Soft bottom benthos of the Baltic Sea

The zoobenthos of the Baltic Sea is characterized by arctic boreal species (the glacial relict like Saduria entomon or the Astarte species) and lusitanian forms. The species richness of the benthic community declines from the Skagerak / Kattegatt via the Belt Sea to start raising again in the area dominated by the freshwater communities. A lot of species living in the central Baltic Sea reach their reproduction limits at the western entrances, while other species need to be drifted into the Baltic Sea as larvae from the Kattegatt because the reduced salinity inhibits their reproduction in the Kiel Bay. Lime building species have thinner shells in the eastern part of the Baltic Sea and in the Gulf of Bothnia in comparison to their relatives from the North Sea, also as an indicator of physiological stress due to reduced salinity.

In the shallow areas of the Kiel Bay you will find a Macoma balthica dominated community, while the community in the deeper areas is dominated by the Abra alba or Arctica islandica (Abra alba may dissapear sometimes according to unfavourable environmental conditions). This is more or less a description of a dynamic process and not a fixed organisation in the sense of Petersen. Zenkevitch tryed to classify the baltic benthos community as a whole, but this is considered out-dated. The characteristic species of the deep basins have vanished for casual colonisation by opportunistic species. These species will explosively increase their population by the first improvement of environmental conditions. Normally these species have extended pelagic larval stages supporting a wide spreading colonisation. Enduring improved environmental conditions discriminate the opportunistic species and support seasonal species, which can build up huge amounts of biomass in short time (e.g. the large ocean quahog Arctica islandica). 'conservative' species represent the back bone of a community in consistent conditions. Right now these species are on the retreat while conditions fluctuate. In general, a community is seeking for a state of higher complexity, species richness and biomass and a higher stage of maturity. The dynamics and disturbing environmental conditions cause draw back and the community starts from the scratch. The 'resilience' and ability to recover after disturbing circumstances is a characteristic feature of the Belt Sea benthic fauna.