By Richard Ziert
The subject of Thermoclines has been worked and reworked by fishermen so often there may be a “turnover” as we read this offering. You will be better fishermen for the effort. O.K. lets grind it out, and possibly bring to life some things those other reports and you may not have known.
A thermocline is a part of a lake’s summer period water stratification. There are normally three layers to the consequential total stratification. These layers of water are called:
• Epilimnion
• Metalimnion
• Hypolimnion
A thermocline usually sets up semi-horizontally in the middle layer and is part of a stratum of water with greater practical stability than any other part of the lake. Yes “practical” because we wouldn’t normally think of going there or, do too much fishing below it. This setting up is not a straight line affair, but dynamic in relationship to what is happening, where, and when in the whole of the ecosystem. In times of seasonal wind direction, force, mix, surface and sub-surface cross currents, etc., there may even be multiple thermoclines one on top of another. . . Confused yet? Hang in there!
The set up and depth of the thermocline (if any) can be shallower or deeper, sooner or later in the season, and long lived or not as long lived depending on:
• Depth of the water
• Wind Fetch, force of the wind and water current action
• Draw down and flooding situations
• Temperature and volume of water in each stratified layer
• Water clarity
• Direction, duration, and angle of the Sun’s Rays
• Lake Basin formation and layout
All of these elements and their interplay will tell us a lot about what is going on there. For example, and on a diversity note, depending on when, how often, and how much the authority pulls water from some reservoirs, thermoclines nearer the release point can be non existent, or they can appear, disappear, or change virtually overnight.
Cold water sinks because it’s more dense and heavier than warm water. Deeper water is under a great deal more pressure - PSI - than the warmer water above it. Anything under pressure wants to escape that pressure if it can. (Please see note on deeper dissolved oxygen appearing later in this report) When the shallow water temperature reaches a depth where it decrease’s in temperature to a point where it no longer has an offsetting factor –holding power - to the water volume part of the equation, the shallower but larger volume of water gains weight so to speak, and pushes downward (gravity), mixing rapidly with whatever is beneath it. At that point these layers would then be of equal temperature, density, and viscosity – the fall turnover.
The steeper, narrower, and less wind directed the part of the lakes basin, the shallower, the thermocline will set up, and sooner and the faster the Fall Turnover will occur there. As an example, when a constant strong spring and early summer breeze blows across the surface water windward, there is heat generated in energy transfer and the thermocline slopes downward at and near the windward shore. The thermocline set up mid lake can undulate depending on the forces affecting same.
The exception to this is stained water. While the thermocline will set up shallower in stained water, the suspended flotsam induced heat retention quality of such water will make the time until turnover longer. In some cases thermoclines do not set up at all; given windy conditions for example, shallow lakes or areas with thermoclines turnover quickly. Alternately, some shallow areas within lakes never stratify and a strong wind can mix through shallow thermoclines (if any) over several days/weeks, affecting an atypical turnover.
The dissolved oxygen content of water is essential to plant and fish life. While surface water action produces dissolved oxygen, warm surface water holds less dissolved oxygen than cold water. Almost as a paradox, the deeper we figuratively look into the water column the more dissolved oxygen dissipates. Aside from dead and rotting veggie and animal material consuming any oxygen found deep, wind and wave causing oxygen can’t mix deep without a very strong and constant wind. Colder water is more dense and viscous; acting as a barrier, and deep water pressure squeezes oxygen molecules toward the path of least resistance. Depending on the time of year, the specific area of the lake, and other factors, water molecules are generally but not always squeezed upward. Squeezed the upper or adjoining layer of water slows their entry due to differing temperatures, density, or over crowding of oxygen content already there. All of these actions develop a changeable, rising and falling, oscillating oxycline just as real and attractive as the ways and means of tentatively approaching the thermocline. Other clines such as pH, light penetration, algae-clines, etc., react similarly, relative to hydrodynamic, and thermodynamic principals. All of which affects bass in specific ways.
