Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach

The Seine river discharges over 700 Gg of carbon (C) every year into the sea mostly under the form of dissolved inorganic carbon (DIC) and emits 445 Gg under the form of carbon dioxide (CO2) to the atmosphere over its entire river network. The watershed, which drains 76,000 km2, is heavily populated with 18 106 inhabitants and is thus submitted to large anthropic pressure. The offline coupling of two Reactive Transport Models is used to understand the complex spatial and temporal dynamics of carbon, oxygen and nutrients and quantify the CO2 exchange at the air-water interface along the main axis of the river. The estuarine section of the Seine is simulated by the generic estuarine model C-GEM (for Carbon Generic Estuarine Model), while the upstream part of the network, devoid of tidal influence is simulated by the pyNuts-Riverstrahler modeling platform which also includes an explicit representation of the drainage network ecological functioning. Our simulations provide a process-based representation of nutrients, oxygen, total organic carbon (TOC) and the carbonate system (DIC and alkalinity) over the entire year 2010. Our coupled modeling chain allows quantifying the respective contributions of the estuarine and freshwater sections of the system in the removal of carbon as well as following the fate of TOC and DIC along the river network. Our results also allow calculating an integrated carbon budget of the Seine river network for year 2010

The community-centered freshwater biogeochemistry model unified RIVE v1.0: a unified version for water column

Research on mechanisms of organic matter degradation, bacterial activities, phytoplankton dynamics, and other processes has led to the development of numerous sophisticated water quality models. The earliest model, dating back to 1925, was based on first-order kinetics for organic matter degradation. The community-centered freshwater biogeochemistry model RIVE was initially developed in 1994 and has subsequently been integrated into several software programs such as Seneque-Riverstrahler, pyNuts-Riverstrahler, ProSe/ProSe-PA, and Barman. After 30 years of research, the use of different programming languages including QBasic, Visual Basic, Fortran, ANSI C, and Python, as well as parallel evolution and the addition of new formalisms, raises questions about their comparability. This paper presents a unified version of the RIVE model for the water column, including formalisms for bacterial communities (heterotrophic and nitrifying), primary producers, zooplankton, nutrients, inorganic carbon, and dissolved oxygen cycles. The unified RIVE model is open-source and implemented in Python 3 to create pyRIVE 1.0 and in ANSI C to create C-RIVE 0.32. The organic matter degradation module is validated by simulating batch experiments. The comparability of the pyRIVE 1.0 and C-RIVE 0.32 software is verified by modeling a river stretch case study. The case study considers the full biogeochemical cycles (microorganisms, nutrients, carbon, and oxygen) in the water column, as well as the effects of light and water temperature. The results show that the simulated concentrations of all state variables, including microorganisms and chemical species, are very similar for pyRIVE 1.0 and C-RIVE 0.32. This open-source project highly encourages contributions from the freshwater biogeochemistry community to further advance the project and achieve common objectives.</p

Carbon cycling across the human-impacted Seine River basin : from the modeling of carbon dioxide outgassing to the assessment of greenhouse gas emissions

