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In the Helsinki Commission Red List project 2009-2012, taxonomic and distributional data of benthic (macro) invertebrates were compiled by the present authors in a comprehensive checklist of the Baltic Sea fauna. Based on the most recent and comprehensive data, this paper presents the diversity patterns observed among benthic invertebrates in the Baltic Sea. As expected, the total number of species per sub-region generally declined along the salinity gradient from the Danish Straits to the northern Baltic Sea. This relationship is well known from the Baltic Sea and has resulted in a general assumption of an exponentially positive relationship between species richness and salinity for marine species, and a negative relationship for freshwater species. In 1934, Remane produced a diagram to describe the hypothetical distribution of benthic invertebrate diversity along a marine-freshwater salinity gradient. Our results clearly indicated the validity of this theory for the macrozoobenthic diversity pattern within the Baltic Sea. Categorisation of sub-regions according to species composition showed both separation and grouping of some sub-regions and a strong alignment of similarity patterns of zoobenthic species composition along the salinity gradient. [ABSTRACT FROM AUTHOR]

Many studies worldwide have emphasized the need to mop the potential of commercial ports to function as receiver and donor areas for nonindigenous species (NIS). In this study, the macroinvertebrate composition of hard and soft sea bottoms, hydrographical conditions, and traffic connections of fonr Finnish coastal harbours (Naantali, Koverhar, Porvoo, Hamina) were investigated and compared with 'natural' reference areas lacking anthropogenic constructions. The harbours hosted macroinvertebrate communities fairly similar to those found in adjacent reference areas, but sediment community diversity was slightly higher in harbour basins relative to in surrounding areas. A total of eight NIS were registered in the study. With regard to hydrography, NIS that tolerate low salinities, low temperatures and eutrophic conditions are most likely to survive local conditions and hence pose a potential risk for larger areas of the Baltic Sea. A ship traffic analysis revealed that the most important donor and recipient areas of ballast water to and from the study harbours are coastal harbours of other parts of the Baltic Sea and of the North Sea. Some suggestions are made for future harbour surveys along the Finnish coast. [ABSTRACT FROM AUTHOR]

After the mid-1990s the phosphorus concentrations and budget of the Gulf of Finland have been strongly affected by the enhanced benthic P input caused by reduced conditions at the sediment-water interface. Especially in late summers and autumns of 1996 and 2001 the total areas of reduced bottom sediments were large enough to affect the P concentrations of the whole gulf. The total benthic release of phosphate-P, on an annual level, can be several times higher than the bioavailable P load from the catchment of the gulf. The area of reduced surface sediments varies considerably both spatially and from year to year, especially in the relatively shallow eastern gulf. The intensity of the internal P loading is greatly controlled by vertical stratification, bottom topography, organic matter content of the sediment surface layer, and the abundance of benthic fauna. Evidently the sediment efflux can strongly affect the annual P balance and dynamics of the Gulf of Finland and counteract the decreases in the external P loading. This is verified both by inter-annual concentration changes and experimental results. The only way to reduce the benthic release of nutrients is to decrease primary production and the succeeding sedimentation of organic matter via further cutting external nutrient inputs both directly to the Gulf of Finland and to the Baltic Proper, which is an important source of nutrients for the gulf. [ABSTRACT FROM AUTHOR]

In 1928, Sven G. Segerstråle made detailed analyses of benthic communities in the Tvärminne region (SW coast of Finland). In addition to traditional presentation, he included illustrative photographs where the collected macroinvertebrates were laid on black background, in natural density, and reproduced in their natural size. This approach was replicated in the present study. The photographs highlight changes that have taken place during the past century in the benthic communities. In the deeper area there has been a change from an amphipod (Monoporeia affinis) dominated community to one dominated by the Baltic clam Macoma baltica. In the shallower study area the diverseCorophium volutator-M. baltica-chironomid community has been replaced by a community dominated by Macoma and a North American invader, the polychaeteMarenzelleria viridis. Furthermore, the density of macroinvertebrates has declined considerably. The causes for these changes remain unclear, but are most likely connected to eutrophication related factors in the coastal ecosystem and concurrent interspecific interactions. [ABSTRACT FROM AUTHOR]