corresponding to a mean concentration of 9.5 mg l −1 and 226 mg l −1 , and to a mean yield of 3.3 t km −2 y −1 and 77 t km −2 y −1 , respectively. DSi yields exceeding 6.6 t km −2 y −1 , i.e. 〉2× the global average, represent 17.4% of the global continental ice-free exorheic area but correspond to 56.0% of DSi fluxes. Pacific catchments hold most of the hyper-active areas (〉5× global average), suggesting a close connection between tectonic activity and DSi fluxes resulting from silicate weathering. The macro-filters of regional and marginal seas intercept 33% and 46% of the total dissolved and particulate silica fluxes. The mass of DSi received from rivers per unit square area of various oceans ranges over more than one order of magnitude. When expressed per unit volume and when individual regional seas are considered this figure ranges over two to three orders of magnitude, an illustration of the heterogeneity of the land to sea connection.

Description: Integrating field sampling, geostatistics and remote sensing to map wetland vegetation in the Pantanal, Brazil Biogeosciences, 8, 667-686, 2011 Author(s): J. Arieira, D. Karssenberg, S. M. de Jong, E. A. Addink, E. G. Couto, C. Nunes da Cunha, and J. O. Skøien Development of efficient methodologies for mapping wetland vegetation is of key importance to wetland conservation. Here we propose the integration of a number of statistical techniques, in particular cluster analysis, universal kriging and error propagation modelling, to integrate observations from remote sensing and field sampling for mapping vegetation communities and estimating uncertainty. The approach results in seven vegetation communities with a known floral composition that can be mapped over large areas using remotely sensed data. The relationship between remotely sensed data and vegetation patterns, captured in four factorial axes, were described using multiple linear regression models. There were then used in a universal kriging procedure to reduce the mapping uncertainty. Cross-validation procedures and Monte Carlo simulations were used to quantify the uncertainty in the resulting map. Cross-validation showed that accuracy in classification varies according with the community type, as a result of sampling density and configuration. A map of uncertainty derived from Monte Carlo simulations revealed significant spatial variation in classification, but this had little impact on the proportion and arrangement of the communities observed. These results suggested that mapping improvement could be achieved by increasing the number of field observations of those communities with a scattered and small patch size distribution; or by including a larger number of digital images as explanatory variables in the model. Comparison of the resulting plant community map with a flood duration map, revealed that flooding duration is an important driver of vegetation zonation. This mapping approach is able to integrate field point data and high-resolution remote-sensing images, providing a new basis to map wetland vegetation and allow its future application in habitat management, conservation assessment and long-term ecological monitoring in wetland landscapes.

Description: Stable carbon isotope discrimination and microbiology of methane formation in tropical anoxic lake sediments Biogeosciences, 8, 795-814, 2011 Author(s): R. Conrad, M. Noll, P. Claus, M. Klose, W. R. Bastos, and A. Enrich-Prast Methane is an important end product of degradation of organic matter in anoxic lake sediments. Methane is mainly produced by either reduction of CO 2 or cleavage of acetate involving different methanogenic archaea. The contribution of the different methanogenic paths and of the diverse bacteria and archaea involved in CH 4 production exhibits a large variability that is not well understood. Lakes in tropical areas, e.g. in Brazil, are wetlands with high potential impact on the global CH 4 budget. However, they have hardly been studied with respect to methanogenesis. Therefore, we used samples from 16 different lake sediments in the Pantanal and Amazon region of Brazil to measure production of CH 4 , CO 2 , analyze the content of 13 C in the products and in intermediately formed acetate, determine the abundance of bacterial and archaeal microorgansisms and their community composition and diversity by targeting the genes of bacterial and archaeal ribosomal RNA and of methyl coenzyme M reductase, the key enzyme of methanogenic archaea. These experiments were done in the presence and absence of methyl fluoride, an inhibitor of acetoclastic methanogenesis. While production rates of CH 4 and CO 2 were correlated to the content of organic matter and the abundance of archaea in the sediment, values of 13 C in acetate, CO 2 , and CH 4 were related to the 13 C content of organic matter and to the path of CH 4 production with its intrinsic carbon isotope fractionation. Isotope fractionation was small (average 10‰) for conversion of C org to acetate-methyl, which was hardly further fractionated during CH 4 production. However, fractionation was strong for CO 2 conversion to CH 4 (average 75‰), which generally accounted for 〉50% of total CH 4 production. Canonical correspondence analysis did not reveal an effect of microbial community composition, despite the fact that it exhibited a pronounced variability among the different sediments.

Description: Dissolved and particulate primary production along a longitudinal gradient in the Mediterranean Sea Biogeosciences, 8, 815-825, 2011 Author(s): D. C. López-Sandoval, A. Fernández, and E. Marañón We have determined the photosynthetic production of dissolved (DOCp) and particulate organic carbon (POCp) along a longitudinal transect in the Mediterranean Sea during the summer stratification period. The euphotic layer-integrated rates of DOCp and POCp ranged between approximately 50–130 and 95–210 mgC m −2 d −1 , respectively, and showed an east to west increasing trend. For the whole transect, the relative contribution of DOCp to total, euphotic layer-integrated primary production (percentage of extracellular release, PER) averaged ~37% and did not show any clear longitudinal pattern. In spite of the relatively high PER values, the measured DOCp rates were much lower than the estimated bacterial carbon demand, suggesting a small degree of coupling between phytoplankton exudation and bacterial metabolism. Our results, when compared with previous measurements obtained with the same methods in several ecosystems of contrasting productivity, support the view that the relative importance of DOCp increases under strong nutrient limitation.

Description: Quantifying nitrate dynamics in an oligotrophic lake using Δ 17 O Biogeosciences, 8, 687-702, 2011 Author(s): U. Tsunogai, S. Daita, D. D. Komatsu, F. Nakagawa, and A. Tanaka The stable isotopic compositions of nitrate, including the 17 O anomalies (Δ 17 O), were determined twice in 1 yr (June and August 2007) in the oligotrophic water column of Lake Mashu, Japan. These data were then used to quantify the geochemical dynamics of nitrate in the lake, by using the deposition rate of the atmospheric nitrate onto the entire catchment area of the lake. The total amount of nitrate in the lake water decreased from 4.2 to 2.1 Mmol during the period between the observations, while the average Δ 17 O values remained uniform at +2.5‰. The Δ 17 O values corresponded to an small and uniform mixing ratio of atmospheric nitrate to total nitrate of 9.7 ± 0.8%. These results indicate that 0.52 ± 0.34 Mmol of the remineralized nitrate was fed into the water column through nitrification, while 2.6 ± 0.4 Mmol of nitrate was simultaneously removed from the water column by assimilation, during the period between the observations. The lake water dissolved nitrate was characterized by rapid removal through assimilation during summer until it was almost completely removed from the euphotic layer, as well as continuous feeding into the lake through nitrification (3.2 ± 0.3 Mmol a −1 ) and deposition (0.35 ± 0.2 Mmol a −1 ), regardless of the seasons. The 15 N-depleted nitrogen isotopic compositions of nitrate were as low as −6.5‰ in June, which also indicates that in-lake nitrification is the major source of nitrate in the lake and suggests that there is low potential for denitrification in and around the lake. Atmospheric nitrate deposited into the lake will be assimilated quickly, having a mean residence time of 1.2 ± 0.1 yr. In addition, more than 90% of the assimilated nitrate will be remineralized to nitrate and re-assimilated via active nitrogen cycling in the lake.

Description: Short-term dynamics of microplankton abundance and diversity in NW Mediterranean Sea during late summer conditions (DYNAPROC 2 cruise; 2004) Biogeosciences, 8, 743-761, 2011 Author(s): S. Lasternas, A. Tunin-Ley, F. Ibañez, V. Andersen, M.-D. Pizay, and R. Lemée Changes in marine phytoplankton communities at short-time scales have rarely been examined. As a part of the DYNAPROC 2 cruise (14 September–17 October 2004), conducted in the NW Mediterranean Sea, we daily sampled and determined taxonomic composition and abundance of the microphytoplankton and the dominant microzooplanktonic groups from both net and bottle sampling, during 4 cycles of 5 days. Hydrological conditions were characterised by the dominance of a stratified water column and nutrient-depleted conditions. However the stratification index revealed a destabilization of the water column from the beginning of the second cycle, related to a wind stress event accompanied with coastal water intrusion. This conducted to an increase of taxonomic richness and a general decrease of evenness, depicting the dominance of species with the best fitness. We also emphasised on the great interest of taxonomic studies, as able to provide valuable information on biogeochemical-important groups of species, potential water masses indicators and trophic aspects of the community that are ignored or largely underestimated with other kind of phytoplankton studies, such as pigment analysis.

Description: Isotopic fractionation during soil uptake of atmospheric hydrogen Biogeosciences, 8, 763-769, 2011 Author(s): A. Rice, A. Dayalu, P. Quay, and R. Gammon Soil uptake of atmospheric hydrogen (H 2 ) and the associated hydrogen isotope effect were studied using soil chambers in a Western Washington second-growth coniferous forest. Chamber studies were conducted during both winter and summer seasons to account for large natural variability in soil moisture content (4–50%) and temperature (6–22 °C). H 2 deposition velocities were found to range from 0.01–0.06 cm s −1 with an average of 0.033 ± 0.008 cm s −1 (95% confidence interval). Consistent with prior studies, deposition velocities were correlated with soil moisture below 20% soil moisture content during the summer season. During winter, there was considerable variability observed in deposition velocity that was not closely related to soil moisture. The hydrogen kinetic isotope effect with H 2 uptake was found to range from −24‰ to −109‰. Aggregate analysis of experimental data results in an average KIE of −57 ± 5‰ (95% CI). Some of the variability in KIE can be explained by larger isotope effects at lower ( 30%) soil moisture contents. The measured KIE was also found to be correlated with deposition velocity, with smaller isotope effects occurring at higher deposition velocities. If correct, these findings will have an impact on the interpretation of atmospheric measurements and modeling of δD of H 2 .

Description: Changes in ocean circulation and carbon storage are decoupled from air-sea CO 2 fluxes Biogeosciences, 8, 505-513, 2011 Author(s): I. Marinov and A. Gnanadesikan The spatial distribution of the air-sea flux of carbon dioxide is a poor indicator of the underlying ocean circulation and of ocean carbon storage. The weak dependence on circulation arises because mixing-driven changes in solubility-driven and biologically-driven air-sea fluxes largely cancel out. This cancellation occurs because mixing driven increases in the poleward residual mean circulation result in more transport of both remineralized nutrients and heat from low to high latitudes. By contrast, increasing vertical mixing decreases the storage associated with both the biological and solubility pumps, as it decreases remineralized carbon storage in the deep ocean and warms the ocean as a whole.

Description: Near-ubiquity of ice-edge blooms in the Arctic Biogeosciences, 8, 515-524, 2011 Author(s): M. Perrette, A. Yool, G. D. Quartly, and E. E. Popova Ice-edge blooms are significant features of Arctic primary production, yet have received relatively little attention. Here we combine satellite ocean colour and sea-ice data in a pan-Arctic study. Ice-edge blooms occur in all seasonally ice-covered areas and from spring to late summer, being observed in 77–89% of locations for which adequate data exist, and usually peaking within 20 days of ice retreat. They sometimes form long belts along the ice-edge (greater than 100 km), although smaller structures were also found. The bloom peak is on average more than 1 mg m −3 , with major blooms more than 10 mg m −3 , and is usually located close to the ice-edge, though not always. Some propagate behind the receding ice-edge over hundreds of kilometres and over several months, while others remain stationary. The strong connection between ice retreat and productivity suggests that the ongoing changes in Arctic sea-ice may have a significant impact on higher trophic levels and local fish stocks.

Description: Lack of P-limitation of phytoplankton and heterotrophic prokaryotes in surface waters of three anticyclonic eddies in the stratified Mediterranean Sea Biogeosciences, 8, 525-538, 2011 Author(s): T. Tanaka, T. F. Thingstad, U. Christaki, J. Colombet, V. Cornet-Barthaux, C. Courties, J.-D. Grattepanche, A. Lagaria, J. Nedoma, L. Oriol, S. Psarra, M. Pujo-Pay, and F. Van Wambeke We investigated the identity of the limiting nutrient of the pelagic microbial food web in the Mediterranean Sea using nutrient manipulated microcosms during summer 2008. Experiments were carried out with surface waters at the center of anticyclonic eddies in the Western Basin, the Ionian Basin, and the Levantine Basin. In situ, the ratio of N to P was always higher in both dissolved and particulate organic fractions compared to the Redfield ratio, suggesting a relative P-starvation. In each experiment, four different treatments in triplicates (addition of ammonium, phosphate, a combination of both, and the unamended control) were employed and chemical and biological parameters monitored throughout a 3–4 day incubation. Temporal changes of turnover time of phosphate and ATP, and alkaline phosphatase activity during the incubation suggested that the phytoplankton and heterotrophic prokaryotes (Hprok) communities were not P-limited at the sites. Furthermore, statistical comparison among treatments at the end of the incubation did not support a hypothesis of P-limitation at the three study sites. In contrast, primary production was consistently limited by N, and Hprok growth was not limited by N nor P in the Western Basin, but N-limited in the Ionian Basin, and N and P co-limited in the Levantine Basin. Our results demonstrated the gap between biogeochemical features (an apparent P-starved status) and biological responses (no apparent P-limitation). We question the general notion that Mediterranean surface waters are limited by P alone during the stratified period.

