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<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%