By Michael Scott
Unbeknownst to many Michigan residents, a large percentage of our lakes go through a transformative process every year that is both natural and necessary for the good of the hyperlocal environment and lake life in general.
This natural process is called stratification, and it occurs when warm water mixes with cold waters in a lake. The mixing process replenishes dissolved oxygen levels at different layers in the water column which in in turn supports plant and biological life, water supply quality and more. Other environmental factors help to ensure this process happens, such as sunlight, wind and waves.
Stratification occurs in most inland lakes with depth maximum depths of 20 feet or more in the state and parts of the Great Lakes, according to Sarah Holden, environmental quality specialist in the Lake Michigan Unit and Water Resources Division of the state’s Department of Environment, Great Lakes, and Energy (EGLE). It is primarily driven by chemical or thermal processes.
Stratification will cause significant changes in all three layers of a lake, the epilimnion (the top layer), the metalimnion (middle layer) and the bypulimnion (bottom layer). Lakes in southeastern Michigan and other parts of the country that experience four relatively distinct seasons usually “turnover,” which is another way to refer to the seasonal mixing of the lake’s water column. The process generally happens twice each year – in the spring as winter ice melts and in the fall before ice forms.
During turnover, warmer and less dense water floats towards the top of the lake while cooler, denser water falls to the bottom of the lake. From there, temperature layering weakens as it gets colder in the fall, Holden said. This allows the lake to mix, or stratify, when air temperature matches water temperatures. This phenomenon occurs because cold water is denser than warm water, and because the top later of a lake generally consists of water that is not as dense as water that sits in its bottom layer.
Lake stratification is not a guaranteed occurrence. As mentioned, lake depth is an important factor in determining whether the process occurs, and lake depth can vary year-over-year. For most lakes with a minimum of 20 feet of depth, there’s a “profound” change in the temperature at different depths in the lake, Holden said.
“Most lakes in Michigan mix in the fall when the top of the lake cools off and approaches the temperature at the bottom. Then a storm or strong winds can cause the lake to turn over or start mixing,” Holden said.
For example, if the water at the top approaches 39 degrees Fahrenheit it may become denser than the water below it and cause lake turnover, she added. Besides lake depth, thermal stratification is also impacted by the shape of a lake, This can impact when turnover occurs or the extent to which it stratifies.
Seasonal changes in temperature, including sunlight intensity, help to change surface water temperature in both the spring and fall, according to Clean Lakes Alliance Deputy Director & Chief Science Officer Paul Dearlove. This causes the temperature of the top and bottom layers of the lake to equalize. With the help of strong winds, this new equilibrium breaks the thermal stratification, allowing for lake levels to naturally mix.
Oxygen from the surface then mixes with the bottom, while nutrients trapped near the bottom are free to mix throughout the lake. While environmentally beneficial, this turnover can cause an unpleasant smell for anglers and local property owners because decomposing organic materials are often churned up from the bottom of the lake. The smell, according to the Clean Lakes Alliance, is a sign that turnover is working, “just like Mother Natured intended,” he said.
Some lakes can develop unstable stratification if they are near that threshold and stratify, then mix, several times a year following storms or strong winds, Holden said.
Even larger bodies of water that have shallow areas may not turn over. Examples include the part of the western base of Lake Erie, which is known to be the shallowest of all five Great Lakes, and parts of Saginaw Bay, according to Wayne State University Director of Environmental Sciences and Professor of Biology Donna Kashian.
“From late spring to fall, the water at the top of the lake is warmer and less dense than the water deeper in the lake,” Kashian said. “Once this type of stratification develops different biological and chemical processes occur in the separate lake layers.”
According to Kashian, wind and solar energy are two notable variables that can affect when lake turnover occurs. Given the timing of the seasons in Michigan, stratification most often happens in May before Memorial Day in the spring, and in September or October in the fall. But several factors can alter the timeline by a period of days or weeks besides a lake’s shape.
When the conditions are right, it doesn’t take much for stratification to happen. In fact, the “flip” may occur within just a few hours. “High wind can absolutely influence a flip,” Kashian said. “When you have waves overlapping onto a shore it can also cause (the lake to flip).”
She believes that the spring turnover is particularly critical in Michigan, especially for fish and other living organisms. Evolution is dependent on this stratification and turnover process in Michigan because that is how these bodies of water have acted for centuries. “The entire food chain is tied to this process,” Kashian said. “We may not realize how important it is. It’s not just the aquatic life but this matters to all of us.”
Experienced anglers know about this process and recognize when it’s happening. “When a lake is turning over, you often get this brown color rising to the top,” Kashian said. “That signifies the (movement) of silt.” More than anything normal turnover is a natural and healthy part of the ecological system.