Description: The influence of iron and light on net community production in the Subantarctic and Polar Frontal Zones Biogeosciences, 8, 227-237, 2011 Author(s): N. Cassar, P. J. DiFiore, B. A. Barnett, M. L. Bender, A. R. Bowie, B. Tilbrook, K. Petrou, K. J. Westwood, S. W. Wright, and D. Lefevre The roles of iron and light in controlling biomass and primary productivity are clearly established in the Southern Ocean. However, their influence on net community production (NCP) and carbon export remains to be quantified. To improve our understanding of NCP and carbon export production in the Subantarctic Zone (SAZ) and the northern reaches of the Polar Frontal Zone (PFZ), we conducted continuous onboard determinations of NCP as part of the Sub-Antarctic Sensitivity to Environmental Change (SAZ-Sense) study, which occurred in January–February 2007. Biological O 2 supersaturation was derived from measuring O 2 /Ar ratios by equilibrator inlet mass spectrometry. Based on these continuous measurements, NCP during the austral summer 2007 in the Australian SAZ was approximately 43 mmol O 2 m −2 d −1 . NCP showed significant spatial variability, with larger values near the Subtropical front, and a general southward decrease. For shallower mixed layers (

Description: Flow cytometric assessment of specific leucine incorporation in the open Mediterranean Biogeosciences, 8, 253-265, 2011 Author(s): A. Talarmin, F. Van Wambeke, P. Catala, C. Courties, and P. Lebaron The surface of the Mediterranean Sea is a low-phosphate-low-chlorophyll marine area where marine heterotrophic prokaryotes significantly contribute to the biogeochemical cycles of all biogenic elements such as carbon, notably through the mineralization of dissolved organic compounds. Cell-specific leucine incorporation rates were determined in early summer in the open stratified Mediterranean Sea. The bulk leucine incorporation rate was on average 5 ± 4 pmol leu l −1 h −1 ( n =30). Cell-specific 3 H-leucine incorporation rates were assayed using flow cytometry coupled to cell sorting. Heterotrophic prokaryotes (Hprok) were divided into cytometric groups according to their side scatter and green fluorescence properties: high nucleic acid containing cells (HNA) with high scatter (HNA-hs) and low scatter (HNA-ls) and low nucleic acid containing cells (LNA). Cell-specific leucine incorporation rates of these cytometric groups ranged from 2 to 54, 0.9 to 11, and 1 to 12 × 10 -21 mol cell −1 h −1 , respectively. LNA cells represented 45 to 63% of the Hprok abundance, and significantly contributed to the bulk leucine incorporation rates, from 12 to 43%. HNA/LNA ratios of cell-specific leucine incorporation were on average 2.0 ± 0.7 ( n =30). In surface layers (from 0 m down to the deep chlorophyll depth, DCM), cell-specific rates of HNA-hs were elevated (7 and 13 times greater than LNA and HNA-ls, respectively). Nevertheless, on average HNA-hs (26%) and LNA (27%) equally contributed to the bulk leucine incorporation in these layers. Prochlorococcus cells were easily sorted near the DCM and displayed cell-specific leucine incorporation rates ranging from 3 to 55 × 10 -21 mol leu cell −1 h −1 , i.e. as high as HNA-hs'. These sorted groups could therefore be defined as key-players in the process of leucine incorporation into proteins. The mixotrophic features of certain photosynthetic prokaryotes and the high contribution of LNA cells to leucine incorporation within the microbial communities of the Mediterranean could be reinforced.

Description: Distribution and host diversity of Amoebophryidae parasites across oligotrophic waters of the Mediterranean Sea Biogeosciences, 8, 267-278, 2011 Author(s): R. Siano, C. Alves-de-Souza, E. Foulon, El M. Bendif, N. Simon, L. Guillou, and F. Not Sequences affiliated to Syndiniales (Marine alveolate, MALV) regularly dominate 18S rDNA genetic libraries of nearly all marine ecosystems investigated so far. Among them, Amoebophryidae (MALV group II) is composed of numerous and genetically distant environmental sequences, where Amoebophrya is the only known and formally described genus. Amoebophrya species include virulent pathogens for a wide range of dinoflagellate species. Beside their regular occurrence in marine ecosystems, their quantitative distribution and the environmental factors triggering host infection have barely been studied in open oligotrophic waters. In order to understand the functional role of these parasites in natural environments, we studied the distribution and contribution to the eukaryotic community of the small free-living stage of Amoebophryidae (the dinospores) along a transect in the Mediterranean Sea, as well as their host diversity at three oligotrophic stations. Dinospores were more abundant at a coastal station (max. 1.5 × 10 3 cells ml −1 ) than in oligotrophic waters (max. 51 ± 16.3 cells ml −1 ), where they represented 10.3 to 34.9% of the total eukaryotic community at 40 and 30 m depth, respectively and 21.2% on average along the water column. Positive correlation was found between dinospore occurrence and higher concentration of NO 3 + NO 2 at the coastal station. At selected stations, out of 38 different dinoflagellates taxa identified, 15 were infected, among which a majority were not recognized as Amoebophryidae host so far. Prevalences (percentage of infected cells) generally varied between 1% and 10%, with a notable exception for Blepharocysta paulsenii for which 25% of cells were infected at the most oligotrophic station. The present study shows that dinospores are able to thrive and infect dinoflagellates both in coastal and ultra-oligotrophic open waters. Our results emphasize the role of parasitism in microbial food web dynamics and ultimately on biogeochemical cycles.

Description: Detection of pore space in CT soil images using artificial neural networks Biogeosciences, 8, 279-288, 2011 Author(s): M. G. Cortina-Januchs, J. Quintanilla-Dominguez, A. Vega-Corona, A. M. Tarquis, and D. Andina Computed Tomography (CT) images provide a non-invasive alternative for observing soil structures, particularly pore space. Pore space in soil data indicates empty or free space in the sense that no material is present there except fluids such as air, water, and gas. Fluid transport depends on where pore spaces are located in the soil, and for this reason, it is important to identify pore zones. The low contrast between soil and pore space in CT images presents a problem with respect to pore quantification. In this paper, we present a methodology that integrates image processing, clustering techniques and artificial neural networks, in order to classify pore space in soil images. Image processing was used for the feature extraction of images. Three clustering algorithms were implemented (K-means, Fuzzy C-means, and Self Organising Maps) to segment images. The objective of clustering process is to find pixel groups of a similar grey level intensity and to organise them into more or less homogeneous groups. The segmented images are used for test a classifier. An Artificial Neural Network is characterised by a great degree of modularity and flexibility, and it is very efficient for large-scale and generic pattern recognition applications. For these reasons, an Artificial Neural Network was used to classify soil images into two classes (pore space and solid soil). Our methodology shows an alternative way to detect solid soil and pore space in CT images. The percentages of correct classifications of pore space of the total number of classifications among the tested images were 97.01%, 96.47% and 96.12%.

Description: Free atmospheric CO 2 enrichment increased above ground biomass but did not affect symbiotic N 2 -fixation and soil carbon dynamics in a mixed deciduous stand in Wales Biogeosciences, 8, 353-364, 2011 Author(s): M. R. Hoosbeek, M. Lukac, E. Velthorst, A. R. Smith, and D. L. Godbold Through increases in net primary production (NPP), elevated CO 2 is hypothesized to increase the amount of plant litter entering the soil. The fate of this extra carbon on the forest floor or in mineral soil is currently not clear. Moreover, increased rates of NPP can be maintained only if forests can escape nitrogen limitation. In a Free atmospheric CO 2 Enrichment (FACE) experiment near Bangor, Wales, 4 ambient and 4 elevated [CO 2 ] plots were planted with patches of Betula pendula , Alnus glutinosa and Fagus sylvatica on a former arable field. After 4 years, biomass averaged for the 3 species was 5497 (se 270) g m −2 in ambient and 6450 (se 130) g m −2 in elevated [CO 2 ] plots, a significant increase of 17% ( P = 0.018). During that time, only a shallow L forest floor litter layer had formed due to intensive bioturbation. Total soil C and N contents increased irrespective of treatment and species as a result of afforestation. We could not detect an additional C sink in the soil, nor were soil C stabilization processes affected by elevated [CO 2 ]. We observed a decrease of leaf N content in Betula and Alnus under elevated [CO 2 ], while the soil C/N ratio decreased regardless of CO 2 treatment. The ratio of N taken up from the soil and by N 2 -fixation in Alnus was not affected by elevated [CO 2 ]. We infer that increased nitrogen use efficiency is the mechanism by which increased NPP is sustained under elevated [CO 2 ] at this site.

Description: Influence of bioturbation on the biogeochemistry of littoral sediments of an acidic post-mining pit lake Biogeosciences, 8, 339-352, 2011 Author(s): S. Lagauzère, S. Moreira, and M. Koschorreck In the last decades, the mining exploitation of large areas in Lusatia (Eastern Germany) but also in other mining areas worldwide has led to the formation of hundreds of pit lakes. Pyrite oxidation in the surrounding dumps makes many such lakes extremely acidic (pH 〈 3). The biogeochemical functioning of these lakes is mainly governed by cycling of iron. This represents a relevant ecological problem and intensive research has been conducted to understand the involved biogeochemical processes and develop bioremediation strategies. Despite some studies reporting the presence of living organisms (mostly bacteria, algae, and macro-invertebrates) under such acidic conditions, and their trophic interactions, their potential impact on the ecosystem functioning was poorly investigated. The present study aimed to assess the influence of chironomid larvae on oxygen dynamics and iron cycle in the sediment of acidic pit lakes. In the Mining Lake 111, used as a study case since 1996, Chironomus crassimanus (Insecta, Diptera) is the dominant benthic macro-invertebrate species and occurs at relatively high abundances in shallow water. A 16-day laboratory experiment using microcosms combined with high resolution measurements (DET gel probes and O 2 microsensors) was carried out. The burrowing activity of C. crassimanus larvae induced a 3-fold increase of the diffusive oxygen uptake by sediment, indicating a stimulation of the mineralization of organic matter in the upper layers of the sediment. The iron cycle was also impacted (e.g. lower rates of reduction and oxidation, increase of iron-oxidizing bacteria abundance, stimulation of mineral formation) but with no significant effect on the iron flux at the sediment-water interface, and thus on the water acidity budget. This work provides the first assessment of bioturbation in an acidic mining lake and shows that its influence on biogeochemistry cannot be neglected.

Description: CO 2 maximum in the oxygen minimum zone (OMZ) Biogeosciences, 8, 239-252, 2011 Author(s): A. Paulmier, D. Ruiz-Pino, and V. Garçon Oxygen minimum zones (OMZs), known as suboxic layers which are mainly localized in the Eastern Boundary Upwelling Systems, have been expanding since the 20th "high CO 2 " century, probably due to global warming. OMZs are also known to significantly contribute to the oceanic production of N 2 O, a greenhouse gas (GHG) more efficient than CO 2 . However, the contribution of the OMZs on the oceanic sources and sinks budget of CO 2 , the main GHG, still remains to be established. We present here the dissolved inorganic carbon (DIC) structure, associated locally with the Chilean OMZ and globally with the main most intense OMZs (O 2 2225 μmol kg −1 , up to 2350 μmol kg −1 ) have been reported over the whole OMZ thickness, allowing the definition for all studied OMZs a Carbon Maximum Zone (CMZ). Locally off Chile, the shallow cores of the OMZ and CMZ are spatially and temporally collocated at 21° S, 30° S and 36° S despite different cross-shore, long-shore and seasonal configurations. Globally, the mean state of the main OMZs also corresponds to the largest carbon reserves of the ocean in subsurface waters. The CMZs-OMZs could then induce a positive feedback for the atmosphere during upwelling activity, as potential direct local sources of CO 2 . The CMZ paradoxically presents a slight "carbon deficit" in its core (~10%), meaning a DIC increase from the oxygenated ocean to the OMZ lower than the corresponding O 2 decrease (assuming classical C/O molar ratios). This "carbon deficit" would be related to regional thermal mechanisms affecting faster O 2 than DIC (due to the carbonate buffer effect) and occurring upstream in warm waters (e.g., in the Equatorial Divergence), where the CMZ-OMZ core originates. The "carbon deficit" in the CMZ core would be mainly compensated locally at the oxycline, by a "carbon excess" induced by a specific remineralization. Indeed, a possible co-existence of bacterial heterotrophic and autotrophic processes usually occurring at different depths could stimulate an intense aerobic-anaerobic remineralization, inducing the deviation of C/O molar ratios from the canonical Redfield ratios. Further studies to confirm these results for all OMZs are required to understand the OMZ effects on both climatic feedback mechanisms and marine ecosystem perturbations.