Several recent studies have highlighted significant fluxes of carbon dioxide (CO2) from inland waters in the global carbon cycling. The first main objective of this thesis was to quantify and understand carbon dynamics in the Seine River basin, which is deeply impacted by human activities. For this purpose a new inorganic carbon (IC) module was implemented in the biogeochemical Riverstrahler model, to simulate spatial and temporal variations in carbon forms in the drainage work. A second major objective was to size both aquatic and terrestrial emissions as a part of a joint assessment of three main GHGs (CO2, methane –CH4, and nitrous oxide –N2O). Field campaigns in rivers draining various land uses in different hydrological seasons, showed a supersaturation in CO2 of the Seine hydrosystem leading to CO2.emissions to the atmosphere. The main factor controlling the CO2 partial pressure (pCO2) was the concentration of dissolved organic carbon (DOC) (R2=0.56, n=119, p&lt;0.05), modulated by hydro-climatic conditions and groundwater contribution. In small streams, DOC concentrations were dependent on the soil organic carbon stock. For the main stem, a long-term analysis (1970-2015) showed that pCO2 tracked urban pollution, decreasing from the 2000s after improvement of wastewater treatment. The validation of the IC module newly implemented in Riverstrahler showed that IC inputs to the Seine River dominated the overall carbon budget (1138 ktC yr-1 on average for the period 2010-2013) of which less than 2% was produced from biogeochemical processes (27 ktC yr-1). In addition, CO2 outgassing represented 30% of IC outputs while exports to the estuary represented 69% of IC outputs. OC inputs were comparatively lower, accounting only for 104 ktC yr-1. Analysis of the biogeochemical processes of the Seine River showed a negative net ecosystem production (NEP), the river being mostly heterotrophic. In order to complete the modeling of the fate of carbon in the Seine River, the Riverstrahler model was combined with the estuarine C-GEM model, towards an integrated approach to the Land-to-Ocean Aquatic continuum. Representing 34% of the river mirror area, the estuary thus contributes ~23% of the CO2 emitted from the whole estuary-river aquatic continuum (estimated at 445 kt C for the year 2010). In addition, analyses of available institutional databases and measurements of other GHGs (CH4 and N2O) enabled estimation of aquatic emissions at 3.7% of the Seine basin total emissions (2,276 ktCO2 equivalent yr-1), dominated by CO2 (95.3%), while agricultural (14,295 ktCO2 equivalent yr-1) and urban emissions (44,713 ktCO2 equivalent yr-1) accounted for 23.3% and 73.0%, respectively. A historical reconstruction of agricultural emissions for the whole of France (1850-2014) estimated that, among the 114,000 ktCO2 equivalent yr-1 emitted by the agricultural sector, 22% were represented by CO2, 49% by CH4 and 29% by N2O. Finally, two contrasting scenarios were explored (horizon 2040). The first, characterized by the current trend towards specialization and intensification, predicted an almost 1.5-fold increase in agricultural emissions. While the second, characterized by a transition to organic agriculture and dietary change, would reduce current emissions by about 50%

Le cycle du carbone dans le bassin anthropisé de la Seine : de la modélisation du dioxyde de carbone à l’évaluation des émissions des gaz à effet de serre

