Centuries of change: Salinity and carbon dynamics in a large shallow lake

This study examines over a century of salinity changes in Lake Balaton, the largest shallow lake in Central Europe. Total ion concentrations have increased from a historical baseline of 450 mg/L to approximately 650 mg/L today, with an annual rise of 5.0 mg/L in the largest basin since the 1970s. Notable annual increases include magnesium (0.7 mg/L), sodium (0.6 mg/L), chloride (0.7 mg/L), sulfate (1.8 mg/L), and bicarbonate-carbonate ions (1.0 mg/L). In contrast, calcium levels have remained stable due to substantial calcite precipitation, which reduces the calcium content of inflowing waters by over half. Calcite precipitation in Lake Balaton varies significantly, ranging from 25,000 to 125,000 tons/year, with an average of 75,000 tons/year between 2010 and 2022. Biologically induced calcite precipitation, driven by photosynthesis, occurs seasonally during summer but dissolves again in autumn and winter. However, the majority of calcite precipitation is non-biogenic, driven by the equilibration of CO 2 -supersaturated inflows with atmospheric CO 2 . This process releases approximately 33,000 tons of CO 2 annually, underscoring Lake Balaton’s contribution to global carbon cycling. These findings highlight the combined impact of natural processes and anthropogenic influences on Lake Balaton’s salinity and biogeochemical dynamics, emphasizing its importance as a model for understanding the broader implications of freshwater salinization and carbon flux in shallow lakes.