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. CO, 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 CO,. 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. From tropical coral reefs to temperate-zone marl lakes and streams, calcareous plants and photosynthetic symbiosis abound in alkaline, hard-water environments and are largely responsible for carbonate sedimentation (Dean and Fouch 1983), raising questions about how calcification and photosynthesis interact. We explore this problem for the case of a mildly alkaline, closed basin, north-temperate lake. In the process, we investigate carbon sources for lake production, including atmospheric and groundwater exchanges and recycling of sedimentary organic and inorganic C. The chemical budget approach follows in the tradition of Otsuki and Wetzel (1974) and Murphy and Wilkinson (1980). Most of the important characteristics of typical marly lakes have been appreciated for a long time (e.g. Davis 1900). Dissolved calcium and bicarbonate usually derive from dissoluAcknowledgments Several landowners generously allowed access to the lake and permitted drilling and monitoring of groundwater sampling wells. Thomas Winter and Dennis Merk contributed greatly to this study and to the interdisciplinary research initiative program which made it possible. tion of limestones in up-gradient glacial tills. Davis (1900) argued against authigenic precipitation as the major cause of lacustrine calcification and also observed that snail shells and other animal remains generally comprise a small part of the marl. Instead, aquatic plants, including various angiosperms, cyanobacteria, and especially the alga Chara, appeared to be the dominant calcifiers. Davis also noted that some plants calcify far more than others and speculated that simple photosynthetic CO2 removal from the water was not the only cause of plant calcification. The induction of calcification by aquatic photosynthesis can be viewed as a consequence of the reaction 2HC03-D CH20 + O2 + C032-, (1) which raises C032concentrations and therefore CaCO, saturation levels. Smith (1973), Smith and Veeh (1989), and McConnaughey (1994a) discuss some ramifications of this mechanism, and simple calculations illustrate its limits. Starting with water initially saturated with respect to CaCO, and in CO2 partial pressure equilibrium with the atmosphere (350 ppm pC02), CO2 withdrawal can the-