When the Lake Turns Wintry Gray…

Bill Foster
Watershed Educator

Three-level summertime lake stratification.
Fall turnover completely mixes the lake.

Unless you’re a limnologist or an avid angler, you’ve never associated the word "turnover" with a lake. But, you might recall one particular day during a past fall when you looked out over the Lake and thought to yourself — "Gee, the lake sure looks wintry and gray today." That was probably the work of the Fall Turnover, and it signals approaching winter on Cayuga Lake.

The seasonal remixing of waters, or turnover, is an important process in most lakes that aids maintaining oxygen levels and recycling nutrients. Turnover is caused principally by seasonal temperature changes in a lake's surface waters, and may happen during the spring or fall. In Cayuga Lake, however, turnover occurs only in the fall. Here is what's happening:

During the summer season, lake waters stratify into three distinct layers. Topmost is the "epilimnion", which receives heat from sun, and oxygen from growing plants — this is where most biological activity occurs. On the bottom is the "hyplimnion", where colder water remains. Because colder water is denser, it stays on the bottom, and a zone of rapidly changing temperatures and conditions forms between the top and bottom layers. This transitional zone is called the "thermocline".

Over the course of the summer, these zones become more and more distinguished from one another as the surface level heats up and essentially floats on the colder water beneath. As cooler fall winds and night-time temperatures cause the surface temperature of the lake to drop, however, this divide begins to disappear. When the surface layer cools into the mid-fifties, its density approaches that of the colder water, and the two layers are ready to mix. A windy fall day, or a well-timed frost, can make this happen almost overnight.

Look for turnover in Cayuga Lake near the end of November or early December, depending on the weather conditions. There is quite a bit of variation in turnover dates amongst the Finger Lakes. The smaller lakes tend to turn earlier in the season, since they have less volume and store less heat. Once fall turnover begins, Cayuga Lake waters are completely mixed. Cold bottom waters reaching the surface take on oxygen and release other gases, which sometimes cause a slightly swampy, or "rotten egg" smell. The natural decay processes that produce these gases also enrich the up-welling waters with nutrients that are distributed throughout the lake during turnover. Additionally, sediment is brought up from the lake bottom, lending a decidedly cold, gray appearance to the lake for a period of time. But take heart — its a sign that our lake is functioning as designed!