ABSTRACT
Because of its biogeographic history, the Baltic Sea is unique as an ecological formation. It originated as recently as 10,000 to 12,000 years ago, after the retreat of the last great glaciers from north-western Europe. The Baltic basin is relatively isolated from the main body of the Atlantic Ocean, to which it is connected through the Danish Straits. The water is brackish, with a salinity gradient of 0.6 to 0.8 per cent, to almost zero on the surface from south to north; this is a severe stress to both marine and freshwater organisms. The Baltic draws its waters from a huge and mostly densely populated catchment area that extends over eastern Scandinavia, Finland, north-western Russia, the Baltic states and western Poland; the total human population was approximately 80 million in 1989 (Jansson and Velner, 1995). This is a perfect set-up for serious pollution problems; Jansson and Velner (1995, p332) describe the particular fragility of the Baltic ecosystem as follows:
The large catchment area; the semi-closed cold water body, heavily stratified without tide; and a water residence time of several decades make the Baltic a very sensitive area. The intermittent pulses of oxygenated North Sea water seem to come in long intervals, leaving periods of stagnation and low oxygen levels in between.
