When Does Lake Turnover Occur


Nature constantly keeps changing making itself better and repairing itself in a finely balanced way that just is amazing and the more time you spend outdoors the more you understand how complicated and fragile it is like how a few degrees in temperatures can move huge amounts of water in the deepest lake not only once but twice a year every Spring and Fall. When does lake turnover occur?

Spring Turnover of a Lake:

  • Wind & Sun heat & Mix surface to about 50°F.
  • Warm H2O sinks Down
  • Bottom layer is displaced & moves up
  • Temp & Oxygen remain uniform

Fall turnover of a Lake:

  • Cold Winds mix & cool surface
  • Cooler Dense H2O sinks down
  • Bottom layer is displaced & moves up
  • Temp & Oxygen remain uniform

 

Water is a wonderous element here on earth, when water freezes to ice, it floats. When most compounds change from liquid to solid they become heavier. But not water. Water molecules become less dense spread further apart as water freezes. If not for this unique property of water, lakes would freeze solid from the bottom up, and there would be little if any chance for living things to exist in them.

Thermal Stratification In Lakes

 

Because of the wide range of temperatures in lake water, Thermal Stratification is a typical natural occurrence.  that happens in late Spring and Summer. The lake will have 3 different layers of temperature. The sun warms the uppermost layer and cooler water separates into 2 more layers.

In Winter there is no Stratification as the whole lake cools and the overall temperature of the water becomes uniform. For most of the year, there are layers separated by temperatures and density differences.

  • Epilimnion level-is the top level that is constantly mixed by wind and waves and is warmed by the sun from the late spring to late Fall.
  • Metalimnion level– is the middle level that is characterized by a steep gradient in temperature and is the barrier that prevents mixture and the heat exchange between the Epilimnion level and the Hypolimnion levels in the lake. This layer is also called the Thermocline because of its rapid decrease in temperature and O2 concentration levels.
  • Hypolimnion level-is the deepest and coldest of water levels and does not mix with the other layers and it has low circulation. It also contains water with a maximum density and temperatures of 4°C or 39.2°

Oxygen levels will also decrease beneath this line. Above the thermocline, fish survive comfortably in the oxygen-rich water. Below that line, the Hypolimnion which is a level with the lowest oxygen levels discourages fish from living there. The decomposing organic matter at the lake bottom contributes to the low dissolved oxygen content. The coolest water at this level is the densest and weighs the most while the warmer water in the Epilimnion is less dense.

The deeper the lake the larger the gradients become in the Thermal Stratification. If you have ever been out on a boat in a deep lake and decide to jump out and go for a swim you can really feel the difference in the layers from the top warm water level to the Thermocline level if you dive deep enough and hit the top of it.

There is a huge difference in temperature. In fact the top few feet depending on how deep the lake is can be quite warm almost the same temperature as the air on top in the boat. The water drops right down to cool not far down off the surface of the lake. That cooled level of water is the Thermocline level of the lake. 

Lake Turnover Temperature

The density of water depends on its temperature. The warmer the water, the less dense it becomes. Water is at its densest closest just before it freezes. Liquids of different densities often don’t mix easily. The bigger the difference in density, the harder it is to get the liquids to mix. If you think about oil and vinegar in a salad dressing, the same type of resistance to mixing also occurs in water that has different temperatures.

In the Winter Stratification, water temperatures range from about 39 degrees Fahrenheit at the bottom of a body of water. Colder water will hold more oxygen dissolved in the solution. Oxygen levels usually are higher during cool weather, especially in early winter because colder water has the ability to hold more oxygen in dilution than warmer water can.
The exceptions are shallow lakes or rivers that are full of weeds, which can develop dissolved oxygen problems during prolonged ice cover. When a body of water is covered with ice, it does not allow water’s surface to circulate and pick up oxygen from the atmosphere as it would in warmer temperatures.
 
ScanIf the lake is snow-covered ice, that will prevent 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.
Fall turnover in lake water begins to circulate when surface water reaches about 50°F. Oxygen levels are higher from the surface to the bottom. Fish are located from the shallows to deeper water, especially around structures in the water.  The turnover in a lake happens in the Fall and the Spring and is caused by the seasonal changes that come with them.
When a Spring Turnover happens, it mixes the whole water column, where the oxygen content and the temperature is completely the same throughout the lake.
It 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.
When the ice melts in the springtime solar radiation warms the water on the top of the lake much faster than the deeper parts of the lake. Sunlight only penetrates a few feet down into the lake before it dissipates. This water is warmer and lighter so it stays above the
cooler water that is denser.

Spring Lake Turnover

Spring turnover may last a week or so 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, after the lake starts to settle down to normal, it begins to stratify or develop temperature layers once again and settles into Summer Stratification.
In the Summer the upper layer of the body of water along with the warmest water that is exposed to the sun will also have the highest amount of oxygen because of a few different reasons:
  • Water is closer & opened up to the atmosphere
  • Water is exposed to the wind
  • Water is exposed to oxygen producers like plants, algae, and other microorganisms using Photosynthesisers exchanging carbon dioxide and expelling Oxygen 

Spring turnover in a body of water is important for the same reason as Fall Turnover. Once thermal stratification sets up, the deeper Hypolimnion level does not get mixed to the surface and whatever dissolved oxygen exists in the Epilimnion level of the lake is all that is available until lake turnover in the fall.