Several recent studies have highlighted significant fluxes of carbon dioxide (CO2) from inland waters in the global carbon cycling. The first main objective of this thesis was to quantify and understand carbon dynamics in the Seine River basin, which is deeply impacted by human activities. For this purpose a new inorganic carbon (IC) module was implemented in the biogeochemical Riverstrahler model, to simulate spatial and temporal variations in carbon forms in the drainage work. A second major objective was to size both aquatic and terrestrial emissions as a part of a joint assessment of three main GHGs (CO2, methane –CH4, and nitrous oxide –N2O). Field campaigns in rivers draining various land uses in different hydrological seasons, showed a supersaturation in CO2 of the Seine hydrosystem leading to CO2.emissions to the atmosphere. The main factor controlling the CO2 partial pressure (pCO2) was the concentration of dissolved organic carbon (DOC) (R2=0.56, n=119, p<0.05), modulated by hydro-climatic conditions and groundwater contribution. In small streams, DOC concentrations were dependent on the soil organic carbon stock. For the main stem, a long-term analysis (1970-2015) showed that pCO2 tracked urban pollution, decreasing from the 2000s after improvement of wastewater treatment. The validation of the IC module newly implemented in Riverstrahler showed that IC inputs to the Seine River dominated the overall carbon budget (1138 ktC yr-1 on average for the period 2010-2013) of which less than 2% was produced from biogeochemical processes (27 ktC yr-1). In addition, CO2 outgassing represented 30% of IC outputs while exports to the estuary represented 69% of IC outputs. OC inputs were comparatively lower, accounting only for 104 ktC yr-1. Analysis of the biogeochemical processes of the Seine River showed a negative net ecosystem production (NEP), the river being mostly heterotrophic. In order to complete the modeling of the fate of carbon in the Seine River, the Riverstrahler model was combined with the estuarine C-GEM model, towards an integrated approach to the Land-to-Ocean Aquatic continuum. Representing 34% of the river mirror area, the estuary thus contributes ~23% of the CO2 emitted from the whole estuary-river aquatic continuum (estimated at 445 kt C for the year 2010). In addition, analyses of available institutional databases and measurements of other GHGs (CH4 and N2O) enabled estimation of aquatic emissions at 3.7% of the Seine basin total emissions (2,276 ktCO2 equivalent yr-1), dominated by CO2 (95.3%), while agricultural (14,295 ktCO2 equivalent yr-1) and urban emissions (44,713 ktCO2 equivalent yr-1) accounted for 23.3% and 73.0%, respectively. A historical reconstruction of agricultural emissions for the whole of France (1850-2014) estimated that, among the 114,000 ktCO2 equivalent yr-1 emitted by the agricultural sector, 22% were represented by CO2, 49% by CH4 and 29% by N2O. Finally, two contrasting scenarios were explored (horizon 2040). The first, characterized by the current trend towards specialization and intensification, predicted an almost 1.5-fold increase in agricultural emissions. While the second, characterized by a transition to organic agriculture and dietary change, would reduce current emissions by about 50%.Des études récentes ont souligné l’importance des émissions de dioxyde de carbone (CO2) par les eaux continentales, replaçant ainsi l’hydro-système comme compartiment actif du bilan carbone. Un premier objectif de cette thèse a été de comprendre et quantifier la dynamique du C aquatique le long du continuum aquatique de la Seine, empreint d’une très forte activité anthropique. Pour cela, un module de carbone inorganique (CI) a été développé au sein du modèle de fonctionnement biogéochimique des écosystèmes aquatiques, Riverstrahler, permettant de simuler les variations spatio-temporelles du C. Le second objectif était de quantifier les émissions aquatiques et terrestres afin de proposer une évaluation conjointe des trois principaux gaz à effet de serre (GES: CO2, méthane – CH4, protoxyde d’azote- N2O) à l’échelle du bassin. Les mesures de la pression partielle de CO2 (pCO2) dans des rivières drainant différentes occupations du sol, à différentes saisons, attestent que l’hydro-système Seine est sursaturé et une source d’émission de CO2 vers l’atmosphère. Le principal facteur de contrôle de pCO2 est la concentration en carbone organique dissout (COD) (R2 = 0,56, p < 0,05), modulée par les conditions hydro-climatiques et les contributions d'eaux souterraines. Dans les rivières amont, les concentrations en COD semblent reliées au stock de CO des sols, alors que sur l’axe principal de la Seine, elles dépendent des effluents de stations d’épuration. Sur le long terme (1970-2015) la pCO2 a clairement évolué conjointement à l’amélioration du traitement des eaux usées. Les bilans par modélisation (moyenne 2010-2013) montrent l’importance du CI apporté à l’hydro-système Seine (1138 ktC an-1) et une faible contribution des processus biogéochimiques (27 ktC an-1). Si une grande part du CI est exportée vers l’estuaire (69%), les émissions de CO2 dépassent 360 ktC an-1 (soit 30%). Les apports de carbone organique ne représentent que 104 ktC an-1. La production nette de l’écosystème (NEP) apparait négative, et indique le caractère hétérotrophe de la Seine. Cette nouvelle version du modèle Riverstrahler a été couplée au modèle estuarien C-GEM afin de proposer une description complète de la cascade du carbone dans le continuum rivière-estuaire. L’estuaire représente 34 % de la surface miroir de la Seine et contribue à hauteur de 23% des émissions aquatiques de CO2 du bassin, estimée à 445 kt C (année 2010). Les émissions de CO2 complétées par celles de N2O et CH4 montrent que les émissions aquatiques de GES représentent 3.7% des émissions totales du bassin de la Seine (2,276 kt CO2 équivalent an-1 dont 95,3% de CO2). Les émissions agricoles (14,295 ktCO2 équivalent an-1) et urbaines (44,713 ktCO2 équivalent an-1) contribuent respectivement pour 23.3 et 73.0%. Une reconstruction historique des émissions agricoles en France montre une augmentation par 4 de 1850 à 2014, soit 114,000 kt CO2 équivalent an-1 actuellement (CO2:22%, CH4:49%, N2O: 29%). Un scénario prolongeant la tendance actuelle à la spécialisation et l’intensification à l’horizon 2040, prédit une augmentation par 1.5 des émissions agricoles, alors qu’un second scenario, proposant un changement profond de l’agriculture française, réduirait les émissions actuelles de 50%

