THE HYPERBENTHIC AMPHIPODA AND ISOPODA OF THE VOORDELTA AND THE WESTERSCHELDE ESTUARY

Data are reported on the amphipods and isopods taken in a sledge-type hyperbenthos sampler in the subtidal of the Voordelta, a shallow coast al area and the Westerschelde, the last true estuary of the delta area (SW Netherlands). More than 300 samples were collected between 1988 a nd 1991. Two salt marshes in the brackish part of the estuary were sam pled monthly with a flume net in 1990 and 1991. A total of 46 amphipod species (42 gammarideans, 3 caprellids and 1 hyperiid), 19 of which a re new for the area were recorded. The dominant amphipod species in th e Voordelta and the marine part of the Westerschelde estuary are Atylu s swammerdami and Gammarus crinicornis. Estimated densities of both sp ecies are high comparison to these estimated from macrobenthos samples taken in the same area. This suggests that an important part of the p opulation swims above the bottom and is subsequently overlooked in ben thos surveys using only grabs or corers. Some amphipod species that ha d not been previously recorded from the area were found to be quite co mmon inhabitants of the hyperbenthal.The dominant species in the brack ish part of the estuary are Corophium volutator, Gammarus salinus, Ple usymtes glaber and Bathyporeia species. In the salt marshes the same s pecies are dominant but densities are much higher than in the main est uarine channel. Gammarus zaddachi is only present in the salt marshes. The only abundant Isopoda are Idotea linearis in the Voordelta and Pa ragnathia formica, Eurydice pulchra and Lekanesphaera rugicauda in the salt marshes. From a total of 8 species, there were no new records fo r the study area.