Description: Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model Biogeosciences, 8, 387-414, 2011 Author(s): N. Mahowald, K. Lindsay, D. Rothenberg, S. C. Doney, J. K. Moore, P. Thornton, J. T. Randerson, and C. D. Jones Coupled-carbon-climate simulations are an essential tool for predicting the impact of human activity onto the climate and biogeochemistry. Here we incorporate prognostic desert dust and anthropogenic aerosols into the CCSM3.1 coupled carbon-climate model and explore the resulting interactions with climate and biogeochemical dynamics through a series of transient anthropogenic simulations (20th and 21st centuries) and sensitivity studies. The inclusion of prognostic aerosols into this model has a small net global cooling effect on climate but does not significantly impact the globally averaged carbon cycle; we argue that this is likely to be because the CCSM3.1 model has a small climate feedback onto the carbon cycle. We propose a mechanism for including desert dust and anthropogenic aerosols into a simple carbon-climate feedback analysis to explain the results of our and previous studies. Inclusion of aerosols has statistically significant impacts on regional climate and biogeochemistry, in particular through the effects on the ocean nitrogen cycle and primary productivity of altered iron inputs from desert dust deposition.

Description: Influence of distributary channels on sediment and organic matter supply in event-dominated coastal margins: the Po prodelta as a study case Biogeosciences, 8, 365-385, 2011 Author(s): T. Tesi, S. Miserocchi, M. A. Goñi, M. Turchetto, L. Langone, A. De Lazzari, S. Albertazzi, and A. Correggiari From November 2008 through May 2009, the Po river (Italy) experienced several floods exceeding 5000 m 3 s −1 . This long series of events ended with a large flood in early May 2009 (~8000 m 3 s −1 ). An event-response sampling was carried out in the Po prodelta in April–May 2009 to characterize the preservation of this series of floods in the sediment record and to describe the event-supply and deposition of riverborne particulate material during the May 2009 flood. The water sampling was carried out early in the event under conditions of moderate river flow (~5000 m 3 s −1 ) and 24 h later during the peak discharge (~8000 m 3 s −1 ). Sediment cores were collected in the prodelta before and after the peak flood. At each station, profiles of conductivity, transmittance, and fluorescence were acquired. Surface and bottom waters were sampled to collect sediments in suspension. In addition, a few days before the May 2009 event, suspended sediments were collected at Pontelagoscuro gauging station, ~90 km upstream from the coast. Biogeochemical compositions and sedimentological characteristics of suspended and sediment samples were investigated using bulk and biomarker analyses. Furthermore, 7 Be and radiographs were used to analyze the internal stratigraphy of sediment cores. During moderate flow, the water column did not show evidence of plume penetration. Stations re-occupied 24 h later exhibited marked physical and biogeochemical changes during the peak flood. However, the concentration of terrestrially-derived material in surface waters was still less than expected. These results suggested that, since material enters the Adriatic as buoyancy-driven flow with a reduced transport capacity, settling and flocculation processes result in trapping a significant fraction of land-derived material in shallow sediments and/or within distributary channels. Although numerous discharge peaks occurred from November 2008 through April 2009 (4000–6000 m 3 s −1 ), sediment cores collected in late April 2009 showed lack of event-strata preservation and reduced 7 Be penetrations. This suggested that only a small fraction of the sediment supply during ordinary events reaches the deepest region of the prodelta (12–20 m water depth). As a result, these event-strata have a thickness not sufficient to be preserved in the sediment record because of post-depositional processes that destroy the flood signal. Stations in the northern and central prodelta were re-occupied after the peak of the May 2009 flood. Based on 7 Be and radiographs, we estimated event layers of 17 and 6 cm thickness, respectively. Selective trapping of coarse material occurred in the central prodelta likely because of the geomorphologic setting of the central outlet characterized by an estuary-like mouth. Despite these settling processes, lignin-based parameters indicated that the composition of the terrigenous OC was fairly homogenous throughout the network of channels and between size-fractions.

Description: Deep silicon maxima in the stratified oligotrophic Mediterranean Sea Biogeosciences, 8, 459-475, 2011 Author(s): Y. Crombet, K. Leblanc, B. Quéguiner, T. Moutin, P. Rimmelin, J. Ras, H. Claustre, N. Leblond, L. Oriol, and M. Pujo-Pay The silicon biogeochemical cycle has been studied in the Mediterranean Sea during late summer/early autumn 1999 and summer 2008. The distribution of nutrients, particulate carbon and silicon, fucoxanthin (Fuco), and total chlorophyll- a (TChl- a ) were investigated along an eastward gradient of oligotrophy during two cruises (PROSOPE and BOUM) encompassing the entire Mediterranean Sea during the stratified period. At both seasons, surface waters were depleted in nutrients and the nutriclines gradually deepened towards the East, the phosphacline being the deepest in the easternmost Levantine basin. Following the nutriclines, parallel deep maxima of biogenic silica (DSM), fucoxanthin (DFM) and TChl- a (DCM) were evidenced during both seasons with maximal concentrations of 0.45 μmol L −1 for BSi, 0.26 μg L −1 for Fuco, and 1.70 μg L −1 for TChl- a , all measured during summer. Contrary to the DCM which was a persistent feature in the Mediterranean Sea, the DSM and DFMs were observed in discrete areas of the Alboran Sea, the Algero-Provencal basin, the Ionian sea and the Levantine basin, indicating that diatoms were able to grow at depth and dominate the DCM under specific conditions. Diatom assemblages were dominated by Chaetoceros spp., Leptocylindrus spp., Pseudonitzschia spp. and the association between large centric diatoms ( Hemiaulus hauckii and Rhizosolenia styliformis ) and the cyanobacterium Richelia intracellularis was observed at nearly all sites. The diatom's ability to grow at depth is commonly observed in other oligotrophic regions and could play a major role in ecosystem productivity and carbon export to depth. Contrary to the common view that Si and siliceous phytoplankton are not major components of the Mediterranean biogeochemistry, we suggest here that diatoms, by persisting at depth during the stratified period, could contribute to a large part of the marine primary production as observed in other oligotrophic areas.

Description: Quality or decomposer efficiency – which is most important in the temperature response of litter decomposition? A modelling study using the GLUE methodology Biogeosciences, 8, 477-487, 2011 Author(s): J. Å. M. Wetterstedt and G. I. Ågren We still lack full mechanistic understanding of how the temperature history affects the future decomposition rate of litter and soil organic matter. To explore that, we used the GLUE modelling framework together with the Q-model and data from a needle litter incubation experiment to compare a differential temperature response of litter qualities to a temperature-dependent decomposer efficiency. The needle litter incubation was a full factorial design with the initial and final temperatures 5, 15 and 25 °C. Samples were moved from the initial to the final temperature when approximately 12% of the initial carbon had been respired and the experiment terminated when an additional 12% had been lost. We used four variations of the Q-model; the litter was described as having one or two initial quality values and the decomposer efficiency was either fixed or allowed to vary with temperature. All variations were calibrated with good fits to the data subsets with equal initial and final temperatures. Evaluation against temperature shift subsets also showed good results, except just after the change in temperature where all variations predicted a smaller response than observed. The effects of having one or two initial litter quality values (fixed decomposer efficiency) on end-of-experiment litter quality and respiration were marginal. Letting decomposer efficiency vary with temperature resulted in a decrease in efficiency between 5 and 15 °C but no change between 15 and 25 °C and in substantial differences in litter quality at the end of the initial incubation in response to incubation temperature. The temperature response of decomposition through temperature dependent decomposer efficiency proved, therefore, to be more important than the differential response to different substrate qualities. These results suggests that it may be important to consider other factors (e.g. microbial efficiency, changing substrate composition) than the temperature sensitivity coupled to substrate quality when evaluating effects of temperature changes on soil organic matter stability.

Description: Evidence for aggregation and export of cyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone Biogeosciences, 8, 203-216, 2011 Author(s): M. W. Lomas and S. B. Moran Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus , and nano-plankton contribute significantly to POC export, via formation and gravitational settling of aggregates and/or consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (〉0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, 〉53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus , Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to 〉53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates likely by gravitational settling and/or convective mixing. Using available pigment and ancillary data from these cruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export via formation of aggregates in this oligotrophic ocean gyre. We suggest that aggregate formation and fate should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.

Description: Spatially explicit analysis of gastropod biodiversity in ancient Lake Ohrid Biogeosciences, 8, 175-188, 2011 Author(s): T. Hauffe, C. Albrecht, K. Schreiber, K. Birkhofer, S. Trajanovski, and T. Wilke The quality of spatial analyses of biodiversity is improved by (i) utilizing study areas with well defined physiogeographical boundaries, (ii) limiting the impact of widespread species, and (iii) using taxa with heterogeneous distributions. These conditions are typically met by ecosystems such as oceanic islands or ancient lakes and their biota. While research on ancient lakes has contributed significantly to our understanding of evolutionary processes, statistically sound studies of spatial variation of extant biodiversity have been hampered by the frequently vast size of ancient lakes, their limited accessibility, and the lack of scientific infrastructure. The European ancient Lake Ohrid provides a rare opportunity for such a reliable spatial study. The comprehensive horizontal and vertical sampling of a species-rich taxon, the Gastropoda, presented here, revealed interesting patterns of biodiversity, which, in part, have not been shown before for other ancient lakes. In a total of 284 samples from 224 different locations throughout the Ohrid Basin, 68 gastropod species, with 50 of them (= 73.5%) being endemic, could be reported. The spatial distribution of these species shows the following characteristics: (i) within Lake Ohrid, the most frequent species are endemic taxa with a wide depth range, (ii) widespread species (i.e. those occurring throughout the Balkans or beyond) are rare and mainly occur in the upper layer of the lake, (iii) while the total number of species decreases with water depth, the proportion of endemics increases, and (iv) the deeper layers of Lake Ohrid appear to have a higher spatial homogeneity of biodiversity. Moreover, gastropod communities of Lake Ohrid and its feeder springs are both distinct from each other and from the surrounding waters. The analysis also shows that community similarity of Lake Ohrid is mainly driven by niche processes (e.g. environmental factors), but also by neutral processes (e.g. dispersal limitation and evolutionary histories of species). For niche-based mechanisms it is shown that large scale effects such as type of water body or water depth are mainly responsible for the similarity of gastropod communities, whereas small scale effects like environmental gradients affect gastropod compositions only marginally. In fact, neutral processes appear to be more important than the small scale environmental factors, thus emphasizing the importance of dispersal capacities and evolutionary histories of species.

Description: Synoptic relationships between surface Chlorophyll- a and diagnostic pigments specific to phytoplankton functional types Biogeosciences, 8, 311-327, 2011 Author(s): T. Hirata, N. J. Hardman-Mountford, R. J. W. Brewin, J. Aiken, R. Barlow, K. Suzuki, T. Isada, E. Howell, T. Hashioka, M. Noguchi-Aita, and Y. Yamanaka Error-quantified, synoptic-scale relationships between chlorophyll- a (Chl- a ) and phytoplankton pigment groups at the sea surface are presented. A total of ten pigment groups were considered to represent three Phytoplankton Size Classes (PSCs, micro-, nano- and picoplankton) and seven Phytoplankton Functional Types (PFTs, i.e. diatoms, dinoflagellates, green algae, prymnesiophytes (haptophytes), pico-eukaryotes, prokaryotes and Prochlorococcus sp.). The observed relationships between Chl- a and PSCs/PFTs were well-defined at the global scale to show that a community shift of phytoplankton at the basin and global scales is reflected by a change in Chl- a of the total community. Thus, Chl- a of the total community can be used as an index of not only phytoplankton biomass but also of their community structure. Within these relationships, we also found non-monotonic variations with Chl- a for certain pico-sized phytoplankton (pico-eukaryotes, Prokaryotes and Prochlorococcus sp.) and nano-sized phytoplankton (Green algae, prymnesiophytes). The relationships were quantified with a least-square fitting approach in order to enable an estimation of the PFTs from Chl- a where PFTs are expressed as a percentage of the total Chl- a . The estimated uncertainty of the relationships depends on both PFT and Chl- a concentration. Maximum uncertainty of 31.8% was found for diatoms at Chl- a = 0.49 mg m −3 . However, the mean uncertainty of the relationships over all PFTs was 5.9% over the entire Chl- a range observed in situ (0.02 〈 Chl- a 〈 4.26 mg m −3 ). The relationships were applied to SeaWiFS satellite Chl- a data from 1998 to 2009 to show the global climatological fields of the surface distribution of PFTs. Results show that microplankton are present in the mid and high latitudes, constituting only ~10.9% of the entire phytoplankton community in the mean field for 1998–2009, in which diatoms explain ~7.5%. Nanoplankton are ubiquitous throughout the global surface oceans, except the subtropical gyres, constituting ~45.5%, of which prymnesiophytes (haptophytes) are the major group explaining ~31.7% while green algae contribute ~13.9%. Picoplankton are dominant in the subtropical gyres, but constitute ~43.6% globally, of which prokaryotes are the major group explaining ~26.5% ( Prochlorococcus sp. explaining 22.8%), while pico-eukaryotes explain ~17.2% and are relatively abundant in the South Pacific. These results may be of use to evaluate global marine ecosystem models.

