Benthic assemblages associated with rocks in a brackish environment of the southern Baltic Sea

Abstract

Sandy bottoms, with local patches of rocks, dominate the southern Baltic Sea coast. These rock patches create three-dimensional habitats with additional niches that can support diverse assemblages of organisms. In this study we investigated macrofaunal assemblages associated with the boulder field in the brackish Gulf of Gdansk. Of the 30 recorded taxa three animal species (Mytilus trossulus, Balanus improvisus and Electra crustulenta) together with five species of algae were directly attached to rocks. These engineering organisms provided habitats for a further 22 taxa. Among the fauna directly associated with rocks, barnacles (76%) were the most abundant while among indirectly associated biota, oligochaetes were the dominant group (45%). Rock size and algal biomass explained most variance in macrofaunal structure of the assemblages investigated. There were statistical differences in assemblage structure between two separate localities within the rocky patch, despite environmental homogeneity (salinity, water temperature and structure of sea bottom). These differences in assemblage structure were mostly due to differences in dominance of particular species rather than in species composition. Our results show that rocky patches in an otherwise soft sediment habitat provide additional living space for macrofauna leading to an increase in local biodiversity and organismal abundance.