RESULTS OF THE STUDY OF LAKE ONEGO ECOSYSTEM IN THE PERIOD OF SUMMER STRATIFICATION BASED ON MEASURED DATA FROM A 2017 EXPEDITION

New data characterizing the hydrophysical features (water temperature), water quality (water transparency, dissolved oxygen, and number of saprophytic bacteria), trophic status (chlorophyll a), and impact of the catchment (water color) of Lake Onego ecosystem wereobtained in August 2017. The vertical profiles of these parameters along lake transects from northwestern polluted near-shore areas receiving river runoff (Petrozavodskaya,Kondopozhskaya bays) to the southern tip of the lake are shown. The results of the surveysshow that the central and southern parts of Onego Lake are still characterized byhigh water quality and oligotrophic state. The epilimnion was about 10 m thick, the watertemperature of this layer did not exceed 14.8–16.7 °C. The thermocline in the deepwaterpart of the lake was located at a depth of 10–20 m. The transparency of the waterin pelagic parts of the lake reached 3.5–4.0 m, water color was 23–31 degrees on Pt-Coscale, oxygen saturation of the water was 87–92 %, chlorophyll a concentration in the photic layer did not exceed 3.2 μg/l, and saprophytic bacterioplankton numbers were within n∙102 CFU/ml. A significant reduction in water color and abundance of saprophytic bacteria, and an increase in dissolved oxygen concentrations were observed along the transect from Petrozavodskaya and Kondopozhskaya bays towards the pelagic part of the lake. This confirms that natural (humic substances) and anthropogenic (municipal and industrial wastewaters) allochthonous organic matter is effectively transformed in the bays. The abundances of saprophytic bacteria, the oxygen saturation of water, and the concentration of chlorophyll a in some areas of the lake were slightly lower than in previous decades. Water color, on the contrary, showed a tendency for an increase.