Mining lakes are within the focus of limnological and public interest in many countries because they have unusal mineral content and can comprise a great portion of standing waters in certain areas. Due to pyrite oxidation, many mining lakes are extremely acidic and therefore differ considerably from natural circumneutral lakes in their chemical and biological characteristics (GELLER et al. 1998, LESSMANN & NIXDORF 2000). In central Europe deep lakes are usually regarded as dimictic. A presupposition for stable winter stagnation is the formation of ice cover, which depends on the duration of the frost period. Within the last ten years central Europe has seen several mild winters that inhibited the formation of a long-lasting ice cover and thus the occurrence of a stable winter stagnation. The importance of occurrence and duration of winter ice cover and winter stagnation for the phytoplankton development is shown by the example of Mining Lake (ML) Plessa 117 of the Lusatian lignite mining district (Germany). This study compares the relatively mild winter 2001/2003 with the strong winter 2002/2003 and analyzes phytoplankton development in winters of 1997 to 2000
