Date: Tue May 18 00:25:01 EDT 1999
Subject: A.Word.A.Day--hypolimnion
hypolimnion (hy-puh-LIM-nee-on, -uhn) noun

The layer of water in a thermally stratified lake that lies below the
thermocline, is noncirculating, and remains perpetually cold.

[Hypo- + Greek limnion, diminutive of limne, lake, pool.]

Biology Dictionary

Definition: Zone of water in a stratified lake in which temperature and oxygen concentration drop precipitously with depth.
Layer of water between the warmer surface zone and the colder deep zone of a thermally stratified body of water. In the thermocline, temperature decreases rapidly with depth.