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The hyperbenthic Amphipoda and Isopoda of the Voordelta and the Westerschelde estuary

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Data are reported on the amphipods and isopods taken in a sledge-type hyperbenthos sampler in the subtidal of the Voordelta, a shallow coastal area in the Westerschelde, the last estuary of the delta area (SW Netherlands). More than 300 samples were collected between 1988 and 1991. Two salt marches in the brackish part of the estuary were sampled 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 are new for the area were recorded. The dominant amphipod species in the Voordelta and the marine part of the Westerschelde estuary are Atylus swammerdami and Gammarus crinicornis. Estimated densities of both species are high comparison to these estimated from macrobenthos samples taken in the same area. This suggests that an important part of the population swims above the bottom and is subsequently overlooked in benthos surveys using only grabs or corers. Some amphipod species that had not been previously recorded from that area were found to be quite common inhabitants of the hyperbenthal. The dominant species in the brackish part of the estuary are Corophium volutator, Gammarus salinus, Pleusymtes glaber and Bathyporeia species. In the salt marshes the same species are dominant but densities are much higher than in the main estuarine channel. Gammarus zaddachi is only present in the salt marshes. The only abundant Isopoda are Idotea linearis in the Voordelta and Paragnathia formica, Eurydice pulchra and Lekanesphaera rugicauda in the salt marshes. From a total of 8 species, there were no new records for the study area.

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