Response of zoobenthic communities to changing eutrophication in the northern Baltic Sea

The relationships between the concentration of water nutrients and the biomass of benthic invertebrate feeding guilds were examined at 46 sites in the northern Baltic Sea during 1993-2003. We analysed whether and how degree of exposure, presence of fronts, salinity, hypoxia, nutrient concentrations, depth, sediment type and structure of invertebrate communities contributed to these relationships. In general macrozoobenthos did not respond to the changing nutrient concentrations in the areas that were regularly impacted by fronts (river estuaries, bank slopes, straits). Macrobenthic species diversity, depth, 11-year average of nutrient concentration and sediment type explained best how strong the nutrient-invertebrate relationships were. The deposit feeders, that inhabited more diverse communities, were less sensitive to the increased concentration of nutrients than those in less diverse communities. On the other hand, the sensitivity of suspension feeders to rising nutrient load increased with benthic diversity. The response of macrozoobenthos to nitrogen level decreased with increasing depth. Our data did not support the hypothesis that there was a significant difference in the occurrence of nutrient-invertebrate relationships between hypoxic and normoxic conditions. The probability of finding negative nutrient- invertebrate relationships increased with depth. The results pointed to nitrogen limitation in the coarse and fine sediments and phosphorus limitation in the mixed sediments. Increased nitrogen values strengthened the response of suspension feeders to the concentration of phosphorus. Increasing phosphorus level dampened the relationships between benthic functions and concentration of phosphorus. This study confirmed that depth and sediment type were the best regularly monitored abiotic variables that could be used to determine the type areas within the northern Baltic Sea in sensu the European Community Water Framework Directive. As the nutrient-invertebrate relationships were significantly modified by macrobenthic diversity, the environmental classification should incorporate specific biological measures such as benthic diversity in order to better describe the quality status of the water body.