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International audience; We report on diurnal, tidal, and seasonal variations of dissolved inorganic carbon (DIC), water partial pressure of CO2 (pCO2), and associated water–air CO2 fluxes in a tidal creek of a temperate coastal lagoon with 70% of intertidal flats, during eight tidal/diurnal cycles and two consecutive years covering all seasons. Surface waters of the lagoon were always slightly oversaturated in CO2 with respect to the atmosphere with an average pCO2 value of 496 ± 36 ppmv. Seasonally, subsurface water pCO2 values were controlled by both temperature and biological/tidal advection effects that compensated each other and resulted in weak annual variations. High-resolution temporal pCO2 records reveal that the highest fluctuations (192 ppmv) occurred at the tidal/diurnal scale as a result of biological activity, advection from the tidal flat, and porewater pumping that all contributed to water pCO2 and carbonate chemistry variations. Total alkalinity (TA) versus salinity plots suggest a net production of alkalinity in the lagoon attributed to benthic carbonate dissolution and/or anaerobic degradation of organic matter. We specifically highlighted that for the same salinity range, during flooding, daytime pCO2 were generally lower than nighttime pCO2 values because of photosynthesis, whereas during ebbing, daytime pCO2 were higher than nighttime pCO2 values because of heating. Waters in the lagoon were a relatively weak CO2 source to the atmosphere over the year compared to other estuarine and lagoon waters elsewhere, and to sediment-air fluxes measured simultaneously by atmospheric Eddy Covariance (EC) in the Arcachon lagoon. Because of low values and small variations of the air-sea pCO2 gradient, the variability of fluxes calculated using the piston velocity parameterization was greatly controlled by the wind speed at the diurnal and, to a lesser extent, seasonal time scales. During the emersion, the comparison of these pCO2 data in the tidal creek with EC fluxes measured 1.8 km away on the tidal flat suggests high heterogeneity in air-sea CO2 fluxes, both spatially and at short time scales according to the inundation cycle and the wind speed. In addition to tidal pumping when the flat becomes emerged, our data suggest that lateral water movement during the emersion of the flat generates strong spatial heterogeneity in water–air CO2 flux.

In Brazil and in many other tropical countries, large urban cities and populations are still growing on the coast and coverage in terms of sewage treatments is far from desirable. Cultural eutrophication is not solely a threat for the coastal ocean; it is now acting as one of its major biogeochemical and ecological driver. Along the littoral of the state of Rio de Janeiro, semi-enclosed marine bays and lagoons show clear spatial and temporal pattern of increasing concentrations of chlorophyll a (Chl a), organic carbon, and nutrients in their waters and sediments in urbanized regions. Acting as a buffer, the nearshore ecosystems have turned highly eutrophic and their autotrophic metabolism has been enhanced creating strong carbon dioxide (CO2 ) sinks. We compile here data of CO2 fluxes recently gathered in four coastal marine ecosystems in the state of Rio de Janeiro: the Guanabara Bay and the Araruama, Saquarema and Jacarepagua lagoons. We observed intense CO2 sources in restricted areas at the vicinity of sewage loads, where microbial degradation of organic matter predominates, and large CO2 sinks in confined and nearshore brackish, marine and hypersaline waters, where phytoplankton blooms occur. We also report a correlation across the four ecosystems between the partial pressure of CO2 in waters and the Chl a concentration. Chl a satellite data all along the Brazilian coast suggest that the CO2 sink induced by eutrophication probably occurs in many coastal ecosystems including bays, lagoon and shelf waters, and could contribute to an additional blue carbon. Part of the additional organic carbon is stored in sediments, and part is exported offshore. However, this additional blue carbon has dramatic environment impacts as it would evolve toward the formation of marine dead zones, and could contribute to a production of methane (CH4) a more powerful greenhouse gas. We emphasize an urgent need for multidisciplinary research to promote simultaneously the storage of atmospheric carbon, and the preservation of biodiversity and socio-economic goods in the eutrophic tropical coastal ocean. No Brasil e em muitos países tropicais, os grandes centros urbanos estão em crescimento na zona costeira, porém a cobertura das Estações de Tratamento de Esgoto ainda está muito longe do ideal. A eutrofização cultural não é somente uma ameaça ao oceano costeiro; a eutrofização é atualmente uma das maiores forçantes de processos biogeoquímicos e ecológicos. Ao longo da zona costeira do estado do Rio de Janeiro, baías e lagunas costeiras semienclausuradas mostram um padrão espaçotemoral muito claro de concentrações crescentes de clorofila a (Chl a), carbono orgânico, nutrientes em suas águas e sedimentos de regiões urbanizadas. Atuando como uma região tampão, os ecossistemas costeiros se tornaram altamente eutrofizados e o seu metabolismo autotrófico tem sido estimulado, criando fortes sumidouros de dióxido de carbono (CO2 ) da atmosfera. No presente trabalho, compilamos dados de fluxos de CO2 recentemente coletados em quatro ecossistemas marinhos costeiros do estado do Rio de Janeiro: a Baía de Guanabara e as lagunas de Araruama, Saquarema e Jacarepaguá. Observamos fontes intensas de CO2 para a atmosfera em áreas restritas nas proximidades de descargas de esgoto doméstico, onde predomina a degradação microbiana da matéria orgânica, e grandes sumidouros de CO2 em águas rasas e confinadas, salinas e hipersalinas, onde ocorrem florações de fitoplâncton. Também relatamos uma correlação entre a pressão parcial de CO2 nas águas e a concentração de Chl a nos quatro ecossistemas analisados. Os dados de satélite de toda a costa brasileira sugerem que o sumidouro de CO2 induzido pela eutrofização provavelmente ocorre em muitos outros ecossistemas costeiros, incluindo baías, lagunas e águas de plataforma continental e pode contribuir com um estoque adicional de carbono azul. Parte do carbono orgânico adicional é armazenada nos sedimentos e parte é exportada para o oceano costeiro adjacente. No entanto, esse carbono azul adicional tem impactos ambientais dramáticos, pois evoluiria em direção à formação de zonas marinhas mortas e poderia contribuir para a produção de metano (CH4 ), um gás de efeito estufa mais poderoso. Enfatizamos a necessidade urgente de pesquisas multidisciplinares para promover simultaneamente o armazenamento de carbono atmosférico e a preservação da biodiversidade e dos bens socioeconômicos no oceano costeiro tropical eutrofizado.

International audience; The carbonate chemistry was investigated in the semiarid eutrophic Araruama Lagoon (Brazil), one of the largest hypersaline coastal lagoons in the world. Spatial surveys during winter and summer periods were performed, in addition to a diurnal sampling in summer. The hypersaline waters have higher concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) than the seawater that feed the lagoon, due to evaporation. However, TA and DIC concentrations were lower than those expected from evaporation. Calcium carbonate (CaCO3) precipitation partially explained these deficits. The negative correlation between the partial pressure of CO2 (pCO2) and chlorophyll a (Chl a) indicated that DIC was also consumed by primary producers. The uptake by photosynthesis contributes to 57–63% of DIC deviation from evaporation, the remaining credited to CaCO3 precipitation. Marked pCO2 undersaturation was prevalent at the innermost region with shallow, confined, and phytoplankton-dominated waters, with a strong enrichment of heavier carbon isotope (δ13C-DIC up to 5.55‰), and highest pH (locally counter-acting the process of ocean acidification). Oversaturation was restricted to an urbanized region, and during night-time. The lagoon behaved as a marked CO2 sink during winter (− 15.32 to − 10.15 mmolC m−2 day−1), a moderate sink during summer (− 5.50 to − 4.67 mmolC m−2 day−1), with a net community production (NCP) of 93.7 mmolC m−2 day−1 and prevalence of net autotrophic metabolism. A decoupling between CO2 and O2 exchange rate at the air–water interface was attributed to differences in gas solubility, and high buffering capacity. The carbonate chemistry reveals simultaneous and antagonistic actions of CaCO3 precipitation and autotrophic metabolism on CO2 fluxes, and could reflect future conditions in populated and semiarid coastal ecosystems worldwide.

