Petersen's benthic macrofauna stations revisited in the Öresund area (southern Sweden) and species composition in the 1990s - signs of decreased biological variation

Six stations in Öresund and Skälderviken between Kattegat and the Baltic were revisited in 1990. These were originally established in 1910-1912 before large-scale industrialization and modern agriculture began. Total biomass and abundance were generally higher in the 1990 samples and the species' average size was significantly smaller. Fewer benthic fauna communities, fewer species and increased dominance of the ophiuroid Amphiura filiformis were noted. Generally, the changes indicate a transition from great biological variation, with different dominants, to conditions where a few species dominate, especially Amphiura filiformis and deposit-feeding polychaetes. The changes in species composition were supported by other data from early in the 1900s and from surveys performed in 1989-2000. These confirmed several of the observations made in 1990. The observed fauna changes correspond in several aspects with results from the Oslo Fjord, Skagerrak, Kattegat and the Belts, suggesting certain largescale changes and emphasizing the need to study the biological diversity and distribution of species. Some of these changes seem to have taken place during the last decades, parallel with increased nutrient load to the coastal waters, with subsequent high sedimentation of organic material and oxygen deficiency. High concentrations of different xenobiotics, e.g. organic tin compounds, might explain why some species of gastropods are very rare today. Trawling or changed hydrographical conditions are not likely to have caused the observed great structural changes. Considering the high turnover of water in Öresund, an impact in this area is particularly alarming.