What Is Lake Turnover?
It happens in spring and fall, and anglers know it affects the location of sport fishes.
Tell someone that the lake you are fishing has recently turned over and most likely that will produce some
strange looks and perhaps an image of a lake flip flopping in the dark of night. But turnover is what happens to
lake water in the spring and fall. Anglers recognize the effect if not the actual process.
Lake turnover is caused principally by seasonal temperature changes of a lake's surface water. Turnover
occurs in the spring when water begins to warm and again in the fall when water cools.
Water circulates and stratifies in a lake throughout the year. Since temperature and circulation regulate the
amount of oxygen that water can hold in solution, water temperatures and oxygen levels often dictate where
fish are found in a lake. Anglers who adapt their angling to the temperatures and oxygen levels preferred by
sport fishes can improve their success.
In winter, water temperatures range from about 39 degrees Fahrenheit at the bottom. Water is densest and
heaviest at 39 degrees (4ºC); colder and warmer waters are lighter. Colder water holds more oxygen in
solution. Consequently, oxygen levels usually are high during cool weather, especially in early winter.
The exceptions are shallow lakes or lakes choked with weeds, which can develop oxygen problems during
prolonged ice cover. Ice cover does not allow lake water to circulate and pick up oxygen from the atmosphere
at the surface. In addition, snow-covered ice prevents sunlight from entering the water, and this slows oxygen
production by aquatic plants. Under these conditions, oxygen levels can become depressed due to respiration
by aquatic animals and plants, as well as from the decomposition of organic materials such as dead plants. A
fish-kill will result if oxygen levels become excessively low.
During winter, most fish tend to seek the warmer water near the bottom.
When surface water reaches 39 degrees in spring, it becomes heavier than the water immediately below it and
therefore sinks. The downward movement of surface water forces water in the deeper parts of the lake upward.
The resulting circulation of water is called spring turnover. Spring winds also help promote water circulation.
Spring turnover may last several weeks if the weather is cool and windy, or a matter of days if the weather
suddenly turns hot and calm. When the water reaches about 50 degrees, it begins to stratify or develop
temperature layers.
WINTER
No circulation of water during ice cover. Oxygen levels are usually high. However, during ice cover, oxygen
levels in shallow lakes can become very low. Fish are usually located near the bottom in deep water.
SPRING
Spring turnover - lake water begins to circulate when surface water reaches 39°F. Oxygen levels are high from
surface to bottom. Fish are located from shallow to deep water, especially around structures.
SUMMER
Epilimnion - circulating warm water. Oxygen levels are moderate to high. Fish often are confined to the zone
which may be up to 20 feet deep.
Thermocline - zone of rapid temperature and oxygen change. Fish will select this zone if oxygen is present.
Hypolimnion - cold, non-circulating water. Oxygen levels are low or oxygen is absent. Fish cannot inhabit this
zone for any length of time due to low oxygen levels.
FALL
Fall turnover - lake water begins to circulate when surface water reaches about 50°F. Oxygen levels are high
from surface to bottom. Fish are located from shallow to deep water, especially around structures.
During spring turnover, oxygen is continually absorbed at the surface as the water circulates. This, plus the fact
that the water is cold, results in high oxygen levels. Theoretically, fish can inhabit all areas of the lake, but they
tend to lie near the bottom and around submerged structures such as old stream channels, trees and brush.
Since most Missouri sport fishes spawn during the spring in relatively shallow water, fishing these waters can
be productive.
Water temperature continues to rise through spring into summer, and distinct temperature layers form - a
phenomenon called "thermal stratification". Surface water, which is lighter because it's warmer, eventually
forms a layer above a zone of colder water. This layer of warm water, called the epilimnion, circulates primarily
due to wind at the surface. In deep, clear lakes, this layer may extend 20 feet down.
Since water in the epilimnion continually comes into contact with the surface, it continues to heat up as air
temperature rises. This upper layer also tends to have higher oxygen levels due to aeration at the surface and
also because oxygen is produced by aquatic plants which live in this zone.
The hypolimnion is the cold, bottom layer of water. In most Missouri lakes, this layer loses most, if not all of its
oxygen due to the decomposition of organic materials and the consumption of oxygen by aquatic animals. The
density barrier which forms between the cold and warm layers prevents the water in the hypolimnion from
circulating to the surface. Little or no sunlight penetrates to the hypolimnion, so oxygen-producing plants
cannot survive in this zone. As summer progresses, oxygen levels become depleted, so fish and most other
aquatic animals must move to shallower waters which contain adequate oxygen.
Between the warm, upper water and cold, bottom water, a narrow layer called the thermocline or metalimnion
forms. Water temperature and oxygen within this layer change dramatically from its upper to its lower levels.
During the hot summer months, many sport fishes prefer the cool water in the thermocline, but they may not be
able to live in this zone if oxygen levels are low.
When surface water reaches about 50 degrees Fahrenheit in the fall, it becomes as dense as deeper water
and sinks. The downward movement of surface water forces deeper water upward. As happens in spring, the
water begins to circulate, and this is called fall turnover. Circulation is also enhanced by fall winds. Since the
circulating water continually comes into contact with the surface, oxygen levels become high throughout the
lake.
The onset of fall turnover sometimes produces a sulfurous or rotten-egg odor. It comes from sulfur dioxide gas
produced by decomposition during the summer and trapped within the hypolimnion. When the deep water
reaches the surface during fall turnover, the sulfur dioxide is released into the atmosphere.
As in spring, fish can be found throughout the lake. Sport fishes will tend to gather near the bottom and around
submerged structures.
Some degree of water circulation can continue until ice covers the lake. When ice forms, wind can no longer
provide the driving force to circulate the water. Minor stratification then sets in, with the dense, 39-degree water
at the bottom and water as cold as 32 degrees at the surface.
Any lake will have similar turnover and stratification patterns from year to year, assuming weather conditions
are near normal. Also, lakes which have similar depths, fertility and exposures to prevailing winds will tend to
have similar turnover and stratification patterns. All lakes turn over, but not all of them stratify strongly. Some
shallow lakes contain warm water from the surface to the bottom, and even deep lakes do not significantly
stratify if they are exposed to strong winds which can continually circulate the water.
Probably the most accurate way to determine lake turnover and stratification patterns throughout the year is to
measure temperature and oxygen profiles from top to bottom. Although electronic temperature and oxygen
meters are becoming more popular, their high cost keeps them on the wish lists of most anglers. However, if a
fishing friend has such equipment, have him/her determine seasonal temperature and oxygen profiles on your
favorite lakes. If the lakes you fish have good water quality; water temperature, dissolved oxygen and
submerged structures will be the three most important factors affecting the location of sport fishes. Your ability
to find good structures in waters which contain preferred temperatures and adequate oxygen will improve your
odds of catching fish.