Answer(s) from Grok, which includes a variety of sources. All the sources (many from fishing forums) are included here for completeness. Most are from lakes in the northern tier of states, which share issues with New Hampshire (and Ontario in Canada).
Such AI-generated answers request that details be checked for accuracy.
"When a lake turns over, it refers to the process where the stratified layers of water—typically the warm, oxygen-rich surface layer (epilimnion), the rapidly changing temperature layer (thermocline), and the cold, oxygen-poor bottom layer (hypolimnion)—mix together due to cooling surface temperatures in the fall.
This mixing occurs when the surface water cools to a temperature similar to the deeper water, making it denser and causing it to sink, which disrupts the thermocline and leads to a complete vertical mixing of the water column.
This event is crucial for oxygen distribution, as it replenishes oxygen in the deeper layers that were depleted during summer stratification.
The change in lake color during turnover is often due to the resuspension of organic matter, sediments, and nutrients from the lake bottom. This can cause the water to appear murky, brown, or green.
The discoloration is not solely due to turnover but can also result from runoff, algal blooms, or diatom blooms that occur as water temperatures shift.
For instance, diatoms often dominate after ice-out, giving water a brownish hue, and their dominance can shift as the thermocline forms and sunlight is blocked from reaching deeper layers.
Additionally, the presence of hydrogen sulfide (H₂S), which smells like rotten eggs, can indicate turnover, as it is produced by anaerobic bacteria in oxygen-depleted bottom waters and is released into the surface when mixing occurs.
While a change in color may signal turnover, it is not definitive, as rain or wind can also muddy the water.
Therefore, color changes are often a symptom of the broader physical and biological changes occurring during the turnover process.
June 12, 2024 - Lake turnover is a phenomenon whereby the entire volume of water in a lake is mixed by wind. This can only happen when the entire lake is the same temperature (and density!), which in northwestern Ontario (home to IISD-ELA) generally occurs ...
water, but each night the thermocline get destroyed as the nightly convective mixing from evaporative ... The full depth mixing after turnover destroys any particular algae dominance in the lake, so cyanobacteria · HABs are destroyed. And as winter approaches and the water turn even colder, cold water diatoms still thrive, resulting in the water having a brown color.
Michigan lakes are usually dimictic in their seasonal patterns – meaning they mix twice a year. Lakefront property owners are familiar with what many describe as spring or fall “turnover.” At these times the temperature and density of lake water are so similar from top to bottom that winds blowing over the surface can actually cause a vertical mixing of the entire lake.
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a distinct layer based on temperature within a large body of fluid (e.g. … Wikipedia
Thermoclines can also be observed in lakes. In colder climates, this leads to a phenomenon called stratification. During the summer, warm water, which is less dense, will sit on top of colder, denser, deeper water with a thermocline separating them. The warm layer is called the epilimnion and the cold layer is called the hypolimnion.
June 16, 2025 - During periods of stratification, a lake will develop distinct layers based on temperature. The top layer, the epilimnion, is warm and less dense due to solar heating. Below that lies the thermocline, a zone of rapid temperature change.
November 1, 2024 - You may also notice small bubbles or a film on top of the water. This signifies a lake turnover, which happens when the surface water temperature falls lower than the sub-surface water temperature.
Eventually the heat will make its way to the bottom and the bottom of the lake will warm up and break-up as the methane and carbon dioxide trapped in the silt start to expand. This
Spring lake turnover is the process by which a lake mixes itself, thereby replenishing its oxygen supply. Many lakes and ponds are changing colors – from pleasant blue or clear to murky brown or green, caused by runoff of nutrients and carbon, coupled with warmer temperatures.
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.
Well, if you picture a cross section of a lake, you can divide it roughly into three layers: the hypolimnion on the bottom, the thermocline as a middle layer, and the epilimnion as the top layer of water (the name of the process that forms these layers is thermal stratification). During the summer, the epilimnion will be the warmest since it is closest to the sun and the dense cold water will stay at the bottom. However, in the fall and spring, a critical process called lake turnover occurs.
A thermocline is the transition ... to tell when you have reached the thermocline in a body of water because there is a sudden change in temperature...., 2025 - In temperate latitudes, many lakes that become stratified during the summer months de-stratify during cooler windier weather, with surface mixing by wind being a significant driver in this process. This is often referred to as "autumn turn-over". The mixing of the hypolimnium into the mixed water body of the lake recirculates nutrients, particularly phosphorus compounds, trapped in the hypolimnion during the warm weather.
Because of the warmth and the wind action, the water in the epilimnion is generally more turbulent than the layers below. Beneath the epilimnion is the thermocline, a transition zone where temperature decreases rapidly with increasing
This causes the cold water below the thermocline to mix with the warm water above the thermocline. This essentially equalizes the lakes water temperature from top to bottom. No major temperature differences will exist in the lake again until ...
A thermocline is a transition layer of water that has a dramatically different temperature than the fluid above or below it. Did you know? Not only do thermoclines exist, but there are also haloclines and pycnoclines. Haloclines are layers of water containing different salinity contents and pycnoclines are layers of water with distinct differences in their densities. Thermoclines occur in most large bodies of water including lakes and oceans.
The density of water changes with temperature and that is the underlying cause (plus seasonal temperature changes). The most dense that water gets is when temperatures are in the high 30s (Fahrenheit). So, this means: Ice floats. Ice is at most 32 degrees F. This affects your glass of ice water as well as lakes/ice fishing. In the summer, the hottest water is on the surface of a lake. Fish that like cold water will be deeper, assuming there is enough oxygen in a lake. In addition, in the summer, many/most lakes form a “thermocline”, which is basically a thin layer where the water gets a lot colder very quickly. Below the thermocline, the water is likely at its most dense temperature (high 30s F). Because the thermocline represents a pretty substantial change in water density, it is really unusual for a deepish lake to de-stratify due to a weather event (I have seen it happen during a typhoon, but there aren’t many typhoons in Nebraska, for example). The only way that a lake loses its thermocline is for the water above the thermocline to get cold enough that its density is similar to the water below the thermocline. When this happens, you have a turnover event. This would be a fall turnover, as I have described it. There is a similar/opposite turnover in the spring. Assuming that ice forms on the lake, the warmest water will be on the bottom and the coldest water (ice) is on the top of the lake. As the “cold” water on top gets warm (into the high 30s), it gets more dense and sinks to the bottom. Eventually, the entire lake is about the same temperature and it mixes at that time. Most lakes that do this are called “dimictic” lakes, meaning they mix twice per year. There are some interesting lakes that have a salty layer on the bottom (salty water is more dense than freshwater). These lakes are called “meromictic” lakes. Some lakes/ponds are very shallow (less than 5 feet) such that heavy winds can cause mixing and destroy a thermocline. These would be called “polymictic” lakes.
This is an area (layer) of water that may or may not have increased oxygenation and nutrients. Most lakes "turn over" where the lighter, warmer water will rise to the surface and displace the colder water from winter.
Its a small layer of water where the water temperature rapidly changes from warm to cold. The thermocline layer prevents mixing of the warm (epilimnion) and cold (hypolimnion) water. In the fall, the surface waters cool until they are as dense as the bottom waters. With the help of the wind, the lake can mix and this is called "fall turnover."
Lakes turn over when the thermocline is eliminated and oxygen levels/temperature is fairly constant top to bottom. The thermocline can be eliminated by colder water on top becoming dense (heavier) and displacing deeper water."
Maybe this should be moved to the fishing forum?
IDK.
.