Understanding the effect of reservoirs on suspended sediments and biogeochemical fluxes over seasonal and storm scales

International audienceReservoirs such as dams modify the transport, the retention and the remobilization of sediments and nutrientsin rivers. Such modifications can lead to strong degradation of downstream ecosystems as consequence ofdisturbances of biogeochemical cycles or habitat. When such negative impacts or cost of upgrading safety systemsoutweigh the benefits of dams, removal operation may be considered, e.g. Service (2011) inventoried more than500 removals in USA in 10 years. The modification on River fluxes induced by the presence of a reservoir as wellas the changes likely to occur in the case of its removal will depend on the fluxes from the catchment, on the functioningof the reservoir itself, and on the linkage between both systems. Therefore, we argue that characterizing themodification of hydrological, sediment and dissolved fluxes induced by dams should be achieved (i) on a varietyof time scales (storms, seasons, years), and (ii) within an integrative approach that combine sediments, associatedelements, and biological variables to enable understanding the biogeochemical processes impacted by the reservoir.We conducted such an approach on the Sélune River, a 91 km river long, flowing to the Mont Saint-Michel Bay(France), draining a catchment of 1 083 km2, and regulated by two hydroelectric dams:, La-Roche-qui-Boit andVézin (16 and 36 m respectively). A set of station has been equipped to monitor water, sediment and dissolvedelements fluxes from upstream to downstream the 2 dams since 2015. Sensors recorded every hour or less streamflow and physical and chemical parameters such as turbidity, temperature and conductivity on 4 stations. Watersamples were collected every week and at higher frequency during storms on 2 stations to determine anions,dissolved nitrogen and silica, dissolved and total phosphorus concentrations and Carbone and nitrogen content ofsuspended solids. Results over 2 contrasted water years and more than 30 sampled storm events, showed that thetwo dams have 2 main effects:(i) They act as buffer regarding suspended sediment and phosphorus annual and storm fluxes by trappingsuspended solids. They also buffer concentration variations during storm events by mixing all dissolved elementswith the water they store.(ii) They act as biogeochemical reactors seasonally: in summer when light and temperature conditions areenhancing phytoplankton production into the reservoirs, the dissolved phosphorus and silica nutrients areconsumed in the reservoirs whereas lower C/N content of downstream suspended material suggest exportation ofautochthonous products from the reservoir.No effect on stream flow dynamics is observed due to specific regulation strategy. Annual nitrate loads upstreamand downstream to the dams do not show significant difference.Service, R.F. (2011) Will Busting Dams Boost Salmon? Science 334, 888–892