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The carbon budgets in terrestrial ecosystems in China are strongly coupled with climate changes. Over the past decade, China has experienced dramatic climate changes characterized by enhanced summer monsoon and decelerated warming. However, the changes in the trends of terrestrial net ecosystem production (NEP) in China under climate changes are not well documented. Here, we used three ecosystem models to simulate the spatiotemporal variations in China's NEP during 1982–2010 and quantify the contribution of the strengthened summer monsoon and warming hiatus to the NEP variations in four distinct climatic regions of the country. Our results revealed a decadal-scale shift in NEP from a downtrend of –5.95 Tg C/yr2 (reduced sink) during 1982–2000 to an uptrend of 14.22 Tg C/yr2 (enhanced sink) during 2000–10. This shift was essentially induced by the strengthened summer monsoon, which stimulated carbon uptake, and the warming hiatus, which lessened the decrease in the NEP trend. Compared to the contribution of 56.3% by the climate effect, atmospheric CO2 concentration and nitrogen deposition had relatively small contributions (8.6 and 11.3%, respectively) to the shift. In conclusion, within the context of the global-warming hiatus, the strengthening of the summer monsoon is a critical climate factor that enhances carbon uptake in China due to the asymmetric response of photosynthesis and respiration. Our study not only revealed the shift in ecosystem carbon sequestration in China in recent decades, but also provides some insight for understanding ecosystem carbon dynamics in other monsoonal areas.

The South-East Madagascar Bloom, one of the most compelling biogeochemical features of the Indian Ocean, occurs sporadically during austral summer in the oligotrophic waters south-east of Madagascar, where it can cover up to 1% of the global ocean surface area. Its spatial extension and its timing are highly variable. A high-resolution biophysical model is used to investigate a previous hypothesis that the onset of a particular circulation of the South-East Madagascar Current advects fresher and nutrient-rich waters eastward, feeding the bloom. The model is able to reproduce an intermittent phytoplankton bloom with large spatial variability but in the subsurface layers, as well as the presence of an irregular retroflection of the South-East Madagascar Current. The simulated bloom occurs within a shallow stratified mixed layer, with fresher waters at the surface, parallel to the water mass in an observed bloom. The model results suggest, from a nutrient flux analysis, that horizontal advection of low-salinity nutrient-rich Madagascan coastal waters can indeed trigger a phytoplankton bloom. The coupled model is also able to resolve a bloom that is atmospherically forced by cyclonic activity.

Highlights • Fixed nutrient ratio not appropriate to simulate the chemical conditions in the Mediterranean Sea. • A simple approach allows to apply the ‘line of frugality’ to biogeochemical models. • Nutrient and plankton simulations improve with the variable internal nutrient ratio., The relative abundance of nitrate (N) over phosphate (P) measured as a molar ratio (N:P) is typically considered to determine the macronutrient limiting marine primary production. In low-complexity biogeochemical models, a simple threshold value is usually applied based on the canonical Redfield ratio (N:P = 16). However, the N:P ratio is not constant in many oceanic areas, especially marginal, semi-enclosed seas, such as the Mediterranean basin. In this work, a flexible definition of the N:P ratio based on the capacity of phytoplankton to modulate phosphate uptake according to its availability in seawater, the so-called Line of Frugality, is incorporated into the biogeochemical model MedERGOM. This modification allows the acquisition of a more realistic representation of the stoichiometry of nutrients in the Mediterranean basin and allows to better reproduce the observed phytoplankton biomass in productive areas such as the Gulf of Gabes and the Adriatic Sea. This approach is, thus, especially suitable for coastal areas in which basin-scale biogeochemical models fail to reproduce patterns observed by remote sensing or in situ measurements. Our results show that implementation of the stoichiometric flexibility of phytoplankton in a low-complexity biogeochemical model enhances the reproducibility of ecosystem dynamics without increasing the computational demand, representing a simple approximation easily implemented in models aiming to describe regions with a Non-Redfieldian stoichiometry.

