THE MACROBENTHOS OF THE NORTH-SEA

The potential threat to the North Sea ecosystem due to pollution by organic and chemical wastes, has directed attention to the benthic assemblages in the area. Because these assemblages comprise a wide range of ecological groups and species, many of which have a life span surpassing one year, they integrate effects of pollution over long periods of time. Knowledge on the spatial distribution of different types of assemblages allows assessment of the extension of possible effects over large areas. Analysis of the macrobenthos collected in the Dutch, German and Danish sectors of the North Sea during the synoptical ICES NORTH SEA BENTHOS SURVEY, showed that the samples from this area can be grouped into four clusters on the basis of macrofaunal similarity. Because the geographical distribution of the clusters correlated well with sediment characteristics and depth, for convenience the clusters can be described as shallow coarse- (medium-)sand, shallow fine-sand, shallow muddy-sand and deep muddy-sand. These four clusters differ not only with respect to species composition, diversity and biomass, but also with respect to their consumption and production of organic material. The differences between the structure and functioning of the clusters are interpreted in the light of local differences in the production and deposition of organic material and physical stress on the bottom. Because of the paucity of long-term series of observations, the important question as to whether the present situation shows signs of eutrophication cannot be addressed for the entire investigated area. In the German Bight, where nutrient levels have increased during the past decades and heavy algal blooms have been reported, the macrobenthic communities have not shown a major change. Even after mass mortality among benthic animals due to oxygen deficiency following the breakdown of algal blooms, the benthic community rapidly regained its former structure. This is explained by the dominance of physical factors in shaping the community of this shallow sand habitat. In the more protected muddy-sand habitats, on the other hand. similar events may lead to more permanent changes due to the importance of biological interactions. The available data from studies on the effects of single-point organic inputs on macrobenthic species can be used to predict the sort of changes that would take place in the case of large-scale eutrophication and can help to determine whether this is taking place.