Description: Relation between methanogenic archaea and methane production potential in selected natural wetland ecosystems across China Biogeosciences, 8, 329-338, 2011 Author(s): D. Y. Liu, W. X. Ding, Z. J. Jia, and Z. C. Cai Methane (CH 4 ) emissions from natural wetland ecosystems exhibit large spatial variability at regional, national, and global levels related to temperature, water table, plant type and methanogenic archaea etc. To understand the underlying factors that induce spatial differences in CH 4 emissions, and the relationship between the population of methanogenic archaea and CH 4 production potential in natural wetlands around China, we measured the CH 4 production potential and the abundance of methanogenic archaea in vertical soil profiles sampled from the Poyang wetland in the subtropical zone, the Hongze wetland in the warm temperate zone, the Sanjiang marsh in the cold temperate zone, and the Ruoergai peatland in the Qinghai-Tibetan Plateau in the alpine climate zone. The top soil layer had the highest population of methanogens (1.07–8.29 × 10 9 cells g −1 soil) in all wetlands except the Ruoergai peatland and exhibited the maximum CH 4 production potential measured at the mean in situ summer temperature. There is a significant logarithmic correlation between the abundance of methanogenic archaea and the soil organic carbon ( R 2 = 0.72, P 〈 0.001, n = 13) and between the abundance of methanogenic archaea and the total nitrogen concentrations ( R 2 = 0.76, P 〈 0.001, n = 13) in wetland soils. This indicates that the amount of soil organic carbon may affect the population of methanogens in wetland ecosystems. While the CH 4 production potential is not significantly related to methanogen population ( R 2 = 0.01, P 〉 0.05, n = 13), it is related to the dissolved organic carbon concentration ( R 2 = 0.31, P = 0.05, n = 13). This suggests that the methanogen population might be not an effective index for predicting the CH 4 production in wetland ecosystems. The CH 4 production rate of the top soil layer increases with increasing latitude, from 273.64 μg CH 4 kg −1 soil d −1 in the Poyang wetland to 664.59 μg CH 4 kg −1 soil d −1 in the Carex lasiocarpa marsh of the Sanjiang Plain. We conclude that CH 4 production potential in the freshwater wetlands of Eastern China is mainly affected by the supply of methanogenic substrates rather than temperature; in contrast, low summer temperatures at high elevations in the Ruoergai peatland of the Qinghai–Tibetan Plateau result in the presence of dominant species of methanogens with low CH 4 production potential, which in turn suppresses CH 4 production.

Description: Remote sensing of ecosystem light use efficiency with MODIS-based PRI Biogeosciences, 8, 189-202, 2011 Author(s): A. Goerner, M. Reichstein, E. Tomelleri, N. Hanan, S. Rambal, D. Papale, D. Dragoni, and C. Schmullius Several studies sustained the possibility that a photochemical reflectance index (PRI) directly obtained from satellite data can be used as a proxy for ecosystem light use efficiency (LUE) in diagnostic models of gross primary productivity. This modelling approach would avoid the complications that are involved in using meteorological data as constraints for a fixed maximum LUE. However, no unifying model predicting LUE across climate zones and time based on MODIS PRI has been published to date. In this study, we evaluate the effectiveness with which MODIS-based PRI can be used to estimate ecosystem light use efficiency at study sites of different plant functional types and vegetation densities. Our objective is to examine if known limitations such as dependence on viewing and illumination geometry can be overcome and a single PRI-based model of LUE (i.e. based on the same reference band) can be applied under a wide range of conditions. Furthermore, we were interested in the effect of using different faPAR (fraction of absorbed photosynthetically active radiation) products on the in-situ LUE used as ground truth and thus on the whole evaluation exercise. We found that estimating LUE at site-level based on PRI reduces uncertainty compared to the approaches relying on a maximum LUE reduced by minimum temperature and vapour pressure deficit. Despite the advantages of using PRI to estimate LUE at site-level, we could not establish an universally applicable light use efficiency model based on MODIS PRI. Models that were optimised for a pool of data from several sites did not perform well.

Description: Influence of the Amazon River on dissolved and intra-cellular metal concentrations in Trichodesmium colonies along the western boundary of the sub-tropical North Atlantic Ocean Biogeosciences, 8, 217-225, 2011 Author(s): A. Tovar-Sanchez and S. A. Sañudo-Wilhelmy Despite the ecological importance of Trichodesmium spp. for the global oceanic nitrogen budget, there is limited information on their trace metal composition in field samples. We report dissolved (

Description: Influence of seasonal monsoons on net community production and CO 2 in subtropical Hong Kong coastal waters Biogeosciences, 8, 289-300, 2011 Author(s): X.-C. Yuan, K. Yin, W.-J. Cai, A. Y. Ho, J. Xu, and P. J. Harrison Data from seven cruises in three different environments including the Pearl River estuary, sewage discharge outfall, and eastern coastal/shelf waters were used to examine the seasonal variations in net community production (NCP) and the biologically active gases O 2 and CO 2 . In the winter dry season, when monsoon-induced downwelling was dominant, NCP was negative (−84 ± 50 mmol C m −2 d −1 ) in all three regions. The negative NCP corresponded to O 2 influxes of 100 ± 50 mmol O 2 m −2 d −1 and CO 2 effluxes of 24 ± 10 mmol C m −2 d −1 . In the summer wet season, when upwelling brought the deep oceanic waters to the coast due to the southwest monsoonal winds, there was a 2 to 15-fold increase in integrated primary production (IPP) compared to winter. The increase in IPP was likely due to the favorable conditions such as stratification and the nutrient inputs from upwelled waters and the Pearl River estuary. NCP in the mixed layer reached up to 110 ± 48 mmol C m −2 d −1 in the wet season. However, accompanying the high positive NCP, we observed an O 2 influx of 100 ± 60 mmol O 2 m −2 d −1 and CO 2 efflux of 21 ± 15 mmol C m −2 d −1 . The contradictory observation of positive NCP and CO 2 release and O 2 uptake in the mixed layer could be explained by the influence of the southwest monsoon-induced upwelling along with the influence of the Pearl River, as the upwelling brought cold, low dissolved oxygen (DO, 160 ± 30 μM) and high dissolved inorganic carbon (DIC, 1960 ± 100 μatm) water to the surface in the wet season. Hence, the subtropical Hong Kong coastal waters are generally a CO 2 source due to the monsoonal influence during both the dry-heterotrophic and wet-autotrophic seasons.

Description: Detection and phylogenetic analysis of coastal bioaerosols using culture dependent and independent techniques Biogeosciences, 8, 301-309, 2011 Author(s): R. Urbano, B. Palenik, C. J. Gaston, and K. A. Prather Bioaerosols are emerging as important yet poorly understood players in atmospheric processes. Microorganisms can impact atmospheric chemistry through metabolic reactions and can potentially influence physical processes by participating in ice nucleation and cloud droplet formation. Microbial roles in atmospheric processes are thought to be species-specific and potentially dependent on cell viability. Using a coastal pier monitoring site as a sampling platform, culture-dependent (i.e. agar plates) and culture-independent (i.e. DNA clone libraries from filters) approaches were combined with 18S rRNA and 16S rRNA gene targeting to obtain insight into the local atmospheric microbial composition. From 13 microbial isolates and 42 DNA library clones, a total of 55 sequences were obtained representing four independent sampling events. Sequence analysis revealed that in these coastal samples two fungal phyla, Ascomycota and Basidiomycota, predominate among eukaryotes while Firmicutes and Proteobacteria predominate among bacteria. Furthermore, our culture-dependent study verifies the viability of microbes from all four phyla detected through our culture-independent study. Contrary to our expectations and despite oceanic air mass sources, common marine planktonic bacteria and phytoplankton were not typically found. The abundance of terrestrial and marine sediment-associated microorganisms suggests a potential importance for bioaerosols derived from beaches and/or coastal erosion processes.

Description: Sensitivity of pelagic calcification to ocean acidification Biogeosciences, 8, 433-458, 2011 Author(s): R. Gangstø, F. Joos, and M. Gehlen Ocean acidification might reduce the ability of calcifying plankton to produce and maintain their shells of calcite, or of aragonite, the more soluble form of CaCO 3 . In addition to possibly large biological impacts, reduced CaCO 3 production corresponds to a negative feedback on atmospheric CO 2 . In order to explore the sensitivity of the ocean carbon cycle to increasing concentrations of atmospheric CO 2 , we use the new biogeochemical Bern3D/PISCES model. The model reproduces the large scale distributions of biogeochemical tracers. With a range of sensitivity studies, we explore the effect of (i) using different parameterizations of CaCO 3 production fitted to available laboratory and field experiments, of (ii) letting calcite and aragonite be produced by auto- and heterotrophic plankton groups, and of (iii) using carbon emissions from the range of the most recent IPCC Representative Concentration Pathways (RCP). Under a high-emission scenario, the CaCO 3 production of all the model versions decreases from ~1 Pg C yr −1 to between 0.36 and 0.82 Pg C yr −1 by the year 2100. The changes in CaCO 3 production and dissolution resulting from ocean acidification provide only a small feedback on atmospheric CO 2 of −1 to −11 ppm by the year 2100, despite the wide range of parameterizations, model versions and scenarios included in our study. A potential upper limit of the CO 2 -calcification/dissolution feedback of −30 ppm by the year 2100 is computed by setting calcification to zero after 2000 in a high 21st century emission scenario. The similarity of feedback estimates yielded by the model version with calcite produced by nanophytoplankton and the one with calcite, respectively aragonite produced by mesozooplankton suggests that expending biogeochemical models to calcifying zooplankton might not be needed to simulate biogeochemical impacts on the marine carbonate cycle. The changes in saturation state confirm previous studies indicating that future anthropogenic CO 2 emissions may lead to irreversible changes in Ω A for several centuries. Furthermore, due to the long-term changes in the deep ocean, the ratio of open water CaCO 3 dissolution to production stabilizes by the year 2500 at a value that is 30–50% higher than at pre-industrial times when carbon emissions are set to zero after 2100.

Description: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels: paleoceanographic and paleoecological significance Biogeosciences, 8, 415-431, 2011 Author(s): D. Gallego-Torres, F. Martinez-Ruiz, P. A. Meyers, A. Paytan, F. J. Jimenez-Espejo, and M. Ortega-Huertas We have studied a suite of 35 sapropel sequences from a transect of four ODP sites across the Eastern Mediterranean to explore for paleoproductivity patterns and provide new insights on ecological changes during their deposition. Paleoproductivity variations were identified using TOC and Ba bio mass accumulation rates and δ 15 N total and δ 13 C org values. Elevated Ba/Al and TOC mass accumulation rates record periods of basin-wide amplified productivity. Our data further support that sapropels were formed by cyclic increases in primary production of marine organic matter largely sustained by N-fixing bacteria. This productivity increase was triggered by climate factors leading to increased fluvial discharge and amplified nutrient input that also favored the establishment of N-fixing bacteria. Enhanced productivity led to depletion of deepwater dissolved oxygen and consequently improved organic matter preservation. Primary production was more intense during the middle to Late Pleistocene compared to Pliocene equivalents, coinciding with increasing total sedimentation rates. δ 15 N values are dramatically lower in the sapropels than in TOC-poor background sediments, indicating a major contribution from nitrogen-fixing bacteria to the higher productivity during sapropel deposition. Additionally, different degrees of denitrification occurred as a consequence of water column oxygenation which in turns evolved from stagnant anoxic bottom waters during Pliocene sapropels to oxygen depleted and sluggish circulation in late Quaternary layers. These differences between sapropel layers provide new evidences for the general evolution of the Eastern Mediterranean basin during the last 3 Mys in terms of paleoceanographic conditions and the intensity of climate variability leading to sapropel deposition.

