A. Cattrijsse

Spatial patterns of the hyperbenthos of subtidal sandbanks in the southern North Sea

Abstract The hyperbenthos of the Belgian continental shelf, the Dutch continental shelf off Zeeland and the Westerschelde estuary was sampled. Seven biotic communities were identified and the distribution was correlated with the presence of isolated sandbanks. A community in the Westerschelde estuary was different from the shelf hyperbenthos and was characterised by the highest density and biomass and lowest diversity. Two gradients in community structure were detected on the continental shelf: the principal onshore-offshore gradient perpendicular to the coastline and a less pronounced east-west gradient parallel to the coastline. The first gradient mainly indicated differences in density and biomass, while the second reflected species richness and diversity. The holohyperbenthos reached highest densities onshore and consisted nearly exclusively of mysids; Schistomysis spiritus and S. kervillei were the most common species. The high levels of suspended matter and mud in this area probably advantage this motile fauna in terms of food. Community composition altered with distance from the shore and planktonic species increased in abundance. Estuarine influence may have caused a decline of species richness eastward. The merohyperbenthos was most abundant at a certain distance from the coast and was mainly composed of a variety of larval decapods. Biomass peaked onshore with a dominance of postlarval gobies. A strong diversity gradient was found for the merohyperbenthos with a richer fauna off the Belgian coast than in the waters off Zeeland. Current direction and habitat heterogeneity are believed to be the most important structuring factors.