Petersen's benthic stations revisited. II. Is the Oslofjord and eastern Skagerrak enriched?

Abstract

In summer 1985, 23 of the stations established in 1914 by C.G.J. Petersen in the northern Skagerrak and the Oslofjord, were revisited to compare the present state of the macrobenthic assemblages with the previous condition. The methods used were similar to those of the original survey. The total benthic biomass was significantly higher in 1985 by, on average, a factor of 1.8 over the whole area. Most of this change was due to a highly significant increase of worms, notably polychaetes, and a significant increase of ophiuroid and echinoid echinoderms. Similarity values in terms of biomass suggested that community composition had changed considerably over the 70-yr period, in particular in the inner and central Oslofjord (11% similarity) but also in the outer Oslofjord and the Skagerrak (33 % similarity). There was no difference in the comparative size of individual organisms over the period. The observed increase in biomass is greater than the increase along the Swedish Skagerrak coast observed in the late 1970s, and well beyond the observed limits of annual variability in such areas. Different causes for the observed changes are discussed and it is suggested that a general organic enrichment has taken place in the Oslofjord.