Le cycle du carbone dans le bassin anthropisé de la Seine : de la modélisation du dioxyde de carbone à l’évaluation des émissions des gaz à effet de serre

Several recent studies have highlighted significant fluxes of carbon dioxide (CO2) from inland waters in the global carbon cycling. The first main objective of this thesis was to quantify and understand carbon dynamics in the Seine River basin, which is deeply impacted by human activities. For this purpose a new inorganic carbon (IC) module was implemented in the biogeochemical Riverstrahler model, to simulate spatial and temporal variations in carbon forms in the drainage work. A second major objective was to size both aquatic and terrestrial emissions as a part of a joint assessment of three main GHGs (CO2, methane –CH4, and nitrous oxide –N2O). Field campaigns in rivers draining various land uses in different hydrological seasons, showed a supersaturation in CO2 of the Seine hydrosystem leading to CO2.emissions to the atmosphere. The main factor controlling the CO2 partial pressure (pCO2) was the concentration of dissolved organic carbon (DOC) (R2=0.56, n=119, p<0.05), modulated by hydro-climatic conditions and groundwater contribution. In small streams, DOC concentrations were dependent on the soil organic carbon stock. For the main stem, a long-term analysis (1970-2015) showed that pCO2 tracked urban pollution, decreasing from the 2000s after improvement of wastewater treatment. The validation of the IC module newly implemented in Riverstrahler showed that IC inputs to the Seine River dominated the overall carbon budget (1138 ktC yr-1 on average for the period 2010-2013) of which less than 2% was produced from biogeochemical processes (27 ktC yr-1). In addition, CO2 outgassing represented 30% of IC outputs while exports to the estuary represented 69% of IC outputs. OC inputs were comparatively lower, accounting only for 104 ktC yr-1. Analysis of the biogeochemical processes of the Seine River showed a negative net ecosystem production (NEP), the river being mostly heterotrophic. In order to complete the modeling of the fate of carbon in the Seine River, the Riverstrahler model was combined with the estuarine C-GEM model, towards an integrated approach to the Land-to-Ocean Aquatic continuum. Representing 34% of the river mirror area, the estuary thus contributes ~23% of the CO2 emitted from the whole estuary-river aquatic continuum (estimated at 445 kt C for the year 2010). In addition, analyses of available institutional databases and measurements of other GHGs (CH4 and N2O) enabled estimation of aquatic emissions at 3.7% of the Seine basin total emissions (2,276 ktCO2 equivalent yr-1), dominated by CO2 (95.3%), while agricultural (14,295 ktCO2 equivalent yr-1) and urban emissions (44,713 ktCO2 equivalent yr-1) accounted for 23.3% and 73.0%, respectively. A historical reconstruction of agricultural emissions for the whole of France (1850-2014) estimated that, among the 114,000 ktCO2 equivalent yr-1 emitted by the agricultural sector, 22% were represented by CO2, 49% by CH4 and 29% by N2O. Finally, two contrasting scenarios were explored (horizon 2040). The first, characterized by the current trend towards specialization and intensification, predicted an almost 1.5-fold increase in agricultural emissions. While the second, characterized by a transition to organic agriculture and dietary change, would reduce current emissions by about 