Description: Spatial and temporal variation of CO 2 efflux along a disturbance gradient in a miombo woodland in Western Zambia Biogeosciences, 8, 147-164, 2011 Author(s): L. Merbold, W. Ziegler, M. M. Mukelabai, and W. L. Kutsch Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol CO 2 m −2 s −1 ) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol CO 2 m −2 s −1 ) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO 2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO 2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and could not be explained by variables such as leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration ( R eco ) from eddy covariance measurements were compared to two bottom-up estimates of R eco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of R eco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.

Description: Twentieth century δ 13 C variability in surface water dissolved inorganic carbon recorded by coralline algae in the northern North Pacific Ocean and the Bering Sea Biogeosciences, 8, 165-174, 2011 Author(s): B. Williams, J. Halfar, R. S. Steneck, U. G. Wortmann, S. Hetzinger, W. Adey, P. Lebednik, and M. Joachimski The oxygen isotopic composition and Mg/Ca ratios in the skeletons of long-lived coralline algae record ambient seawater temperature over time. Similarly, the carbon isotopic composition in the skeletons record δ 13 C values of ambient seawater dissolved inorganic carbon. Here, we measured δ 13 C in the coralline alga Clathromorphum nereostratum to test the feasibility of reconstructing the intrusion of anthropogenic CO 2 into the northern North Pacific Ocean and Bering Sea. The δ 13 C was measured in the high Mg-calcite skeleton of three C. nereostratum specimens from two islands 500 km apart in the Aleutian archipelago. In the records spanning 1887 to 2003, the average decadal rate of decline in δ 13 C values increased from 0.03‰ yr −1 in the 1960s to 0.095‰ yr −1 in the 1990s, which was higher than expected due to solely the δ 13 C-Suess effect. Deeper water in this region exhibits higher concentrations of CO 2 and low δ 13 C values. Transport of deeper water into surface water (i.e., upwelling) increases when the Aleutian Low is intensified. We hypothesized that the acceleration of the δ 13 C decline may result from increased upwelling from the 1960s to 1990s, which in turn was driven by increased intensity of the Aleutian Low. Detrended δ 13 C records also varied on 4–7 year and bidecadal timescales supporting an atmospheric teleconnection of tropical climate patterns to the northern North Pacific Ocean and Bering Sea manifested as changes in upwelling.

Description: The Texas-Louisiana shelf in the Northern Gulf of Mexico receives large inputs of nutrients and freshwater from the Mississippi/Atchafalaya River system. The nutrients stimulate high rates of primary production in the river plume, which contributes to the development of a large and recurring hypoxic area in summer. The mechanistic links between hypoxia and river discharge of freshwater and nutrients are complex as the accumulation and vertical export of organic matter, the establishment and maintenance of vertical stratification, and the microbial degradation of organic matter are controlled by a non-linear interplay of factors. We present results from a realistic, 3-dimensional, physical-biological model that includes the processes thought to be of first order importance to hypoxia formation and demonstrate that the model realistically reproduces many features of observed nitrate and phytoplankton dynamics including observed property distributions and rates. We then contrast the environmental factors and phytoplankton source and sink terms characteristic of three model subregions that represent an ecological gradient from eutrophic to oligotrophic conditions. We analyze specifically the reasons behind the counterintuitive observation that primary production in the light-limited plume region near the Mississippi River delta is positively correlated with river nutrient input. We find that, while primary production and phytoplankton biomass are positively correlated with nutrient load, phytoplankton growth rate is not. This suggests that accumulation of biomass in this region is not primarily controlled bottom up by nutrient-stimulation, but top down by systematic differences in the loss processes. We hypothesize that increased retention of river water in high discharge years explains this phenomenon.

Description: Importance of crop varieties and management practices: evaluation of a process-based model for simulating CO 2 and H 2 O fluxes at five European maize ( Zea mays L.) sites Biogeosciences, 8, 1721-1736, 2011 Author(s): L. Li, N. Vuichard, N. Viovy, P. Ciais, T. Wang, E. Ceschia, W. Jans, M. Wattenbach, P. Béziat, T. Gruenwald, S. Lehuger, and C. Bernhofer This paper is a modelling study of crop management impacts on carbon and water fluxes at a range of European sites. The model is a crop growth model (STICS) coupled with a process-based land surface model (ORCHIDEE). The data are online eddy-covariance observations of CO 2 and H 2 O fluxes at five European maize cultivation sites. The results show that the ORCHIDEE-STICS model explains up to 75 % of the observed daily net CO 2 ecosystem exchange (NEE) variance, and up to 79 % of the latent heat flux (LE) variance at five sites. The model is better able to reproduce gross primary production (GPP) variations than terrestrial ecosystem respiration (TER) variations. We conclude that structural deficiencies in the model parameterizations of leaf area index (LAI) and TER are the main sources of error in simulating CO 2 and H 2 O fluxes. A number of sensitivity tests, with variable crop variety, nitrogen fertilization, irrigation, and planting date, indicate that any of these management factors is able to change NEE by more than 15 %, but that the response of NEE to management parameters is highly site-dependent. Changes in management parameters are found to impact not only the daily values of NEE and LE, but also the cumulative yearly values. In addition, LE is shown to be less sensitive to management parameters than NEE. Multi-site model evaluations, coupled with sensitivity analysis to management parameters, thus provide important information about model errors, which helps to improve the simulation of CO 2 and H 2 O fluxes across European croplands.

Description: Widespread release of old carbon across the Siberian Arctic echoed by its large rivers Biogeosciences, 8, 1737-1743, 2011 Author(s): Ö. Gustafsson, B. E. van Dongen, J. E. Vonk, O. V. Dudarev, and I. P. Semiletov Over decadal-centennial timescales, only a few mechanisms in the carbon-climate system could cause a massive net redistribution of carbon from land and ocean systems to the atmosphere in response to climate warming. The largest such climate-vulnerable carbon pool is the old organic carbon (OC) stored in Arctic permafrost (perennially frozen) soils. Climate warming, both predicted and now observed to be the strongest globally in the Eurasian Arctic and Alaska, causes thaw-release of old permafrost carbon from local tundra sites. However, a central challenge for the assessment of the general vulnerability of this old OC pool is to deduce any signal integrating its release over larger scales. Here we examine radiocarbon measurements of molecular soil markers exported by the five Great Russian-Arctic Rivers (Ob, Yenisey, Lena, Indigirka and Kolyma), employed as natural integrators of carbon release processes in their watersheds. The signals held in estuarine surface sediments revealed that average radiocarbon ages of n -alkanes increased east-to-west from 6400 yr BP in Kolyma to 11 400 yr BP in Ob. This is consistent with westwards trends of both warmer climate and more degraded organic matter as indicated by the ratio of high molecular weight (HMW) n -alkanoic acids to HMW n -alkanes. The dynamics of Siberian permafrost can thus be probed via the molecular-radiocarbon signal as carried by Arctic rivers. Old permafrost carbon is at present vulnerable to mobilization over continental scales. Climate-induced changes in the radiocarbon fingerprint of released permafrost carbon will likely depend on changes in both permafrost coverage and Arctic soil hydraulics.

Description: East Siberian Sea, an Arctic region of very high biogeochemical activity Biogeosciences, 8, 1745-1754, 2011 Author(s): L. G. Anderson, G. Björk, S. Jutterström, I. Pipko, N. Shakhova, I. Semiletov, and I. Wåhlström Shelf seas are among the most active biogeochemical marine environments and the East Siberian Sea is a prime example. This sea is supplied by seawater from both the Atlantic and Pacific Oceans and has a substantial input of river runoff. All of these waters contribute chemical constituents, dissolved and particulate, but of different signatures. Sea ice formation during the winter season and melting in the summer has a major impact on physical as well as biogeochemical conditions. The internal circulation and water mass distribution is significantly influenced by the atmospheric pressure field. The western region is dominated by input of river runoff from the Laptev Sea and an extensive input of terrestrial organic matter. The microbial decay of this organic matter produces carbon dioxide (CO 2 ) that oversaturates all waters from the surface to bottom relative to atmospheric level, even when primary production, inferred from low surface water nutrients, has occurred. The eastern surface waters were under-saturated with respect to CO 2 illustrating the dominance of marine primary production. The drawdown of dissolved inorganic carbon equals a primary production of ~0.8 ± 2 mol C m −2 , which when multiplied by half the area of the East Siberian Sea, ~500 000 km 2 , results in an annual primary production of 0.4 (± 1) × 10 12 mol C or ~4 (± 10) × 10 12 gC. Microbial decay occurs through much of the water column, but dominates at the sediment interface where the majority of organic matter ends up, thus more of the decay products are recycled to the bottom water. High nutrient concentrations and fugacity of CO 2 and low oxygen and pH were observed in the bottom waters. Another signature of organic matter decomposition, methane (CH 4 ), was observed in very high but variable concentrations. This is due to its seabed sources of glacial origin or modern production from ancient organic matter, becoming available due to sub-sea permafrost thaw and formation of so-called taliks. The decay of organic matter to CO 2 as well as oxidation of CH 4 to CO 2 contribute to a natural ocean acidification making the saturation state of calcium carbonate low, resulting in under-saturation of all the bottom waters with respect to aragonite and large areas of under-saturation down to 50 % with respect to calcite. Hence, conditions for calcifying organisms are very unfavorable.

Description: Drought-associated changes in climate and their relevance for ecosystem experiments and models Biogeosciences, 8, 1121-1130, 2011 Author(s): H. J. De Boeck and H. Verbeeck Drought periods can have important impacts on plant productivity and ecosystem functioning, but climatic conditions other than the lack of precipitation during droughts have never been quantified and have therefore not been considered explicitly in both experimental and modeling studies. Here, we identify which climatic characteristics deviate from normal during droughts and how these deviations could affect plant responses. Analysis of 609 years of daily data from nine Western European meteorological stations reveals that droughts in the studied region are consistently associated with more sunshine (+45 %), increased mean (+1.6 °C) and maximum (+2.8 °C) air temperatures and vapour pressure deficits that were 51 % higher than under normal conditions. These deviations from normal increase significantly as droughts progress. Using the process-model ORCHIDEE, we simulated droughts consistent with the results of the dataset analysis and compared water and carbon exchange of three different vegetation types during such natural droughts and droughts in which only the precipitation was affected. The comparison revealed contrasting responses: carbon loss was higher under natural drought in grasslands, while increased carbon uptake was found especially in decidious forests. This difference was attributed to better access to water reserves in forest ecosystems which prevented drought stress. This demonstrates that the warmer and sunnier conditions naturally associated with droughts can either improve growth or aggravate drought-related stress, depending on water reserves. As the impacts of including or excluding climatic parameters that correlate with drought are substantial, we propose that both experimental and modeling efforts should take into account other environmental factors than merely precipitation.

Description: Exploring the sensitivity of soil carbon dynamics to climate change, fire disturbance and permafrost thaw in a black spruce ecosystem Biogeosciences, 8, 1367-1382, 2011 Author(s): J. A. O'Donnell, J. W. Harden, A. D. McGuire, and V. E. Romanovsky In the boreal region, soil organic carbon (OC) dynamics are strongly governed by the interaction between wildfire and permafrost. Using a combination of field measurements, numerical modeling of soil thermal dynamics, and mass-balance modeling of OC dynamics, we tested the sensitivity of soil OC storage to a suite of individual climate factors (air temperature, soil moisture, and snow depth) and fire severity. We also conducted sensitivity analyses to explore the combined effects of fire-soil moisture interactions and snow seasonality on OC storage. OC losses were calculated as the difference in OC stocks after three fire cycles (~500 yr) following a prescribed step-change in climate and/or fire. Across single-factor scenarios, our findings indicate that warmer air temperatures resulted in the largest relative soil OC losses (~5.3 kg C m −2 ), whereas dry soil conditions alone (in the absence of wildfire) resulted in the smallest carbon losses (~0.1 kg C m −2 ). Increased fire severity resulted in carbon loss of ~3.3 kg C m −2 , whereas changes in snow depth resulted in smaller OC losses (2.1–2.2 kg C m −2 ). Across multiple climate factors, we observed larger OC losses than for single-factor scenarios. For instance, high fire severity regime associated with warmer and drier conditions resulted in OC losses of ~6.1 kg C m −2 , whereas a low fire severity regime associated with warmer and wetter conditions resulted in OC losses of ~5.6 kg C m −2 . A longer snow-free season associated with future warming resulted in OC losses of ~5.4 kg C m −2 . Soil climate was the dominant control on soil OC loss, governing the sensitivity of microbial decomposers to fluctuations in temperature and soil moisture; this control, in turn, is governed by interannual changes in active layer depth. Transitional responses of the active layer depth to fire regimes also contributed to OC losses, primarily by determining the proportion of OC into frozen and unfrozen soil layers.

