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This chapter illustrates the role of watersheds and estuaries in the biogeochemical functioning of coastal zones, specifically in terms ofeutrophication, which is a widespread phenomenon. Along the land–ocean aquatic continuums, estuaries are receptacles of thepollution generated by human activities (e.g. domestic wastewater, agriculture) in watersheds; they can further modify the riverinenutrient delivery before it reaches the marine zone. We present several applications of a chain of modeling tools able to simulate waterquality transfers and transformations for a reference situation in a variety of systems to be compared with scenarios involvingimprovement of domestic wastewater and structural changes in agriculture., info:eu-repo/semantics/published

Estuaries are key reactive ecosystems along the land–ocean aquatic continuum, with significant ecological and economic value. However, they have been facing strong morphological management changes and increased nutrient and contaminant inputs, possibly leading to ecological problems such as coastal eutrophication. Therefore, it is necessary to quantify the import and export fluxes of the estuaries, their retention capacity, and estuarine eutrophication potential. The 1-D Carbon-Generic Estuary Model (C-GEM) was used to simulate the transient hydrodynamics, transport, and biogeochemistry for estuaries with different sizes and morphologies along the French Atlantic coast during the period 2014–2016 using readily available geometric, hydraulic, and biogeochemical data. These simulations allowed us to evaluate the budgets of the main nutrients (phosphorus – P; nitrogen – N; silica – Si) and total organic carbon (TOC), and their imbalance, providing insights into their eutrophication potential. Cumulated average annual fluxes to the Atlantic coast from the seven estuaries studied were 9.6 kt P yr−1, 259 kt N yr−1, 304 kt Si yr−1, and 145 kt C yr−1. Retention rates varied depending on the estuarine residence times, ranging from 0 %–27 % and 0 %–34 % to 2 %–39 % and 8 %–96 % for total phosphorus (TP), total nitrogen (TN), dissolved silica (DSi), and TOC, respectively. Large-scale estuaries had higher retention rates than medium and small estuaries, which we interpreted in terms of estuarine residence times. As shown by the indicator of eutrophication potential (ICEP), there might be a risk of coastal eutrophication, i.e., the development of non-siliceous algae that is potentially harmful to the systems studied due to the excess TN over DSi. This study also demonstrates the ability of our model to be applied with a similar setup to several estuarine systems characterized by different sizes, geometries, and riverine loads.

Depuis la révolution industrielle, les émissions de dioxyde de carbone (CO2) dues à l’activité humaine ont fortement augmenté la concentration de carbone dans l’atmosphère, perturbant le cycle naturel du carbone (C). Cette augmentation a un effet direct sur le climat de la Terre et a de nombreuses conséquences telles que l’augmentation de la température, la modification des courants océaniques et l’augmentation du niveau des mers. Pour répondre à cette perturbation anthropogénique, les océans et la biosphère terrestre ont vu leurs stocks de C augmenter, agissant ainsi en tant qu’important atténuateur du changement climatique. La quantification et la compréhension de la variabilité spatiale et temporelle de ces puits océanique et terrestre sont donc essentielles. Une partie du C fixée par la photosynthèse n’est en réalité pas stockée in-situ mais est à la place transférée vers les eaux continentales. Cette perte de C terrestre se passe à travers le lessivage du C organique dissous (COD) et du C inorganique dissous (CID) ainsi que par érosion du C organique particulaire (COP). Lors de son passage dans ces eaux, le C peut être minéralisé et réémis vers l’atmosphère sous forme de CO2 et de méthane (CH4) ou enterré dans les sédiments, le reste étant amené jusqu’aux océans. Cependant la quantification et la variabilité spatio-temporelle de ces processus ne sont pas encore tout à fait comprises. Dans ma thèse, j’ai utilisé trois méthodes différentes afin d’améliorer notre compréhension de la dynamique du C dans le réseau hydrographique Européen, en me concentrant sur le destin du COD dans le système hydrographique.Tout d’abord, j’ai évalué et appliqué ORCHILEAK, un modèle du système terre, à l’échelle Européenne pour estimer et étudier la variabilité spatio-temporelle du transfert du COD des terres jusqu’aux rivières. A ma connaissance, ceci est la première estimation paneuropéenne des flux de DOC à travers l’interface terre-eaux continentales. La performance du modèle est, Since the industrial revolution, emissions of carbon dioxide (CO2) due to human activities have drastically increased carbon concentration in the atmosphere, perturbing the natural cycle of carbon (C). This increase directly impacts Earth climate and has numerous consequences such as increase of air temperature, modification of the oceanic circulation and sea-level rise. As a response to this anthropogenic perturbation, oceans and the land biosphere have seen their C stocks increase, thereby acting as important mitigators of climate change. The quantification and the understanding of the spatio-temporal variability of those ocean and terrestrial C sinks are thus essential. Part of the terrestrial C fixed by photosynthesis is however not stored in-situ but is instead transferred to the inland waters. This terrestrial C loss occurs through leaching of dissolved organic C (DOC) and dissolved inorganic C (DIC) and through erosion of particulate organic C (POC). During transit, C can be mineralized and re-emitted back to the atmosphere in the form of CO2 and methane (CH4) or buried in the sediments, the leftovers being exported to the ocean. However, the quantification and spatio-temporal variability of these processes are not yet fully understood and remain poorly constrained. In my thesis, I used three different methods to improve our understanding if the C dynamics in the European inland waters, with a focus on the fate of DOC in the river network.Firstly, I evaluated and applied the land surface model ORCHILEAK at the European scale with the aim to investigate the present-day spatio-temporal variability of DOC leaching from land to river. To my knowledge, this is the first pan-European assessment of DOC fluxes through the land-inland water interface. The model performance is evaluated against the observations of DOC leaching, of DOC fluxes and DOC reactivity in rivers, providing evidence that our simulated DOC leaching fluxes are realistic. I estimated that around 14, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Estuaries are key reactive ecosystems along the land–ocean aquatic continuum, with significant ecological and economic value. However, they have been facing strong morphological management changes as well as increased nutrient and contaminant inputs, possibly leading to ecological problems such as coastal eutrophication. Therefore, it is necessary to quantify the ingoing and outgoing fluxes of the estuaries, their retention capacity, and estuarine eutrophication potential. A 1-D Carbon–Generic Estuary Model (C-GEM) was used to simulate the transient hydrodynamics, transport, and biogeochemistry for estuaries with different sizes and morphologies along the French Atlantic coast during the period 2014–2016 using readily available geometric, hydraulic, and biogeochemical data. These simulations allowed us to evaluate the budgets of the main nutrients (phosphorus [P], nitrogen [N], silica [Si]) and total organic carbon (TOC), and their imbalance with respect to estuarine eutrophication potential. Cumulated average annual fluxes to the Atlantic coast from the seven estuaries studied were 9.6 kt P yr−1, 259 kt N yr−1, 304 kt Si yr−1, and 145 kt C yr−1. Retention rates varied depending on the estuarine residence times, ranging from 0–27 %, 0–34 %, 2–39 %, and 8–96 % for TP, TN, DSi, and TOC, respectively. Large-scale estuaries had higher retention rates than medium and small estuaries, which we interpreted in terms of estuarine residence times. As shown by the indicator of eutrophication potential (ICEP), there might be a risk of coastal eutrophication, i.e., the development of nonsiliceous algae that is potentially harmful to the systems studied due to the excess TN over DSi.

