Variation in the sublittoral macrozoobenthos of the Baltic Sea along environmental gradients: A functional-group approach

The enclosed Baltic Sea, one of the world's largest brackish water basins, resembles a large estuary with steep horizontal and vertical environmental gradients. Thus, salinities range from 25 to 30 ppt in the Danish Sound area in the south to 1-3 ppt in the inner reaches of the Gulfs of Bothnia and Finland, and a persistent pycnocline in the Baltic basin causes stagnation of bottom waters for long periods, with periodic hypoxia/anoxia as a consequence, over an area covering up to 100 000 km(2). Further, climatic variation from boreal to subarctic causes additional stress on the ecosystem. In recent decades, eutrophication and pollution have also significantly affected the biota of the Baltic Sea. The soft bottom infauna is poor in terms of species composition, and functional complexity is considered to be low. This paper examines the estuarine soft bottom infauna of the Baltic Sea along some principal environmental gradients using a functional-group perspective. We have used the functional-group concept (primarily feeding type, mobility and microhabitat), designed for polychaetes by Fauchald and Jumars (1979), to analyze and illustrate if and how the environmental gradients are reflected in the zoobenthos. A total of 25 functional groups were identified, forming dines from complex functional communities in the south and west, towards functionally poor assemblages in the north and east. The shift in functional groups indicates a loss of carnivores, tentaculate sessile organisms, and burrowers from areas beyond the Baltic and its marine approaches towards the inner bays. On the other hand, suspension feeders and surface deposit feeders increase in importance. In the northernmost areas of the Baltic only 1-3 functional groups are found, compared to 8-20 in the south.