This perspective paper aims at presenting the current knowledge on the processes of ocean acidification (OA) and coastal acidification (CA) in the Brazilian coastal ocean. We define and differentiate the processes of OA and CA: the first driven by the actual global increase of atmospheric carbon dioxide (CO2); the second driven by a combination of ocean uptake of atmospheric CO2 and other local/regional chemical additions or subtractions in aquatic ecosystems at the land-ocean interface. Regarding OA, we have centered our analysis on the data available for the main water masses along the Brazilian coast: South Atlantic Central Water (SACW), Tropical Water (TW), and Coastal Water (CW). The few data available for the shallow coastal waters (< 200 m depth) of the continental shelf reveal an increase in the anthropogenic component of the total dissolved inorganic carbon (DIC) pool in the SACW, with a decline in the ocean pH (over two decades; 1993-2013), and in the saturation state of calcium carbonate (CaCO3) minerals. We could not find OA trends for TW and CW because no data was available. Overall, the colder water masses (SACW, Plata Plume) have lower buffering capacity and simulations show that will potentially experience earlier negative OA impacts than the warmer waters masses (TW, Amazon Plume). Regarding CA, we have identified some local/regional studies investigating the carbonate chemistry in nearshore/estuarine ecosystems, particularly on the quantification of sources and sinks of CO2 , and determining short-term variabilities. Apparently, spreading coastal eutrophication in Brazil can enhances or reduces the process of OA, depending on the net ecosystem metabolism in combination with other chemical alterations. However, we could not find medium-long term acidification trends due to the limited data. There is a limited capacity to produce long time-series of carbonate chemistry parameters in key ecosystems and regions along the Brazilian coast. This lack of past information hinders and impairs the scientific community for identifying potential patterns of acidification along the Brazilian coast. We call for an urgent action in Brazil, with emphasis on the establishment of moored buoys/stations and/or scientific programs in the long term with continuous, real-time measurements of the main carbonate chemistry parameters. Keywords: ocean acidification, coastal acidification, coastal eutrophication, continental shelf, Southwestern Atlantic Ocean

International audience; Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH4) and carbon dioxide (CO2), are still poorly understood. Here, we aim to evaluate the relationships between CO2 and CH4 efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO2 and CH4 fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δ13C and δ15N), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δ15N (proxy of anthropogenic N enrichment), less negative δ13C, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO2 efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m−2 h−1 from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO2 fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH4 emissions were relatively low (three orders of magnitude lower than CO2 fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.

International audience

Since the establishment of the first radiocarbon accelerator mass spectrometry facility in Latin America in 2009, the Radiocarbon Laboratory team of Universidade Federal Fluminense (LAC-UFF) has worked to improve sample preparation protocols and increase the range of environmental matrices to be analyzed. We now present the preliminary results for DIC sample preparation protocols. The first validation tests include background evaluation with pMC value (0.35 ± 0.04) using bicarbonate dissolved in water. We also analyzed surface seawater resulting in pMC value (101.38 ± 0.38) and a groundwater previously dated from LEMA AMS-Laboratory with pMC value (12.30 ± 0.15).

International audience; Macrophyte detritus is one of the main sources of organic carbon (OC) in inland waters, and it is potentially available for methane (CH4) production in anoxic bottom waters and sediments. However, the transformation of macrophyte‐derived OC into CH4 has not been studied systematically, thus its extent and relationship with macrophyte characteristics remains uncertain. We performed decomposition experiments of macrophyte detritus from 10 different species at anoxic conditions, in presence and absence of a freshwater sediment, in order to relate the extent and rate of CH4 production to the detritus water content, C/N and C/P ratios. A significant fraction of the macrophyte OC was transformed to CH4 (mean = 7.9%; range = 0–15.0%) during the 59‐d incubation, and the mean total C loss to CO2 and CH4 was 17.3% (range = 1.3–32.7%). The transformation efficiency of macrophyte OC to CH4 was significantly and positively related to the macrophyte water content, and negatively to its C/N and C/P ratios. The presence of sediment increased the transformation efficiency to CH4 from an average of 4.0% (without sediment) to 11.8%, possibly due to physicochemical conditions favorable for CH4 production (low redox potential, buffered pH) or because sediment particles facilitate biofilm formation. The relationship between macrophyte characteristics and CH4 production can be used by future studies to model CH4 emission in systems colonized by macrophytes. Furthermore, this study highlights that the extent to which macrophyte detritus is mixed with sediment also affects CH4 production.

Estuarine carbonate chemistry predicts that thermodynamic equilibration during the mixing of freshwater with seawater will generate a carbon dioxide (CO 2 ) sink in the case of warm and poorly buffered tropical rivers. The Sao Francisco River estuary has historically become oligotrophic after the construction of a series of hydroelectric dams in its watershed, where organic matter and nutrients are retained. During two cruises in late winter (Aug. 2014) and early summer (Nov. 2015), dissolved inorganic carbon (DIC) and total alkalinity (TA) were found to increase linearly with salinity in the main estuarine channel, where water mixing required half a day, and showed nearly conservative behaviour. In the main channel, the water partial pressure of CO 2 (pCO2) recorded at a 1-min frequency followed an asymmetric bell-shaped trend versus salinity, similar to the curve predicted by the thermodynamic conservative mixing of freshwater DIC and TA with seawater DIC and TA. The low (0–3) salinity region was always a source of atmospheric CO2, where despite low chlorophyll concentrations, a pCO2 diurnal change of approximately 60 ppmv suggested the occurrence of photosynthesis in summer. At salinities above 3, undersaturated pCO2 values (down to 225 ppmv in winter and neap tides) and invasion of atmospheric CO2 of 0.38–1.70 mmol m−1 h−1 occurred because of the predominating thermodynamics during estuarine mixing. In winter and neap tides, the higher river discharge, intense estuarine mixing, lower temperatures and limited tidal pumping resulted in observed pCO2 differences from the theoretical conservative pCO2 by less than 3 ppmv at salinities >3. Conversely, in summer and spring tides, the recorded pCO2 values were on average + 43 ± 35 ppmv above the conservative mixing curve, when tidal pumping, CO2 invasion and surface heating were more significant in the mixing zone but not sufficient to offset the thermodynamic uptake of atmospheric CO2. By combining carbonate chemistry with estuarine mixing modelling and gas exchange calculations, we estimate that heating contributed to approximately 15% and gas exchange contributed to approximately 10% of the positive pCO2 deviation from conservative mixing during summer. The remaining 75% of the deviation reached its maximum at ebb tides and within salinity ranges consistent with the occurrence of tidal pumping from marches and mangrove soils. Indeed, in the mangrove channel, water was supersaturated, with pCO2 values of 976 ± 314 ppmv, while in the main channel, the highest positive pCO2 deviations from conservative mixing (up to +100 ppmv for several hours) occurred at ebb tides. An important finding was that in Sao Francisco, the thermodynamic and biological processes compete with each other for CO2 fluxes both at low salinities where evasion and autotrophy occur and at high salinities where invasion, heterotrophy and tidal pumping occur. Our study suggests that carbonate thermodynamics during mixing is a key process that has been overlooked in estuarine studies, although they can generate important air-water CO 2 exchange and significantly contribute to the carbon budget of estuaries and river plumes .

International audience; Increasing eutrophication of coastal waters generates disturbances in greenhouse gas (GHG) concentrations and emissions to the atmosphere that are still poorly documented, particularly in the tropics. Here, we investigated the concentrations and diffusive fluxes of carbon dioxide (CO2) and methane (CH4) in the urban-dominated Jacarepagua Lagoon Complex (JLC) in Southeastern Brazil. This lagoonal complex receives highly polluted freshwater and shows frequent occurrences of anoxia and hypoxia and dense phytoplankton blooms. Between 2017 and 2018, four spatial surveys were performed (dry and wet conditions), with sampling in the river waters that drain the urban watershed and in the lagoon waters with increasing salinities. Strong oxygen depletion was found in the rivers, associated with extremely high values of partial pressure of CO2 (pCO2; up to 20,417 ppmv) and CH4 concentrations (up to 288,572 nmol L−1). These high GHG concentrations are attributed to organic matter degradation from untreated domestic effluents mediated by aerobic and anaerobic processes, with concomitant production of total alkalinity (TA) and dissolved inorganic carbon (DIC). In the lagoon, GHG concentrations decreased mainly due to dilution with seawater and degassing. In addition, the phytoplankton growth and CH4 oxidation apparently consumed some CO2 and CH4, respectively. TA concentrations showed a marked minimum at salinity of ~20 compared to the two freshwater and marine end members, indicating processes of re-oxidation of inorganic reduced species from the low-salinity region, such as ammonia, iron, and/or sulfides. Diffusive emissions of gases from the entire lagoon ranged from 22 to 48 mmol C m−2 d−1 for CO2 and from 2.2 to 16.5 mmol C m−2 d−1 for CH4. This later value is among the highest documented in coastal waters. In terms of global warming potential (GWP) and CO2 equivalent emissions (CO2-eq), the diffusive emissions of CH4 were higher than those of CO2. These results highlight that highly polluted coastal ecosystems are hotspots of GHG emissions to the atmosphere, which may become increasingly significant in future global carbon budgets.