Description: The relative importance of seed competition, resource competition and perturbations on community structure Biogeosciences, 8, 1107-1120, 2011 Author(s): K. Bohn, J. G. Dyke, R. Pavlick, B. Reineking, B. Reu, and A. Kleidon While the regional climate is the primary selection pressure for whether a plant strategy can survive, however, competitive interactions strongly affect the relative abundances of plant strategies within communities. Here, we investigate the relative importance of competition and perturbations on the development of vegetation community structure. To do so, we develop DIVE (Dynamics and Interactions of VEgetation), a simple general model that links plant strategies to their competitive dynamics, using growth and reproduction characteristics that emerge from climatic constraints. The model calculates population dynamics based on establishment, mortality, invasion and exclusion in the presence of different strengths of perturbations, seed and resource competition. The highest levels of diversity were found in simulations without competition as long as mortality is not too high. However, reasonable successional dynamics were only achieved when resource competition is considered. Under high levels of competition, intermediate levels of perturbations were required to obtain coexistence. Since succession and coexistence are observed in plant communities, we conclude that the DIVE model with competition and intermediate levels of perturbation represents an adequate way to model population dynamics. Because of the simplicity and generality of DIVE, it could be used to understand vegetation structure and functioning at the global scale and the response of vegetation to global change.

Description: Relating ring width of Mediterranean evergreen species to seasonal and annual variations of precipitation and temperature Biogeosciences, 8, 1141-1152, 2011 Author(s): W. Nijland, E. Jansma, E. A. Addink, M. Domínguez Delmás, and S. M. De Jong Plant growth in Mediterranean landscapes is limited by the typical summer-dry climate. Forests in these areas are only marginally productive and may be quite susceptible to modern climate change. To improve our understanding of forest sensitivity to annual and seasonal climatic variability, we use tree-ring measurements of two Mediterranean evergreen tree species: Quercus ilex L. and Arbutus unedo L. We sampled 34 stems of these species on three different types of substrates in the Peyne study area in southern France. The resulting chronologies were analysed in combination with 38 yr of monthly precipitation and temperature data to reconstruct the response of stem growth to climatic variability. Results indicate a strong positive response to May and June precipitation, as well as a significant positive influence of early-spring temperatures and a negative growth response to summer heat. Comparison of the data with more detailed productivity measurements in two contrasting years confirms these observations and shows a strong productivity limiting effect of low early-summer precipitation. The results show that tree-ring data from Q.ilex and A.unedo can provide valuable information about the response of these tree species to climate variability, improving our ability to predict the effects of climate change in Mediterranean ecosystems.

Description: Seasonal variations of belowground carbon transfer assessed by in situ 13 CO 2 pulse labelling of trees Biogeosciences, 8, 1153-1168, 2011 Author(s): D. Epron, J. Ngao, M. Dannoura, M. R. Bakker, B. Zeller, S. Bazot, A. Bosc, C. Plain, J. C. Lata, P. Priault, L. Barthes, and D. Loustau Soil CO 2 efflux is the main source of CO 2 from forest ecosystems and it is tightly coupled to the transfer of recent photosynthetic assimilates belowground and their metabolism in roots, mycorrhiza and rhizosphere microorganisms feeding on root-derived exudates. The objective of our study was to assess patterns of belowground carbon allocation among tree species and along seasons. Pure 13 CO 2 pulse labelling of the entire crown of three different tree species (beech, oak and pine) was carried out at distinct phenological stages. Excess 13 C in soil CO 2 efflux was tracked using tuneable diode laser absorption spectrometry to determine time lags between the start of the labelling and the appearance of 13 C in soil CO 2 efflux and the amount of 13 C allocated to soil CO 2 efflux. Isotope composition (δ 13 C) of CO 2 respired by fine roots and soil microbes was measured at several occasions after labelling, together with δ 13 C of bulk root tissue and microbial carbon. Time lags ranged from 0.5 to 1.3 days in beech and oak and were longer in pine (1.6–2.7 days during the active growing season, more than 4 days during the resting season), and the transfer of C to the microbial biomass was as fast as to the fine roots. The amount of 13 C allocated to soil CO 2 efflux was estimated from a compartment model. It varied between 1 and 21 % of the amount of 13 CO 2 taken up by the crown, depending on the species and the season. While rainfall exclusion that moderately decreased soil water content did not affect the pattern of carbon allocation to soil CO 2 efflux in beech, seasonal patterns of carbon allocation belowground differed markedly between species, with pronounced seasonal variations in pine and beech. In beech, it may reflect competition with the strength of other sinks (aboveground growth in late spring and storage in late summer) that were not observed in oak. We report a fast transfer of recent photosynthates to the mycorhizosphere and we conclude that the patterns of carbon allocation belowground are species specific and change seasonally according to the phenology of the species.

Description: What are the main climate drivers for shrub growth in Northeastern Siberian tundra? Biogeosciences, 8, 1169-1179, 2011 Author(s): D. Blok, U. Sass-Klaassen, G. Schaepman-Strub, M. M. P. D. Heijmans, P. Sauren, and F. Berendse Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r = 0.73, p 〈 0.001) and B. nana (Pearson's r = 0.46, p 〈 0.001). No effect of winter precipitation on shrub growth was observed. In contrast, summer precipitation of the previous year correlated positively with B. nana ring width (Pearson's r = 0.42, p 〈 0.01), suggesting that wet summers facilitate shrub growth in the following growing season. S. pulchra ring width correlated positively with peak summer NDVI, despite the small coverage of S. pulchra shrubs ( 〈 5 % surface cover) in our research area. We provide the first climate-growth study on shrubs for Northeast Siberia, the largest tundra region in the world. We show that two deciduous shrub species with markedly different growth forms have a similar growth response to changes in climate. The obtained shrub growth response to climate variability in the past increases our understanding of the mechanisms underlying current shrub expansion, which is required to predict future climate-driven tundra vegetation shifts.

Description: The role of endophytic methane-oxidizing bacteria in submerged Sphagnum in determining methane emissions of Northeastern Siberian tundra Biogeosciences, 8, 1267-1278, 2011 Author(s): F. J. W. Parmentier, J. van Huissteden, N. Kip, H. J. M. Op den Camp, M. S. M. Jetten, T. C. Maximov, and A. J. Dolman The role of the microbial processes governing methane emissions from tundra ecosystems is receiving increasing attention. Recently, cooperation between methanotrophic bacteria and submerged Sphagnum was shown to reduce methane emissions but also to supply CO 2 for photosynthesis for the plant. Although this process was shown to be important in the laboratory, the differences that exist in methane emissions from inundated vegetation types with or without Sphagnum in the field have not been linked to these bacteria before. In this study, chamber flux measurements, an incubation study and a process model were used to investigate the drivers and controls on the relative difference in methane emissions between a submerged Sphagnum /sedge vegetation type and an inundated sedge vegetation type without Sphagnum . It was found that methane emissions in the Sphagnum -dominated vegetation type were 50 % lower than in the vegetation type without Sphagnum . A model sensitivity analysis showed that these differences could not sufficiently be explained by differences in methane production and plant transport. The model, combined with an incubation study, indicated that methane oxidation by endophytic bacteria, living in cooperation with submerged Sphagnum , plays a significant role in methane cycling at this site. This result is important for spatial upscaling as oxidation by these bacteria is likely involved in 15 % of the net methane emissions at this tundra site. Our findings support the notion that methane-oxidizing bacteria are an important factor in understanding the processes behind methane emissions in tundra.

Description: Optimizing models of the North Atlantic spring bloom using physical, chemical and bio-optical observations from a Lagrangian float Biogeosciences, 8, 1291-1307, 2011 Author(s): W. Bagniewski, K. Fennel, M. J. Perry, and E. A. D'Asaro The North Atlantic spring bloom is one of the main events that lead to carbon export to the deep ocean and drive oceanic uptake of CO 2 from the atmosphere. Here we use a suite of physical, bio-optical and chemical measurements made during the 2008 spring bloom to optimize and compare three different models of biological carbon export. The observations are from a Lagrangian float that operated south of Iceland from early April to late June, and were calibrated with ship-based measurements. The simplest model is representative of typical NPZD models used for the North Atlantic, while the most complex model explicitly includes diatoms and the formation of fast sinking diatom aggregates and cysts under silicate limitation. We carried out a variational optimization and error analysis for the biological parameters of all three models, and compared their ability to replicate the observations. The observations were sufficient to constrain most phytoplankton-related model parameters to accuracies of better than 15 %. However, the lack of zooplankton observations leads to large uncertainties in model parameters for grazing. The simulated vertical carbon flux at 100 m depth is similar between models and agrees well with available observations, but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatom aggregation. While none of the models can be formally rejected based on their misfit with the available observations, the model that includes export by diatom aggregation has a statistically significant better fit to the observations and more accurately represents the mechanisms and timing of carbon export based on observations not included in the optimization. Thus models that accurately simulate the upper 100 m do not necessarily accurately simulate export to deeper depths.

Description: Modeling biogeochemical processes in sediments from the Rhône River prodelta area (NW Mediterranean Sea) Biogeosciences, 8, 1351-1366, 2011 Author(s): L. Pastor, C. Cathalot, B. Deflandre, E. Viollier, K. Soetaert, F. J. R. Meysman, C. Ulses, E. Metzger, and C. Rabouille In situ oxygen microprofiles, sediment organic carbon content, and pore-water concentrations of nitrate, ammonium, iron, manganese, and sulfides obtained in sediments from the Rhône River prodelta and its adjacent continental shelf were used to constrain a numerical diagenetic model. Results showed that (1) the organic matter from the Rhône River is composed of a fraction of fresh material associated to high first-order degradation rate constants (11–33 yr −1 ); (2) the burial efficiency (burial/input ratio) in the Rhône prodelta (within 3 km of the river outlet) can be up to 80 %, and decreases to ~20 % on the adjacent continental shelf 10–15 km further offshore; (3) there is a large contribution of anoxic processes to total mineralization in sediments near the river mouth, certainly due to large inputs of fresh organic material combined with high sedimentation rates; (4) diagenetic by-products originally produced during anoxic organic matter mineralization are almost entirely precipitated (〉97 %) and buried in the sediment, which leads to (5) a low contribution of the re-oxidation of reduced products to total oxygen consumption. Consequently, total carbon mineralization rates as based on oxygen consumption rates and using Redfield stoichiometry can be largely underestimated in such River-dominated Ocean Margins (RiOMar) environments.

Description: Rapid transfer of photosynthetic carbon through the plant-soil system in differently managed species-rich grasslands Biogeosciences, 8, 1131-1139, 2011 Author(s): G. B. De Deyn, H. Quirk, S. Oakley, N. Ostle, and R. D. Bardgett Plant-soil interactions are central to short-term carbon (C) cycling through the rapid transfer of recently assimilated C from plant roots to soil biota. In grassland ecosystems, changes in C cycling are likely to be influenced by land use and management that changes vegetation and the associated soil microbial communities. Here we tested whether changes in grassland vegetation composition resulting from management for plant diversity influences short-term rates of C assimilation and transfer from plants to soil microbes. To do this, we used an in situ 13 C-CO 2 pulse-labelling approach to measure differential C uptake among different plant species and the transfer of the plant-derived 13 C to key groups of soil microbiota across selected treatments of a long-term plant diversity grassland restoration experiment. Results showed that plant taxa differed markedly in the rate of 13 C assimilation and concentration: uptake was greatest and 13 C concentration declined fastest in Ranunculus repens , and assimilation was least and 13 C signature remained longest in mosses. Incorporation of recent plant-derived 13 C was maximal in all microbial phosopholipid fatty acid (PLFA) markers at 24 h after labelling. The greatest incorporation of 13 C was in the PLFA 16:1ω5, a marker for arbuscular mycorrhizal fungi (AMF), while after 1 week most 13 C was retained in the PLFA18:2ω6,9 which is indicative of assimilation of plant-derived 13 C by saprophytic fungi. Our results of 13 C assimilation and transfer within plant species and soil microbes were consistent across management treatments. Overall, our findings suggest that plant diversity restoration management may not directly affect the C assimilation or retention of C by individual plant taxa or groups of soil microbes, it can impact on the fate of recent C by changing their relative abundances in the plant-soil system. Moreover, across all treatments we found that plant-derived C is rapidly transferred specifically to AMF and decomposer fungi, indicating their consistent key role in the cycling of recent plant derived C.

