Dynamics of benthic communities state and the sediment chemical composition in lake Onega under the influence of anthropogenic and natural factors

We consider the dynamics of the sediment chemical composition and benthic community state in different areas of Lake Onega in the last 20–25 years. It is shown that the combined effect of anthropogenic factors and climate changes has caused a profound transformation of deepwater benthic communities. The upper parts of the northwestern bays (Kondopozhskaya and Petrozavodskaya bays), where the effect of anthropogenic factors is the most manifested, are referred to the most contaminated (first) zone. In these areas, structural adjustments of bottom cenoses are noted; they are the dominance of resistant representatives of oligochaetes and chironomids and the absence of relict crustaceans. The sediments of the first zone are characterized by the greatest content of organic matter, reduced values of pH and Eh, the presence of toxic agents (in Kondopozhskaya Bay). In the buffer areas of the north-western bays referred to the second zone the anthropogenic effect is weakened by the influence of oxygen-saturated waters passing from the adjacent open areas of the lake. The bottom sediments of the buffer zones are characterized by a deeper oxidized layer and the maximum development of amphipods. The central deep-water areas of Lake Onega (the third zone) are currently the least affected by the anthropogenic factor. The sediments in the central zone are characterized by the minimum content of organic matter, the presence of ore crusts (containing iron and manganese), which determine the poor development of benthos here. In 1998–2016 in all studied areas of Lake Onega classified as three zones a decrease in the numbers and biomass of benthos occurred, which led to a decrease in its production by 2–4 times. It is suggested that the decline in benthic community indicators is caused by the suppressive action of iron, which is accumulated in sediments. The reason of that is an increasing input of this element with river waters as a part of humic substances due to climatic changes in the catchment area.