Substantiation of the Possible Nutrient Load on Lake Onezhskoe and Prognostic Assessments of Changes in Its Ecosystem under Different Climatic and Socio-Economic Scenarios

The possible nutrient load on Lake Onego and its catchment area by the middle of the 21st century is substantiated, taking into account the RCP climate change scenarios and socioeconomic conditions. The assessments of socioeconomic development were derived from 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 water balance of the lake are considered. It is shown that the implementation of the RCP 2.6 scenario may reduce the nutrient load on the lake from the main tributaries due to a decrease in river runoff, and if the RCP 8.5 scenario is implemented, an increase in runoff and a corresponding increase in the export of phosphorus and nitrogen by 14.0–22.5% should be expected. Calculations based on the ILLM model (Institute of Limnology of theRussian Academy of Sciences) for assessing the diffuse nutrient load showed that the introduction of the best available technologies in agricultural production will not have a noticeable effect on the ecological conditions of Lake Onego. Estimates of a possible increase in the input of nitrogen and phosphorus from the catchment area and its sub-basins into the lake by 2050 at the current growth rate of fish farming in water bodies of the catchment area, show that a slight increase in the nutrient load on the lake in terms of phosphorus by <4% compared to 2021 can be expected. However, if the rate of production growth increases, the nutrient load may increase up to 30% compared to the current level. The permissible nutrient load and the assimilation potential of the lake for nutrients to maintain the trophic status of the lake are estimated. The obtained calculations formed the basis for assigning the nutrient load on the lake until 2050 to assess the possible changes in the lake ecosystem on a 3-D SPLEM model of 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.