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

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 elastic 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: Autochthonous formation and subsequent precipitation of calcium carbonate is an important process in the carbon and calcium cycling in many hard waters. Through coprecipitation calcium carbonates affect nutrient budgets (Rossmann 1980; Rossknecht 1980; Murphy et al. 1983) and by adsorption they remove dissolved organic carbon (DOC) from the water (Suess 1970; Chave and Suess 1970; Otsuki and Wetzel 1972). Hence, several studies have focused on the formation, reactions, settling processes, and deposition of CaCO, in lakes (cf. Minder 1922; Kelts and Hsii 1978; Effler 1984; House 1984). Various calcium carbonate minerals may occur in the aquatic environment either as primary carbonates or as the results of diagenetic processes in the sediments. Whether low Mg-calcites, calcite, aragonite, or dolomite phases form seems highly dependent on the ratio of Mg : Ca in the water (Miiller et al. 1972). On the basis of these studies, calcite is the most common mineral deposited in low salinity lakes in temperate zones. The calcite solution equilibria of freshwaters are computed from the activities of the calcium and carbonate ions and the sol’ This work was supported by the Deutsche Forschungsgemeinschaft (DFG Az: Ti 115/2). ubility constant with a set of equations outlined by Stumm and Morgan (198 1). Most of the parameters affecting these equilibria can be measured easily (i.e. pH, temperature, alkalinity, calcium concentrations, and conductivity) and thus periods of supersaturation with respect to calcite can be calculated. Surprisingly, no comprehensive work has yet been done to compare changes in lake water supersaturation with actual changes in the sedimentation fluxes of calcium carbonates. I have computed calcite saturation indices (following Berner 197 1) for the limnetic zone of an arm of Lake Constance, Uberlinger See. To evaluate factors which trigger calcite precipitation, I calculated mean indices for the euphotic zone and related these to changes in the daily sedimentation rates of calcium obtained from concomitant sedimentation trap experiments. Moreover, a quantitative separation of allochthonous from autochthonous fluxes of Ca was attempted and the impact of certain algal species on the precipitation of calcite in Lake Constance is discussed. I acknowledge the help of K. Wiedemann, captain of RV Robert Lauterborn in the fieldwork. P. Rebholz constructed the mooring station and sediment traps. Laboratory data were evaluated in part by G.