Meromictic

                                                        Meromictic references a type of lake which is constantly stratified.  The surface and bottom waters do not ever mix.  In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer. 

The Black Sea is the largest meromictic lake (NASA).

 Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) (fishionary.fisheries.org/meromictic) and may be formed because they are:

1. small and unusually deep, or
2. the lower layer is denser and more saline than the surface layer.