The Baltic Sea and benthic live

There are some premises for the development of benthic live in the Baltic Sea. On one hand is the short geological existence (approximatly 8000 years; in its current form partly only 1500 years). This short time can be seen in the relativly young communities. On the other hand is the benthos of the Baltic Sea influenced by strong oceanographic effects. The western Baltic is so to say an estuary and the salinity gradient is very strong and variable caused by inflow situations, wind drift and convection. The benthos is also influenced by a very strong temperature variability within the normal annual cycle with hot summer and icewinters. The reduced salinity is caused by the freshwater overspill  from rivers al around the Baltic. The light freshwater on top of the water column is flowing outward to the North Sea and heavy saltwater is flowing into the Baltic Sea. This needs the right conditions to achieve an event with enough strength that this new saltwater is able to reach into the large basins in the Baltic proper. The periode between the last large inflow situations 1976 and 1993 were dominated by continuesly decline of oxygen concentration in the deep basins of the Baltic Sea. This decline in the watercolumn had direct influence on the oxygen concentration in the sediments with extensiv consequences for the benthic organisms. Oxygen depletion is a commen event in the western Baltic since the 70ies. Deficit situations mainly happen in the deeper areas with muddy sediments and low current like the Mecklenburg Bay. From time to time it can happen that forced by wind drift water with low oxygen concentration comes into shallow areas. This may result in a fish kills. During the summer season it can be seen that dying macro algae cause local oxygen depletion and influence the water quality.

80 Mio people in the direct watershed of the nearly enclosed Baltic Sea have an influence on the ecosystem. The sewage of 50 Mio people flow nearly direct into the Baltic Sea. The sedimentation of this organic matter causes the consumption of the whole oxygen in this area. Additionally the rivers bring a lot of pollutants into the Baltic Sea. Today it is believed that the eutrophication is responsible for the oxygen depletion in the deep water basins of the Baltic.

But it is necessary to say, that a sea like the Baltic Sea has already experienced such situations before. During the 11. centurary it has been recorded already. It appearce that oxygen depletion is not a new phenomenon to the Baltic, but the frequence of these situations is increasing.