Thermocline "Sweet Spots"

[Bonefish]

Around the end of May or the first of June, the thermocline, a thin layer of water sandwiched between an upper epilimnion layer and lower hypolimnion layer, sets up when the water temperature reaches aproximately 73 degrees. During the hot summer months, there are three distinct temperature ranges from 0-12 ft, 12-22 ft and 22-45 ft with an approximate 10 degree drop for each depth range. Lakes of 15 ft and less do not have a thermocline because there is enough sunlight to support photosynthesis and phytoplankton growth at all depths. Lake clarity and strong winds can change the depth of the thermocline.

Because the lack of sunlight and decay of organic materials on the bottom, the hypolimnion lacks oxgyen and vegetation growth. Plant life doesn't grow well below 30 ft. However, there are times when the hypolimnion can hold fish because currents, strong winds and feeder streams can bring high levels of oxygen to this layer and fish will follow. In the fall when the surface temperature drops, the surface water sinks to the bottom and the warmer bottom water comes to the surface and is called "turnover." The water may smell like rotten eggs (hydrogen sulfide gas) and there will most likely be a shad kill. This is nature's way of fertilizing the upper levels of water.

Why is this important to the bass fishman? First of all bass like the thermocline because there is too much light in the epilimnion and too little oxygen in the hypolimnion. In late spring, early summer after the thermocline sets up, and post-spawn is over, predictable fishing returns in water depths of 12 to 22 ft. Bass fishermen should only be concerned with thermocline and above, and more specifically, the "sweet spot" from the thermocline to 5 ft above. Usually, bait fish and bass will be together in this zone.

October is the month with the most temperture changes. Water temperatures become more uniform from the surface to approximately 25 ft, varying from 70 to 65 degrees around 22 ft. In early October, bass like to be in the upper region of the thermocline, and the thermocline can be as much as 7 to 10 ft thick at that level. When this happens, bass fishing improves greatly. After turnover, bass become active and can be found anywhere from chasing shad in coves to suspending over humps and points in pre-winter patterns.

In closing, if you can find where the thermocline touches a point or deep structure, you are in a "true thermocline sweet spot." In the coming year, play around with your gragh in increasing the sensivity function of your sonar to around 90% and track the location of fish to the thermocline. A lot of times the fish we see do not bite, but I would rather be fishing where I know they are than beat the banks with out fish. Good fishing everyone!

[Moose1am]

Two thumbs up. Wish we had more fishermen that were intereseted in the science of the water environment.

[Rayman]

Would that theory apply to a lake such as Kentucky Lake which is a large river system? Would the curent or change in water levels make a difference?

[Bonefish]

Since KY Lake is a river supplied lake the current will affect the thermocline especially if heavy rains cause the curent to be strong. However in the summer months when the lake remains stable and if the water is 20 ft and deeper a thermocline will set up. It also depends upon how much sunlight gets through. During the summer months try to see if your graph will pick it up.

[Bonefish]

For the guys who fish farm ponds during the hot summer months when the water is hot and a storm drops lots of cold water in a short time, it will also cause the cold water on the surface to exchange with the warmer on the bottom and can cause unfortunately large fish kills. That is a sad situation when you see 7 and 8 lb bass lining the banks as well as bluegill, catfish and grass carp.

[bassin101]

Just wondering when and what water temp the turnover usaully begins. I fish Rough, Nolin and Barren mostly. How long do fish shutdown during the turnover? I've never had to fish a lake at the time of a turnover.

[Bonefish]

The turnover takes place whenever the surface temperatures reaches anywhere from 65 to 73 degrees and it takes a week or so before fish acclimate to the new conditions.

[Moose1am]

The temperature will vary. It all depends on how cold the Hypolimnion waters get before the lake stratifies. In oligotrophic lakes the hypolimnion can get down to 4 deg C (3.94 Deg C to be more specific). at the very bottom of the lake while the water above the bottom varies from 3.94 deg c up to zero and forms ice on the surface during the winter months.

The water column is divided into three zones if there is just one thermocline (metalimnion).

The water at different temperatures varies in water density. The water density can also be effected by the water's salt content also. Substances dissolved in the water can effect the water's density slightly. Lake water has different densities at the same temperature as distilled water at the same temperature.

As summer progresses into fall the angle of the sun's rays striking the Earth changes and becomes more oblique. The sun's rays have to travel though more of the Earth's atmosphere before it can reach the water's surface. The earth is farther from the sun at this time of the year. The earth's orbit around the sun is not circular but elliptical.

