Something strange and wonderful happens twice a year when Tennessee lake water reaches a temperature of approximately 39 degrees Fahrenheit (40 Celsius). Well, it is wonderful for lakes and the life in them, not so much for the roads on which we drive! We call it “Lake Turnover”!
In any case, as water cools to that temperature, it does, as predicted, contract and become more dense, ultimately sinking to the bottom of the lake and pushing the water it has displaced to the surface, where it too can cool. With continued cooling at the surface, the decreasing temperature should (or so it would seem) eventually cause all the water to eventually freeze solid and sink to the bottom. This means that the lake would freeze from the bottom up — eventually destroying all life beneath the waves.
Why then does this not happen? The chemistry of the water molecule dictates that at 39 degrees Fahrenheit, water actually expands and becomes less dense, allowing it to float above the warmer water! The water that cools below that temperature, to 32 degrees, freezes and stays on the top, effectively capping the lake. It also stops further energy loss from the lake. Everything beneath the surface of the ice never gets any colder than 39 degrees Fahrenheit. Cool Right?
For those who own Tennessee Lakefront Homes, how does all of this relate to lake turnover? Let’s first consider the fall turnover:
Starting in the spring and over the course of the summer, surface waters absorb a lot of the sun’s energy and can heat extensively, causing them to become quite buoyant. Winds and storms can cause some mixing and do add some oxygen; atmospheric oxygen is added by the air-water interaction to the oxygen produced within the water by aquatic plants.
But there is too much difference in temperature between the surface water and that at depth to allow for complete mixing of all the water in the lake. Because of the density-temperature relationship, many lakes in temperate climates tend to stratify, that is, they separate into distinct layers.
The middle layer, known as the Thermocline, acts as an effective barrier to any mixing of the deeper waters. Toward the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place.
As the days get shorter and cooler, and energy is transported away from and out of the lake, mixing becomes easier. At about 50 degrees, the cooler water (with a higher oxygen content) at the surface begins to sink into and through the Thermocline, forcing warmer and less dense water to the surface, eventually erasing the temperature stratification built up over the summer.
At some point, the majority of the water in the lake reaches an approximately uniform temperature. Now, storms and sustained high winds can begin to perform the task of overturning and mixing all of the water in the lake — referred to as “Fall Turnover”.
The deep water contains an abundance of decaying matter and sulfurous gases; when it reaches the surface, it produces a telltale odor that indicates the process has begun. Eventually the turnover mixes fresh oxygen into the entire lake mass, replenishing the deep waters with the life-giving stuff and cleansing the sulfurous fumes from the water, allowing fish to return to the depths where they will spend the winter months.
But not all lakes experience turnover to the same extent or in the exact same way, due to things like depth, bottom structure and size.
Very shallow lakes — with little or no Thermocline — may experience little if any noticeable turnover. Large and very deep lakes will obviously take longer for the phenomenon to be completed.
As winter approaches, the water that has now reached 39 degrees sinks to the bottom, allowing colder and less dense, buoyant water to remain at the surface to freeze. The ice thickens because it is not a good insulator; water in contact with the underside of the ice cools further and freezes, adding to the surface layer.
A stratification similar to that of the summer months will occur in the water column during the winter months, but not to as great an extent. With the advent of spring, the warming/melting of the ice layer at the surface and the much smaller temperature differences in the water column, winds and storms are able to create a spring turnover with little difficulty.
As the waters continue to warm, stratification begins again and the endless cycle continues. Mother Nature has performed her timeless and never-ending task, adding additional life-giving oxygen to the lake and readying it for the onslaught of another season of fishermen.
Fall turnover can come quickly to a lake and one day you can be catching your favorite
fish and the next day these same fish can be gone from your favorite spot. It’s difficult for the angler to find fish after turnover, so they must try to cover as much water as possible. So to catch fish during this unique time period, you must be willing to fish both shallow and deep, move quickly from one location to another, and use lures and Crankbaits that can cover water quickly.
Lake Turnover Information gathered from www.OnTheLake.net!
Posted by Karen Millikan, Lakefront Living Realty, Tennessee Lakefront Specialist