The Dynamic Lake Environment
The most misunderstood lake concept that I encounter is lake turnover and the mixing of shallow lakes that do not stratify. To understand turnover and its consequences you need to understand three concepts. One concept is that cold water is denser than warm water. The second is that energy moves from high temperature to low temperature. The third is that algae is a plant that grows quickly when fertilized with nutrients.
Water is densest at 39 degrees Fahrenheit . When ice is covering the lake, the densest water is on the bottom which is 39 degrees. If the lake is shallow ,the lake may have cooled off to the point where the bottom temperature is colder than 39 degrees. As the ice melts, the surface water warms to 39 degrees and sinks. This displaces the colder water at the bottom which rises to the surface and brings organic matter to the surface. This process continues until the entire water column is 39 degrees and is known as spring turnover. It happens fairly quickly as spring air temperatures rise and isn’t as obvious as water clears quickly. It should be noted that air temperatures rise much quicker than water temperatures . It takes about four times more energy to change the temperature of water compared to air. This is why the air temperature can fluctuate much more than water temperature does. As previously mentioned, energy always moves from high temperature to low temperature so in the spring the air is giving its energy to the water.
As air temperatures continue to increase the water temperatures also increase. Winds will push the less dense warmer water to the windward side of the lake and warm water will “stack up” on the windward side of the lake. This is important to note for fisherman as fish will move and feed in the warmest water in the spring and early summer. Water temperatures will continue to increase to approximately the depth that sunlight penetrates. If conditions are stable and air temperatures steadily increase, a distinct barrier will develop at the lowermost extent of sunlight penetration. There will be a large temperature difference between the warmer water above this level and the colder water below. This is called the thermocline. It can be measured as a temperature change of 10 degrees Fahrenheit over 5 feet. Above this level, photosynthesis by algae and rooted weeds can take place. Below this level very little photosynthesis takes place and decomposition of organic matter uses up oxygen. In all but very clear lakes like Trout lake and Lake Tomahawk, this creates an area of very low oxygen. You can sometimes see this on your fish locator as a shaded area. This is helpful to the fisherman to know as fish cannot survive below the thermocline for long. Don’t fish below the thermocline unless you are on a very clear Oligotrophic lake. These types of lakes have little weed growth and therefore little decomposing organic matter at depth. They will have enough dissolved oxygen in the water below the thermocline for fish to survive.
The water column is not a static environment, it is constantly changing. If water temperatures are high, the thermocline might be pushed downward. If the lake is relatively shallow ( less than 30 feet), and it is oriented so the wind blows the length of the lake(fetch) the lake may completely mix. This occurred on Big Saint Germain around mid July and the entire water column measured 69 degrees from top to bottom. It then stabilized and a thermocline set up around 22 feet in early August.
Usually in August the lakes in northern Wisconsin experience cold nights. The water will lose energy as evidenced by “steam” over the water in the morning. This indicates that the lake surface is cooling. As this happens, the cooler surface water will sink and warmer water at depth will upwell until water temps are stabilized. This usually doesn’t break the thermocline. However on some shallow sections of lakes and some shallow lakes without a thermocline this cooling may cause the organic material on the bottom to upwell to the surface. This, in essence, fertilizes the water column. Algae acts quickly to take advantage of the increased nutrients and algae blooms occur. This is further aided by the breakdown of submergent weeds as the summer progresses. Weeds die and start decomposing adding more nutrients to the system creating algae blooms that may occur until much later in the fall. This mixing of the layer of water above the thermocline (epilimnion) occurs on a daily basis when air temperatures fluctuate.
The term turnover refers to the destruction of the thermocline. Water above the thermocline is warmer and less dense. Water below the thermocline has a density different enough so an invisible barrier develops. When surface water temperatures drop below the temperature of the water column below the thermocline (hypolimnion) surface water will sink through the thermocline and mix the entire water column. This is what is known as turnover. It doesn’t occur at the same time on all lakes as all lakes do not have the same water temperature at the bottom. Little Arbor Vitae may have a bottom temperature of 54 degrees and will turn much earlier than Trout lake that has 41 degree water at the bottom. This time is a turbulent time for fish. Fish crave stability so they may temporarily move very shallow or suspend in the water column. Generally move to a different lake if you see the tell tale gorp that shows up at the surface at this time.
After turnover the entire water column is equal in temperature and oxygen briefly. Fish can move anywhere and be comfortable. As temperatures at night decrease and then warm during the day fish become uncomfortable with this daily change and they will seek stability. This generally means deeper water adjacent to weeds or shallow rock. They will continue this behavior until the cover of ice changes everything.
With the cover of ice preventing drastic changes in lake temperatures fish move back into the shallows to feed as this is the only cover left in the lake. Deep wood can be an alternative source of cover for baitfish and can be very good. Fish like walleye may slip deeper during the day and move into the shallows when they have more of a sight advantage or they may sink into the weeds. Fish will stay near weeds until they start decomposing and using up oxygen. This occurs naturally later in the winter or can happen earlier if heavy slush develops early and persists like it did in the winter of 2022-23. If this happens fish move either under the ice or into deeper water that contains better oxygen. In some very weedy lakes, sunlight penetration can be reduced so much that weeds die, decompose and use up oxygen and cause winter fish kills. I can only assume that the last two winters had severe winter kills in those shallow lakes.
As you can see, lakes are a very dynamic environment. Knowing this information can help you locate fish throughout the year and help you catch more fish.