Long-term assessment of nutrient budgets for the four reservoirs of the Seine Basin (France).

Artificial reservoirs represent one of the most significant human disturbances of water flows and associated water quality, including nutrients and SM (suspended matter). However, most of the previous studies were only focused on few years or even single year, and the long-term dynamics of nutrient retention in reservoir are under explored. In this study, we present the long-term (1998-2018) hydrological characteristics and water quality in four reservoirs (Marne, Aube, Seine, and Pannecière reservoirs) and their related rivers (Marne, Aube, Seine, and Yonne rivers) of the Seine Basin, France. Based on the hydrology and water quality data, the long-term budgets of nutrients and SM were evaluated in these reservoirs according to mass balance calculation. The results indicated that the four reservoirs play important roles in the retention/elimination of nutrients and SM, and the retention/elimination rates may be affected by hydrophysical and biogeochemical processes. The mean annual retention rates accounted for 16-53% of the inputs of DIN (dissolved inorganic nitrogen), 26-48% of PO-P (orthophosphates), 22-40% of Si (dissolved silicon), and 36-76% of SM in the four reservoirs during the 1998-2018 period. Further analysis suggested that the annual residence time and the percentage of water released from reservoirs during the filling period significantly correlated with DIN retention rates in the four reservoirs (p < 0.01), which highlights the importance of reservoir water management strategies for the DIN concentrations in the downstream rivers. Interestingly, the Wilcoxon test results also revealed that the three diverted reservoirs (Marne, Aube, and Seine reservoirs) indeed lowered the nutrient concentrations in their downstream rivers during the emptying period, thereby modifying the biogeochemical functioning in the downstream river networks. Finally, these results emphasized the importance of hydrological characteristics in better understanding nutrient retention in reservoirs.