| Abstract: | Sedimentation in the estuary of the Neva River (Gulf of Finland) has been significantly altered in recent years by the construction of a flood prevention barrier for St Petersburg. Concentrations of toxic organic substances and heavy metals have increased 1-5-3-0 fold. a shift in sediment redox potential has occurred in most of the fine-grained particles and the concentration of dissolved oxygen in the bottom waters has decreasd to 0-5-1-0 mg 1-1. the changing environmental conditions and intensive human activities in the area have led to the remobilization of the sedimentary material, threatening the ecosystem with secondary pollution. Not only direct industrial discharges are responsible for the contaminants identified; human activities throughout the catchment area and atmospheric deposition affect the capacity of the ecosystems to absorb pollutants and result in the increased migration of persistent pollutants into the Neva estuary. The functions, factors and processes contributing to the transport and sedimentation of toxic substances in the drainage basin of the Lake Ladoga/Neva River system is discussed. Physical (deforestation, drying of marshes) and chemical factors (industrial pollution, agrochemical use) are reviewed. the role of eutrophication in Lake Ladoga in influencing the migration and bioavailability of toxic substances is discussed with respect to its function as a filter and bioreactor. the temporal and spatial variations in the distribution of suspended and dissolved matter in the lake, which is dependent on its morphometric and hydrological characteristics, change the patterns of transport of toxic substances into the Neva estuary. the ecological status of the Neva estuary is reviewed with particular emphasis on the concentration of pollutants in sediments, redox shifts and dumping and drainage activities. the role of gas production and evolution in the remobilization of lipophilic organic substances is considered to be of major importance in shallow estuaries such as this. |
