Lake Turnover
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.
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, and the bottom layer is displaced & moves up
Temp & Oxygen remain uniform
Fall turnover of a Lake:
Cold Winds mix & cool the surface
Cooler, Dense H2O sinks down
The bottom layer is displaced & moves up
Temp & Oxygen remain uniform
Water is a wonderful element here on Earth. When water freezes into ice, it floats.
When most compounds change from liquid to solid, they become heavier. But not water.
Water molecules become less dense and 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 temperature 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.
The metalimnion level is the middle level that is characterized by a steep gradient in temperature and is the barrier that prevents mixture and heat exchange between the Epilimnion level and the Hypolimnion level 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 temperature of 4°C or 39.2°F
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 decided to jump out and go for a swim, you can 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 temperature difference.
The top few feet, depending on how deep the lake is, can be quite warm, almost the same temperature as the air on top of 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 the water’s surface to circulate and pick up oxygen from the atmosphere as it would in warmer temperatures.
If 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 are 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 in 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 & open to the atmosphere
Water is exposed to the wind
Water is exposed to oxygen producers like plants, algae, and other microorganisms, using photosynthesis, 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 Turnover is Coming heavier and causing them to sink.
This will cause the cause of Turnover and make the whole lake mix with the water column of the entire lake, called the Fall Turnover.
Water at lower temperatures and denser conditions can mix much more easily.
This effect will result in making the lake’s water entirely uniform in temperature and oxygen.
After a few days or weeks, the lake settles down into Winter Stratification
If you are on this body of water during the year, you will notice an obvious smell, and decomposed material that was on the bottom of the lake is 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 are 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 the 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, the transition continues as the upper layers get colder and approach the freezing mark of 32°F.
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 Summer stratification occurs, with the warmer layer of water on top and the cooler layer of water below.
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.
Lakes can contain numerous types of ecosystems, all nearby.
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 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.
Photosynthetic sources from the Sun cannot survive the 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 a higher temperature, more sunlight, and more Oxygen.
Even though this is 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.
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 keeps you on the fish!