Marine biogeochemical processes can strongly interact with processes occurring in adjacent ice and sediments. This is especially likely in areas with shallow water and frequent ice cover, both of which are common in the Arctic. Modeling tools are therefore required to simulate coupled biogeochemical systems in ice, water, and sediment domains. We developed a 1D sympagic&ndash, pelagic&ndash, benthic transport model (SPBM) which uses input from physical model simulations to describe hydrodynamics and ice growth and modules from the Framework for Aquatic Biogeochemical Models (FABM) to construct a user-defined biogeochemical model. SPBM coupled with a biogeochemical model simulates the processes of vertical diffusion, sinking/burial, and biogeochemical transformations within and between the three domains. The potential utility of SPBM is demonstrated herein with two test runs using modules from the European regional seas ecosystem model (ERSEM) and the bottom-redox model biogeochemistry (BROM-biogeochemistry). The first run simulates multiple phytoplankton functional groups inhabiting the ice and water domains, while the second simulates detailed redox biogeochemistry in the ice, water, and sediments. SPBM is a flexible tool for integrated simulation of ice, water, and sediment biogeochemistry, and as such may help in producing well-parameterized biogeochemical models for regions with strong sympagic&ndash, benthic interactions.

The rate at which organic contaminants can be degraded in aquatic environments is not only dependent upon specific degrading bacteria, but also upon the composition of the microbial community, mass transfer of the contaminant, and abiotic processes that occur in the environment. In this study, we present three-phase batch experiments of tetrachloroethene (PCE) degradation by a consortium of organohalide-respiring bacteria, cultivated alone or in communities with iron- and/or sulfate-reducers. We developed a modeling approach to quantitatively evaluate the experimental results, comprised of chemical and biomolecular time series data. The model utilizes the IPhreeqc module to couple multiphase mass transfer between gaseous, organic and aqueous phases with microbial and aquatic geochemical processes described using the geochemical code PHREEQC. The proposed approach is able to capture the contaminant degradation, the microbial population dynamics, the effects of multi-phase kinetic mass transfer and sample removal, and the geochemical reactions occurring in the aqueous phase. The model demonstrates the importance of aqueous speciation and abiotic reactions on the bioavailability of the substrates. The model-based interpretation allowed us to quantify the reaction kinetics of the different bacterial guilds. The model further revealed that the inclusion of sulfate-reducing bacteria lowers the rate of PCE degradation and that this effect is moderated in the presence of iron-reducing bacteria. (C) 2019 Elsevier Ltd. All rights reserved.

Identificadors digitals: Digital object identifier for the 'European Research Council' (http://dx.doi.org/10.13039/501100000781) and Digital object identifier for 'Horizon 2020' (http://dx.doi.org/10.13039/501100007601) Unidad de excelencia María de Maeztu MdM-2015-0552 The long-term trend of increasing phytoplankton net primary production (NPP) in the Arctic correlates with increasing light penetration due to sea ice loss. However, recent studies suggest that enhanced stormy wind mixing may also play a significant role enhancing NPP. Here, we isolate the role of sea ice and stormy winds (hereafter high-frequency winds) using an eddy-permitting ice-ocean-biogeochemical model configured for the North Atlantic and the Arctic. In the model, the presence of high-frequency winds stimulates nutrient upwelling by producing an earlier and longer autumn-winter mixing period with deeper mixing layer. The early onset of autumn mixing results in nutrients being brought-up to near-surface waters before the light becomes the dominant limiting factor, which leads to the autumn bloom. The enhanced mixing results in higher nutrient concentrations in spring and thus a large spring bloom. The model also shows significant iron limitation in the Labrador Sea, which is intensified by high-frequency winds. The effect of sea ice loss on NPP was found to be regionally dependent on the presence of high-frequency winds. This numerical study suggests high-frequency winds play significant role increasing NPP in the Arctic and sub-Arctic by alleviating phytoplankton nutrient limitation and that the isolated effect of sea ice loss on light plays a comparatively minor role.

