Most lakes in the temperate zone turn their waters over every autumn as the water cools. Water reaches its greatest density at 4 degrees above freezing, so it sinks when it cools to that temperature. The sinking water displaces the water below, no matter what temperature it's at, and the result is a complete mixing of the lake. The freshly oxygenated deep water sustains fish throughout the winter even when the surface is frozen over. See the Freshwater Fishing Guide for more about the fall turnover.
The rocks around Round and Green Lakes contain beds of salt, making their bottom waters a layer of strong brine. Their surface waters are devoid of fish, instead supporting an unusual community of bacteria and algae that give the water a peculiar milky blue-green color.
Because the bottom of meromictic lakes is so stable, the sediments that accumulate there are exceptionally well-preserved records of the plant species growing in the region as well as the changing aquatic community in the surface layers. Geographically, Round and Green Lakes sit on the border between two great weather systems separated by a jet stream in the upper atmosphere. This makes them very sensitive to subtle climate changes that have occurred during the last 10,000 years since the glaciers left.Other meromictic lakes in New York include Ballston Lake near Albany, Glacier Lake in Clark Reservation State Park, and Devil's Bathtub in Mendon Ponds State Park. Other examples in the U.S.A. are Soap Lake in Washington state and Utah's Great Salt Lake.
