Ann Dewicke

Seasonal composition and spatial distribution of hyperbenthic communities along estuarine gradients in the Westerschelde

The hyperbenthic fauna of the Westerschelde estuary was sampled in spring, summer and winter of 1990 at 14 stations along the salinity gradient. Mysids dominated the hyperbenthos in each season. Other important species, either permanently (e.g. amphipods and isopods) or temporarily (e.g. fish larvae and decapod larvae) hyperbenthic, belong to a variety of faunistic groups. Spatial structure was stable through time: the estuary could be divided in the same geographically defined zones in each season. Each zone had a characteristic fauna. Gradients in salinity, dissolved oxygen and turbidity correlated strongly with the observed variation in community structure. The spatial patterns dominated over the temporal patterns, especially in the brackish part of the estuary. In the marine part, seasonal differences in the communities were more pronounced due to the occurrence of a series of temporary hyperbenthic species in spring and summer. In each season, the upstream (brackish) communities were characterized by few species occurring in very high numbers, whereas the downstream (marine) communities were composed of many species but at lower densities.

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.

Evidence for an enriched hyperbenthic fauna in the Frisian front (North Sea)

The hyperbenthos of the Frisian front, an enriched benthic area of the southern North Sea, and surrounding waters was investigated along two cross-frontal transects in August 1994 and April 1996. A total of 111 species was recorded, mainly peracarid crustaceans and decapods. On average, Calanus spp. (Copepoda), Schistomysis ornata (Mysidacea) and Scopelocheirus hopei (Amphipoda) represented more than 40% of the total density, while S. ornata constituted 30% of the biomass. Community structure differed strongly between the two months, as shown by the species composition and the much lower densities and biomasses in April. In August, the density of the holohyperbenthic fraction reached pronounced peaks in the Frisian front area: densities for chaetognaths, copepods, amphipods and mysids were one order of magnitude higher than in the surrounding waters. Diversity was also highest at the Frisian front. The high abundances may be explained by active migration and/or by passive transport to the food-enriched area. Merohyperbenthic species showed a less distinct increase in density in the front zone, but a clear south-to-north change in community structure was observed. These species are more heavily subjected to the prevailing tidal flow. The Frisian front fauna seems to be transitional between two merohyperbenthic communities established in late summer, one belonging to the environmentally different sandy Southern Bight to the south, the other to the silty Oyster Ground to the north. In spring, no such hyperbenthic enrichment over the Frisian front was observed. This is most probably due to the strong seasonality of the hyperbenthic fauna and the low water temperature, suggesting that production and subsequent recruitment had not started yet. Alternatively, winter storms could have resuspended particulate organic matter, followed by a rapid migration of the motile hyperbenthos to more northern depositional areas and thus leading to a temporal density decline. We conclude that the Frisian front is an enriched area for the hyperbenthos at the end of summer, as has already been reported for the benthic system. D 2002 Published by Elsevier Science B.V.

Aberrant individuals of Neomysis integer and other Mysidacea: intersexuality and variable telson morphology

Intersexuality in the mysidNeomysis integer is described from samples collected in the Elbe (north Germany), Eems-Dollard (north Netherlands), Westerschelde (south-west Netherlands) and Gironde (southwest France) estuaries. Individuals which had an irregularly shaped or nearly symmetrically rounded telson, rather than a typically truncated one, were also recorded from the four populations studied. A culture experiment with damaged specimens revealed that all types of abnormal telson morphology found in the field can result from regeneration of damaged parts. It is concluded that both intersexuality and aberrant telson morphology are widespread phenomena among estuarineN. integer populations. Both abnormalities seem to be rare in the other common mysid species in the study area: only one intersexualGastrosaccus spinifer and oneSchistomysis kervillei with an aberrant telson were recorded.