| Summary: | Water chemistry and sediment trap data from the 18.9-m-deep, high-altitude, hardwater lake Hagelseewli (2339 m asl.) indicates that biogenic induced calcite precipitation occurs in a water depth of 6 to 9 m at a temperature of 4°C. Our data indicates that calcite precipitation takes place as a short pulse that lasts less than 14 d in response to photosynthetic CO 2 uptake in late summer probably by cyanobacterial picoplankton. In up to 8.4 times supersaturated water 30 μm large calcite crystals precipitate that show surface features typical for eutrophic lakes (rough surface and enhanced columnar growth). Subsequently, during the long period of ice coverage calcite is completely dissolved in strongly undersaturated bottom waters. Sediment calcite concentrations therefore are below 0.3%, although the comparison of trap fluxes and sediment accumulation rates indicates that they could be as high as 25%. The theoretical annual accumulation of calcite in Hagelseewli is similar in magnitude to a single spring precipitation event in a low-altitude temperate, hardwater lake. In the most recent sediments the occurrence of higher amounts of organic carbon and sulfur indicates increasing bottom water anoxia during the last 30 to 40 yr.
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