 

Fall Lake Turnover

When the Fall Turnover happens, prevailing winds over the surface start to get colder and slowly cool down the top layer which makes the surface water with the help of these cold Fall Lake Turnover is Coming! | Ozarks Water Watch at Beaver Lakewinds,  heavier and causes them to sink. This will cause the cause Turnover and makes the whole lake mix the water column of the entire lake called the Fall Turnover. Water at lower temperatures and denser can mix much easier.

This effect will end result will make the lake’s water entirely uniform in temperature and oxygen. After a few day or weeks, the lake settles down into Winter Stratification

If you are on this body of water during the year you will notice a smell that is obvious and decomposed material that was on the bottom of the lake, now on top. The smell comes from hydrogen sulfide gases that were trapped at the bottom of the lake as aquatic plants and materials break down and collect on the lake bottom then released throughout the water column by the turnover.

This is a dramatic event for the lake as well as the fish that live in the lake. During winter months, a temperature differentiation of only seven degrees is within the lake, 32 degrees just under the ice to the consistent 39 degrees at the bottom which will remain stable, as ice cover prevents winter winds from mixing the water.

As the Fall Turnover continues into Winter transition continues as the upper layers get colder and approach the freezing mark of 32°. Not all lakes freeze and you don’t have to have a frozen surface for a lake to go through winter stratification. The body of water or this lake just needs to have the colder water on the top layer at 4° celsius. This means that in Winter the inverse stratification happens then what happens in the Summer stratification occurs with the warmer layer of water on top and the cooler layer of water below. For a great Lake Turnover explanation and Diagram watch this video:

 

 

 

Even when ice develops across the top surface of a lake, some sunlight can penetrate the ice into the water providing just enough oxygen and food for fish and other organisms, and somehow for the most part fish and other aquatic living things make it through another cold winter in Nature.

 

Thermocline Fishing

 

Give yourself an edge in the science of fishing. The more knowledge you can get your hands on the better and more successful you will be fishing in different zones and water columns with any type of fish. Reading through this article you may have read and hopefully learned that determining factors like the Sun, temperature, and oxygen levels make changes in different layers, especially in lakes.

Lakes have the ability to contain numerous types of ecosystems all near each other location. Oceans saltwater and big freshwater lakes within a hundred feet from each other can have the layers in. For fishing, this could be the Secret Sauce you need to step up your game. You can step up your tackle and baits the tools of the trade along with your basic knowledge of what you learned over the years. “Let’s get Scientific-Yall!”

A thermocline is a thin but distinct layer in a large body of fluid in which temperature changes more drastically with depth than it does in the layers above(Epilimnion level) or below (Metalimnion level)  In the ocean, the thermocline divides the upper mixed layer from the calm deep water below but most bodies of water have them. Check some of these electronic portable water testers through Amazon for Ice Fishing or in the spring when trout then Largemouth seasons are open. Optical Dissolved Oxygen Meter Tester Water Quality Monitor DO Meter with DO Probe they can also measure test like Ph and Salinity that are good water test for fishing for Largemouth Bass in Brackish waters or rivers where systems change.

Most lakes will have similar turnover and stratification patterns from year to year, assuming weather conditions are near normal. Also, lakes that have similar depths, fertility, and exposures to prevailing winds will tend to have your favorite lake act almost the same every season.  Photosynthesis is the driving force along with the sunlight radiation that can penetrate the upper level of a body of water to provide Oxygen for life on and in the deep and shallow freshwater lakes. Another source of Oxygen is wind and air from the atmosphere above the surface of the water.
Photosynthesis sources from the Sun cannot survive deeper depths of the water. Then the further you go the less light will penetrate. That also means that there will be less Oxygen at that spot too. So this means that the upper level of the lake will contain higher temperature, more sunlight, and more Oxygen. Even though this the general rule there are some lakes that have winds that can help mix the upper parts of the lake with some Oxygen and warmer temperatures in the lower deeper parts. The fish know but you won’t unless you can test for it.
If you are fishing, knowing where these layers are can help you find more fish. In the upper-level temperature, Dissolved Oxygen, and nutrients are well mixed and pretty much stay the same all through this Epilimnion level of the lake. You know about Largemouth Bass then you should know that They thrive in high Dissolved Oxygen water as high as 5- 8mg/l  range and can tolerate as low as 3 mg/ l which is milligrams per liter a measurement used to measure Oxygen same as ppm or parts per million.
Below 2 mg/l would be lethal and that would be in the Thermocline layer in the lake.  They can also tolerate salinity that can be found in Brackish water but at a minimum. I wrote this article a few months back on the subject and you would be surprised  called Do Largemouth Bass Live in Brackish Water  
Bass do hang around the Thermocline level but never in it or below it. During the summer months when the water temperatures are hot, Bass will be somewhere just above the Thermocline level so it’s good information to know where it’s at. It could be 10 feet down or 20 ft down and 3 ft wide. You just need a thermometer to see it.
If you ask any fisherman they may tell you all different answers. Having the portable test equipment with you available in your tackle box you’ll be the one everyone will be asking plus you’ll have the fish to prove it. Some Sonar equipment can also pick up the radical temperature difference of the layers in a deep lake.
Once you know how far down the fish are at the spot just above the Thermocline level where it’s rich in Oxygen, some food source, and a cooler temperature that will keep their activity level up, which in turn keeps their appetite up-which in turn keep you on the fish!

 

 

JimGalloway Author/Editor

       MyWaterEarth&Sky

 

references:

IISD- How and Why Lakes Stratify and Turn Over: We explain the science behind the phenomena

Outdoor News- Catching fish around the challenging “fall turnover”

 

 

 

 

 

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