Calcite precipitation in Lake Constance: Chemical equilibrium, sedimentation, and nucleation by algae1

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Abstract
Seasonal shifts in calcium carbonate solution equilibria in the limnetic zone of Lake Constance were documented between 1981 and 1983. Except during mixing times, lake water was supersaturated with respect to calcite; epilimnetic supersaturation maxima were closely related to changes in pH. Sedimentation of calcium from the lower boundary of the euphotic zone to the lake bottom was also assessed. The daily sedimentation flux for calcium during the season varied over three orders of magnitude. Differences in the calcium content of sediments trapped at various depths were insignificant. Sedimentation rates of titanium were used as tracers for the fluxes of calcium bound on allochthonous clastic material. Lacustrine calcite precipitation (39–49% of the total sedimentation rate) was obtained by subtracting fluxes of allochthonous Ca from the rates of total Ca sedimentation.Calcium supersaturation maxima were not in phase seasonally with maximum fluxes of autochthonous Ca. Instead, calcite precipitation was associated with the occurrence of Stephanodiscus hantzschii, Fragilaria crotonensis, Asterionella formosa, Chlorella sp., Pandorina morum, and Mougeotia thylespora, indicating that calcite formation was triggered by heterogeneous nucleation induced by certain algal species.

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