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WatersBefore the 1960's Lake Ladoga was an oligotrophic water body setting the standard of highest water quality. Intensive development of economic activities in the catchment area and the lack of water protection measures in the following 25-30 years deteriorated the status of the lake. Early in the 1990's the lake was classed as weakly mesotrophic in the central and northern parts with some eutrophic features in the southern part. Intensive cyanobacterial blooms were observed in the summer. Lately however, as some pulp-and-paper mills were closed and the general activity of industrial and agricultural production reduced self-purification processes in the lake intensified, and Ladoga is currently recovering its oligotrophic status. Changes in the Lake Ladoga ecosystem can be traced to two main causes: anthropogenic eutrophication and pollution. At present, concentrations of nickel, copper, zinc, lead, cadmium and cobalt in the bottom water layers near the southern shore of Lake Ladoga and in the rivers emptying into it are close to the average concentrations in the rivers of the world. Iron content in the lake water is 3 times higher than the average in the world, in the river water - 13 times higher. The concentrations of most microelements decrease where the river and lake waters mix. The content of suspended metals drops 10-300 times, of dissolved metals - 2-20 times. Two localities with the dissolved heavy metal concentrations abnormal for Lake Ladoga were found. The first one is the Morje river estuary zone, where the concentrations of nickel, cadmium and copper are 20, 10 and 3 times higher than the background concentrations respectively. The other locality is the western part of the Petrokrepost bay, where copper and zinc concentrations exceed the background 20 and 10 times respectively. The sanitary-toxicological assessment of the near-shore waters in the lake (1989 - 1991) demonstrated that the areas noted for particularly heavy pollution alongside with eutrophication and microbial pollution were the Volkhov bay, eastern shore areas (estuaries of the rivers Olonka, Tuloksa, Tulemajoki) and some northern shore areas (towns of Priozersk, Pitaranta, Lahdenpohja). Heavy microbial and toxic pollution with the lower level of eutrophication was observed in the Svir bay and a number of the eastern shore areas (estuaries of the rivers Obzhanka and Vidlitsa). |
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