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. |