50%.Des études récentes ont souligné l’importance des émissions de dioxyde de carbone (CO2) par les eaux continentales, replaçant ainsi l’hydro-système comme compartiment actif du bilan carbone. Un premier objectif de cette thèse a été de comprendre et quantifier la dynamique du C aquatique le long du continuum aquatique de la Seine, empreint d’une très forte activité anthropique. Pour cela, un module de carbone inorganique (CI) a été développé au sein du modèle de fonctionnement biogéochimique des écosystèmes aquatiques, Riverstrahler, permettant de simuler les variations spatio-temporelles du C. Le second objectif était de quantifier les émissions aquatiques et terrestres afin de proposer une évaluation conjointe des trois principaux gaz à effet de serre (GES: CO2, méthane – CH4, protoxyde d’azote- N2O) à l’échelle du bassin. Les mesures de la pression partielle de CO2 (pCO2) dans des rivières drainant différentes occupations du sol, à différentes saisons, attestent que l’hydro-système Seine est sursaturé et une source d’émission de CO2 vers l’atmosphère. Le principal facteur de contrôle de pCO2 est la concentration en carbone organique dissout (COD) (R2 = 0,56, p < 0,05), modulée par les conditions hydro-climatiques et les contributions d'eaux souterraines. Dans les rivières amont, les concentrations en COD semblent reliées au stock de CO des sols, alors que sur l’axe principal de la Seine, elles dépendent des effluents de stations d’épuration. Sur le long terme (1970-2015) la pCO2 a clairement évolué conjointement à l’amélioration du traitement des eaux usées. Les bilans par modélisation (moyenne 2010-2013) montrent l’importance du CI apporté à l’hydro-système Seine (1138 ktC an-1) et une faible contribution des processus biogéochimiques (27 ktC an-1). Si une grande part du CI est exportée vers l’estuaire (69%), les émissions de CO2 dépassent 360 ktC an-1 (soit 30%). Les apports de carbone organique ne représentent que 104 ktC an-1. La production nette de l’écosystème (NEP) apparait négative, et indique le caractère hétérotrophe de la Seine. Cette nouvelle version du modèle Riverstrahler a été couplée au modèle estuarien C-GEM afin de proposer une description complète de la cascade du carbone dans le continuum rivière-estuaire. L’estuaire représente 34 % de la surface miroir de la Seine et contribue à hauteur de 23% des émissions aquatiques de CO2 du bassin, estimée à 445 kt C (année 2010). Les émissions de CO2 complétées par celles de N2O et CH4 montrent que les émissions aquatiques de GES représentent 3.7% des émissions totales du bassin de la Seine (2,276 kt CO2 équivalent an-1 dont 95,3% de CO2). Les émissions agricoles (14,295 ktCO2 équivalent an-1) et urbaines (44,713 ktCO2 équivalent an-1) contribuent respectivement pour 23.3 et 73.0%. Une reconstruction historique des émissions agricoles en France montre une augmentation par 4 de 1850 à 2014, soit 114,000 kt CO2 équivalent an-1 actuellement (CO2:22%, CH4:49%, N2O: 29%). Un scénario prolongeant la tendance actuelle à la spécialisation et l’intensification à l’horizon 2040, prédit une augmentation par 1.5 des émissions agricoles, alors qu’un second scenario, proposant un changement profond de l’agriculture française, réduirait les émissions actuelles de 50%

Le cycle du carbone dans le bassin anthropisé de la Seine :de la modélisation de l'évasion du dioxyde de carbone à la quantification des émissions de gaz à effet de serre