Estuarine carbonate chemistry predicts that thermodynamic equilibration during the mixing of freshwater with seawater will generate a carbon dioxide (CO2) sink in the case of warm and poorly buffered tropical rivers. The S ̃ao Francisco River estuary has historically become oligotrophic after the construction of a series of hydroelectric dams in its watershed, where organic matter and nutrients are retained. During two cruises in late winter (Aug. 2014) and early summer (Nov. 2015), dissolved inorganic carbon (DIC) and total alkalinity (TA) were found to increase linearly with salinity in the main estuarine channel, where water mixing required half a day, and showed nearly conservative behaviour. In the main channel, the water partial pressure of CO2 (pCO2) recorded at a 1-min frequency followed an asymmetric bell-shaped trend versus salinity, similar to the curve predicted by the thermodynamic conservative mixing of freshwater DIC and TA with seawater DIC and TA. The low (0–3) salinity region was always a source of atmospheric CO2, where despite low chlorophyll concentrations, a pCO2 diurnal change of approxi-mately 60 ppmv suggested the occurrence of photosynthesis in summer. At salinities above 3, undersaturated pCO2 values (down to 225 ppmv in winter and neap tides) and invasion of atmospheric CO2 of 0.38–1.70 mmol mL.

International audience; High-energy tidal beaches are exposed to strong physical forcings. The submarine groundwater discharge (SGD) that occurs in intertidal sandy sediments includes both terrestrial, fresh groundwater flow and seawater recirculation, and plays a significant role in regulating biogeochemical cycles in some coastal zones. In this transition zone between land and sea, complex biogeochemical reactions alter the chemical composition of pore waters that discharge to the coastal ocean. Recent studies highlight that SGD can be a significant source of carbon to the coastal ocean but very few have investigated SGD in high-energy environments. We have characterized the dissolved carbon dynamics in such a high-energy environment (Truc Vert Beach, SW France) through pore water sampling in key compartments of the SGD system. Dissolved organic carbon (DOC), pH, total alkalinity (TA), and the isotopic composition of dissolved inorganic carbon (d 13 C-DIC) were measured in pore waters sampled at regular intervals between 2011 and 2014 in the intertidal zone of the beach, the mixing zone of the subterranean estuary (STE), and the freshwater aquifer upstream from the beach. Results reveal that SGD exports dissolved carbon mostly as DIC to the Aquitaine coast some of which originates from the aerobic respiration of marine organic matter within the beach aquifer. This is highlighted by the opposite spatial trend of DOC, which is consumed, and DIC, which is produced. Saline pore waters expelled from the beach through tidally-driven recirculation of seawater provide about 4400 tons of carbon per year to the coastal zone of the 240 km-long Aquitaine sandy coast. Terrestrial groundwater, characterized by high pCO2 values, is also a significant contributor to the DIC flux to the coastal ocean (16200 tons per year). This flux is abated by CO2 evasion in the upper beach, at the onset of the salinity gradient in the STE, and within the surficial freshwater aquifer along the forest-beach transect below the coastal foredune. Accordingly, the DIC:TA ratio evolves to below 1, suggesting that this SGD increases the buffer capacity of coastal seawater against acidification. This study demonstrates that high-energy beaches are active vectors of DIC from the land to the coastal ocean as well as significant sources of CO2 to the atmosphere, and must therefore be taken into consideration in SGD carbon budgets.

International audience; Tropical estuarine deltas generally present poorly buffered waters in their freshwaters. Carbonate chemistry predicts that mixture of such warm freshwater with seawater will create rapid consumption of the freshwater carbon dioxide (CO2) by the carbonate buffering capacity of the seawater. In this study, we used the Paraiba do Sul River as a laboratory to investigate how thermodynamics compare with biological processes, gas exchange, and tidal advection from mangrove. We conducted three spatial surveys covering the salinity gradient of the main channel and surrounding mangrove waters and one 24-h mooring in a mangrove creek. In the main channel, dissolved inorganic carbon (DIC) and total alkalinity (TA) showed closely conservative distribution along the salinity gradient, increasing seaward. The partial pressure of CO2 (pCO2) followed a bell-shaped curve predicted by carbonate chemistry for conservative mixing of river and ocean endmembers. During high river flow, pCO2 sharply decreased between salinities 0 and 5 (1800 to 390 ppmv), a pCO2 drawdown attributed to riverine outgassing and thermodynamics. Indeed, the mixing of TA-poor freshwater (363 ± 16 μmol kg−1) with TA-rich seawater creates a deficit of dissolved CO2 not related to biotic processes. During low river flow, the entire mixing zone was undersaturated in pCO2 with an increasing trend seaward. However, observed pCO2 values were slightly above those predicted by conservativity. Approximately half of this deviation was attributed to biological activity (net heterotrophy), and remaining deviation was assigned to the effects of gas exchange (18%) and water heating (36%). The effect of gas exchange was higher in fresh and low salinity waters, reflecting the higher outgassing/ingassing of CO2, and lower buffering capacity. Water heating was more important in mid- to high-salinities as a result of diel patterns of solar irradiance. Heterotrophy was slight and not able to outcompete thermodynamics and generate outgassing during estuarine mixing. Consistently, stable isotopic signatures of DIC (δ13C-DIC) presented slight deviations below the conservative mixing, corroborating net heterotrophy in the main channel. Areas of CO2 uptake due to phytoplankton activity were identified but restricted to the freshwater endmember during low river flow, with lowest pCO2 (up to 41 ppmv) and the highest chlorophyll a (up to 21.3 μg L−1). The estuary was a CO2 sink during low river flow (−1.34 to −5.26 mmolC m−2 d−1) and a source during high river flow (5.71 to 19.37 mmolC m−2 d−1). In the mangrove creek, the pCO2, DIC, δ13C-DIC and TA presented deviations from the conservativity, with slopes between TA and DIC demonstrating organic carbon degradation mediated by aerobic respiration and sulphate reduction. Mangrove creek waters were a CO2 source (average of 134.81 mmolC m−2 d−1), exhibiting high values of pCO2 (up to 21,720 ppmv). The results reveal that the low buffering capacity in the main channel of tropical estuarine deltas can be the predominant driver of pCO2, generating CO2 undersaturation along the mixing zone, a process overlooked in estuarine systems. Moreover, air-water CO2 exchange, thermal variability, and biological activities contribute to deviation of the carbonate system from conservative mixing in specific estuarine areas, also modulating pCO2 variability.

Increasing eutrophication of coastal waters generates disturbances in greenhouse gas (GHG) concentrations and emissions to the atmosphere that are still poorly documented, particularly in the tropics. Here, we investigated the concentrations and diffusive fluxes of carbon dioxide (CO

In this paper, we quantify CO2 and N2O emissions from denitrification over the Amazonian wetlands. The study concerns the entire Amazonian wetland ecosystem with a specific focus on three focal locations: the Branco Floodplain, the Madeira Floodplain and the floodplains alongside the Amazon River. We adapted a simple denitrification model to the case of tropical wetlands and forced it by open water surface extent products from the Soil Moisture and Ocean Salinity (SMOS) satellite. A priori model parameters were provided by in situ observations and gauging stations from the HyBAm observatory. Our results show that the denitrification and emissions present a strong cyclic pattern linked to the inundation processes that can be divided into three distinct phases: activation – stabilization – deactivation. We quantify the average yearly denitrification and associated emissions of CO2 and N2O over the entire watershed at 17.8 kgN/ha/yr, 0.37 gC/m2/yr and 0.18 gN/m2/yr respectively. When compared to local observations, it was found that the CO2 emissions accounted for 0.01 % of the integrated ecosystem, which emphasis the fact that minor changes to the land cover may induce strong impacts to the Amazonian carbon budget. Our results are quite consistent with the state of the art global nitrogen models with a positive bias of 28 %. When compared to other wetlands in different pedo-climatic environments we found that the Amazonian wetlands have close emissions of N2O to the tropical Congo wetlands and lower emissions than the tropical and temperate anthropogenic wetlands of the Garonne river, the Rhine river, and south-eastern Asia rice paddies. In summary our paper shows that a data driven approach can be successfully applied to quantify N2O and CO2 fluxes associated with denitrification over the Amazon basin. In the future, the use of higher resolution remote sensing product from sensor fusion or new sensors like the SWOT mission will permit the transposition to other large scale watersheds in tropical environment.

