Alkalinity generation in a soft-water lake: Watershed and in-lake processes

Alkalinity mass balance studies of the oligo‐mesotrophic soft‐water Piburger See (913 m ASL) over a period of 11 yr showed annual rates of in‐lake alkalinity generation from 24 to 144 µeq liter‒1, or 0.33 to 1.97 eq m‒2. From nitrogen, sulfur, and iron budgets, alkalinity generation rates of 21–38 µeq liter‒1 yr‒1 (0.37–0.52 eq m‒2 yr‒1) were inferred. Reduction of the artificial deep‐water discharge by ∼40% led to doubling of the rates of in‐lake alkalinity production. The processes which generated alkalinity in the lake were release of calcium (37%) and magnesium (18%) from sediment or suspended matter and the reduction of sulfate (20%), nitrate (16%), and iron (9%). Budget studies based on alkalinity measurements showed 12–21% lower values than the sum of alkalinity calculated from dissimilation and release. The difference can be explained by uncertainties in the measurement of elemental cycles in the water column (nitrogen fixation vs. denitrification) and sediments (Fe: S stoichiometry of iron sulfide precipitation) and budget errors. The areal rate of net alkalinity generation in the watershed of Piburger See accounted for 5–27% of that produced in the lake. As a whole, the catchment produced 100–500% of the in‐lake alkalinity.