Several recent studies have highlighted significant fluxes of carbon dioxide (CO2) from inland waters in the global carbon cycling. The first main objective of this thesis was to quantify and understand carbon dynamics in the Seine River basin, which is deeply impacted by human activities. For this purpose a new inorganic carbon (IC) module was implemented in the biogeochemical Riverstrahler model, to simulate spatial and temporal variations in carbon forms in the drainage work. A second major objective was to size both aquatic and terrestrial emissions as a part of a joint assessment of three main GHGs (CO2, methane –CH4, and nitrous oxide –N2O).Field campaigns in rivers draining various land uses in different hydrological seasons, showed a supersaturation in CO2 of the Seine hydrosystem leading to CO2.emissions to the atmosphere. The main factor controlling the CO2 partial pressure (pCO2) was the concentration of dissolved organic carbon (DOC) (R2=0.56, n=119, p<0.05), modulated by hydro-climatic conditions and groundwater contribution. In small streams, DOC concentrations were dependent on the soil organic carbon stock. For the main stem, a long-term analysis (1970-2015) showed that pCO2 tracked urban pollution, decreasing from the 2000s after improvement of wastewater treatment.The validation of the IC module newly implemented in Riverstrahler showed that IC inputs to the Seine River dominated the overall carbon budget (1124 ktC yr-1 on average for the period 2010-2013) of which less than 2% was produced from biogeochemical processes (17 ktC yr-1). In addition, CO2 outgassing represented 31% of IC outputs while exports to the estuary represented 68% of IC outputs. OC inputs were comparatively lower, accounting only for 105 ktC yr-1. Analysis of the biogeochemical processes of the Seine River showed a negative net ecosystem production (NEP), the river being mostly heterotrophic.In order to complete the modeling of the fate of carbon in the Seine River, the Riverstrahler model was combined with the estuarine C-GEM model, towards an integrated approach to the Land-to-Ocean Aquatic continuum. Representing 34% of the river mirror area, the estuary thus contributes ~23% of the CO2 emitted from the whole estuary-river aquatic continuum (estimated at 445 kt C for the year 2010).In addition, analyses of available institutional databases and measurements of other GHGs (CH4 and N2O) enabled estimation of aquatic emissions at 3.7% of the Seine basin total emissions (2,276 ktCO2 equivalent yr-1), dominated by CO2 (95.3%), while agricultural (14,295 ktCO2 equivalent yr-1) and urban emissions (44,713 ktCO2 equivalent yr-1) accounted for 23.3% and 73.0%, respectively. A historical reconstruction of agricultural emissions for the whole of France (1850-2014) estimated that, among the 114,000 ktCO2 equivalent yr-1 emitted by the agricultural sector, 22% were represented by CO2, 49% by CH4 and 29% by N2O. Finally, two contrasting scenarios were explored (horizon 2040). The first, characterized by the current trend towards specialization and intensification, predicted an almost 1.5-fold increase in agricultural emissions. While the second, characterized by a transition to organic agriculture and dietary change, would reduce current emissions by about 50%.Des études récentes ont souligné l’importance des émissions de dioxyde de carbone (CO2) par les eaux continentales, replaçant ainsi l’hydro-système comme compartiment actif du bilan carbone. Un premier objectif de cette thèse a été de comprendre et quantifier la dynamique du C aquatique le long du continuum aquatique de la Seine, empreint d’une très forte activité anthropique. Pour cela, un module de carbone inorganique (CI) a été développé au sein du modèle de fonctionnement biogéochimique des écosystèmes aquatiques, Riverstrahler, permettant de simuler les variations spatio-temporelles du C. Le second objectif était de quantifier les émissions aquatiques et terrestres afin de proposer une évaluation conjointe des trois principaux gaz à effet de serre (GES: CO2, méthane – CH4, protoxyde d’azote- N2O) à l’échelle du bassin.Les mesures de la pression partielle de CO2 (pCO2) dans des rivières drainant différentes occupations du sol, à différentes saisons, attestent que l’hydro-système Seine est sursaturé et une source d’émission de CO2 vers l’atmosphère. Le principal facteur de contrôle de pCO2 est la concentration en carbone organique dissout (COD) (R2 = 0,56, p < 0,05), modulée par les conditions hydro-climatiques et les contributions d'eaux souterraines. Dans les rivières amont, les concentrations en COD semblent reliées au stock de CO des sols, alors que sur l’axe principal de la Seine, elles dépendent des effluents de stations d’épuration. Sur le long terme (1970-2015) la pCO2 a clairement évolué conjointement à l’amélioration du traitement des eaux usées.Les bilans par modélisation (moyenne 2010-2013) montrent l’importance du CI apporté à l’hydro-système Seine (1124 ktC an-1) dont une faible contribution des processus biogéochimiques (17 ktC an-1). Si une grande part du CI est exportée vers l’estuaire (68%), les émissions de CO2 dépassent 360 ktC an-1 (soit 31%). Les apports de carbone organique ne représentent que 105 ktC an-1. La production nette de l’écosystème (NEP) apparait négative, et indique le caractère hétérotrophe de la Seine. Cette nouvelle version du modèle Riverstrahler a été couplée au modèle estuarien C-GEM afin de proposer une description complète de la cascade du carbone dans le continuum rivière-estuaire. L’estuaire représente 34 % de la surface miroir de la Seine et contribue à hauteur de 23% des émissions aquatiques de CO2 du bassin, estimée à 445 kt C (année 2010).Les émissions de CO2 complétées par celles de N2O et CH4 montrent que les émissions aquatiques de GES représentent 3.7% des émissions totales du bassin de la Seine (2,276 kt CO2 équivalent an-1 dont 95,3% de CO2). Les émissions agricoles (14,295 ktCO2 équivalent an-1) et urbaines (44,713 ktCO2 équivalent an-1) contribuent respectivement pour 23.3 et 73.0%. Une reconstruction historique des émissions agricoles en France montre une augmentation par 4 de 1850 à 2014, soit 114,000 kt CO2 équivalent an-1 actuellement (CO2:22%, CH4:49%, N2O: 29%). Un scénario prolongeant la tendance actuelle à la spécialisation et l’intensification à l’horizon 2040, prédit une augmentation par 1.5 des émissions agricoles, alors qu’un second scenario, proposant un changement profond de l’agriculture française, réduirait les émissions actuelles de 50%

