Bottom sediments and biocoenoses of northern Ladoga and their changes under human impact

Abstract

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.