Zoobenthos as indicators of marine habitats in the northern Baltic Sea

Zoobenthos are beneficial indicators of habitat quality and environmental stress and change due to their longevity and how they react species-specifically to abiotic factors. This PhD, consisting of four published papers, investigates the effects of physical and chemical variables on zoobenthic species in space and time, as well as the indicator value of the distinct zoobenthic taxa studied. Furthermore, I investigate the linkages between zoobenthos and their habitats in the northern Baltic Sea. The individual studies of this thesis were conducted at different spatial and/or temporal scales. Specific species were found to be indicative of different habitat types. Sediment type (clay, mud, silt, sandy silt, fine sand, coarse sand, gravel, stones) was found to be an important driving factor for zoobenthic species distribution, especially on medium (between ~100 – 1000 m) spatial scales. On a large landscape scale (several kilometres), seabed topographical factors were more significant drivers. The number of species generally increased with increasing topographical complexity, but the most complex habitats were not the most species rich. Physical and chemical factors (temperature, oxygen, nutrients and salinity) in the water significantly impacted species distribution along the Finnish coastal zone of the Gulf of Finland. On the temporal scale of a few decades, drastic changes have occurred in species assemblage composition. The greatest change occurred in the beginning of the 1990s, as the invasive polychaete Marenzelleria spp. arrived and rapidly exponentially increased in abundance. At the same time the crustaceans Monoporeia affinis and Pontoporeia femorata decreased or nearly vanished from the study sites. These changes have had undeniable consequences for the functioning of the ecosystem, due to the different functional traits of the species. The changes in species composition are simultaneous with long-term temperature increases as well as with decreases in salinity and oxygen concentrations in the area. The results of the studies can be applied as a tool to assess species responses to habitat and water quality changes. In addition, the results provide information on the status of the biological diversity in the sediment water interface in the Gulf of Finland during previous decades. In the future it is critical to assess the impacts of the changes in species composition on the functioning of the ecosystem.