Eutrophication is the major threat for marine ecosystems in coastal and archipelago areas of the Baltic Sea. Eutrophication-induced effects on benthic macrofauna were studied over a 30 year period in the Aland Islands, northern Baltic Sea. This study describes the present soft-bottom community and analyses changes in community structure in definable archipelago zones, and relates them to environmental changes since the 1970s. Similar analyses are made for changes in the benthic communities in the transition zone between the outer archipelago and the open sea over the last decade. Abundance and biomass peaked in the late 1980s after which a general decrease has taken place, but in different ways in the various archipelago zones. This decline seems to be closely linked with local nutrient loads, rather than to the overall trends in the Baltic Sea. A new species in this low-diversity area is the polychaete Marenzelleria viridis, which since its first observation in 1993 has rapidly spread over the entire region and is now among the five most dominant species in the benthic assemblage.
