Meromixis as a part of lake evolution — observations and a revised classification of true meromictic lakes in Finland

Based on Finnish lakes, meromixis is viewed from a paleolimnological perspective as a part of lake evolution. The study concludes that theoretical search of meromictic lakes is almost impossible, due to inconsistent and insufficient basic data and the complexity of meromixis as a phenomenon. An estimation of possible Finnish meromictic lakes turned out to be a few dozen. Meromictic lakes are more numerous than formerly expected, but still rare. Based on the present estimation, only one lake in 800 is truly meromictic. Their most probable geographical location, besides the coastal lakes, are the Salpausselkä end-moraine zone and the areas between the Salpausselkä zone, Ostrobothnia, the Kainuu Region, and the eastern border of Finland. Terminology concerning holomixis/meromixis is presently confusing and this study favors a strict use of the term meromictic and grouping complete and incomplete holomictic lakes as one group and true meromictic as another. The revised classification recognizes meromixis that has resulted from (1) flow/precipitation of saline water over freshwater or freshwater over saline water, (2) superficial diffuse nutrient load and/or turbidity currents from the catchment, (3) subsurface inflow of groundwater, (4) inadequate mixing due to the lake morphology and surrounding topography.