Modelling thermal stratification and calcite precipitation of Lake Belau (northern Germany)

Abstract

Abstract Some aspects of the approach of the Bornhoved Lakes Ecosystem Research Project to modelling Lake Belau are presented. The concept includes a base model which is extended by several submodels to account for special relevant processes in Lake Belau. Two of them and some results are discussed in detail: The model for thermocline dynamics and the model for the carbondioxide equilibrium system together with calcite precipitation. The first includes a submodel for the heat energy balance and especially accounts for the effect of wind mixing by a successive comparison of potential and kinetic energy. The overall agreement with measured values is good, the model is even capable to detect a secondary thermocline. The results underline the sensitivity of the processes within the water body of Lake Belau to wind mixing. The kernel of the inorganic carbon cycle is the calcite equilibrium where the respective system of algebraic equations is solved iteratively. Several adaptions to the hydrochemical conditions of Lake Belau have been made. Calcite precipitation including coprecipitation of phosphate is a mechanism controlling eutrophication of Lake Belau. About 100 kg P are removed from the epilimnion by this process during spring and summer. The amount of 1 kg P/ha removed from the epilimnion underlines the importance of this self-purification process. During summer the carbondioxide concentration in Lake Belau decreases to values of 0.3 · 10 6 M/l so that algae are forced to use HCO 3 − , which still has a concentration of 0.4 · 10 3 M/l, as a carbon source. Therefore carbon limitation of primary production does not occur.