A large reservoir as a source of zooplankton for the river: structure of the populations and influence of fish predation

Within an extensive programme on the Seine river, including the largereservoirs in the upstream basin, zooplankton populations were investigatedat the reservoir-river interface to find out their fate when released intothe river. Zooplankton structure, abundance and biomass were analysed inthe river upstream and downstream from the reservoir, in the release canaland in the reservoir. Whereas the river zooplankton are dominated byrotifers, microcrustaceans represent a large part of the population in thereservoir, because of the longer residence time (6 months). We have shownthat the reservoir is a source of zooplankton; we calculated a biomassexport of 30-60 tons C year^-1, which is, however,rapidly lost in the river. One of the main loss factors was found to beselective fish predation on larger zooplankton species developed in thereservoir.