Community dynamics and development of soft bottom macrozoobenthos in the German Bight (North Sea) 1969 - 2000

Abstract

The development and interannual variability of sublitoral soft bottom communities of the German Bight has been studied continuously at four representative permanent stations.A large interannual variability was combined with some changes on roughly decadal time scale. In accordance with large-scale system changes documented for the North Sea, also the composition of benthic communities changed since the late 1960ies. Possible climatic, oceanographic and anthropogenic influences were evaluated by correlating the faunal development with environmental data. The community development at all stations showed clear correlations to the NAOI. The most dramatic changes followed the severe winters of 1970, 1979, 1986 and 1996, when reductions in species number and abundance were discernible at all stations. The shallower stations are characterised by larger interannual changes, and the situation following severe winters is not as clearly different from other years as it is at the deeper stations. The main factors affecting benthic community development are biotic interactions as well as climatic conditions, food supply and the disturbance regime. The most common forms of disturbances are extremely cold winters, hypoxia following algal blooms in stratified waters, and physical disturbance of the sediment by turbulent wave erosion during strong storms or by demersal fishing gear.A clear distinction between the effects of climate, eutrophication, pollution or bottom trawling is hardly possible, not only because various factors are predicted to produce similar effects, but also because the observed changes are a result of the synergistic effects of all factors. The vicinities of the stations were extensively sampled to evaluate the spatial variability and the sample-size-dependencies of community descriptors. Recommendations are derived for offshore monitoring of North Sea soft-bottom macrozoobenthos.