Justification of the Possible Biogenic Load on Onegskoe Lake and Prognostic Assessments of Changes in Its Ecosystem Under Different Climatic and Socio-Economic Scenarios

The article presents a substantiation of the possible nutrient load on Lake Onego and its catchment area by the middle of the 21st century, taking into account the RCP climate change scenarios Onegskoe Lake (Onego) and socio-economic conditions. The socio-economic development estimates are obtained based on the analysis of official statistical data and model estimates for the seven largest regions included in the lake catchment area. Possible climate changes and their impact on the lake water balance are considered. It is shown that the implementation of the RCP 2.6 scenario can lead to a decrease in the nutrient load on the lake from the main tributaries due to a decrease in river runoff, and with the implementation of the RCP 8.5 scenario, an increase in runoff and a corresponding increase in the removal of phosphorus and nitrogen by 14.0–22.5% should be expected. Calculations based on the ILLM model (Institute of Limnology) for assessing the diffuse nutrient load showed that the introduction of the best available technologies (BAT) in agricultural production will not have a noticeable impact on the ecological state of Lake Onego. Estimates of the possible increase in nitrogen and phosphorus input from the catchment area and its sub-basins to the lake by 2050, while maintaining the growth rate of fish farming in the water bodies of the catchment area, show that a slight increase in the nutrient load on the lake for phosphorus of less than 4% can be expected compared to 2021. However, if the production growth rate increases, the nutrient load may increase to 30% compared to the current level. Estimates of the permissible load (PL) and assimilation potential (AP) of the lake for nutrients to maintain the trophic status of the lake are presented. The obtained calculations formed the basis for assigning the biogenic load to the lake until 2050 to obtain estimates of possible changes in the lake ecosystem on a 3-D SPLEM model with high spatial resolution (1 km), which showed that, if the considered scenarios are implemented, the main increase in primary production of phytoplankton until 2050 will be observed in the bays and gulfs of Lake Onego, while in the open areas of the lake a slight increase in primary production may be observed.