Projections for the next 50 years predict a widespread distribution of hypoxic zones in the open and coastal ocean due to environmental and global changes. The Tidal Garonne River (SW France) has already experienced few episodic hypoxic events. However, predicted future climate and demographic changes suggest that summer hypoxia could become more severe and even permanent near the city of Bordeaux in the next few decades. A 3D model, which couples hydrodynamic, sediment transport, and biogeochemical processes, is applied to assess the impact of factors submitted to global and regional climate changes on oxygenation in the turbidity maximum zone (TMZ) of the Tidal Garonne River during low-discharge periods. The model simulates an intensification of summer hypoxia with an increase in temperature, a decrease in river flow or an increase in the local population, but not with sea level rise, which has a negligible impact on dissolved oxygen. Different scenarios were tested by combining these different factors according to the regional projections for 2050 and 2100. All the simulations showed a trend toward a spatial and temporal extension of summer hypoxia that needs to be considered by local water authorities to impose management strategies to protect the ecosystem.

Dissolved inorganic carbon (DIC), its stable isotope composition (δ13C-DIC) and ancillary parameters of the water column were investigated in a eutrophic tropical marine-dominated estuary surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil). Most negative δ13C-DIC signatures (down to −6.1‰) were found in polluted regions affected by direct sewage discharges where net heterotrophy induces high partial pressure of CO2 (pCO2) and DIC concentrations. Keeling plot was applied to this polluted region and determined the δ13C-DIC sewage signature source of −12.2‰, which is very consistent with isotopic signature found in wastewater treatment plans. These negative δ13C-DIC signatures (i.e., DIC depleted in 13C) were restricted to the vicinity of urban outlets, whereas in the largest area of the bay δ13C-DIC signatures were more positive (i.e., DIC enriched in 13C). The most positive δ13C-DIC signatures (up to 4.6‰) were found in surface waters dominated by large phytoplankton blooms, with positive correlation with chlorophyll a (Chl a). In the largest area of the bay, the preferential uptake of the lighter stable carbon isotope (12C) during photosynthesis followed the Rayleigh distillation, and appeared as the most important driver of δ13C-DIC variations. This reveals an important isotopic fractionation (e) by phytoplankton due to successive algal blooms that has turned the remaining DIC pool enriched with the heavier stable carbon isotope (13C). The calculated diel apparent e showed higher values in the morning (18.7‰–21.6‰) and decreasing in the afternoon (6.8‰–11.1‰). e was positively correlated to the pCO2 (R2 = 0.88, p = 0.005) and DIC concentrations (R2 = 0.73, p = 0.02), suggesting a decline in carbon assimilation efficiency and decreasing uptake of the lighter carbon under CO2 limiting conditions. The eutrophic coastal waters of Guanabara Bay have δ13C-DIC signatures well above that found in estuaries, shelf and ocean waters worldwide.

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O-2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3-; on average 1140 +/- 485 mu mol L-1) and low methane (CH4; 40 +/- 25 nmol L-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3- (25 +/- 40 mu mol L-1) and higher CH4 (1770 +/- 1830 nmol L-1) concentrations. The partial pressure of carbon dioxide (pCO(2); 30,650 +/- 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 +/- 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO(2) was not significantly different in crop (4480 +/- 2680 ppmv) and forest (4900 +/- 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3- concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O-2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO(2) was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

International audience; In coastal environments, oscillating redox conditions represent a functional state affecting organic matter mineralization. Such transient diagenetic processes remain difficult to study in situ, and we therefore designed a specific reactor to provide experimental results that are environmentally relevant in this context. Here, we present the results of two independent experiments carried out with sediment having contrasting Fe, Mn contents, collected from a coastal tidal lagoon (the Arcachon bay) and a mesotidal estuary (Adour river). Sediment and overlying water were mixed to form slurries that were submitted to redox oscillations to assess the diagenetic mechanisms that affect N, P, Fe, Mn, and S. Changing from anoxic to oxic conditions, we observed a rapid oxidation of dissolved Fe(II) and dissolved inorganic phosphorus (DIP) was apparently trapped by the newly formed Fe-oxyhydroxides (Fe-ox). DIP was totally titrated in the coastal lagoon sediment, but not in estuarine sediment, where the initial amount of Fe available was lower. In both experiments, Mn(II) was only slowly oxidized during the oxidation events and a major part of Mn(II) was adsorbed on new Fe-ox. In coastal lagoon sediment, ammonium remained constant in oxic conditions while nitrate was produced from organic-N mineralization. On the contrary, in estuarine sediment, ammonium was quantitatively oxidized to nitrate. When the conditions became anoxic again, direct reduction of nitrate to ammonium occurred in coastal lagoon sediment. Anaerobic production of nitrate occurred in estuarine sediment, probably because Mn-oxides (Mn-ox), which had a high concentration, acted as an oxidant for ammonium. Consequently, nitrate production prevented Fe(II) accumulation. The Mn-N-Fe coupling outlined here is an apparent indirect oxidation of Fe(II) by Mn-ox through anaerobic nitrification (with Mn-ox) and denitrification (with Fe-ox). This coupling also implied P availability because of the strong control of P by Fe. These experimental results show that nutrient dynamics in oscillatory redox environments such as the estuarine turbidity zone, bioturbated sediment, or tidal permeable sediments highly depends on Mn-and Fe-ox availability.

In view of future coastal hypoxia widespreading, it is essential to define management solutions to preserve a good quality of coastal ecosystems. The lower Tidal Garonne River (TGR, SW France), characterized by the seasonal presence of a turbidity maximum zone and urban water discharges, is subject to episodic hypoxia events during summer low river flow periods. The future climatic conditions (higher temperature; summer droughts) but also an increasing urbanization could enhance hypoxia risks near the city of Bordeaux in the next decades. A 3D model of dissolved oxygen (DO), which couples hydrodynamics, sediment transport and biogeochemical processes, is used to assess the efficiency of different management solutions on TGR oxygenation during summer low-discharge periods. We have runned different scenarios of reduction of urban sewage overflows, displacement of urban discharges downstream from Bordeaux, and/or temporary river flow support during summer period. The model shows that each option limits hypoxia, but with variable efficiency over time and space. Sewage overflow reduction improves DO levels only locally near the city of Bordeaux. Downstream relocation of wastewater discharges allows to reach better oxygenation level in the lower TGR. The support of low river flow limits the upstream TMZ propagation and dilutes TGR waters with well-oxygenated river waters. Scenarios combining wastewater network management and low water replenishment indicate an improvement in water quality over the entire TGR. These modelling outcomes constitute important tools for local water authorities to develop the most appropriate strategies to limit hypoxia in TGR.

International audience; We conducted diurnal sampling in a tidal creek (Ras Dege, Tanzania) to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, followed the tidal variations and was highest at low tide. Stable isotope data of POC and DOC exhibit large variations in both pools, and followed tidal variations. Although the variation of d13CDOC (-23.8 to -13.8‰) was higher than that of d13CPOC (-26.2 to -20.5‰) due to the different end-member pool sizes, the d13C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, on the other hand, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH4, and partial pressure of CO2 (pCO2) were similarly governed by tidal influence and were up to 7- and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. Furthermore, this illustrates that constraining an ecosystem-level budget of these greenhouse gases in tidal systems requires a careful appraisal of tidal variations. When assuming that the high dissolved inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, d13C data on DIC indicate that the source of the mineralized organic matter has a signature of -22.4‰, which shows that imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data show a striking example of how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter exchange are fundamentally different. The estimated export of DIC through porewater exchange appears considerably larger than for DOC, suggesting that if this mechanism is indeed a major driver of solute exchange, benthic mineralization and subsequent export as DIC could represent a very significant and previously unaccounted fate of mangrove-derived C. Budgeting efforts should therefore pay attention to understanding the mechanisms and quantification of different pathways of exchange within and between both zones.

