Late Summer Trophic Conditions in the North-east Gulf of Finland and the River Neva Estuary, Baltic Sea

Abstract

Trophic conditions and hydrography of the whole north-east Gulf of Finland in both the Finnish and the Russian waters, including the Neva Estuary, were for the first time extensively explored in August 1990. The Neva Estuary was defined on the basis of hydrographical and geomorphological characteristics. The results revealed extensive and strong increasing gradients in nutrients, bio-masses of phytoplankton (especially filamentous blue-green algae) and primary productivity, as well as in the bio-masses of several heterotrophic organisms from the open Gulf towards the inner Neva Estuary. The basic cause of these phenomena is the high and continuous load of nutrients and organic matter from St Petersburg and the River Neva, together with the typical estuarine hydrography and huge reserves of inorganic nutrients immediately below the mixed surface layer. Under typical late summer conditions immediate effects of the River Neva and the St Petersburg region cover the whole estuary ( c. 3000 km 2), i.e. 10% of the surface area of the whole Gulf, but do not reach the open parts of the Gulf or the Finnish territorial waters. However, recycled effects, as well as effects of the huge sub-thermocline nutrient resources, clearly increase the productivity of the whole north-east Gulf and cause the general tendency of increasing eutrophy towards east in the whole Gulf of Finland.