THE BENTHOS COMMUNITY STRUCTURE ANOMALY IN THE LATE 1970S AND EARLY 1980S - A RESULT OF A MAJOR FOOD PULSE

Previous work has suggested a transition in benthic community structure in the late 1970s and early 1980s at two widely spaced sites, one in the western and one in the eastern part of the North Sea. Human impact, such as eutrophication, was suggested to be responsible for this phenomenon. In this paper we further explore this issue by presenting additional time series data from both the benthic and the pelagic environments in the Skagerrak-Kattegat area covering the transition period. Total abundance and biomass data from six stations were analysed for trends with smoothing average technique and changes in species abundance composition was assessed using nonmetric multidimensional scaling. Biomass and abundance at all stations and somatic growth of the opiuroid Amphiura filiformis at three stations showed a marked increase in connection with the transition period. There was no time lag between abundance and biomass at individual stations. Changes in species composition were greater in the transition period than in adjacent periods. Parallel to, and preceeding the faunal changes, land runoff increased dramatically. Winter inorganic nitrogen concentrations as well as chlorophyll-a concentrations in the top 15 m also showed increases in the transition period and were significantly positively correlated with run-off. Benthic abundance, biomass and Amphiura - size seemed to show similar trends as run-off and the best correlation was obtained when the benthic variables lagged run-off with 1 and 2 years. The results are consistent with the hypothesis that sedimentation of pelagic production was particularily high in the transition period and that some of the faunal changes were a response to this event.