At the global scale, inland waters are a significant source of atmospheric carbon (C), particularly in the tropics. The active pipe concept predicts that C emissions from streams, lakes and rivers are largely fuelled by terrestrial ecosystems. The traditionally recognized C transfer mechanisms from terrestrial to aquatic systems are surface runoff and groundwater drainage. We present here a series of arguments that support the idea that land flooding is an additional significant process that fuels inland waters with C at the global scale. Whether the majority of CO2 emitted by rivers comes from floodable land (approximately 10 % of the continents) or from well-drained land is a fundamental question that impacts our capacity to predict how these C fluxes might change in the future. Using classical concepts in ecology, we propose, as a necessary step forward, an update of the active pipe concept that differentiates floodable land from drained land. Contrarily to well-drained land, wetlands combine strong hydrological connectivity with inland waters, high productivity assimilating CO2 from the atmosphere, direct transfer of litter and exudation products to water and waterlogged soils, a generally dominant allocation of ecosystem respiration below the water surface and a slow gas exchange rate at the water-air interface. These properties force plants to pump atmospheric C to wetland waters and, when hydrology is favourable, to inland waters as organic C and dissolved CO2. This wetland CO2 pump may contribute disproportionately to CO2 emissions from inland waters, particularly in the tropics, and consequently at the global scale. In future studies, more care must be taken in the way that vertical and horizontal C fluxes are conceptualized along watersheds and 2D-models that adequately account for the hydrological export of all C species are necessary. In wetland ecosystems, significant effort should be dedicated to quantifying the components of primary production and respiration in air, water and waterlogged soils, and these metabolic rates should be used in coupled hydrological-biogeochemical models. The construction of a global typology of wetlands also appears necessary to adequately integrate continental C fluxes at the global scale.

We report a data set of biogeochemical variables related to carbon cycling obtained in the three branches (M y Tho, Ham Luong, C o ^ ´ Chien) of the Mekong delta (B e ^ ´ n Tre province, Vietnam) in December 2003, April 2004, and October 2004. Both the inner estuary (upstream of the mouth) and the outer estuary (river plume) were sampled, as well as side channels. The values of the partial pressure of CO 2 (pCO 2 ) ranged between 232 and 4085 ppm, O 2 saturation level (%O 2) between 63 and 114 %, and CH 4 between 2 and 2217 nmol L −1 , within the ranges of values previously reported in temperate and tropical meso- and macro-tidal estuaries. Strong seasonal variations were observed. In the upper oligohaline estuary, low pCO 2 (479–753 ppm) and high %O 2 (98–106 %) values were observed in April 2004 most probably related to freshwater phytoplankton growth owing to low freshwater discharge (1400 m 3 s −1) and increase in water residence time; during the two other sampling periods with a higher freshwater discharge (9300–17 900 m 3 s −1) , higher pCO 2 (1895–2664 ppm) and lower %O 2 (69–84 %) values were observed in the oligohaline part of the estuary. In October 2004, important phytoplankton growth occurred in the offshore part of the river plume as attested by changes in the contribution of particulate organic carbon (POC) to total suspended matter (TSM) (%POC) and the stable isotope composition of POC ( δ13 C-POC), possibly related to low TSM values (improvement of light conditions for phytoplankton development), leading to low pCO 2 (232 ppm) and high %O 2 (114 %) values. Water in the side channels in the Mekong delta was strongly impacted by inputs from the extensive shrimp farming ponds. The values of pCO 2 , CH 4 , %O 2 , and the stable isotope composition of dissolved inorganic carbon ( δ13 C-DIC) indicated intense organic matter degradation that was partly mediated by sulfate reduction in sediments, as revealed by the slope of total alkalinity (TA) and DIC covariations. The δ13 C-POC variations also indicated intense phytoplankton growth in the side channels, presumably due to nutrient enrichment related to the shrimp farming ponds. A data set in the mangrove creeks of the Ca Mau province (part of the Mekong delta) was also acquired in April and October 2004. These data extended the range of variability in pCO 2 and %O 2 with more extreme values than in the Mekong delta (B e ^ ´ n Tre), with maxima and minima of 6912 ppm and 37 %, respectively. Similarly, the maximum CH 4 concentration (686 nmol L −1) was higher in the Ca Mau province mangrove creeks than in the Mekong delta (B e ^ ´ n Tre, maximum 222 nmol L −1) during the October 2004 cruise (rainy season and high freshwater discharge period). In April 2004 (dry season and low freshwater discharge period), the CH 4 values were much lower than in October 2004 (average 19 ± 13 and 210 ± 158 nmol L −1 , respectively) in the Ca Mau province mangrove creeks, owing to the higher salinity (average 33.2 ± 0.6 and 14.1 ± 1.2, respectively) that probably led to higher sediment sulfate reduction, leading to inhibition of sediment methanogenesis and higher anaerobic CH 4 oxidation. In the inner estuarine region (three branches of the Mekong delta), CO 2 emissions to the atmosphere averaged 121 mmol m −2 d −1 , and the CH 4 emissions averaged 118 µ mol m −2 d −1 . The CO 2 emission to the atmosphere from the Mekong inner estuary was higher than reported in the Yangtze and Pearl river inner estuaries. This was probably due to the lower salinity in the Mekong delta branches, possibly due to different morphology: relatively linear channels in the Mekong delta versus funnel-shaped estuaries for the Yangtze and Pearl river inner estuaries.

Streams and rivers emit significant amounts of CO2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO2 degassing based on the water-air CO2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (delta C-13-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on DIC concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO(2) and DIC and an increase in delta C-13-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater C-12-DIC compared to C-13-DIC. This downstream enrichment in C-13-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of C-13 in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO2 concentrations, we quantified CO2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2-4, our CO2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO(2), gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.

Given the relative scarcity of stable isotope data on dissolved organic carbon (DOC) in the Amazon Basin, we hypothesized that the variability in DOC sources may be underestimated in such major river basins. To explore the links between the mainstem and tributaries and the floodplain, particular efforts were made during five distinct cruises at different stages of the hydrograph between October 2008 and January 2011, to document the spatial and temporal variation of DOC concentrations and delta C-13-DOC in the central Amazon River system (Brazil). Based on more than 200 data, the spatial and temporal variability of delta C-13-DOC values was found to be larger than previously reported in the same area. Although a small range of variation was observed throughout the hydrological cycle in the upper reach of the studied section (-29.2 to -29.5 parts per thousand in the Rio Negro and -28.7 to -29.0 parts per thousand in the Rio Solimoes), a much larger one (-28.0 to -34.6 parts per thousand) was found in the lower reach of the river, as the proportion of open lakes increased downstream in the floodplains. The low variability in the upper reaches suggests constant and homogeneous DOC sources from upland soils and flooded forest, while lower delta C-13-DOC values recorded in the lower reach mainstem at high and falling waters can be attributed to a greater export of plankton-derived C-13-depleted DOC from flooded lakes. Noteworthy are the higher delta C-13-DOC values measured in the Rio Madeira and the associated flooded lakes (-26.5 to -28.8 parts per thousand, which may reflect the imprint from upland headwaters and a weaker density of flooded forest in the watershed. The higher delta C-13-DOC values observed in the lower reach during low waters are still not fully understood. Floating meadows principally consisting of C-4 macrophytes were found to increase delta C-13-DOC values by similar to 1.5 parts per thousand in their vicinity, but this impact was no longer noticeable at distances of similar to 10 m from the plant rafts. This rather modest C-13-enrichment suggests rapid decomposition and/or dilution of this wetland-derived DOC.

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO2 undersaturation, and DOC reached 82 mg L−1, POC 152 mg L−1, and Chl a 800 μg L−1. Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m−2 year−1, representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.

In contrast to its small surface area, the coastal zone plays a disproportionate role in the global carbon cycle. Carbon production, transformation, emission and burial rates at the land–ocean interface are significant at the global scale but still poorly known, especially in tropical regions. Surface water pCO2 and ancillary parameters were monitored during nine field campaigns between April 2013 and April 2014 in Guanabara Bay, a tropical eutrophic to hypertrophic semi-enclosed estuarine embayment surrounded by the city of Rio de Janeiro, southeast Brazil. Water pCO2 varied between 22 and 3715 ppmv in the bay, showing spatial, diurnal and seasonal trends that mirrored those of dissolved oxygen (DO) and chlorophyll a (Chl a). Marked pCO2 undersaturation was prevalent in the shallow, confined and thermally stratified waters of the upper bay, whereas pCO2 oversaturation was restricted to sites close to the small river mouths and small sewage channels, which covered only 10 % of the bay's area. Substantial daily variations in pCO2 (up to 395 ppmv between dawn and dusk) were also registered and could be integrated temporally and spatially for the establishment of net diurnal, seasonal and annual CO2 fluxes. In contrast to other estuaries worldwide, Guanabara Bay behaved as a net sink of atmospheric CO2, a property enhanced by the concomitant effects of strong radiation intensity, thermal stratification, and high availability of nutrients, which promotes phytoplankton development and net autotrophy. The calculated CO2 fluxes for Guanabara Bay ranged between −9.6 and −18.3 mol C m−2 yr−1, of the same order of magnitude as the organic carbon burial and organic carbon inputs from the watershed. The positive and high net community production (52.1 mol C m−2 yr−1) confirms the high carbon production in the bay. This autotrophic metabolism is apparently enhanced by eutrophication. Our results show that global CO2 budgetary assertions still lack information on tropical, marine-dominated estuarine systems, which are affected by thermal stratification and eutrophication and behave specifically with respect to atmospheric CO2.