The reduction of riverine nutrients inputs is considered the means of choice to improve the eutrophication status of the southern North Sea. With the European Union's Water Framework Directive (WFD) reduction measures presently under debate, two questions arise: (1) What changes in eutrophication indicators can be expected? (2) How do the reductions by the individual member states contribute to these? We combine an element tracing method (TBNT) with a biogeochemical model to analyze the effects of WFD-compliant nitrogen reductions proposed by OSPAR's North Sea member states. We first analyze changes in selected OSPAR assessment parameters relative to a reference simulation. Second, we quantify the source-specific contributions to total nitrogen (TN) in different regions. An overall nitrogen load reduction of 14 % is achieved. However, the response shows significant spatial variations due to strong differences between the countries' load reductions. TN and dissolved inorganic nitrogen reductions up to 60 % and 35 % are simulated near the Bay of Seine (France) and in the German Bight, respectively. Along the Dutch coast, reductions are below 10 %, and no changes occur along the British coast. Reductions in chlorophyll-a are generally lower. The TBNT analysis for the German Exclusive Economic Zone shows a TN reduction in the coastal region comparable to the N reductions in the German rivers (~25 %). In the offshore region, TN is reduced by only 6 % due to the strong influence of riverine sources with only low reductions and non-riverine sources. Our analysis reveals that non-linear responses in the biogeochemistry cause a faster removal of N from rivers with strong reductions by benthic denitrification, which enhances indirectly the removal of N from less reduced sources. Consequently, reductions in remote sources in non-problem areas can have a relevant positive effect on problem areas. This demonstrates that the TBNT method is an ideal tool to put in practice the “source-oriented approach” advocated by OSPAR, and to inform stakeholders about the effects of defined reduction strategies. However, an assessment framework is required to efficiently use it in management and for decision making, either by OSPAR, or in the context of WFD or Marine Strategy Framework Directive.

We present an analysis of the spatial distributions and seasonal dynamics of phosphate and nitrate in the Mediterranean Sea. The analysis involves observations under present conditions and relies on statistical analysis of the output of a state-of-the-art corroborated numerical model. Model results are corroborated with available data from cruises and fixed monitoring stations. The results capture and quantitatively detail the main spatial patterns observed in the Mediterranean Sea, with an east–west gradient in the surface layer for phosphate and a north–south gradient for nitrate. The model results highlight that plankton growth in Mediterranean waters is usually nutrient limited and that such limitation is less frequent in the western sub-basin and is usually due to a lack of phosphate, rather than nitrate, with the exception of the areas close to the Atlantic Ocean, where nitrogen limitation is important. The model simulation illustrates the benefit of a variable stoichiometric formulation of the phytoplankton cell, showing how the ratio of the internal N:P quota adapts to different limiting conditions.

Forests store a large amount of terrestrial carbon, but this storage capacity is vulnerable to wildfire. Combustion, and subsequent tree mortality and soil erosion, can lead to increased carbon release and decreased carbon uptake. Previous work has shown that non-constant fire return intervals over the past 4000 years strongly shaped subalpine forest carbon trajectories. The extent to which fire-regime variability has impacted carbon trajectories in other subalpine forest types is unknown. Here, we explored the interactions between fire and carbon dynamics of 14 subalpine watersheds in Colorado, USA. We tested the impact of varying fire frequency over a ~2000 year period on ecosystem productivity and carbon storage using an improved biogeochemical model. High fire frequency simulations had overall lower carbon stocks across all sites compared to scenarios with lower fire frequencies, highlighting the importance of fire-frequency in determining ecosystem carbon storage. Additionally, variability in fire-free periods strongly influenced carbon trajectories across all the sites. Biogeochemical trajectories (e.g., increasing or decreasing total ecosystem carbon and carbon-to-nitrogen (C:N) ratios) did not vary among forest types but there were trends that they may vary by elevation. Lower-elevations sites had lower overall soil C:N ratios, potentially because of higher fire frequencies reducing carbon inputs more than nitrogen losses over time. Additional measurements of ecosystem response to fire-regime variability will be essential for improving estimates of carbon dynamics from Earth system models.

