Zooplankton distribution across the brackish and freshwater zone of the Scheldt estuary

Zooplankton diversity and distribution in the brackish and freshwater zone of the Scheldt is studied, as a continuation of preceeding studies carried out in the periods 1967-1669 and 1995-1997. We report on species diversity and abundancy of the zooplankters, collected on a monthly basis, in 20 sampling sites along the salinity gradient. Calanoids occur mainly in the brackish part, with Eurytemora affinis, Acartia tonsa and Eudiaptomus gracilis as the dominant species. In the freshwater region, rotifers (i.e. Brachionus calyciflorus, Keratella quadrata, among others), cyclopids (i.e. Acanthocyclops robustus, Cyclops vicinus) and cladocerans (Daphnia longispina, Bosmina longirostris...) are the dominant zooplankters. In addition, we report the presence of several species which were not detected in the former studies, such as the rotifers Brachionus leydigi var. quadratus, Notholca labis, Platyias quadricornis, Euchlanis dilatata, Gastropus hyptopus, Testudinella patina, Polyarthra dolichoptera and Trichotria sp. ; the cladocerans Acroperus harpae and Biapertura affinis, and the harparticoid Bryocamptus (Bryocamptus) minutus