Abundant research has been devoted to understanding the complexity of the biogeochemical and physical processes that are responsible for greenhouse gas (GHG) emissions from hydropower reservoirs. These systems may have spatially complex and heterogeneous GHG emissions due to flooded biomass, river inflows, primary production and dam operation. In this study, we investigated the relationships between the water–air CO2 fluxes and the phytoplanktonic biomass in the Funil Reservoir, which is an old, stratified tropical reservoir that exhibits intense phytoplankton blooms and a low partial pressure of CO2 (pCO2). Our results indicated that the seasonal and spatial variability of chlorophyll concentrations (Chl) and pCO2 in the Funil Reservoir are related more to changes in the river inflow over the year than to environmental factors such as air temperature and solar radiation. Field data and hydro\\-dynamic simulations revealed that river inflow contributes to increased heterogeneity during the dry season due to variations in the reservoir retention time and river temperature. Contradictory conclusions could be drawn if only temporal data collected near the dam were considered without spatial data to represent CO2 fluxes throughout the reservoir. During periods of high retention, the average CO2 fluxes were 10.3 mmol m−2 d−1 based on temporal data near the dam versus −7.2 mmol m−2 d−1 with spatial data from along the reservoir surface. In this case, the use of solely temporal data to calculate CO2 fluxes results in the reservoir acting as a CO2 source rather than a sink. This finding suggests that the lack of spatial data in reservoir C budget calculations can affect regional and global estimates. Our results support the idea that the Funil Reservoir is a dynamic system where the hydrodynamics represented by changes in the river inflow and retention time are potentially a more important force driving both the Chl and pCO2 spatial variability than the in-system ecological factors.

Inland waters have been recognized as a significant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water–air gradient of the partial pressure of CO2 (pCO2). Currently, direct measurements of water pCO2 remain scarce in freshwaters, and most published pCO2 data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO2 in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 μmol L−1), pH (3.94 to 9.17), measured pCO2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L−1). Calculated pCO2 were >10% higher than measured pCO2 in 60% of the samples (with a median overestimation of calculated pCO2 compared to measured pCO2 of 2560 ppmv) and were >100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO2 with respect to measured pCO2 are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO2 to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO2 for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO2 are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.

We report a data-set obtained in the three branches (My Tho, Ham Luong, Co Chien) of the Mekong delta (Bến Tre province, Vietnam) in December 2003, April 2004, and October 2004, of biogeochemical variables related to carbon cycling (pH, total alkalinity (TA), O 2 saturation level (%O 2 ), calculated partial pressure of CO 2 (pCO 2 ), dissolved CH 4 concentration, particulate (POC) and dissolved (DOC) organic carbon concentration and stable isotope composition (δ 13 C-POC, δ 13 C-DOC), particulate nitrogen (PN), dissolved inorganic carbon (DIC) stable isotope composition (δ 13 C-DIC), total suspended matter (TSM)). Both the inner estuary (upstream of the mouth) and the outer estuary (river plume) were sampled, as well as side channels. The values of pCO 2 ranged between 232 and 4,085 ppm, %O 2 between 63 and 114 %, and CH 4 between 2 and 2,217 nmol L −1 , within the ranges of values previously reported in temperate and tropical macro-tidal estuaries. Strong seasonal variations were observed. In the upper oligohaline estuary, low pCO 2 (479–753 ppm) and high %O 2 (98–106 %) values were observed in April 2004 most probably related to freshwater phytoplankton growth owing to low freshwater discharge (1,400 m 3 s −1 ) and increase of water residence time; during the two other sampling periods with a higher freshwater discharge (9,300-17,900 m 3 s −1 ), higher pCO 2 (1,895–2,664 ppm) and lower %O 2 (69–84 %) values were observed in the oligohaline part of the estuary. During the October 2004 sampling, important phytoplankton growth occurred in the off-shore part of the river plume as attested by changes in the contribution of POC to TSM (%POC), δ 13 C-POC, POC : PN ratios, possibly related to low TSM values (improvement of light conditions for phytoplankton development), leading to low pCO 2 (232 ppm) and high %O 2 (114 %) values. Water in the side channels in the Mekong delta was strongly impacted by inputs from the extensive shrimp farming ponds. The values of pCO 2 , CH 4 , %O 2 , δ 13 C-DIC indicated intense organic matter degradation that was partly mediated by sulfate reduction (presumably in sediments), as indicated by the slope of TA and DIC co-variations. The δ 13 C-POC variations also indicated intense phytoplankton growth in the side channels, presumably due to nutrient enrichment related to the shrimp farming ponds. A dataset in the mangrove creeks of the Ca Mau province (part of the Mekong delta) was also acquired in April 2004 and October 2004. These data extended the range of variability of pCO 2 and %O 2 with more extreme values than in the Mekong delta (Bến Tre), with maxima and minima of 6,912 ppm and 37 %, respectively. Similarly, the maximum CH 4 concentration (686 nmol L −1 ) was higher in the Ca Mau province mangrove creeks than in the Mekong delta (Bến Tre, maximum 222 nmol L −1 ), during the October 2004 cruise (rainy season and high freshwater discharge period). In April 2004 (dry season and low freshwater discharge period), the CH 4 values were much lower than in October 2004 (average 19 ± 13 and 210 ± 158 nmol L −1 , respectively) in the Ca Mau province mangrove creeks, owing to the higher salinity (average 33.2 ± 0.6 and 14.1 ± 1.2, respectively) that probably led to higher sediment sulfate reduction, leading to inhibition of sediment methanogenesis and higher anaerobic CH 4 oxidation. In the inner estuarine region (three branches of the Mekong delta), CO 2 emissions to the atmosphere averaged 121 mmol m −2 d −1 , and the CH 4 emissions averaged 118 µmol m −2 d −1 . The CO 2 emission to the atmosphere from the Mekong inner estuary was higher than reported in the Yangtze and Pearl River inner estuaries. This was probably due to the lower salinity in the Mekong delta branches, possibly due to different morphology; relatively linear channels in the Mekong delta versus funnel-shaped estuaries for the Yangtze and Pearl River inner estuaries.

Estuaries are increasingly degraded due to coastal urban development and are prone to hypoxia problems. The macro-tidal Gironde Estuary is characterized by a highly concentrated turbidity maximum zone (TMZ). Field observations show that hypoxia occurs in summer in the TMZ at low river flow and a few days after the spring tide peak. In situ data highlight lower dissolved oxygen (DO) concentrations around the city of Bordeaux, located in the upper estuary. Interactions between multiple factors limit the understanding of the processes controlling the dynamics of hypoxia. A 3D biogeochemical model was developed, coupled with hydrodynamics and a sediment transport model, to assess the contribution of the TMZ and the impact of urban effluents through wastewater treatment plants (WWTPs) and sewage overflows (SOs) on hypoxia. Our model describes the transport of solutes and suspended material and the biogeochemical mechanisms impacting oxygen: primary production, degradation of all organic matter (i.e. including phytoplankton respiration, degradation of river and urban watershed matter), nitrification and gas exchange. The composition and the degradation rates of each variable were characterized by in situ measurements and experimental data from the study area. The DO model was validated against observations in Bordeaux City. The simulated DO concentrations show good agreement with field observations and satisfactorily reproduce the seasonal and neap-spring time scale variations around the city of Bordeaux. Simulations show a spatial and temporal correlation between the formation of summer hypoxia and the location of the TMZ, with minimum DO centered in the vicinity of Bordeaux. To understand the contribution of the urban watershed forcing, different simulations with the presence or absence of urban effluents were compared. Our results show that in summer, a reduction of POC from SO would increase the DO minimum in the vicinity of Bordeaux by 3% of saturation. Omitting discharge from SO and WWTPs, DO would improve by 10% of saturation and mitigate hypoxic events.