There is an urgent need to identify and evaluate management practices for their biophysical potential to maintain productivity under climate change while mitigating greenhouse gas (GHG) emissions from individual cropping systems under specific pedo-climatic conditions. Here, we examined, through DayCent modeling, the long-term impact of soil management practices and their interactions on soil GHG emissions and GHG intensity from Swiss cropping systems. Based on experimental data from four long-term experimental sites in Switzerland (Therwil, Frick, Changins, and Reckenholz), we robustly parameterized and evaluated the model for simulating crop productivity, soil C dynamics and soil N2O emissions across a range of management practices and pedo-climatic conditions. Net soil GHG emissions (NSGHGE) were derived from changes in soil C, N2O emissions and CH4 oxidation. Soils under conventional management acted as a net source of soil GHG emissions (1361–1792 kg CO2eq ha−1 yr−1) and NSGHGE were dominated by N2O (50–63%). Reduced tillage and no-tillage reduced long-term NSGHGE by up to 31 and 58%, respectively. Organic farming, represented by organic fertilization, reduced NSGHGE by up to 31% compared to systems based solely on mineral fertilization. Replacement of slurries with a composted FYM led to an additional reduction in NSGHGE by 46%, although our approach considered only soil GHG emissions and thus did not take into account GHG emissions from the composting process. Cover cropping did not significantly influence NSGHGE, however vetch tended to reduce NSGHGE (-19%). The highest mitigation potential was associated with organic farming plus reduced tillage management, it reduced long-term NSGHGE by up to 128%. Soil C sequestration accounted, on average, for 89% of GHG mitigation potentials, consequently N2O dominated NSGHGE across all treatments and sites (60 − 80%). This indicates that these mitigation potentials are time limited and reversible, if the management is not maintained, in contrast to the reduction in N2O emissions, which is considered permanent. Not all the management practices sustained crop yields. Nevertheless, composting of organic manures, reduced tillage and no-tillage effectively reduced NSGHGE and GHG intensity without a noticeable yield reduction. Our results suggest that implementation of the above soil management practices in Swiss cropping systems have a considerable potential for climate change mitigation, although time-limited.

This study presents a specifically designed Mercury module in a coupled benthic-pelagic reactive-transport model - Bottom RedOx Model (BROM) that allows to study mercury (Hg) biogeochemistry under different conditions. This module considers the transformation of elemental mercury (Hg(O)), divalent mercury (Hg(II)) and methylmercury (MeHg). The behavior of mercury species in the model is interconnected with changes of oxygen, hydrogen sulfide, iron oxides, organic matter, and biota. We simulated the transformation and transport of Hg species in the water column and upper sediment layer under five different scenarios, combining various levels of oxygenation and trophic state in the Berre lagoon, a shallow eutrophic lagoon of the French Mediterranean coast subjected to seasonal anoxia. The first scenario represents the conditions in the lagoon that are compared with experimental data. The four other scenarios were produced by varying the biological productivity, using low and high nutrient (N and P) concentrations, and by varying the redox conditions using different intensity of vertical mixing in the water column. The results of the simulation show that both oxidized and reduced sediments can accumulate Hg, but any shifts in redox conditions in bottom water and upper sediment layer lead to the release of Hg species into the water column. Eutrophication and/or restricted vertical mixing lead to reducing conditions and intensify MeHg formation in the sediment with periodic release to the water column. Oxygenation of an anoxic water body can lead to the appearance of Hg species in the water column and uptake by organisms, whereby Hg may enter into the food web. The comparison of studied scenarios shows that a well-oxygenated eutrophic system favors the conditions for Hg species bioaccumulation with a potential adverse effect on the ecosystem. The research is relevant to the UN Minimata convention, EU policies on water, environmental quality standards and Mercury in particular. VISTA - a basic research program Norwegian Academy of Science and Letters 6164 Statoil 6164 project PREDHYPO A*MIDEX project - Investissements d'Avenir French Government program ANR-11-IDEX-0001-02 Norwegian Research Council SKATTEfunn project Aquatic Modeling Tools 272749 NILU CIMA IMBER Future Earth Coasts Future Earth Ocean KAN info:eu-repo/semantics/publishedVersion

Oxygen (O2) deficiency, i.e., dissolved O2 concentrations below 6 mg O2 L−1, is a common feature in the southern North Sea. Its evolution is governed mainly by the presence of seasonal stratification and production of organic matter, which is subsequently degraded under O2 consumption. The latter is strongly influenced by riverine nutrient loads, i.e., nitrogen (N) and phosphorus (P). As riverine P loads have been reduced significantly over the past decades, this study aims for the quantification of the influence of riverine and non-riverine N inputs on the O2 dynamics in the southern North Sea. For this purpose, we present an approach to expand a nutrient-tagging technique for physical-biogeochemical models — often referred to as ‘trans-boundary nutrient transports’ (TBNT) — by introducing a direct link to the O2 dynamics. We apply the expanded TBNT to the physical-biogeochemical model system HAMSOM-ECOHAM and focus our analysis on N-related O2 consumption in the southern North Sea during 2000–2014. The analysis reveals that near-bottom O2 consumption in the southern North Sea is strongly influenced by the N supply from the North Atlantic across the northern shelf edge. However, riverine N sources — especially the Dutch, German and British rivers — as well as the atmosphere also play an important role. In the region with lowest simulated O2 concentrations (around 56 °N, 6.5 °E), riverine N on average contributes 39% to overall near-bottom O2 consumption during seasonal stratification. Here, the German and the large Dutch rivers constitute the highest riverine contributions (11% and 10%, respectively). At a site in the Oyster Grounds (around 54.5 °N, 4 °E), the average riverine contribution adds up to 41%, even exceeding that of the North Atlantic. Here, highest riverine contributions can be attributed to the Dutch and British rivers adding up to almost 28% on average. The atmospheric contribution results in 13%. Our results emphasize the importance of anthropogenic N inputs and seasonal stratification for the O2 conditions in the southern North Sea. They further suggest that reductions in the riverine and atmospheric N inputs may have a relevant positive effect on the O2 levels in this region.

