LeBlanc: The Thermocline in lake water

It is not unusual at this time of year to see in my fishing report that the Thermocline on Lake Conroe is at 16 or 20 feet, or some other depth in that general range, and many folks have no idea to what I am referring.

I addressed this topic a couple years ago. But since then a lot of people have moved into our area and I have received a number of emails asking what I'm talking about, so once again I will address the Thermocline and what it is all about.

Most serious anglers are well attuned to phenomenon, as the fishing conditions rapidly change with the summer heat. The fishing bite will slow down in the heat of the day and become more active at night and early in the morning and after the sun goes down.

I once picked the brain of Jeff Henson, who was a biologist with the Texas Parks and Wildlife Department, and he filled me in on the Thermocline and the events that lead up to it and that occur when a lake turns over. I will pass along the information at this time.

All fresh water lakes are affected and it is caused by the temperature of the water, so let us start there.

In the wintertime a lake is Isoclinetic, meaning the temperature and oxygen content is virtually the same from the bottom of the lake to the surface of the water.

Then as spring comes around and the sun begins to warm up our world and get all of the plants growing and the animals revitalized after a long winter, the water in our lakes also start to warm. As the water warms it will warm from the top toward the bottom. As spring rolls on into summer the surface water heats up more rapidly than the depths and this will eventually start the water to stratify.

At the peak of summer there will be three distinct layers to the lake. The top layer, which is the Epilimnion, the middle layer named the Thermocline and the bottom layer, which is labeled the Hypolimnion.

The Epilimnion will be the hottest part of the lake due to the sun beating directly on it all of the daylight hours. The oxygen level of the Epilimnion is high, but the high temperature drives the fish into the deeper water.

Go down a little deeper and you will enter the Thermocline and the water will start to decrease in temperature at least one degree Celsius every meter of depth. That temperature decrease will continue throughout the Thermocline until you reach a point where the temperature will stabilize and that will mean you have reached the Hypolimnion. This layer will maintain its temperature throughout the summer and not change.

Most of the fish that we go after will be black bass, crappie and other pan fish, hybrid stripers and catfish. Of these the most temperature and oxygen flexible would be the catfish, but most species prefer water temperatures in the mid 70 degree range. The reason the fish do not hotfoot it to the Hypolimnion and stay there all summer is because everything that dies, plant and animal alike, in a lake will eventually settle to the bottom and decay. That decaying process uses up the oxygen content of the water and the Hypolimnion can end up almost completely devoid of oxygen and the fish cannot survive in that low oxygen level.

Then we reach the fall of the year and the weather fronts start coming though bring with them cooler air, rain and shorter days. The result is the Epilimnion, the top layer, will start to cool. It will also become heavier. At a given point the Epilimnion will sink mixing with the Thermocline and the Hypolimnion and the lake will again become Isoclinetic. The sinking and mixing of the three layers, as the lake becomes Isoclinetic is what is referred to when a lake is identified as “turning over.” So now you know what happens when the lake turns over and why.

To the angler I would like to offer a couple of points that will help those who fish in the summer when the water has stratified. The hotter the water the thinner the Epilimnion layer, consequently the shallower the fish. They will stay as close to this oxygen enriched layer as temperature tolerance will allow, but will be in the Thermocline.

When the surface cools down at sundown the fish will rise from their survival balancing act in the Thermocline into the oxygen rich Epilimnion and do their feeding. That is why you will see in the fishing reports one depth in which one may find fish “early and late” is listed and then another, deeper, location for when the sun is starting to reheat the water from the nights cooling driving them deeper into the Thermocline.

I hope this will help to enlighten the anglers who fish the lakes of our great State and also help them increase the catches even under those difficult summer days.

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