Description: Global fungal spore emissions, review and synthesis of literature data Biogeosciences, 8, 1181-1192, 2011 Author(s): A. Sesartic and T. N. Dallafior The present paper summarizes fungal spore emission fluxes in different biomes. A literature study has been conducted and emission fluxes have been calculated based on 35 fungal spore concentration datasets. Biome area data has been derived from the World Resource Institute. Several assumptions and simplifications needed to be adopted while aggregating the data: results from different measurement methods have been treated equally, while diurnal and seasonal cycles have been neglected. Moreover flux data were aggregated to very coarse biome areas due to scarcity of data. Results show number fluxes per square meter and second of 194 for tropical and subtropical forests, 203 for all other forests, 1203 for shrub, 2509 for crop, 8 for tundra, and 165 for grassland. No data were found for land ice. The annual mean global fluxes amount to 1.69 × 10 –11 kg m −2 s −1 as the best estimates, and 9.01 × 10 –12 kg m −2 s −1 and 3.28 × 10 –11 kg m −2 s −1 as the low and high estimate, respectively.

Description: Large variability in continental shelf production of phytoplankton carbon revealed by satellite Biogeosciences, 8, 1213-1223, 2011 Author(s): B. F. Jönsson, J. E. Salisbury, and A. Mahadevan We estimate the net production of phytoplankton in the Gulf of Maine (GoM) over a 3-yr period using satellite ocean color data in conjunction with surface velocities from a high-resolution operational ocean circulation model. Chlorophyll (Chl- a ) and light attenuation ( K 490 ) products are combined with a carbon to chlorophyll model to estimate the phytoplankton carbon (PC) stock in the euphotic layer. A satellite-based productivity, termed NCP e in analogy with net community production (NCP), is derived by tracking changes in satellite-derived PC from one satellite image to the next, along water parcel trajectories calculated with surface velocities from the ocean circulation model. Such an along-trajectory analysis of satellite data discounts the effect of advection that would otherwise contribute to the temporal change between consecutive images viewed in the fixed reference frame. Our results show a high variability of up to ±500 mg C m −2 d −1 in NCP e on spatial scales of 10–100 km. A region-wide median NCP e of 40–50 mg C m −2 d −1 is often prevalent in the Gulf, while blooms attain peak values of 400 mg C m −2 d −1 for a few days. The spatio-temporal variability of NCP e in this region, though conditioned by seasonality, is dominated by events lasting a few days, which if integrated, lead to large inter-annual variability in the annual carbon budget. This study is a step toward achieving synoptic and time-dependent estimates of oceanic productivity and NCP from satellite data.

Description: Air-sea CO 2 fluxes on the Bering Sea shelf Biogeosciences, 8, 1237-1253, 2011 Author(s): N. R. Bates, J. T. Mathis, and M. A. Jeffries There have been few previous studies of surface seawater CO 2 partial pressure ( p CO 2 ) variability and air-sea CO 2 gas exchange rates for the Bering Sea shelf. In 2008, spring and summertime observations were collected in the Bering Sea shelf as part of the Bering Sea Ecological Study (BEST). Our results indicate that the Bering Sea shelf was close to neutral in terms of CO 2 sink-source status in springtime due to relatively small air-sea CO 2 gradients (i.e., Δ p CO 2 and sea-ice cover. However, by summertime, very low seawater p CO 2 values were observed and much of the Bering Sea shelf became strongly undersaturated with respect to atmospheric CO 2 concentrations. Thus the Bering Sea shelf transitions seasonally from mostly neutral conditions to a strong oceanic sink for atmospheric CO 2 particularly in the " green belt " region of the Bering Sea where there are high rates of phytoplankton primary production (PP)and net community production (NCP). Ocean biological processes dominate the seasonal drawdown of seawater p CO 2 for large areas of the Bering Sea shelf, with the effect partly countered by seasonal warming. In small areas of the Bering Sea shelf south of the Pribilof Islands and in the SE Bering Sea, seasonal warming is the dominant influence on seawater p CO 2 , shifting localized areas of the shelf from minor/neutral CO 2 sink status to neutral/minor CO 2 source status, in contrast to much of the Bering Sea shelf. Overall, we compute that the Bering Sea shelf CO 2 sink in 2008 was 157 ± 35 Tg C yr −1 (Tg = 10 12 g C) and thus a strong sink for CO 2 .

Description: The role of plant functional trade-offs for biodiversity changes and biome shifts under scenarios of global climatic change Biogeosciences, 8, 1255-1266, 2011 Author(s): B. Reu, S. Zaehle, R. Proulx, K. Bohn, A. Kleidon, R. Pavlick, and S. Schmidtlein The global geographic distribution of biodiversity and biomes is determined by species-specific physiological tolerances to climatic constraints. Current vegetation models employ empirical bioclimatic relationships to predict present-day vegetation patterns and to forecast biodiversity changes and biome shifts under climatic change. In this paper, we consider trade-offs in plant functioning and their responses under climatic changes to forecast and explain changes in plant functional richness and shifts in biome geographic distributions. The Jena Diversity model (JeDi) simulates plant survival according to essential plant functional trade-offs, including ecophysiological processes such as water uptake, photosynthesis, allocation, reproduction and phenology. We use JeDi to quantify changes in plant functional richness and biome shifts between present-day and a range of possible future climates from two SRES emission scenarios (A2 and B1) and seven global climate models using metrics of plant functional richness and functional identity. Our results show (i) a significant loss of plant functional richness in the tropics, (ii) an increase in plant functional richness at mid and high latitudes, and (iii) a pole-ward shift of biomes. While these results are consistent with the findings of empirical approaches, we are able to explain them in terms of the plant functional trade-offs involved in the allocation, metabolic and reproduction strategies of plants. We conclude that general aspects of plant physiological tolerances can be derived from functional trade-offs, which may provide a useful process- and trait-based alternative to bioclimatic relationships. Such a mechanistic approach may be particularly relevant when addressing vegetation responses to climatic changes that encounter novel combinations of climate parameters that do not exist under contemporary climate.

Description: MODIS observed phytoplankton dynamics in the Taiwan Strait: an absorption-based analysis Biogeosciences, 8, 841-850, 2011 Author(s): S. Shang, Q. Dong, Z. Lee, Y. Li, Y. Xie, and M. Behrenfeld This study used MODIS observed phytoplankton absorption coefficient at 443 nm (Aph) as a preferable index to characterize phytoplankton variability in optically complex waters. Aph derived from remote sensing reflectance ( R rs , both in situ and MODIS measured) with the Quasi-Analytical Algorithm (QAA) were evaluated by comparing them with match-up in situ measurements, collected in both oceanic and nearshore waters in the Taiwan Strait (TWS). For the data with matching spatial and temporal window, it was found that the average percentage error (ε) between MODIS derived Aph and field measured Aph was 33.8% ( N =30, Aph ranges from 0.012 to 0.537 m −1 ), with a root mean square error in log space (RMSE_log) of 0.226. By comparison, ε was 28.0% ( N =88, RMSE_log = 0.150) between Aph derived from ship-borne R rs and Aph measured from water samples. However, values of ε as large as 135.6% ( N =30, RMSE_log = 0.383) were found between MODIS derived chlorophyll- a (Chl, OC3M algorithm) and field measured Chl. Based on these evaluation results, we applied QAA to MODIS R rs data in the period of 2003–2009 to derive climatological monthly mean Aph for the TWS. Three distinct features of phytoplankton dynamics were identified. First, Aph is low and the least variable in the Penghu Channel, where the South China Sea water enters the TWS. This region maintains slightly higher values in winter (~17% higher than that in the other seasons) due to surface nutrient entrainment under winter wind-driven vertical mixing. Second, Aph is high and varies the most in the mainland nearshore water, with values peaking in summer (June–August) when river plumes and coastal upwelling enhance surface nutrient loads. Interannual variation of bloom intensity in Hanjiang River estuary in June is highly correlated with alongshore wind stress anomalies, as observed by QuikSCAT. The year of minimum and maximum bloom intensity is in the midst of an El Niño and a La Niña event, respectively. Third, a high Aph patch appears between April and September in the middle of the southern TWS, corresponding to high thermal frontal probabilities, as observed by MODIS. Our results support the use of satellite derived Aph for time series analyses of phytoplankton dynamics in coastal ocean regions, whereas satellite Chl products derived empirically using spectral ratio of R rs suffer from artifacts associated with non-biotic optically active materials.

Description: Thermal adaptation of net ecosystem exchange Biogeosciences, 8, 1453-1463, 2011 Author(s): W. Yuan, Y. Luo, S. Liang, G. Yu, S. Niu, P. Stoy, J. Chen, A. R. Desai, A. Lindroth, C. M. Gough, R. Ceulemans, A. Arain, C. Bernhofer, B. Cook, D. R. Cook, D. Dragoni, B. Gielen, I. A. Janssens, B. Longdoz, H. Liu, M. Lund, G. Matteucci, E. Moors, R. L. Scott, G. Seufert, and R. Varner Thermal adaptation of gross primary production and ecosystem respiration has been well documented over broad thermal gradients. However, no study has examined their interaction as a function of temperature, i.e. the thermal responses of net ecosystem exchange of carbon (NEE). In this study, we constructed temperature response curves of NEE against temperature using 380 site-years of eddy covariance data at 72 forest, grassland and shrubland ecosystems located at latitudes ranging from ~29° N to 64° N. The response curves were used to define two critical temperatures: transition temperature ( T b ) at which ecosystem transfer from carbon source to sink and optimal temperature ( T o ) at which carbon uptake is maximized. T b was strongly correlated with annual mean air temperature. T o was strongly correlated with mean temperature during the net carbon uptake period across the study ecosystems. Our results imply that the net ecosystem exchange of carbon adapts to the temperature across the geographical range due to intrinsic connections between vegetation primary production and ecosystem respiration.

Description: Photosynthetic responses of Emiliania huxleyi to UV radiation and elevated temperature: roles of calcified coccoliths Biogeosciences, 8, 1441-1452, 2011 Author(s): K. Xu, K. Gao, V. E. Villafañe, and E. W. Helbling Changes in calcification of coccolithophores may affect their photosynthetic responses to both, ultraviolet radiation (UVR, 280–400 nm) and temperature. We operated semi-continuous cultures of Emiliania huxleyi (strain CS-369) at reduced (0.1 mM, LCa) and ambient (10 mM, HCa) Ca 2+ concentrations and, after 148 generations, we exposed cells to six radiation treatments (〉280, 〉295, 〉305, 〉320, 〉350 and 〉395 nm by using Schott filters) and two temperatures (20 and 25 °C) to examine photosynthesis and calcification responses. Overall, our study demonstrated that: (1) decreased calcification resulted in a down regulation of photoprotective mechanisms (i.e., as estimated via non-photochemical quenching, NPQ), pigments contents and photosynthetic carbon fixation; (2) calcification ( C ) and photosynthesis ( P ) (as well as their ratio) have different responses related to UVR with cells grown under the high Ca 2+ concentration being more resistant to UVR than those grown under the low Ca 2+ level; (3) elevated temperature increased photosynthesis and calcification of E. huxleyi grown at high Ca 2+ concentrations whereas decreased both processes in low Ca 2+ grown cells. Therefore, a decrease in calcification rates in E. huxleyi is expected to decrease photosynthesis rates, resulting in a negative feedback that further reduces calcification.

Description: Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate Biogeosciences, 8, 17-25, 2011 Author(s): C. E. Morris, D. C. Sands, M. Bardin, R. Jaenicke, B. Vogel, C. Leyronas, P. A. Ariya, and R. Psenner For the past 200 years, the field of aerobiology has explored the abundance, diversity, survival and transport of micro-organisms in the atmosphere. Micro-organisms have been explored as passive and severely stressed riders of atmospheric transport systems. Recently, an interest in the active roles of these micro-organisms has emerged along with proposals that the atmosphere is a global biome for microbial metabolic activity and perhaps even multiplication. As part of a series of papers on the sources, distribution and roles in atmospheric processes of biological particles in the atmosphere, here we describe the pertinence of questions relating to the potential roles that air-borne micro-organisms might play in meteorological phenomena. For the upcoming era of research on the role of air-borne micro-organisms in meteorological phenomena, one important challenge is to go beyond descriptions of abundance of micro-organisms in the atmosphere toward an understanding of their dynamics in terms of both biological and physico-chemical properties and of the relevant transport processes at different scales. Another challenge is to develop this understanding under contexts pertinent to their potential role in processes related to atmospheric chemistry, the formation of clouds, precipitation and radiative forcing. This will require truly interdisciplinary approaches involving collaborators from the biological and physical sciences, from disciplines as disparate as agronomy, microbial genetics and atmosphere physics, for example.

