Content of organic matter and nutrients in the hydro-cryogenic system of Lake Onego

The article presents the results of a survey of the content of nutrients, organic matter and suspended substances in the snow-on-ice/ice/under-ice-water system in the Central part of Lake Onego and its largest bays (Petrozavodsk Bay and Kondopoga Bay) carried out in March 2021. The study revealed patterns in the distribution of macro- and micronutrients in a complex hydro-cryogenic system. The highest content of total phosphorus (TP) and total nitrogen (TN) was found in under-ice water. Nitrogen-bearing substances in the snow cover of Petrozavodsk Bay and the central part of the lake were dominated by nitrates, and in Kondopoga Bay by ammonium nitrogen; TP was chiefly represented by mineral forms. The upper layer of ice in the studied areas of the lake is characterized by higher concentrations of nutrients than the lower layer. In under-ice water at the top of Petrozavodsk Bay, where the Shuya River discharges, the prevalent form was total organic nitrogen (TON), in the outer part – nitrates, in the impact area of the Petrozavodsk City water treatment facilities – ammonium nitrogen; the share of inorganic phosphorus decreased from the top of the bay towards its outlet (from 63 to 40 % of TP). In Kondopoga Bay, affected by pulp and paper mill wastewater, nitrogen was represented mainly by mineral forms, their share decreasing from the top towards the outlet, whereas the trend in the spatial distribution of phosphorus forms was the opposite. The content of organic matter (OM) in snow and ice in the studied regions of Lake Onego was low. The higher OM concentration in under-ice water of the bays compared to the central part of the lake is due to the influence of river runoff and wastewater discharges.