As great as the fishing is in the fall, some lakes deal anglers a bad hand for a brief period when the lake “turns over.” Why some lakes have fall turnover, why some don’t, and why it can adversely affect fishing requires an understanding of stratification.
Thermal stratification is the condition of often widely different temperatures in the upper and lower waters of a lake separated by a narrow zone of rapidly changing temperature. Thermal stratification is a result of two physical properties of water.
First, water has a high heat capacity. Skipping the detailed physics, water warms and cools relatively slowly.
Second, the density of water is affected by temperature. Water is most dense at 39 degrees. Density decreases as water warms above 39 degrees or cools below 39 degrees. Fifty-degree water is lighter (less dense) than 40-degree water, and 80-degree water is lighter than 60-degree water.
To understand how lakes become stratified, let’s start in the spring. The water is the same temperature — and density — top to bottom. Lacking any density differences, subtle water currents created by wind and waves common in the spring mix the entire water column.
As the days lengthen, solar energy increases and the air warms. The water follows suit, and the entire water column warms because the lake is mixing. Sometime in late spring or early summer, a rapid warming of the upper water creates a temperature difference between the upper and lower waters, and the lake stratifies.
Although driven by temperature, stratification actually is a result of differences in water density. In the now-stratified lake, the warmer, less dense upper water “floats” on the cooler, denser lower water.
In a stratified lake, the upper, warmer water is called the epilimnion; and lower, cooler water is the hypolimnion. The two layers are separated by the thermocline — a zone of rapidly changing temperature.
As summer progresses, the epilimnion, which is in contact with warm air and receives the energy from the sun, continues to warm. The hypolimnion is isolated from the warm air and sun’s energy. Because stratification prevents mixing of the upper and lower water, the hypolimnion stays cool.
Mixing occurs within the epilimnion, so the temperature throughout the epilimnion generally is the same down to the thermocline. Yes, on those hot, calm dog days, the upper couple feet of water can be warmer than the rest of the epilimnion. On these scorching days, surface water temperature at dawn is a good indication of the water temperature down to the thermocline.
As summer wanes, the heat loosens its grip on Mississippi. With cooler air temperatures, shorter day length and a less intense sun, the stratified lake cools. It cools the same way it warmed — from the top (surface) down. When the epilimnion cools to approximately the temperature of the hypolimnion, there is no density difference to maintain stratification. The entire water column mixes — the lake turns over.
In Mississippi lakes and reservoirs, the water remains cool and the lake mixes throughout the winter and early spring until warming days and increasing solar energy restart the summer stratification process.
So, what’s the problem with turnover?
Mississippi’s lakes are fertile and support abundant phytoplankton — the microscopic algae suspended in the water column. The phytoplankton fuel the food web and produce oxygen. This ultimately leads to good fish production.
But the dense phytoplankton and sometimes turbid water limit light penetration, and the phytoplankton only produce oxygen in the upper few feet of water. Even in Mississippi’s clearest lakes, the zone of oxygen production rarely extends deeper than 15 feet. The thermocline in most of Mississippi’s larger and deeper lakes is usually about 15 to 20 feet deep. The oxygen is produced above the thermocline in the epilimnion.
Oxygen is present throughout the water column when the lake initially stratifies. After the lake is stratified, oxygen produced in the epilimnion keeps this upper water layer aerated.
Decomposition of materials that die and sink into the hypolimnion depletes the oxygen in the lower water, and the thermocline prevents mixing of the aerated epilimnetic water into the hypolimnion.
By late June, the hypolimnion in most stratified Mississippi lakes lacks sufficient oxygen to support fish. Knowledgeable anglers exploit this — they focus their fishing in the thermocline where the water is cooler but contains oxygen.
When stratification breaks down in the fall, the lower, oxygen-depleted water mixes with the upper water. This is the event called turnover. Depending on the oxygen concentration and the amount of decomposing organic matter in the hypolimnion, mixing can cause an oxygen depletion. Anglers may witness murky, brownish water, even chunks of undecomposed organic matter. In extreme cases, the lack of oxygen may cause a fish kill.
Fishery biologists don’t study what makes fish bite, so there is little biological information to support, or refute, fish turning off during turnover. We do know from experience rearing fish in hatcheries and fish farms that fish stop feeding when oxygen concentrations dip below a couple parts per million. Conceivably, the change in water clarity, or maybe dissolved gases like hydrogen sulfide from the anoxic bottom water, may also adversely affect feeding. If so, these changes directly related to turnover generally last only a few days.
Turnover also brings nutrients stored in the hypolimnion into the light-rich surface water, and an algae bloom may occur. A sudden fall algae bloom is an indication that the lake mixed, but turnover occurred 1 to 2 weeks before the algae bloom.
Be aware that not all lakes stratify and, therefore, do not have turnover. Reservoirs with a high flow through, like Tennessee River hydropower reservoirs, may not stratify. The shallow navigation channel on the Tenn-Tom Waterway, churned by commercial barges and large recreational boats, also will not stratify.
On the other hand, large coves and embayments off the channel may stratify even though the channel doesn’t.
Lakes don’t have to be deep. Shallow oxbow lakes and even farm ponds can stratify, and the thermocline may only be 3 or 4 feet deep.
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