John,
Sorry for any confusion! Hypolimnion is lake jargon. The study of lakes is called "limnology". Most large mid-latitude lakes will thermally stratify during the winter and summer. When lakes stratify during the summer, the upper (warmest) part of the lake is called the "epilimnion", the bottom (coldest) part of the lake is called the "hypolimnion", and the transition zone between the two is called the "metalimnion". OK so far? Now it gets interesting. During the winter, it is the other way around, the lake does not stratify into three distinct layers, but has a gradual thermal gradient from warmest at the bottom, and coldest at the top! Confusing, eh?

This seeming contradiction is due to the fact that the maximum density of water is at 4 degrees Centigrade. So, when mid-latitude lakes stratify (whether winter or summer), the water at the bottom of the lakes is 4 degrees C (it is really a density stratification, not a thermal one). So, during the summer, the warmer, lighter water is higher up in the water column. During the winter, the lighter, cooler water (from 3 to 0 degrees C) is higher up in the water column. Isn't physics great... it even extends BEYOND the physics of fly rod loading!

The term "benthic" just means something associated with the bottom of a river, lake, or ocean.

I promise to be better at defining terms as I use them!
-- Rick