Using a global ocean biogeochemical model combined with a forecast of physical oceanic and atmospheric variables from the NASA Global Modeling and Assimilation Office, we assess the skill of a chlorophyll concentrations forecast in the Equatorial Pacific for the period 2012–2015 with a focus on the forecast of the onset of the 2015 El Niño event. Using a series of retrospective 9-month hindcasts, we assess the uncertainties of the forecasted chlorophyll by comparing the monthly total chlorophyll concentration from the forecast with the corresponding monthly ocean chlorophyll data from the Suomi-National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (S-NPP VIIRS) satellite. The forecast was able to reproduce the phasing of the variability in chlorophyll concentration in the Equatorial Pacific, including the beginning of the 2015–2016 El Niño. The anomaly correlation coefficient (ACC) was significant (p < 0.05) for forecast at 1-month (R = 0.33), 8-month (R = 0.42) and 9-month (R = 0.41) lead times. The root mean square error (RMSE) increased from 0.0399 μg chl L−1 for the 1-month lead forecast to a maximum of 0.0472 μg chl L−1 for the 9-month lead forecast indicating that the forecast of the amplitude of chlorophyll concentration variability was getting worse. Forecasts with a 3-month lead time were on average the closest to the S-NPP VIIRS data (23% or 0.033 μg chl L−1) while the forecast with a 9-month lead time were the furthest (31% or 0.042 μg chl L−1). These results indicate the potential for forecasting chlorophyll concentration in this region but also highlights various deficiencies and suggestions for improvements to the current biogeochemical forecasting system. This system provides an initial basis for future applications including the effects of El Niño events on fisheries and other ocean resources given improvements identified in the analysis of these results.

International audience; The Lot river, a major tributary of the downstream Garonne river, the largest river on the Northern side of the Pyrenees Mountains, was intensively studied in the 1970s. A pioneering program called “Lot Rivière Claire” provided a diagnosis of water quality at the scale of the whole watershed and proposed an ambitious program to manage nutrient pollution and eutrophication largely caused by urban wastewater releases. Later on, the implementation of European directives from 1991 to 2000 resulted in the nearly complete treatment of point sources of pollution in spite of a doubling of the basin’s population. At the outlet of the Lot river, ammonium and phosphate contamination which respectively peaked to 1 mg N-NH4 L−1 and 0.3 mg P-PO4 L−1 in the 1980s returned to much lower levels in recent years (0.06 mg N-NH4 L−1 and 0.02 mg P-PO4 L−1), a reduction by a factor 15. However, during this time, nitrate contamination has regularly increased since the 1980s, from 0.5 to 1.2 mg N-NO3 L−1 in average, owing to the intensification of agriculture and livestock farming. Application of the Riverstrahler model allowed us to simulate the water quality of the Lot drainage network for the 2002–2014 period. We showed that, with respect to algal requirements, phosphorus and silica are well balanced, but nitrogen remains largely in excess over phosphorus and silica. This imbalance can be problematic for the ecological status of the water bodies. Using the model, for simulating various scenarios of watershed management, we showed that improvement of urban wastewater treatment would not result in any significant change in the river’s water quality. Even though arable land occupies a rather limited fraction of the watershed area, only the adoption of better farming practices or more radical changes in the agro-food system could reverse the trend of increasing nitrate contamination.

