Lake Ladoga (the largest lake in Europe) was characterized by good water quality some decades ago . Now as a result of human impact, the saprobity and toxicity of water have changed, phytoplankton biomass has increased, and populations of many sensitive species have declined . Morphological deformities of invertebrates are observed in heavily polluted areas and toxic effects are commonly encountered in fishes . The ecological crisis of Lake Ladoga is caused by human activities in its drainage area . This area is characterized by a relatively high degree of industrialization and economic development . The adverse environmental effects appear to be spatially irregular, occurring mostly in isolated bays and near-shore areas, especially when thermal bars are present, but there is also evidence of general increase of nutrient concentrations in the entire lake . Urgent measures should be taken for restoration of Lake Ladoga ecosystem .
Silty and clayey sediments predominate in the bottom sediments of the deep basin of northern Ladoga . Chemical fractionation of organic constituents of the sediments indicate prevalence of chemically stable fractions in most samples . The sedimentary diatom assemblages contain large numbers of littoral taxa, but more than 90% of the total counts of frustules belong to planktonic forms . The macro-and meiozoobenthic communities are characterized by a limited number of species, and the biomasses are low as a result of low temperatures and the small amount of organic matter entering the sediments . There are several restricted areas, e .g. shallow bays, in which pollution by pulp and paper mill or sewage effluents has led into significant changes in the benthic conditions, including increased organic content and heavy metal concentrations of the sediments . There is no apparent change in radioisotope activity, and maximum values do not exceed the natural activity of Karelian granites . Benthic biocoenoses have been affected especially near the sources of anthropogenic pollution, where extensive lifeless areas, polysaprobic zones, and morphological deformities of invertebrates have been observed . Some definite changes have taken place even in the deep profundal benthic communities (e .g. decline of Pallasea quadrispinosa and Gammaracanthus loricatus, increase of Potamothrix hammoniensis and Limnodrilus spp .) and sedimentary diatom assemblages in the open lake (increase of Diatoma elongatum, Asterionella formosa and Aulacoseira italica) since the 1960's . Since the closure of two of the pulp mills and diversion of effluents at a third site, the situation has locally improved, especially at those sites where water exchange is effective . 23
A geochemical study of lacustrine sediments was undertaken as part of a major paleolimnological project concerned with the impact of man on lakes. Factor analysis was applied to the geochemical data obtained from a core from one lake. Three principal factors which explained most of the variance of the initial data were identified. Factor I is related to the organic:mineral component ratio of the sediment, and can be considered as an indirect index of change in lake trophic status through time. Factor II is related to the granulometric composition of the sediment and matches evidence for changes in the hydrological regime of the lake. Factor III reflects changes in redox potential and is more closely related to processes occurring within the lake than other factors. Specific geochemical associations of elements are connected with each factor and are the factor indicators. The correlation of these associations are analyzed as geochemical indices of the variability of sediment accumulation conditions in time.
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