Export of soil carbon to superficial water through the drainage of groundwater is a significant but poorly documented component of the continental carbon budget. We monitored the concentrations of dissolved organic and inorganic carbon (DOC and DIC) in groundwaters and first order streams of a small temperate, forested and sandy watershed where hydrology occurs exclusively through drainage (no surface runoff). The studied watershed was also implemented for continuous measurements of groundwater table, precipitation, evapotranspiration, river discharge, and net ecosystem exchanges of sensible and latent heat fluxes as well as CO2. On a monthly basis, we found a good consistency between precipitation and the sum of evapotranspiration, drainage and groundwater storage. DOC and DIC temporary storage in groundwater and export to streams varied drastically during the hydrological cycle, the residence times of these two carbon forms varying from one month to several years. DOC concentrations in groundwater and streams were maximal at high water table and high stream discharge, when the water table reached the superficial organic rich layer of the soil. A large fraction of this winter DOC maximum was temporarily stored and further mineralized to DIC in the groundwater and only about 15 % was exported to streams during winter periods. In contrast, DIC, which was present in majority in the form of dissolved CO2 in groundwater and streams, was apparently diluted at high water table: DIC concentrations were maximum at low water table and low discharge in late summer and maximum pCO2 in groundwater corresponded to the late summer period of heterotrophic conditions (i.e., Reco > GPP). Groundwater DIC peaked in late summer and was followed by a rapid loss of excess CO2 from stream surface to the atmosphere. Overall, mean carbon export was 7.5 g C m−2 yr−1 (50 % as DOC and 50 % as DIC) and represented only 1.5 % of the NEE. About 65 % of the DIC exported from groundwaters returned to the atmosphere in the form of CO2 in first order streams.

The dynamics of the aragonite saturation state (Ωarag) were investigated in the eutrophic coastal waters of Guanabara Bay (RJ-Brazil). Large phytoplankton blooms stimulated by a high nutrient enrichment promoted the production of organic matter with strong uptake of dissolved inorganic carbon (DIC) in surface waters, lowering the concentrations of dissolved carbon dioxide (CO2aq), and increasing the pH, Ωarag and carbonate ion (CO32 −), especially during summer. The increase of Ωarag related to biological activity was also evident comparing the negative relationship between the Ωarag and the apparent utilization of oxygen (AOU), with a very close behavior between the slopes of the linear regression and the Redfield ratio. The lowest values of Ωarag were found at low-buffered waters in regions that receive direct discharges from domestic effluents and polluted rivers, with episodic evidences of corrosive waters (Ωarag < 1). This study showed that the eutrophication controlled the variations of Ωarag in Guanabara Bay.

Amazonian hydroelectric reservoirs produce abundant carbon dioxide and methane from large quantities of flooded biomass that decompose anaerobically underwater. Emissions are extreme the first years after impounding and progressively decrease with time. To date, only water-to-air fluxes have been considered in these estimates. Here, we investigate in two Amazonian reservoirs (Balbina and Petit Saut) the fate of above water standing dead trees, by combining a qualitative analysis of wood state and density through time and a quantitative analysis of the biomass initially flooded. Dead wood was much more decomposed in the Balbina reservoir 23 years after flooding than in the Petit Saut reservoir 10 years after flooding. Termites apparently played a major role in wood decomposition, occurring mainly above water, and resulting in a complete conversion of this carbon biomass into CO 2 and CH 4 at a timescale much shorter than reservoir operation. The analysis of pre-impounding wood biomass reveals that above-water decomposition in Amazonian reservoirs is a large, previously unrecognized source of carbon emissions to the atmosphere, representing 26–45% of the total reservoir flux integrated over 100 years. Accounting for both below- and above-water fluxes, we could estimate that each km 2 of Amazonian forest converted to reservoir would emit over 140 Gg CO 2 -eq in 100 years. Hydropower plants in the Amazon should thus generate 0.25–0.4 MW h per km 2 flooded area to produce lower greenhouse gas emissions than gas power plants. They also have the disadvantage to emit most of their greenhouse gases the earliest years of operation.

Carbon dioxide (CO2) fluxes from aquatic systems are generally derived from the gradient in the partial pressure of CO2 (pCO2) between air and surface waters. In this study, we compare real-time measurements of water pCO2 using an equilibrator and non-dispersive infrared gas detector, with calculations based on pH and total alkalinity (TA) in two contrasting Brazilian estuaries: Guanabara Bay (Rio de Janeiro) and the São Francisco River Estuary (Alagoas). In Guanabara Bay, the measured and calculated values showed an excellent agreement (R2 = 0.95, p < 0.0001), without significant statistical differences between the two methods. In the São Francisco River Estuary, where the entire gradient from freshwaters to seawater could be sampled, important overestimates were found for the calculated pCO2. The overestimation was on average 71%, and reached up to 737%. This large bias in pCO2 calculation was verified at low pH and TA concentrations in freshwaters (pH < 7.5; TA < 700 µmol kg-1) possibly due to the contribution of organic alkalinity, lowering the buffer capacity of the carbonate system. As such, direct measurements of pCO2 should be considered as a priority for CO2 studies conducted in estuarine systems, particularly tropical systems where physical and biological processes are prone to significant spatial and temporal variability.

The central Amazon floodplains are particularly productive ecosystems, where a large diversity of organic carbon sources are available for aquatic organisms. Despite the fact that C-4 macrophytes generally produce larger biomasses than C-3 macrophytes, food webs in the central Amazon floodplains appear dominantly based on a C-3 carbon source. In order to investigate the respective fate and degradation patterns of C-4 and C-3 aquatic plant-derived material in central Amazon floodplains, we developed a 23-days batch experiment. Fatty acid and carbon concentrations as well as stable isotope compositions were monitored over time in 60 L tanks. These tanks contained Amazon water, with different biomasses of C-3 and C-4 macrophyte, representative of in situ densities occurring in central Amazon floodplains. In the C-4 Paspalum repens treatments, organic (POC, DOC) and inorganic carbon (DIC) got rapidly enriched in C-13, whereas in the C-3 Salvinia auriculata treatments, POC and DOC showed little change in concentration and isotopic composition, and DIC got depleted in C-13. The contribution of P. repens to POC and DOC was estimated to reach up to 94.2 and 70.7%, respectively. In contrast, no differences were reported between the C-3 S. auriculata and control treatments, an observation attributed to the lower C-3 biomass encountered in the field, to a slower degradation rate of C-3 compared to C-4 compounds, and to similar isotopic compositions for river POC and DOC, and C-3 compounds. The C-13 enrichments of POC, DOC, and DIC from P. repens treatments were attributed to an enhanced bacterially-mediated hydrolysis and mineralization of C-4 material. Evolutions of bacterial abundance and branched fatty acid concentrations confirmed the role of heterotrophic microbial communities in the high P. repens decomposition rate. Our experiment highlights the predominant role of C-4 aquatic plants, as a large source of almost entirely biodegradable organic matter available for heterotrophic activity and CO2 outgassing to the atmosphere.

International audience; Lake Pavin is a meromictic maar lake located in the French Massif Central. Its maximum depth is 92 m and its mean diameter is 750 m (0.445 km2). Waters below about 60 m are never mixed with overlying waters and are permanently anoxic. DIC (Dissolved Inorganic Carbon) and CH4 concentrations in the deep monimolimnion are 14 and 4 mmol/L respectively. DIC has two origins: biogenic and volcanic.Data on C species, 13C and 14C isotopes as well as auxiliary data were used to build a carbon mass balance in the lake. Main features of the carbon cycle are: 1. Between 70 and 92 m depth, DIC and methane fluxes coming from the sediment are estimated to 1.5 and 1.0 kmol d−1 respectively. 2. At about 65–70 m depth, a PM14C (percent of modern carbon) minimum suggests the inflow of a mineral water which brought DIC=2.6±0.2 kmol d−1. 3.As suggested by a minimum of δ13C of DIC and very low CH4 concentration at ca. 60 m depth, methane is almost quantitatively transformed mainly into DIC at the redoxcline. The methane escape toward the atmosphere is very low and negligible in the carbon mass balance. 4.The δ13C DIC, DIC vs. O2 plots and some thermal and dissolved oxygen anomalies suggest a fresh water inflow at about 53 m depth; the corresponding input of DIC is estimated to 1.6±0.2 kmol d−1. 5.Between 20 and 50 m depth, mineralization of organic matter produced in the photic zone occurs partially. From a δ13C versus 1/DIC plot, a production of DIC of 0.3±0.2 kmol d−1 can be derived. 6. The maximum of photosynthesis is located within the metalimnion (10–15 m depth). A mean value for the corresponding DIC uptake of 3.5±0.5 kmol d−1 is assumed. During seasonal stratification of the mixolimnion (about April to December), water deeper than 10–12 m are isolated from the atmosphere and superficial inputs as shown by the low PM14C value of 45 % at 10 m depth. 7. In the superficial layer, an input of 1.3±0.2 kmol d−1 of DIC is brought by several small brooks, and an output of 3.0±0.2 kmol d−1 is calculated for the discharge. 8.CO2 exchanges with atmosphere are highly variable seasonally; the lake is strongly under saturated in summer and oversaturated in November. An annual balance is derived from modeling the global lake functioning, leading to a CO2 escape of 4±1 kmol d−1. Budgets for DIC and alkalinity (alk) are proposed. For this latter, we need to distinguish two kinds of dissolved species: “conservative” ions (e.g. Na+, K+, Ca2+, Mg2+, Cl−) brought by streams and other water venues “reactive” species (e.g. Fe2+, NH4+, H2PO4−), essentially produced within the sediment and consumed or precipitated at ca. 60 m depth i.e. at the redox interface.CO2 behaviour is deduced from both DIC and Alk budgets. The dissolved concentration of CO2 in the monimolimnion, associated with those of CH4 and N2, allow assessing the gas outburst natural hazard, which is very low considering the actual conditions.