Geological carbon sequestration captures CO2 from industrial sources and stores the CO2 in subsurface reservoirs, a viable strategy for mitigating global climate change. In assessing the environmental impact of the strategy, a key question is how microbial reactions respond to the elevated CO2 concentration. This study uses biogeochemical modeling to explore the influence of CO2 on the thermodynamics and kinetics of common microbial reactions in subsurface environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results show that increasing CO2 levels decreases groundwater pH and modulates chemical speciation of weak acids in groundwater, which in turn affect microbial reactions in different ways and to different extents. Specifically, a thermodynamic analysis shows that increasing CO2 partial pressure lowers the energy available from syntrophic oxidation and acetoclastic methanogenesis, but raises the available energy of microbial iron reduction, hydrogenotrophic sulfate reduction and methanogenesis. Kinetic modeling suggests that high CO2 has the potential of inhibiting microbial sulfate reduction while promoting iron reduction. These results are consistent with the observations of previous laboratory and field studies, and highlight the complexity in microbiological responses to elevated CO2 abundance, and the potential power of biogeochemical modeling in evaluating and quantifying these responses.

Globally increasing temperatures are likely to have impacts on terrestrial, aquatic and marine ecosystems that are difficult to manage. Quantifying impacts worldwide and systematically as a function of global warming is fundamental to substantiating the discussion on climate mitigation targets and adaptation planning. Here we present a macro-scale analysis of climate change impacts on terrestrial ecosystems based on newly developed sets of climate scenarios featuring a step-wise sampling of global mean temperature increase between 1.5 and 5 K by 2100. These are processed by a biogeochemical model (LPJmL) to derive an aggregated metric of simultaneous biogeochemical and structural shifts in land surface properties which we interpret as a proxy for the risk of shifts and possibly disruptions in ecosystems. Our results show a substantial risk of climate change to transform terrestrial ecosystems profoundly. Nearly no area of the world is free from such risk, unless strong mitigation limits global warming to around 2 degrees above preindustrial level. Even then, our simulations for most climate models agree that up to one-fifth of the land surface may experience at least moderate ecosystem change, primarily at high latitudes and high altitudes. If countries fulfil their current emissions reduction pledges, resulting in roughly 3.5 K of warming, this area expands to cover half the land surface, including the majority of tropical forests and savannas and the boreal zone. Due to differences in regional patterns of climate change, the area potentially at risk of major ecosystem change considering all climate models is up to 2.5 times as large as for a single model.

International audience; The aim of this study is to evaluate the consequences of accounting for variable Chl:C (chlorophyll:carbon) and C:N (carbon:nitrogen) ratios in the formulation of phytoplankton growth in biogeochemical models. We compare the qualitative behaviour of a suite of phytoplankton growth formulations with increasing complexity: 1) a Redfield formulation (constant C:N ratio) without photo-acclimation (constant Chl:C ratio), 2) a Redfield formulation with diagnostic chlorophyll (variable and empirical Chl:C ratio), 3) a quota formulation (variable C:N ratio) with diagnostic chlorophyll, and 4) a quota formulation with prognostic chlorophyll (dynamic variable). These phytoplankton growth formulations are embedded in a simple marine ecosystem model in a 1D framework at the Bermuda Atlantic Time-series (BATS) station. The model parameters are tuned using a stochastic assimilation method (micro-genetic algorithm) and skill assessment techniques are used to compare results. The lowest misfits with observations are obtained when photo-acclimation is taken into account (variable Chl:C ratio) and with non-Redfield stoichiometry (variable C:N ratio), both under spring and summer conditions. This indicates that the most flexible models (i.e., with variable ratios) are necessary to reproduce observations. As seen previously, photo- acclimation is essential in reproducing the observed deep chlorophyll maximum and subsurface production present during summer. Although Redfield and quota formulations of C:N ratios can equally reproduce chlorophyll data the higher primary production that arises from the quota model is in better agreement with observations. Under the oligotrophic conditions that typify the BATS site no clear difference was detected between quota formulations with diagnostic or prognostic chlorophyll.

Globally, forest soils contain twice as much carbon as forest vegetation. Consequently, natural and anthropogenic disturbances affecting carbon accumulation in forest soils can alter regional to global carbon balance. In this study, we evaluate the effects of historic litter raking on soil carbon stocks, a former forest use which used to be widespread throughout Europe for centuries. We estimate, for Switzerland, the carbon sink potential in current forest soils due to recovery from past litter raking (‘legacy effect’). The year 1650 was chosen as starting year for litter raking, with three different end years (1875/1925/1960) implemented for this forest use in the biogeochemical model LPJ-GUESS. The model was run for different agricultural and climatic zones separately. Number of cattle, grain production and the area of wet meadow have an impact on the specific demand for forest litter. The demand was consequently calculated based on historical statistical data on these factors. The results show soil carbon pools to be reduced by an average of 17 % after 310 years of litter raking and legacy effects were still visible 130 years after abandonment of this forest use (2 % average reduction). We estimate the remaining carbon sink potential in Swiss forest due to legacy effects from past litter raking to amount to 158,000 tC. Integrating historical data into biogeochemical models provides insight into the relevance of past land-use practices. Our study underlines the importance of considering potentially long-lasting effects of such land use practices for carbon accounting.

