epilimnion /e-pə-LIM-nee-on/. Noun. The upper, warm layer of a stratified lake. The layer above a lake’s thermocline. More generally, the surface layer of a body of liquid. From Greek limnion, diminutive of limnē (marshy lake). See also: epilimnetic, epiliminial.
“The result of this warming is that, in summer, a warm upper layer of less dense water, the epilimnion, comes to lie over a cold deeper water mass, the hypolimnion.” (Philip Ullyott & Paul Holmes, from Nature)
“Down there the temperature was always an even 4°, no matter what the season, but it was unheard of that a spore should be found there while the high epilimnion was still warm and rich in oxygen.” (James Blish, from “Surface Tension”)