In this study, we investigated the seasonal and spatial pattern of sedimentary organic matter (SOM) in five floodplain lakes of the central Amazon basin (Cabaliana, Janauaca, Canaçari, Miratuba, and Curuai) which have different morphologies, hydrodynamics and vegetation coverages. Surface sediments were collected in four hydrological seasons: low water (LW), rising water (RW), high water (HW) and falling water (FW) in 2009 and 2010. We investigated commonly used bulk geochemical tracers such as the C : N ratio and the stable isotopic composition of organic carbon (δ13Corg). These results were compared with lignin-phenol parameters as an indicator of vascular plant detritus and branched glycerol dialkyl glycerol tetraethers (brGDGTs) to trace the input of soil organic matter (OM) from land to the aquatic settings. We also applied the isoprenoid GDGT (iGDGT) crenarchaeol as an indicator of riverine suspended particulate organic matter (SPOM). Our data showed that during the RW and FW seasons, the surface sediments were enriched in lignin and brGDGTs in comparison to other seasons. Our study also indicated that floodplain lake sediments primarily consisted of allochthonous, C3 plant-derived OM. However, a downstream increase in C4 macrophyte derived OM contribution was observed along the gradient of increasing open waters, i.e. from upstream to downstream. Accordingly, we attribute temporal and spatial difference in SOM composition to the hydrological dynamics between the floodplain lakes and the surrounding flooded forests.

International audience; 1. Eutrophication increases primary productivity and favours the predominance of floating vegetation in wetlands. Carbon (C) fluxes in wetlands are strongly driven by primary productivity and can differ by vegetation type. However, to the best of our knowledge, the role of eutrophication in C fluxes has rarely been assessed. 2. Consequently, we aimed to measure the seasonal variation in carbon dioxide (CO2) and methane (CH4) fluxes at six aquatic sites in four temperate wetlands, ranging along a gradient of sediment total phosphorus content, and determine whether C fluxes correlate with above-ground net primary productivity (ANPP) and plant community composition along this eutrophication gradient. 3. Daytime CO2 emissions were significantly and negatively correlated with wetland net primary productivity as a result of the greater C fixation by photosynthesis during the peak of production. Conversely, CH4 emissions were significantly and positively correlated with wetland ANPP, possibly due to higher litter production and anaerobic decomposition. 4. The highest CH4 emissions were observed above floating vegetation, which favoured hypoxic conditions in the water column. CH4 emissions including ebullition were higher above macroalgal belts than above vascular plants with floating leaves. CH4 emissions without ebullition (i.e. resulting from plant transport and diffusion) better correlated with the abundance of macroalgae than with the abundance of vascular plants with floating leaves. 5. Our results suggest that eutrophication may greatly modify CO2 and CH4 emissions from wetlands through changes in vegetation type and productivity.

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO2 across the water-atmosphere interface were estimated to assess the relative importance of CO2 evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8. 4 ± 3. 4 and 1. 6 ± 0. 6 mg L-1, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1. 3 ± 1. 4 μg L-1) and the relatively constant δ13C-POC value (near -28 ‰) throughout the year reveal this POC originates from terrestrial C3 plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6. 6 ± 2. 2 mg L-1). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5. 9 ± 2. 2 mg L-1). The δ13C-DIC value around -20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO2 between 1,000 and 10,000 ppmv, all watercourses were a source of CO2 to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year-1 or 6 t C km-2 year-1, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO2 degassing to the atmosphere (34 %). © 2012 Springer Basel AG. ispartof: Aquatic Sciences vol:75 issue:2 pages:299-319 status: published

Globally, terrestrial ecosystems have absorbed about 30% of anthropogenic greenhouse gas emissions over the period 2000–2007 and inter-hemispheric gradients indicate that a significant fraction of terrestrial carbon sequestration must be north of the Equator. We present a compilation of the CO2, CO, CH4 and N2O balances of Europe following a dual constraint approach in which (1) a land-based balance derived mainly from ecosystem carbon inventories and (2) a land-based balance derived from flux measurements are compared to (3) the atmospheric data-based balance derived from inversions constrained by measurements of atmospheric GHG (greenhouse gas) concentrations. Good agreement between the GHG balances based on fluxes (1294 ± 545 Tg C in CO2-eq yr−1), inventories (1299 ± 200 Tg C in CO2-eq yr−1) and inversions (1210 ± 405 Tg C in CO2-eq yr−1) increases our confidence that the processes underlying the European GHG budget are well understood and reasonably sampled. However, the uncertainty remains large and largely lacks formal estimates. Given that European net land to atmosphere exchanges are determined by a few dominant fluxes, the uncertainty of these key components needs to be formally estimated before efforts could be made to reduce the overall uncertainty. The net land-to-atmosphere flux is a net source for CO2, CO, CH4 and N2O, because the anthropogenic emissions by far exceed the biogenic sink strength. The dual-constraint approach confirmed that the European biogenic sink removes as much as 205 ± 72 Tg C yr−1 from fossil fuel burning from the atmosphere. However, This C is being sequestered in both terrestrial and inland aquatic ecosystems. If the C-cost for ecosystem management is taken into account, the net uptake of ecosystems is estimated to decrease by 45% but still indicates substantial C-sequestration. However, when the balance is extended from CO2 towards the main GHGs, C-uptake by terrestrial and aquatic ecosystems is offset by emissions of non-CO2 GHGs. As such, the European ecosystems are unlikely to contribute to mitigating the effects of climate change.

At the interface between continent and ocean, estuaries receive particles, and especially particulate organic matter (POM) originating from these two reservoirs, but also produce POM, through autochthonous primary production. The origin and composition of surface POM in the Gironde Estuary (SW France) and the environmental forcing of its variability was investigated using the data set produced by the French Coastal Monitoring Network SOMLIT (Service d'Observation en Milieu LITtoral; monthly like sampling during years 2007–2009). This estuary is considered as a model of macrotidal turbid estuaries. Using elemental and isotopic composition of the POM, we estimated that, at the inner estuary space scale and inter-annual time scale, surface particulate organic carbon (POC) was composed of terrestrial POM originated from the turbidity maximum (96.4%; refractory POC) and flood events (1.6%; labile and refractory POC), and of riverine (0.1%), estuarine (0.8%) and marine (1.1%) phytoplankton, i.e. that POC was 98% and 2% of terrestrial and phytoplankton origin, respectively. However, there was a clear spatial gradient: the phytoplankton contribution increases from ca. 1% in the upper and middle estuary to 8.5% in the lower estuary, where light condition is more favourable to plankton growth. The low contribution of phytoplankton to the POC is a characteristic of the Gironde estuary and contrast with other large temperate estuaries. Statistical analysis indicates that salinity, river flow and SPM concentration, and thus associated hydro-dynamic and sedimentary processes, were the only environmental forcings to the composition of surface POC in this system, at intra- and inter-annual time scale. In contrast, temperature and nutrient concentrations, and thus associated processes, do not force this composition of POC. By combining POC fluxes entering the inner estuary (literature data), POC loss as dissolved organic carbon and CO 2 and as sediment trapping within the inner estuary (literature data), and our estimate of the composition of POC flux at the mouth of the estuary (96% and 4% of terrestrial and phytoplankton origin), a first-order net export of POC originating from the Gironde to the continental shelf was estimated: it amounts 48,150 tC yr −1 , and is composed of 46,200 tC yr −1 of terrestrial material and of 1950 tC yr −1 of estuarine phytoplankton. POC exported by the Gironde Estuary is thus poorly bioavailable for shelf pelagic and benthic food webs.