Carbon dioxide, methane and nitrous oxide emissions from the human-impacted Seine watershed in France

International audienceGreenhouse gas (GHG) emissions from rivers and lakes have been shown to contribute significantly to global carbon and nitrogen cycling. In temperate and human-impacted regions, simultaneous carbon dioxide, methane and nitrous oxide emissions from aquatic systems are poorly documented. We estimated carbon dioxide (CO2) concentrations in the Seine hydrosystem (71,730 km2, France) using direct measurements, and calculations of CO2 partial pressures from 14 field campaigns conducted between 2010 and 2017, and compared them to methane (CH4) and nitrous oxide (N2O) concentrations.In the main stem of the Seine River, CO2 showed the same spatial gradient as N2O and CH4 with peaks in concentration downstream from the arrival of effluents from wastewater treatment plants enriched in organic matter, thus favoring mineralization. It is likely that high CO2 concentrations upstream were due to organic carbon inputs from soils and enriched CO2 groundwater discharges, whereas high N2O and CH4 upstream values were likely due to denitrification in riparian wet areas and anoxic decomposition of organic matter-rich wetlands, respectively. In addition, seasonal variations in all three GHGs were observed with higher concentrations in summer when higher temperatures promote mineralization and low water reduces the dilution of organic matter mainly originating from WWTP effluents.GHG emissions were calculated and compared with agricultural and nonagricultural (urban, transport) fluxes in the basin. In the Seine River network, CO2 emissions dominated riverine GHG emissions, reaching 95.3%, while N2O and CH4 emissions accounted for 4.4% and 0.3%, respectively. These indirect emissions from the hydrosystem were estimated to account for 3.7% of the total GHG emissions from the basin that amounted to 61,284 Gg CO2eq yr−1. Comparatively, direct agricultural and nonagricultural GHG emissions were estimated at 23.3% and 73.0%., respectively

