Trends in the Ionic Composition of Lake Ladoga

The sum of ions of the basic Lake Ladoga water mass has varied quite significantly from 55.6 to 71.6 mg/L over the past 60 years. In the present paper, separate periods of the mineralization growth are identified. It is shown that the change in the mineralization of Lake Ladoga has been always accompanied by a significant change in the ratio of basic ion concentrations. Any significant trends in the total mineralization of Lake Ladoga over the analyzed period have not been identified, but periods of the growth and decrease in the number of ions have been revealed. Until 1998, the periods of increased mineralization, as a rule, were accompanied by an increase in concentrations of  \(\rm SO_4^{2-}\)  and  \(\rm Cl^{-}\) , most likely coming from the catchment area due to industrial and household pollution. During 2009–2019, the mineralization was growing mainly due to an increase in the absolute and relative concentrations of  \(\rm HCO_3^-\) . At the same time, the concentration of dissolved CO₂ in the main water mass of the lake also was increasing (and, consequently, pH was decreasing). Thus, the changes in the bicarbonate ion concentration in the water of Lake Ladoga may be largely determined by an increase in its inflow from the catchment area, whose possible reason is the enhancement of the chemical weathering of carbonate rocks due to the increasing concentrations of dissolved  \(\rm CO_2\)  in the water.