Carbon budget for a groundwater-fed lake: Calcification supports summer photosynthesis

A chemical budget analysis for Williams Lake, Minnesota, tracks the seasonal progression of carbon inputs and outputs. CO2 exchanges with the atmosphere reverse seasonally, with uptake by the lake in summer preceded and followed by larger losses to the atmosphere. Calcium bicarbonate-rich groundwaters seep steadily into the lake, augmented by remobilization of lacustrine marls. Most of the carbon used in summer photosynthesis nevertheless derives from depletion of lake carbon stores, facilitated significantly by plant calcification. Calcification in summer reduces alkalinity and calcium inventories by 15 and 25%, respectively, while generating equal molar quantities of CO2. Marl precipitates mainly on submersed macrophytes, several of which calcify in 1 : 1 ratio to photosynthesis when incubated in Ca-supplemented lake water. Despite calcite supersaturation within the epilimnion, there is little authigenic calcification.