Modeling inorganic carbon dynamics in the Seine River continuum in France

International audienceInland waters are an active component of the carbon cycle where transformations and transports are associated with carbon dioxide (CO 2) outgassing. This study estimated CO 2 emissions from the human-impacted Seine River (France) and provided a detailed budget of aquatic carbon transfers for organic and inorganic forms, including the in-stream metabolism along the whole Seine River network. The existing process-based biogeochemical pyNuts-Riverstrahler model was supplemented with a newly developed inorganic carbon module and simulations were performed for the recent time period 2010-2013. New input constraints for the modeling of riverine inorganic carbon were documented by field measurements and complemented by analysis of existing databases. The resulting dissolved inorganic carbon (DIC) concentrations in the Seine aquifers ranged from 25 to 92 mg C L −1 , while in wastewater treatment plant (WWTP) effluents our DIC measurements averaged 70 mg C L −1. Along the main stem of the Seine River, simulations of DIC, total alkalinity, pH and CO 2 concentrations were of the same order of magnitude as the observations, but seasonal variability was not always well reproduced. Our simulations demonstrated the CO 2 supersaturation with respect to atmospheric concentrations over the entire Seine River network. The most significant outgassing was in lower-order streams while peaks were simulated downstream of the major WWTP effluent. For the period studied (2010-2013), the annual average of simulated CO 2 emissions from the Seine drainage network were estimated at 364 ± 99 Gg C yr −1. Results from metabolism analysis in the Seine hydro-graphic network highlighted the importance of benthic activities in headwaters while planktonic activities occurred mainly downstream in larger rivers. The net ecosystem productivity remained negative throughout the 4 simulated years and over the entire drainage network, highlighting the heterotrophy of the basin

Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach

International audienceThe Seine river discharges over 700 Gg of carbon (C) every year into the sea mostly under the form of dissolved inorganic carbon (DIC) and emits 445 Gg under the form of carbon dioxide (CO2) to the atmosphere over its entire river network. The watershed, which drains 76,000 km2, is heavily populated with 18 106 inhabitants and is thus submitted to large anthropic pressure. The offline coupling of two Reactive Transport Models is used to understand the complex spatial and temporal dynamics of carbon, oxygen and nutrients and quantify the CO2 exchange at the air-water interface along the main axis of the river. The estuarine section of the Seine is simulated by the generic estuarine model C-GEM (for Carbon Generic Estuarine Model), while the upstream part of the network, devoid of tidal influence is simulated by the pyNuts-Riverstrahler modeling platform which also includes an explicit representation of the drainage network ecological functioning. Our simulations provide a process-based representation of nutrients, oxygen, total organic carbon (TOC) and the carbonate system (DIC and alkalinity) over the entire year 2010. Our coupled modeling chain allows quantifying the respective contributions of the estuarine and freshwater sections of the system in the removal of carbon as well as following the fate of TOC and DIC along the river network. Our results also allow calculating an integrated carbon budget of the Seine river network for year 2010

pyRIVE

The pyRIVE model aims at representing the biogeochemical functioning of aquatic system, by simulating concentrations of oxygen and nutrients dissolved (NH4+, NO2-, NO3-, PO43-, SiO2) and particulate (PIP, BSi), suspended matter, dissolved and particulate organic carbon (3 classes of biodegradability), dissolved gases (CO2, N2O et CH4). Biological compartments are represented by 3 taxonomic classes of phytoplankton (diatoms, chlorophyceae, and cyanobacteria), 2 types of zooplankton (rotifers with short generation time and microcrustaceans with long generation time), 2 types of heterotrophic bacteria (small autochthonous and large allochthonous) as well as nitrifying bacteria. Faecal bacteria (free and attached) are also considered as state variables. The model also takes into account benthic variables (benthic organic matter, inorganic particulate phosphorus, benthic biogenic silica)