What is "hypolimnion"

n. (context geography English) the perpetually cold layer of water that lies beneath the thermocline of a thermally stratify lake

The hypolimnion or under lake is the dense, bottom layer of water in a thermally- stratified lake. It is the layer that lies below the thermocline.

Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter. Being at depth, it is isolated from surface wind-mixing during summer, and usually receives insufficient irradiance (light) for photosynthesis to occur.

In deep, temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4 °C throughout the year. The hypolimnion may be much warmer in lakes at warmer latitudes.