BOTTOM SEDIMENTS AND BIOCOENOSES OF NORTHERN LADOGA AND THEIR CHANGESUNDER HUMAN IMPACT

Silty and clayey sediments predominate in the bottom sediments of the deep basin of northern Ladoga. Chemical fractionation of organic const ituents of the sediments indicate prevalence of chemically stable frac tions in most samples. The sedimentary diatom assemblages contain larg e numbers of littoral taxa, but more than 90% of the total counts of f rustules belong to planktonic forms. The macro- and meiozoobenthic com munities are characterized by a limited number of species, and the bio masses are low as a result of low temperatures and the small amount of organic matter entering the sediments. There are several restricted a reas, e.g. shallow bays, in which pollution by pulp and paper mill or sewage effluents has led into significant changes in the benthic condi tions, including increased organic content and heavy metal concentrati ons of the sediments. There is no apparent change in radioisotope acti vity, and maximum values do not exceed the natural activity of Karelia n granites. Benthic biocoenoses have been affected especially near the sources of anthropogenic pollution, where extensive lifeless areas, p olysaprobic zones, and morphological deformities of invertebrates have been observed. Some definite changes have taken place even in the dee p profundal benthic communities (e.g. decline of Pallasea quadrispinos a and Gammaracanthus loricatus, increase of Potamothrix hammoniensis a nd Limnodrilus spp.) and sedimentary diatom assemblages in the open la ke (increase of Diatoma elongatum, Asterionella formosa and Aulacoseir a italica) since the 1960's. Since the closure of two of the pulp mill s and diversion of effluents at a third site, the situation has locall y improved, especially at those sites where water exchange is effectiv e.