Short-term changes in the mesozooplanktonic community in the Seine ROFI (Region of Freshwater Influence) (eastern English Channel)

Discrete-depth, hourly mesozooplankton samples were collected over a 92 h period in May 1992 at an anchor station within the Seine Region of Freshwater Influence (ROFI) (English Channel). The mesozooplankton community defined as a euryhaline marine assemblage was dominated by the calanoid copepods Acartia spp., Temora longicornis and Centropages hamatus, the cladoceran Evadne nordmanni and the appendicularian Oikopleura dioica. The semi-diurnal tidal current was the dominant factor in determining the short-term temporal changes in the community in terms of density and species composition so that zooplankton patches displayed oscillating motion in relation to tidal advection. Although a few species (e.g. Pleurobrachia pileus) exhibited higher densities around low tide, maximum densities were observed for most species (e.g. T. longicornis and E.nordmanni) around high tide, according to salinity variations. Diurnal changes were only reported for cyclopoid copepods (i.e. Halicyclops sp. and Cyclopina sp.) which were mainly endobenthic during the day and moved into the water column at night. Besides temporal changes in depth-averaged densities, most species exhibited vertical migrations at diel and/or tidal periods. Tidal vertical migrations were reported only for a few taxa and could be the result of passive mechanisms (e.g. vertical mixing) rather than of active behaviour. Diel vertical migrations were observed in most of the abundant taxa. While this migration pattern did not appear to be an adaptation to predator avoidance within the Seine ROFI, it could regulate horizontal transport of organisms and promote their retention. The consequences of the short-term mesozooplankton fluctuations for sampling designs are discussed.