Description: ENSO and IOD teleconnections for African ecosystems: evidence of destructive interference between climate oscillations Biogeosciences, 8, 27-40, 2011 Author(s): C. A. Williams and N. P. Hanan Rainfall and vegetation across Africa are known to resonate with the coupled ocean-atmosphere phenomena of El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). However, the regional-scale implications of sea surface temperature variability for Africa's photosyntheis have received little focused attention, particularly in the case of IOD. Furthermore, studies exploring the interactive effects of ENSO and IOD when coincident are lacking. This analysis uses remotely sensed vegetation change plus a land surface model driven with observed meteorology to investigate how rainfall, vegetation, and photosynthesis across Africa respond to these climate oscillations. In addition to the relatively well-known ENSO forcing, the IOD induces large departures of photosynthesis across much of Africa associated with anomalies in rainfall and vegetation greenness. More importantly, sizeable independent effects can be suppressed or even reversed by destructive interferences during periods of simultaneous ENSO and IOD activity. For example, effects of positive IOD on southeastern Africa tended to dominate those of El Niño during their coincidence spanning 1997–1998, with sign reversal of El Niño's typically strong suppression of photosynthesis in this region. These findings call into question past analyses examining teleconnections to ENSO or IOD in isolation, and indicate the need to consider their simultaneous states when examining influences on hydroclimatic and ecological conditions across Africa.

Description: Sensitivity of Holocene atmospheric CO 2 and the modern carbon budget to early human land use: analyses with a process-based model Biogeosciences, 8, 69-88, 2011 Author(s): B. D. Stocker, K. Strassmann, and F. Joos A Dynamic Global Vegetation model coupled to a simplified Earth system model is used to simulate the impact of anthropogenic land cover changes (ALCC) on Holocene atmospheric CO 2 and the contemporary carbon cycle. The model results suggest that early agricultural activities cannot explain the mid to late Holocene CO 2 rise of 20 ppm measured on ice cores and that proposed upward revisions of Holocene ALCC imply a smaller contemporary terrestrial carbon sink. A set of illustrative scenarios is applied to test the robustness of these conclusions and to address the large discrepancies between published ALCC reconstructions. Simulated changes in atmospheric CO 2 due to ALCC are less than 1 ppm before 1000 AD and 30 ppm at 2004 AD when the HYDE 3.1 ALCC reconstruction is prescribed for the past 12 000 years. Cumulative emissions of 69 GtC at 1850 and 233 GtC at 2004 AD are comparable to earlier estimates. CO 2 changes due to ALCC exceed the simulated natural interannual variability only after 1000 AD. To consider evidence that land area used per person was higher before than during early industrialisation, agricultural areas from HYDE 3.1 were increased by a factor of two prior to 1700 AD (scenario H2). For the H2 scenario, the contemporary terrestrial carbon sink required to close the atmospheric CO 2 budget is reduced by 0.5 GtC yr −1 . Simulated CO 2 remains small even in scenarios where average land use per person is increased beyond the range of published estimates. Even extreme assumptions for preindustrial land conversion and high per-capita land use do not result in simulated CO 2 emissions that are sufficient to explain the magnitude and the timing of the late Holocene CO 2 increase.

Description: The use of algorithms to predict surface seawater dimethyl sulphide concentrations in the SE Pacific, a region of steep gradients in primary productivity, biomass and mixed layer depth Biogeosciences, 8, 1-16, 2011 Author(s): A. J. Hind, C. D. Rauschenberg, J. E. Johnson, M. Yang, and P. A. Matrai Dimethyl sulphide (DMS) is an important precursor of cloud condensation nuclei (CCN), particularly in the remote marine atmosphere. The SE Pacific is consistently covered with a persistent stratocumulus layer that increases the albedo over this large area. It is not certain whether the source of CCN to these clouds is natural and oceanic or anthropogenic and terrestrial. This unknown currently limits our ability to reliably model either the cloud behaviour or the oceanic heat budget of the region. In order to better constrain the marine source of CCN, it is necessary to have an improved understanding of the sea-air flux of DMS. Of the factors that govern the magnitude of this flux, the greatest unknown is the surface seawater DMS concentration. In the study area, there is a paucity of such data, although previous measurements suggest that the concentration can be substantially variable. In order to overcome such data scarcity, a number of climatologies and algorithms have been devised in the last decade to predict seawater DMS. Here we test some of these in the SE Pacific by comparing predictions with measurements of surface seawater made during the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. We conclude that none of the existing algorithms reproduce local variability in seawater DMS in this region very well. From these findings, we recommend the best algorithm choice for the SE Pacific and suggest lines of investigation for future work.

Description: Formation and global distribution of sea-surface microlayers Biogeosciences, 8, 121-135, 2011 Author(s): O. Wurl, E. Wurl, L. Miller, K. Johnson, and S. Vagle Results from a study of surfactants in the sea-surface microlayer (SML) in different regions of the ocean (subtropical, temperate, polar) suggest that this interfacial layer between the ocean and atmosphere covers the ocean's surface to a significant extent. New, experimentally-derived threshold values at which primary production acts as a significant source of natural surfactants to the microlayer are coupled with a wind speed threshold at which the SML is presumed to be disrupted, and the results suggest that surfactant enrichment in the SML is greater in oligotrophic regions of the ocean than in more productive waters. Furthermore, surfactant enrichments persisted at wind speeds of up to 10 m s −1 , without any observed depletion above 5 m s −1 . This suggests that the SML is stable enough to exist even at the global average wind speed of 6.6 m s −1 . Using our observations of the surfactant enrichments at various trophic levels and wind states, global maps of primary production and wind speed allow us to extrapolate the ocean's SML coverage . The maps indicate that wide regions of the Pacific and Atlantic Oceans between 30° N and 30° S may be more significantly covered with SML than north of 30° N and south of 30° S, where higher productivity (spring/summer blooms) and wind speeds exceeding 12 m s −1 may prevent extensive SML formation.

Description: Tracing the origin of dissolved silicon transferred from various soil-plant systems towards rivers: a review Biogeosciences, 8, 89-112, 2011 Author(s): J.-T. Cornelis, B. Delvaux, R. B. Georg, Y. Lucas, J. Ranger, and S. Opfergelt Silicon (Si) released as H 4 SiO 4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0.1–10% Si dry weight), Si becomes largely mobile in the soil-plant system. Litter-fall leads to a substantial reactive biogenic silica pool in soil, which contributes to the release of dissolved Si (DSi) in soil solution. Understanding the biogeochemical cycle of silicon in surface environments and the DSi export from soils into rivers is crucial given that the marine primary bio-productivity depends on the availability of H 4 SiO 4 for phytoplankton that requires Si. Continental fluxes of DSi seem to be deeply influenced by climate (temperature and runoff) as well as soil-vegetation systems. Therefore, continental areas can be characterized by various abilities to transfer DSi from soil-plant systems towards rivers. Here we pay special attention to those processes taking place in soil-plant systems and controlling the Si transfer towards rivers. We aim at identifying relevant geochemical tracers of Si pathways within the soil-plant system to obtain a better understanding of the origin of DSi exported towards rivers. In this review, we compare different soil-plant systems (weathering-unlimited and weathering-limited environments) and the variations of the geochemical tracers (Ge/Si ratios and δ 30 Si) in DSi outputs. We recommend the use of biogeochemical tracers in combination with Si mass-balances and detailed physico-chemical characterization of soil-plant systems to allow better insight in the sources and fate of Si in these biogeochemical systems.

Description: Deep plant-derived carbon storage in Amazonian podzols Biogeosciences, 8, 113-120, 2011 Author(s): C. R. Montes, Y. Lucas, O. J. R. Pereira, R. Achard, M. Grimaldi, and A. J. Melfi Equatorial podzols are soils characterized by thick sandy horizons overlying more clayey horizons. Organic matter produced in the topsoil is transferred in depth through the sandy horizons and accumulate at the transition, at a depth varying from 1 to more than 3 m, forming deep horizons rich in organic matter (Bh horizons). Although they cover great surfaces in the equatorial zone, these soils are still poorly known. Studying podzols from Amazonia, we found out that the deep Bh horizons in poorly drained podzol areas have a thickness higher than 1 m and store unexpected amounts of carbon. The average for the studied area was 66.7 ± 5.8 kgC m −2 for the deep Bh and 86.8 ± 7.1 kgC m −2 for the whole profile. Extrapolating to the podzol areas of the whole Amazonian basin has been possible thanks to digital maps, giving an order of magnitude around 13.6 ± 1.1 PgC, at least 12.3 PgC higher than previous estimates. This assessment should be refined by additional investigations, not only in Amazonia but in all equatorial areas where podzols have been identified. Because of the lack of knowledge on the quality and behaviour of the podzol organic matter, the question of the feedback between the climate and the equatorial podzol carbon cycle is open.

Description: Oxygenation variability in Mejillones Bay, off northern Chile, during the last two centuries Biogeosciences, 8, 137-146, 2011 Author(s): J. A. Díaz-Ochoa, S. Pantoja, G. J. De Lange, C. B. Lange, G. E. Sánchez, V. R. Acuña, P. Muñoz, and G. Vargas The Peru Chile Current ecosystem is characterized by high biological productivity and important fisheries. Although this system is likely to be severely affected by climate change, its response to current global warming is still uncertain. In this paper, we analyze 10–166 year-old sediments in two cores collected from Mejillones Bay, an anoxic sedimentary setting favorable for the preservation of proxies. Based on a 166-year chronology, we used proxies of bottom-water oxygenation (Mo, V, S, and the (lycopane + n −C 35 )/ n −C 31 ratio) and surface water productivity (biogenic opal, counts of diatom valves, biogenic Ba, organic carbon, and chlorins) to reconstruct environmental variations in Mejillones Bay. During the last two centuries, a shift took place in the coastal marine ecosystem of Bahia Mejillones at decadal scales. This shift was characterized by intense ENSO-like activity, large-scale fluctuations in biological export productivity and bottom water oxygenation, and increased eolian activity (inferred from Ti/Al and Zr/Al). This short-term variability was accompanied by a gradual increase of sulfidic conditions that has intensified since the early 1960s.

Description: Imprint of past and present environmental conditions on microbiology and biogeochemistry of coastal Quaternary sediments Biogeosciences, 8, 55-68, 2011 Author(s): M. Beck, T. Riedel, J. Graue, J. Köster, N. Kowalski, C. S. Wu, G. Wegener, Y. Lipsewers, H. Freund, M. E. Böttcher, H.-J. Brumsack, H. Cypionka, J. Rullkötter, and B. Engelen To date, North Sea tidal-flat sediments have been intensively studied down to a depth of 5 m below seafloor (mbsf). However, little is known about the biogeochemistry, microbial abundance, and activity of sulfate reducers as well as methanogens in deeper layers. In this study, two 20 m-long cores were retrieved from the tidal-flat area of Spiekeroog Island, NW Germany. The drill sites were selected with a close distance of 900 m allowing to compare two depositional settings: first, a paleo-channel filled with Holocene sediments and second, a mainly Pleistocene sedimentary succession. Analyzing these cores, we wanted to test to which degree the paleo-environmental imprint is superimposed by present processes. In general, the numbers of bacterial 16S rRNA genes are one to two orders of magnitude higher than those of Archaea . The abundances of key genes for sulfate reduction and methanogenesis ( dsr A and mcr A) correspond to the sulfate and methane profiles. A co-variance of these key genes at sulfate-methane interfaces and enhanced ex situ AOM rates suggest that anaerobic oxidation of methane may occur in these layers. Microbial and biogeochemical profiles are vertically stretched relative to 5 m-deep cores from shallower sediments in the same study area, but still appear compressed compared to deep sea sediments. Our interdisciplinary analysis shows that the microbial abundances and metabolic rates are elevated in the Holocene compared to Pleistocene sediments. However, this is mainly due to present environmental conditions such as pore water flow and organic matter availability. The paleo-environmental imprint is still visible but superimposed by these processes.

Description: Annual follow-up of gross diffusive carbon dioxide and methane emissions from a boreal reservoir and two nearby lakes in Québec, Canada Biogeosciences, 8, 41-53, 2011 Author(s): M. Demarty, J. Bastien, and A. Tremblay Surface water p CO 2 and p CH 4 measurements were taken in the boreal zone of Québec, Canada, from summer 2006 to summer 2008 in Eastmain 1 reservoir and two nearby lakes. The goal of this follow-up was to evaluate annual greenhouse gas (GHG) emissions, including spring emissions (N.B. gross emissions for reservoir), through flux calculations using the thin boundary layer model. Our measurements underscored the winter CO 2 accumulation due to ice cover and the importance of a reliable estimate of spring diffusive emissions as the ice breaks up. We clearly demonstrated that in our systems, diffusive CH 4 flux (in terms of CO 2 equivalent) were of minor importance in the GHG emissions (without CH 4 accumulation under ice), with diffusive CO 2 flux generally accounting for more than 95% of the annual diffusive flux. We also noted the extent of spring diffusive CO 2 emissions (23% to 52%) in the annual carbon budget.