Georgia DNR Explains Lake “Turnover”

0
90


“Lake turnover” is an often incorrectly used term to describe a period of the annual cycle of lake stratification (stratification) that affects the water quality of the southeastern reservoirs. Throughout the year, the reservoirs in Georgia’s latitudes and altitudes go through a fairly predictable annual cycle. I will address the annual cycle of Lake Lanier and its impact on water quality downstream in the Chattahoochee River. In general, this pattern is similar in the Carolinas, Tennessee, and most other reservoirs that do not freeze or are not in tropical climates. Sunlight, air and water temperatures as well as the density of the water at different temperatures drive this annual cycle.

During the cold winter months, Lake Lanier’s waters are generally the same temperature from top to bottom. The lake water is cold (around 45-50 degrees F) and clear. The water on the top and bottom of the reservoir has similar densities. Wind action on the surface water rolls the lake and surface water mixes with the groundwater. The exposure of all of the lake water to the surface allows the lake to have plenty of oxygen from top to bottom. In winter, the water temperature and oxygen concentration do not restrict fish movement in the lake. The lake water that drains from the bottom of the lake into the Chattahoochee River below the dam is cold, oxygenated, and clear.

State record brown trout (20 lb, 14 oz),
caught on the Chattahoochee River (07/27/14)

During spring and early summer, the lake begins to gain heat and layer itself into three slightly different layers: the surface layer called the epilimnion, a lower layer called the hypolimnion, and a layer in between called the metalimnion or, as anglers know, the thermocline, that’s what I’ll call it.

In the warm months, high air temperatures and more sunlight heat the lake surface faster than the lake can mix. The warm water, which is less dense, floats to the surface and becomes the epilimnion. This warm layer is fairly uniform in temperature and varies between 15 and 30 feet thick throughout the summer. It is full of oxygen from the action of the wind and from the oxygen production by microscopic algae called phytoplankton through photosynthesis.

The hypolimnion, the cold (45-55 degrees F) lower layer, is isolated and no longer mixes with the warm, oxygen-rich epilimnion. No oxygen is produced in the hypolimnion because this cold, deep layer does not receive any sunlight and does not produce any phytoplankton. At the beginning of the lake’s stratification process, the hypolimnion still contains some oxygen and fish movement is not restricted, but the level of dissolved oxygen decreases in summer as biological and chemical processes consume oxygen. This means that oxygen is consumed in the decomposition of organic substances (nutrients). The amount of nutrients that enter the lake from its watershed is known as the nutrient load. The water released into the Chattahoochee River by the dam comes from this deep water zone. Native river species could not adapt to the changing conditions created by the Buford Dam, but the cold river water, once oxygenated from overflowing shallows, was a great new habitat for trout.

Between the epilimnion and hypolimnion layers is a layer of rapid temperature changes (at least 2 degrees F per yard) called a thermocline. The thermocline, usually 20 to 30 feet thick, does not mix with the surface layer and has little sunlight. Therefore, oxygen production in the thermocline begins to decrease after the lake is stratified.

At the end of summer the lake is heavily stratified. The epilimnion is warm; it receives sunlight and has a lot of oxygen. Both the water temperature and the oxygen concentrations within the thermocline are lower, but still provide an acceptable habitat for cold-water fish species such as striped pikeperch and zander.

In the hypolimnion (deeper than 60 feet), the water is stagnant, cold, and low in oxygen (less than 3 parts per million or ppm). Fish cannot survive in this deepest layer if the dissolved oxygen drops well below 3 ppm. When oxygen levels get low, some metals and sulfides become soluble in the lake sediments. These dissolve in the water and are directed downstream as the water leaves Lake Lanier and enters the river. This first becomes noticeable in late September or early October, when these metals and sulfides give the river water its distinctive fall colors and a rotten egg smell. Although these are stressors for the river fish, low oxygen levels and high metal and sulfide levels are very rarely associated with fish mortality in the river. The river water is quickly re-aerated as it flows downstream, and the fish in the river avoid low-oxygen water by finding seepage, springs, or tributaries that have higher dissolved oxygen and lower metal and sulfide concentrations. Trout fishing in the river near the dam suffers from these water quality conditions in autumn.

Before the 1980s, oxygen levels (more than 5 ppm) and temperatures in the thermocline of Lake Lanier, a then young reservoir, were sufficient for trout to survive. Since then, the entry of organic material into the lake has increased and the trout’s need for oxygen can no longer be met. There is simply not enough oxygen to keep trout alive during this critical summer period. Today striped perch can still find enough oxygen and cool water in the lake’s thermocline to survive the summer; however, they can be stressed by low oxygen conditions (2-4 ppm).

In autumn, when the air temperatures drop, the lake begins to lose heat and the process of stripping begins. The warm water of the epilimnion cools down and becomes deeper and denser. It still has a lot of oxygen. When the epilimnion density approaches the density of the hypolimnion, intermingling of the layers can take place. In this case the stratification is broken up and the bottom water mixes with the surface water and the lake is no longer stratified. This event is called “sea transhipment” and generally takes place around Christmas each year. Once mixed, there will be no layers and the entire lake will have high concentrations of oxygen. Within a few days after the sea change, the dissolved metals become insoluble and settle on the bottom. As a result, the lake water remains clear from top to bottom and the river water also clears. Metals that have settled on the river bed are eventually washed downstream by daily generations.

As spring warms, the stratification process will repeat itself and plankton, fish and other aquatic animals will respond to these changes in their habitat.