Tracing calcite precipitation with specific conductance in a hard water alpine lake (Lake Bourget)

The authors discuss the reliability of a new method used to depict precisel y the authigenic calcite precipitation process in the water column of a mes otrophic alpine lake (Lake Bourget, Savoie, France). Complete chemical anal ysis of dissolved major elements shows that, in the epilimnion, calcium con centration and alkalinity are the only chemical species presenting large va riations. The behaviour of these two parameters is only related to calcite precipitation. The method is based on in situ specific conductance measurem ents (C-25). Experimental values were compared with theoretical calculation s. The results demonstrate clearly that dissolved calcium concentration can be directly related to measured specific conductance. Hence, a mathematica l relationship was developed to compute calcium concentration profiles in t he water column with a centimetre spatial resolution through the whole wate r column. Finally, authigenic calcite production can be observed and quanti fied on a time scale covering two years or experimental data. Calcite produ ction is estimated to 4.8 mol m(-2) yr(-1). Extrapolation to the whole lake surface leads to 16.000 +/- 2000 tons yr(-1) as CaCO3. specific conductanc e is an easy-to-measure in situ parameter which could be used to trace auth igenic calcite production in other hard water alpine lakes. (C) 2000 Elsevi er Science Ltd. All rights reserved.