Impacts of physical environmental characteristics on the distribution of benthic fauna in the northern Baltic Sea

The Baltic Sea is characterized by steep and strong environmental gradients in physical and chemical parameters which contribute to the geographic distribution of biota. At small scales, uneven bottom topography and heterogeneous sediment features induce high habitat diversity for example in the archipelago areas in the northern Baltic Sea. Here we analyse the impact/importance of sediment types and depth on the distribution of benthic animals in a relatively small (ca. 10 km²) archipelago area consisting of a mosaic of different benthic habitats interspaced with islands and skerries. A total of 26 major taxa of benthic macrofauna were found/observed in the study area. Results of the CCA analyses where depth and sediment types (clay, mud, sandy silt, fine sand, coarse sand, gravel and stones) were chosen as the environmental factors, revealed that habitat preferences of zoobenthic species were strongly affected by sediment types. Depth also impacted zonation of the species, but did not change the zonation patterns based on sediment characteristics. Macoma balthica, Saduria entomon and Harmothoe sarsi were identified as the only generalists while all other species/taxa showed clear correlation to different sediment types and depth zones. The animal-sediment patterns found reflected largely differential feeding modes of the species. The results clearly showed that sediment characteristics are of decisive importance for the composition of benthic fauna on soft bottoms.