In the water column of a lake, a condition that may develop during the summer in which the thermocline and pycnocline change over a short vertical distance, at the metalimnion. This prevents mixing between the waters of the epilimnion and hypolimnion. It occurs because of the limited depth to which solar radiation penetrates and wind movement mixes the water. There is almost no temperature gradient within the epilimnion, but in the metalimnion it may be up to 1°C/m; temperature is fairly constant throughout the hypolimnion. Photosynthesis occurs in the epilimnion; organisms residing in the hypolimnion subsist on detrital rain falling from the epilimnion, but dissolved oxygen is rapidly used and is not replenished. As solar warming decreases, thermal stratification breaks down, water mixes fully, and the hypolimnion is recharged with oxygen. This is called ‘overturn’ and occurs in autumn or winter. Inverse stratification occurs where the epilimnion surface freezes. Dense water, at 4°C, sinks to the bottom, and water that is less dense but cooler (about 3.94°C) floats above it. Fish can, therefore, survive the winter in hypolimnion water at temperatures above freezing.
Subjects: Ecology and Conservation.