GEOMAR - Helmholtz-Zentrum für Ozeanforschung Kiel

Zooplankton trade-offs to maximize fitness in highly dynamic environments such as estuaries have long been a question of central importance for understanding the ecology and evolution of estuarine populations. We present here the first comprehensive data set on the population dynamics of the copepod Eurytemmora affinis obtained from 50 h high-frequency sampling in the Seine estuary during spring. Maximum densities of E. affinis were associated with low salinities (0.5-10) and recorded during the ebb in the bottom layer. Vertical variations in population structure were observed between ebb and flood, as well as the spatial distribution of developmental stages. Nauplii were concentrated in the low salinity zone just above salinity 5, and copepodids and adults distributed more widely relative to salinity than nauplii in bottom waters, whereas the opposite pattern was observed in surface waters. The sex-ratio and the proportion "ovigerous females:non-ovigerous females" appeared to be related to tidal cycle and depth, with higher relative densities of non-ovigerous females in bottom waters and around low tide. The vertical variations noticed during the tidal cycle suggest a strategy by the species to avoid flushing by surface currents, although it may incur a cost due to the greater presence of predators in bottom waters.