The impact of reservoir on downstream water quality and pCO2: a case study in Seine Basin, France

The impact of reservoirs on downstream water quality has received widespread attention, but most current studies are based on short-term data only, and less attention has been paid to the impact of reservoirs on downstream carbon dioxide (CO₂) concentrations. In the present study, we assessed the nutrient budgets (DIN: dissolved inorganic nitrogen, PO₄^3-: orthophosphate, DSi: dissolved silica) of the reservoirs (Marne, Aube, Seine, and Pannecière reservoirs) in the Seine Basin using long-term dataset (1998-2018), and we also evaluated the reservoir effect on downstream partial pressure of carbon dioxide (pCO₂) based on field measurements during 2019-2020. The mean annual retention rates accounted for 16%-53%, 26%-48%, and 22%-40%of the inputs of DIN, PO₄^3-, and DSi in the four reservoirs during the period 1998 to 2018, respectively, showing that the four reservoirs play important roles in nutrient retentions. We further identified that three reservoirs (Marne, Aube, and Seine reservoirs) significantly changed downstream water quality during the emptying period, increasing the concentrations of dissolved organic carbon (DOC) and biodegradable DOC, while lowering the concentrations of DIN, DSi, PO₄^3-, and total alkalinity. Interestingly, we found that the three reservoirs notably decreased downstream pCO₂(24%-37%) and enhanced the gas transfer coefficient of CO₂ (21%) in downstream rivers compared to the upstream ones, during the emptying period, which highlights the necessity to consider the potential impact of reservoirs on downstream riverine not only for water quality variables, but also for CO₂ emissions. Finally, the findings of this study highlight the importance of the combination of biogeochemical and hydrological characteristics to understand the biogeochemical functioning of reservoirs to downstream rivers.