SPATIAL AND TEMPORAL PATTERNS OF THE ZOOPLANKTON IN THE WESTERSCHELDE ESTUARY

The invertebrate zooplankton fauna of the Westerschelde (Belgium and The Netherlands) was investigated during 2 yr by means of monthly samples along a salinity gradient. Copepods were usually the most abundant holoplanktonic metazoans except in the freshwater zone where Rotifera were most numerous. The combination of a classification technique and an ordination-regression technique proved to be a valuable tool for the analysis of such an extensive data set. The presence of 4 groups was established, representing spatially distinct populations but with temporally shifting boundaries. Few zooplankton species were truly estuarine in their distribution, but many were derived from nearby coastal waters. This intrusion of marine species started in spring, reaching their most upstream distribution and highest densities in summer-early fall, then declining and retreating from the estuary, disappearing in winter. Fringing this community was a transition group with low densities, but many species. This brackish-water community consisted predominantly of the calanoid copepod Eurytemord affinis. It appeared in late fall, spread out seaward to obtain its maximum density and distribution in winter-early spring. Densities then declined and the community was absent by late summer-early fall. The freshwater zone near the port of Antwerp, Belgium, was characterized by a paucity of large zooplankters, despite the high primary production in this zone. This is probably due to the low oxygen availability in this area. A canonical Correspondence analysis revealed 2 major environmental axes. The salinity gradient (mainly spatial) explained most of the variance. Strongly correlated with this factor were dissolved oxygen content and secchi disc visibility. The temperature gradient (mainly temporal) was almost perpendicular to the salinity axis, indicating little or no correlation. Of lesser importance was the load of suspended matter, which was highest in the brackish area in autumn-winter. Chlorophyll content of the water was unimportant in explaining community structure. Copepod dry weight was maximal in spring in the brackish part (500 mg m-3); a lower maximum (260 mg m-3) was observed in summer in the marine part of the estuary.