Estimating ecosystem metabolism from continuous multi-sensor measurements in the Seine River

Large rivers are important components of the global C cycle. While they are facing an overall degradation of their wate rquality, little remains known about the dynamics of their metabolism. In the present study, we used continuous multi-sensors measurements to assess the temporal variability of gross primary production (GPP) and ecosystem respiration (ER) rates of the anthropized Seine River over an annual cycle. Downstream from the Paris urban area, the Seine River is net heterotrophic at the annual scale (−226 g O₂ m⁻² year⁻¹ or −264 g C m⁻² year⁻¹). Yet, it displays a net autotrophy at the dailyandseasonalscalesduringphytoplanktonbloomsoccurring from late winter to early summer. Multivariate analyses were performed to identify the drivers of river metabolism. Daily GPP is best predicted by chlorophyll a (Chla), water temperature (T), light, and rainfalls, and the coupling of daily GPP and Chla allows for the estimation of the productivity rates of the different phytoplankton communities.ER rates are mainly controlled by T and, to a lesser extent, by Chla. The increase of combined sewer overflows related to storm events. 

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