Understanding the dynamic interplay between physical, biogeochemical and biological processes represents a key challenge in oceanography, particularly in shelf seas where complex hydrodynamics are likely to drive nutrient distribution and niche partitioning of phytoplankton communities. The Iroise Sea includes a tidal front called the 'Ushant Front' that undergoes a pronounced seasonal cycle, with a marked signal during the summer. These characteristics as well as relatively good observational sampling make it a region of choice to study processes impacting phytoplankton dynamics. This innovative modeling study employs a phytoplankton-diversity model, coupled to a regional circulation model to explore mechanisms that alter biogeography of phytoplankton in this highly dynamic environment. Phytoplankton assemblages are mainly influenced by the depth of the mixed layer on a seasonal time scale. Indeed, solar incident irradiance is a limiting resource for phototrophic growth and small phytoplankton cells are advantaged over larger cells. This phenomenon is particularly relevant when vertical mixing is intense, such as during winter and early spring. Relaxation of wind-induced mixing in April causes an improvement of irradiance experienced by cells across the whole study area. This leads, in late spring, to a competitive advantage of larger functional groups such as diatoms as long as the nutrient supply is sufficient. This dominance of large, fast-growing autotrophic cells is also maintained during summer in the productive tidally-mixed shelf waters. In the oligotrophic surface layer of the western part of the Iroise Sea, small cells coexist in a greater proportion with large, nutrient limited cells. The productive Ushant tidal front's region (1800 mgC.m -2 .d -1 between August and September) is also characterized by a high degree of coexistence between three functional groups (diatoms, micro/nano-flagellates and small eukaryotes/cyanobacteria). Consistent with previous studies, the biogeography of phytoplankton functional types at the Ushant front during summer displays an intermediate community composition between contrasted sub-regions on either side of the front. Strong mixing conditions within the frontal sub-region result in a short residence time of water masses, not allowing speciation or long term adaptation to occur. (C) 2016 The Authors. Published by Elsevier B.V.

A coupled physical-biogeochemical model (ROMS-PISCES) forced by climatological fields is used to examine the role of upwelling filaments in the offshore exchange of particulate (POC) and dissolved (DOC) organic carbon in the Canary Current eastern boundary upwelling system (CanC EBUS). In this region, mesoscale filaments at Capes Ghir ( °N) and Juby ( °N) have been frequently described using both observational and numerical data. Due to their semi-permanent presence and unique dynamical characteristics, studies focusing on filaments often provide an incomplete picture of the physical and biological processes at work, and their effects on coast-to-ocean export. The present model experiment confirms the complex three-dimensional structure of the filaments that comprises both offshore and onshore flow components. The model shows strong seasonal variability in the offshore transport mediated by the filaments. Recirculation at the edges of the filaments returns water towards the shore, especially in autumn when they are diverted northwards by the large scale boundary circulation. By contrast, offshore transport peaks during late spring - early summer when onshore recirculation is limited. Overall, the estimated net annual offshore flux of excess total organic carbon (e-TOC, the non-refractory pools of DOC and POC) averages 2.0 kg C y−1, and may increase up to 4.3 kg C y−1 during the peak upwelling season, each filament contributing to export of up to 22.6% of the organic carbon within the first 100 km from shore along the CanC EBUS (between 9.5 and 32 ° N). These results strongly support the inclusion of offshore transport estimates by coastal filaments in regional carbon budgets., This research was supported by projects CAIBEX (CTM2007-66408-CO2-02) and FLUXES (CTM2015- 69392-C3-1-R) funded by the ‘Spanish Plan Nacional de I + D’, and by projects SUMMER (AMD-817806-5) and TRIATLAS (AMD-817578-5) financed by the 'European Commission' to JA. EM acknowledges support from the Spanish Research Agency and the European Regional Development Fund (Award no. CTM2016-78607-P).