thermocline

noun

distinct layer in a large body of fluid in which temperature changes more rapidly with depth than it does in the layers above or below

Wiktionary

noun

Etymology: Etymology tree Proto-Indo-European *gʷʰer-der. Ancient Greek θερμός (thermós)der. English thermo- English -cline English thermocline From thermo- + -cline.

A layer within a body of water or air where the temperature changes rapidly with depth.
“Even among migratory species, only a few (Aglaura hemistoma, Abylopsis tetragona eudoxids, Beroe sp., Thalia democratica, Salpa fusiformis) crossed the thermocline and reached the bottom layer.”