Round Lake National Natural Landmark - Limnology of Round Lake and Green Lake

Limnology of Round Lake and Green Lake

The lake is 180 feet (55 m) deep, which is remarkable given its diameter of 700 feet (213 m). Both Round Lake and Green Lake are meromictic, which means that the bottom waters of the lake are not mixed annually with the surface waters. For both lakes, the waters deeper than 55 feet (17 m) are unmixed and essentially devoid of oxygen. Meromictic lakes are quite rare; nearly all lakes undergo mixing of their deep and shallow waters at least once each year. Green Lake in particular has been the subject of a great deal of limnological research. The lakes are meromictic both because of their depth (relative to their diameters) and because there is an influx of mineral-laden groundwater into them; to some extent the lakes are giant mineral springs. The mineral content also accounts for the lakes' greenish appearance at certain times of year, when small particles of calcium carbonate and other minerals precipitate out of the water; these "whitings" cause the white coating of the lake bottoms that is readily observable at the shoreline.

In addition to the strictly limnological research on the lakes, the sediments at their bottoms are being used to explore the history of the plant and animal life and of the climate around the lake over the last several thousand years (i.e. its paleoclimatology). Because the deep, oxygen-depleted bottoms of the lakes are undisturbed either by annual mixing or by "bioturbation" (plant growth or movement of worms and other animals), each year's sediments are preserved as distinguishable layers (or varves) that can be dated back in time from the present. There is a distinct change in the color of the varves that occurred around 1800; the lower, older varves are dark brown, and the higher, younger varves are grey. The varves thus record the change in the lakes' environment around 1800, when the primeval forests in the region were replaced by open fields and farmland. The Round Lake National Natural Landmark offers a glimpse of the original forest.