One of the important tasks facing the Baltic Sea countries is the reduction of the nutrient loading of the coastal waters. According to recent estimates, 70% of phosphorus and 50% of nitrogen total inputs to the Gulf of Finland are discharged from the River Neva drainage basin (Pitkanen, 2001), covering about 80 % of the whole GOF catchment and populated by 9 million people, including 4.5 million inhabitants in St. Petersburg. However, the basic problem of the eutrophication in the Gulf of Finland is defined by the interaction of natural hydrodynamic and biogeochemical processes with the anthropogenic inputs of nitrogen and phosphorus and the interaction with the Baltic Proper. The complexity of phosphorus cycle in the shallow-water part of the Gulf is determined by a number of processes (Shpaer, 1997), especially in the area of mixing between fresh and brackish waters. Therefore it is still not clear how the Eastern Gulf of Finland ecosystem would react to the nutrient load reductions, how such reductions would affect blue-green algae blooms and other symptoms of eutrophication. This must be taken into account when developing recommendations for the water protection measures.
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