Estuaries are amongst the most productive ecosystems of the land ocean continuum, but they are also under high anthropic pressures due to coastal urbanization. Too sparse observations have hindered the understanding of complex interactions between water quality and estuarine hydrodynamics and biogeochemical transformations. Until now, estuarine modelling studies have mainly focused on temperate estuarine systems in industrialized countries. This study investigates the responses of a tropical estuary to pollution load from a megacity (Ho Chi Minh City, Southern Vietnam) by applying a one-dimensional, biogeochemical estuarine model (C-GEM). The Saigon River Estuary flows through the megacity of Ho Chi Minh (HCMC) and is subject to episodic hypoxia events due to wastewater inputs from urban discharges. Good agreements are found between simulation outputs and observations for tidal propagation, salinity, total suspended sediment, and water quality variables in dry season in Saigon River Estuary. C-GEM reproduces the increases in ammonium, total organic carbon, phytoplankton and dissolved oxygen depletion in the urban section of the Saigon River as an impact of untreated wastewaters from HCMC. The steady-state version of C-GEM also reveals the formation of a pollutant cloud (30-km stretch) resulting from the combined effects of tidal fluctuation and low flushing capacity during the dry season. Furthermore, the quantification of the reaction fluxes simulated by the model demonstrates that nitrification is the main process removing NH4+ from the Saigon River. For the first time in such a type of environment, our study demonstrates the effectiveness of C-GEM at unraveling the complex interplay between biogeochemical reactions and transport in a tropical estuary with a minimized data requirement. This is significant for tropical estuaries in developing countries, where intensive monitoring programs are rare and have thus been rarely the object of modelling investigations., SCOPUS: ar.j, info:eu-repo/semantics/published

Inland waters are an active component of the carbon cycle where transformations and transports are associated with carbon dioxide (CO2) outgassing. This study estimated CO2 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 mgC L-1, while in wastewater treatment plant (WWTP) effluents our DIC measurements averaged 70 mgC L-1. Along the main stem of the Seine River, simulations of DIC, total alkalinity, pH and CO2 concentrations were of the same order of magnitude as the observations, but seasonal variability was not always well reproduced. Our simulations demonstrated the CO2 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 CO2 emissions from the Seine drainage network were estimated at 364±99 GgC yr-1. Results from metabolism analysis in the Seine hydrographic 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. Highlights - CO2 emission from the Seine River, SCOPUS: ar.j, info:eu-repo/semantics/published