Trends in the elements of the carbonate system of Lake Ladoga

During the observation period in 2003 – 2021, it was shown that the total concentration of dissolved inorganic carbon, represented in the water of Lake Ladoga mainly HCO3ˉ and CO2 has been increasing in recent years. The concentration of CO2 has a sufficiently large spatial and temporal variability and, in terms of carbon at the stations under consideration, can range from 0.6 to 47 % of the total concentration of dissolved inorganic carbon, depending on the change in the ratio of production and destruction processes in different seasons at different horizons. At the same time, a significant interannual trend of an increase in the total concentration of dissolved carbon dioxide and, accordingly, a decrease in the hydrogen index was revealed. Calculation of the balance of dissolved in water and atmospheric CO2 showed that, in general, CO2 concentrations in water exceed equilibrium, however, at the peak of the growing season, the surface layer of the lake in some cases, on the contrary, can absorb carbon dioxide from the air. This effect of the «biological pump» is most clearly manifested in the nutrient-rich and heated shallow part of the lake. However, in the main body of water, there is an interannual trend of increasing the calculated concentrations of dissolved carbon dioxide relative to its equilibrium concentrations with the atmosphere. Due to the shortage of nutrients, primarily phosphorus, an increase in temperature does not lead to an increase in productivity and CO2 consumption. Stoichiometric ratios of apparent oxygen and carbon deficiency, in turn, demonstrate that the quantitative change in the content of dissolved carbon in water can be fully explained by oxidation or synthesis of organic matter in only a third of the cases considered. In the remaining 2/3 of cases, the increase in the concentration of dissolved inorganic carbon may be associated with an increase in carbon runoff from the catchment area into Lake Ladoga, or with intra-reservoir processes associated with the peculiarities of accumulation and redistribution of CO2 in the water column.