The Black Sea is another example with exceptionally high concentrations of sulfide caused by river water overlaying source water from the Mediterranean. Lakes vary greatly in their properties depending upon their rate of mixing. Cold Monomictic lakes contain water that never rises above 4 C and which turns over in the summer. Warm monomictic lakes contain water that never falls below 4C and which turns over once in the winter. Oligomictic lakes contain water much warmer than 4C which turn over irregularly.
Meromictic lakes are deep narrow lakes whose bottom waters never mix with waters above.
Turnover is possible in spring and fall in upper waters of meromictic lakes. Oligotrophic lakes are lakes with low concentrations of nutrients. They are deep and have a larger hypolimnion than epilimnion. Eutrophic lakes are lakes with high concentrations of nutrients . Mesotrophic lakes are in between oligo and eutrophic lakes.