Phenomenological Description of Polymixis and Influence on Oxygen Budget and Phosphorus Release in Lake Müggelsee

The polymictic properties of a shallow eutrophic lake (Lake Müggelsee, Berlin) are described by the water column stability (N2) and vertical differences in oxygen saturation at the deepest site (7 m) of the lake from 1979 to 1990. Mixing and stratification changed irregularly up to seven times from April to October. Thermally stable conditions generally lasted 1–2 weeks. A maximum of 5 weeks stratification was observed in 1982. Although the oxygen content above the sediment depleted during stratification periods the lake is well supplied with oxygen in general. As a consequence of oxygen depletion, considerable amounts of phosphate can be released from the sediment leading to differences in the concentration of phosphate between the deepest point and subsurface of more than 1000 μg 1−1. The release of dissolved reactive phosphate from the sediment is mainly controlled by the rate of change of oxygen in the deepest layers of the lake.