Pit lakes are artificial lakes, which form in voids of opencasts. Geochemically different inflows and steep lake basins make pit lakes more prone to meromixis than natural lakes. Mining, environmental legislation and often the poor water quality , mainly due to acidification , require detailed planning and management of pit lakes.

Three cases from different regions are presented. Pit lake Cueva de la Mora has been comprehensively investigated for physical limnology, water chemistry, sediment biogeochemistry and microbial ecology. The connection of the lake with former underground mine galleries and shafts had a decisive impact on the physical and chemical stratification of the waterbody (mixolimnion, chemocline and four distinct monimolimnetic layers). Relatively high primary production in the mixolimnion fuelled intense cycling of iron, other heavy metals and sulphur. In Lake Goitsche (Germany), the disappearance and recovery of meromixis was documented in detail by profile measurements. Groundwater caused the meromixis, which was destroyed by an exceptional flood in 2002. The recovery of the monimolimnion was supported by the lake basin geometry and the protective effect of the thermal summer stratification in 2003.

In Island Copper Mine Pit Lake (Canada), meromixis was created intentionally by capping introduced sea water with brackish water. The monimolimnion was projected for the treatment and deposition of harmful mine waste. The hydrology of the lake was almost completely controlled. Primary production in the mixolimnion was manipulated by year-round fertilization for metal removal via adsorption to algal biomass and subsequent sedimentation. Sulphate reduction in the monimolimnion and burial of metal sulphides in the sediment were boosted successfully.

Finally three management options are discussed: (1) sustaining meromixis, (2) deposition of mine waste and (3) controlled elimination of meromixis. Predictive modelling forms an essential tool for planning and managing meromictic pit lakes . An overview on modelled meromictic lakes is given.