Turnover is a seasonal mixing of these layers. What makes it occur. Talk about ice floats due to the reduction in density as water temperature falls below 4 C. Chemocline are chemical gradients in stable waters formed by chemical influx usually at the bottom of the lake. There are lakes in Africa where subsurface springs deliver large volumes of carbonate saturated brines which layer the bottom of these lakes. 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.