The water's surface is cooling in the upper reaches. Remember that the epilimnion is still free to mix above the thermocline. Winds that accompany the storms help mix the new colder water throughout the upper water column, epilimnion. Water is a good conductor of heat as well. But a lake can break up sooner if the prevailing winds in the fall are allowed to travel the entire length of the lake thus setting up currents that run underwater in the opposite direction of the wind. The lands surrounding the lake can effect how fast a lake cools down. Hills can block the sunlight from reaching the lake or reflect light back onto the waters. Hill-land lakes can be protected from high winds while lowland lake are more exposed to the wind. The lakes latitude has a pronounced effect on the lakes heat gains and losses. Remember that each day the sun warms the waters and at night the lake's heat is radiated out to space. Clean nights can cause a lake to radiate a lot of heat out into space. So a clear cold night will make the water's surface cool down more rapidly than on a cloudy night. Clouds help retain the earth's heat and prevent the heat from radiating out into space.

Bottom line is that once the upper layers of the water, epilimnion, reach the same temperature and DENSITY of the hypolimnion waters below the thermocline the lake can turn over. Currents either from rivers entering a reservoir or those developed by high winds can also help mix the upper layers and get the lake to turn over sooner. It takes energy to mix the waters of different density. But waters of the same density will mix without adding energy in the form of currents.

Remember that hot water floats above the colder water in most cases. This is why lake stratify during the summer months.

The exception is water at 3.94 deg C being denser than water at 3 deg C to zero deg C and ice. Water at temperatures from 3 deg to zero are lighter than water at 3.94 and above. That is why ice floats on the surface of a lake and that's why the lake freeze from the top down and not from the bottom up.

The water in the hypolimnion varies in temperature due to the lake type and the type of winter it was exposed to. Some lakes in the south may have warmer waters in the hypolimnion than lake in the North. The type of lake also effects the water temperature. Deeper lakes take longer to warm and cool than shallower lakes and ponds. The more water volume the longer it takes to heat and cool the body of water.

And not all lakes have depleted oxygen levels in the hypolimnion. Some lakes have a good supply of oxygen in the lower levels. Oxygen is depleted in the lower levels in lakes that have lots of nutrients and lots of organic matter decaying in the lower basin of the lake. Lakes that are devoid of organic material in the hypolimnion will have and adequate oxygen supply all summer long.



Bacteria that require oxygen (Anaerobic bacteria) eat the decaying organic matter in the lake's bottom ooze and they take up the dissolved oxygen in the hypolimnion. Once that oxygen is depleted other types of bacteria (Anaerobic Bacteria) take over and they work without oxygen. These anaerobic bacteria produce methane gas and H2S in the ooze as they consume the organic material without the use of oxygen. These bacteria can fill the hypoliminion's water with H2S gas. The methane can form bubbles and rise up to the lakes surface. I read that this happens faster when there is a low pressure system over the water. (more on that later)

In lakes with low oxygen content in the summer hypolimnion the fall turnover can bring the H2S up into the upper layers of the lake after turnover and this H2S can really fowl up the water quality.

If your lake smells of rotten eggs during the turnover then you have a eutrophic (Possessing lots of nutrients in the water) type lake with low oxygen in the hypolimnion during the summer. If you live up in Canada and you fish mostly oligotrophic (Lacking nutrients) lakes then you have not have this rotten egg smell, since these lakes have plenty of oxygen in the hypolimnion all year round.

Turnover most likely will occur when heavy rains and high winds help to break down the density barrier. The temperature can vary with the lake and with the weather each year. Some years we have ice on KY lake and other years there is no ice.

To know for sure what temperature the lake turns over you would have to take temperature measurements throughout the lake at one foot increments to determine the temperature profile of your lake. This would be done weekly and daily as your approach the fall turnover. The temperature profile survey may need to be conducted in more than one section of the reservoir to get a true picture of the lake at turnover. Some parts of the lake may not turn over at the same time as other parts of a reservoir. Some bays out of the current of the main river channel may not be mixed enough to turn over, while the main lake's river channel flow may allow the main part of the lake to mix and turn over. Maybe that's why the fish head into the mouth of the large bays in the fall?

One more thing. As the water's mix from top to bottom in a lake the amount of oxygen in the bottom waters is replenished and this allows the fish to enter the hypolimnion after the turnover. Water that is full of oxygen at the surface is now free to mix with the bottom waters and the dissolved oxygen profile is a straight line with the same amount of DO at the surface, in the middle and on the bottom of the lake. Fish are not longer bound to stay above the thermocline after the turnover. They are free to roam the entire waters of the reservoir or lake making it harder to find them. The summer patterns are gone and the fish are actively feeding getting ready for winter.

Regards,

Moose1am

[Bonefish]

Excellent! Thanks!

[kc]

I'm no scientist but in the heat of the summer if you go out into the river channel and run your graph there will be oodles of fish 40, 50, 60 feet deep. I know most of these fish are probably not bass or crappie but if there is little oxygen I can't imagine many fish venturing there. My take is that there is always current on KY and I don't believe much if any thermocline sets up in the river channel. Maybe in the bays outside of direct current but the river channel is a different story.

kc

[Bonefish]

In the KY River system I will agree with you. Too much current for a Thermocline to set up.