The traditional fall “Turnover” is a seasonal condition, usually late Fall of the year, where due to colder atmospheric temperatures setting in from above, the distinctive temperature layered segments of the water column upset each other; thereby mixing all water equally.
“Turnovers” do not occur all over the lake at the same time. Stratification and turnovers may not happen at all in some lakes and some parts of lakes are continually stable year round. The mixing process – pulling down from above - creates dissolved oxygen levels in some areas deeper and more uniform than before the build up or breakdown of water stratification. In other words the oxygen in these cases is more or less trapped or log jammed in an underwater layer, until it is used up from or by whatever is the consumer in that layer.
Clines of all kinds and “Turnovers” characterize a change in the lives of fish. All fish are affected. With the Fall Turnover microscopic food are befuddled with rising bottom nutrients; food sources become more spread out and confused with non food items. Little fish groups scatter, and the big ones are seemingly turned off for a time. Turned off is not really the case. They are spread out more; not found in traditional places as often, and they take what they can get as far as food; not knowing where it’s coming from.
Fish slow down because of the uniform colder water temperature affecting their metabolic rate. But, with few exceptions they won't leave their own kind. As with some other unfavorable changes, fish tend to roam more to get more of what they need and to find – bump into - something that suits their needs. What amount of time does it takes for some other form of stabilization to set in? Watch the weather. If the weather is stable for several days just after the turnover, less wind, fewer big drops in atmospheric temperature, the chances of a warmer shallow temporary stabilization can occur before final winter stability sets in.
Spring mixing is a more gradual layering of water from the surface down and is longer in transition. The lake water comes to life in the spring, and puts into motion all that is essential to propagation; leading to the good times of summer; a time of plenty. Remember however, at any time and depth, cooler, stabilized, well oxygenated water sustains more life. This doesn’t mean the bass are not going to be in warmer areas. It means, on average, fish found in cooler climes are going to be more abundant, bigger, and healthier over time. These fish live in a different neighborhood than shallow bass. While they may be all the things I’ve mentioned above, they are also more sophisticated in their approach to life. Angling for these fish is as different as night and day. Moving slower, being more precise, good bait contrast, recognizing that you will catch fewer of them, anatomically correct baits relative to water clarity, and more should be on your mind if you venture there. The rewards are big, but so is the effort.
If I were trying to find fish during fall turnover or spring stratification, I would start at the effective light penetration depth level for your lake and move shallower from there. Say, twice as deep as the mid lake facing weed growth for your lake. For example, if the outer edge of a weed bed is at a 15 feet depth, go no further than 30 feet in your search. Follow what bait may be present first, and see if it relates to conventional structure and any underwater current. Look for those areas which approximate traditional holding bins, deeper sharper drop-offs somewhat adjacent to traditional spawning areas.
Regardless of their shallow or deep niche, fish don't go un-stabilized for long. Sooner rather than later they will end up in these areas. While there are smaller areas that are defined contrary to the rule, for greatest consistency in spring and late fall, go for the side of the lake that is shallower than the other and look for areas that receive the most winter time sunshine. Sometimes you can't find all of these ingredients together in big spots. But, do the best you can with what you have or fish another lake that has what you need. Then, if you fish a lake that freezes enough, you can always ask the Ice Fisherman and bait shops that cater to them, where their historically good spots are. The turnover Bass won't be far away from those spots before it ices up, and they definably will be there just before spawning.
A Final Note on Stabilization:
Water entering the lake through creeks or underwater springs represents stabilization. In general, this water is not long affected by turnovers. In many ways this water is a flowing river or stream, it is often somewhat different in temperature, and has greater oxygen content for some distance into the lake. Underwater springs in and of themselves don’t have too much – on the spot - oxygen. But, if shallow enough and if the flow is strong enough to mix the adjoining water, pulling surface water down around the edges, oxygen mix is created in the process. With subsequent current, you can pinpoint likely fishy areas. These areas have a more or less constant stabilization all of their own.
Peace.