Bloomfield Township Clerk Martin Brook attends many of the township’s lake board meetings during the year and said that lake health is discussed at just about every one of those meetings. Lake board members are familiar with the stratification process, although it affects different lakes in various ways, based on their unique characteristics.
“Many of the lakes in the township are pretty shallow so you may not get (a full turnover),” Brook said. “I think (lakefront property owners) are very aware of it.”
It appears that the timing of the fall turnover varied more in 2024 than usual. Unseasonably warm temperatures that lasted into October meant that this process may have occurred later than usual in many lakes, says John Lutchko, director of the Great Lakes Water Study Institute at Northwestern Michigan College in Traverse City.
While Lutchko doesn’t have objective data to support that claim, he is confident that more solar heat in particular had an impact on many lakes in 2024 and was potentially one of the reasons for a later lake turnover. Kashian agrees with his theory.
Some of the year-long temperature trends in Michigan support it as well. It was a historically warm year statewide in 2024. For example, Saginaw experienced the ninth warmest October on record, according to WNEM-TV. The average maximum temperature in October in metro Detroit was 67.8 degrees, or 5.8 degrees warmer than normal, according to National Weather Service. Virtually the entire state had more days of bright sunlight in September and October than usual.
That warming trend isn’t unusual. Lutchko said that Grand Traverse Bay is regularly experiencing less ice cover each winter, which increases the time that the top layer of water remains warm. Generating data to prove this is difficult given the high cost of tracking weather buoys that use expensive, advanced technologies, but Lutchko is confident there is enough subjective evidence to suggest it’s an ongoing trend.
“We may be seeing a situation where the lakes aren’t not being capped off as early,” Lutchko said. “Even in (Grand Traverse Bay), a quick wind can provide a (significant) impact base on the wind direction.”
One reason for this is that wind can travel over long distances of water to create currents. For example, Lutchko was part of a group of family members who removed a dock at the family’s property in Lake Leelanau, located northwest of Traverse City in mid-October. He believes that Lake Leelanau had already experienced stratification by then because of how cold the lake’s top layer was. However, other lakes in the area or within a short drive may not have turned over yet because of wind patterns.
“You get those colds nights and enough depth like we have (in Lake Leelanau) even with the duration of high temperatures during the day, which can be enough, even at a (hyperlocal) level,” he said. “And remember, if a lake is very shallow you don’t always get that stratification.”
Those unfamiliar with lake stratification and turnover are often surprised to hear how dynamic the process can be, Dearlove said. Many Americans have the perception that lakes remain somewhat “static.” Even more surprising to them is the impact that this turnover has and what it means for aquatic life.
“You can directly see the effects if you are (paying attention),” Dearlove said. “It could be that an unexpected late fall algae bloom is forming. You can often point to the lake turning over as the cause.”
The seasonal effect is real, he confirmed. Cold winters and hot summers, provided with enough water depth, provide the perfect backdrop for lake turnover occurring at least twice per year.
“You’ll have temperature stratification in a deeper lake just by the physics of water and heating of the sun,” Dearlove said.
Lake stratification doesn’t happen everywhere in the U.S. According to Lutchko, having four distinct seasons is an important part of the equation. For example, lakes in southern states with minimal or no snowfall might experience stratified layer movement, but they may not mix or flip, in the same way that most lakes in Michigan experience it. While the upper Midwest and Plains are among the most temperate areas, anywhere there’s enough warmth and depth can experience stratification
Instead of turnover, warmer weather lakes may cause thermocline in the metalimnion layer in lakes, to move vertically while remaining stratified. Ideally there’s some ice at the surface which indicates cold water has moved to the top and warmer water has sunk to the bottom.
“Southern lakes can be unforgiving at their lowest levels,” Lutchko says. “It can be tough to move the oxygen in many of those lakes. As a result, those lakes may not be as conducive to certain types of life like we have here (in Michigan).”
The Clean Lakes Alliance is located in Madison, Wisconsin, and Dearlove said that lakes in that area of the Midwest will often experience turnover simply from natural changes in ambient air temperatures. Water can take a long time to cool and heat up, and lakes require some type of seasonal changes for the turnover, or switch to occur.
Dearlove agrees with Lutchko who stresses that wind can speed up the process as much as any other environmental factor. The natural temperature change, combined with enough wind, can actually nix the entire lake column. Water molecules become rearranged, forming what Dearlove refers to as a “temperature density gradient” that is based on the cooling effects and equalization of temperatures throughout the water column.
“When you have lakes that are too shallow, there’s not enough of a difference for it to occur,” Dearlove said. “The upper layer (of the lake) may be the same temperature as the bottom of the lake, especially if sunlight penetrates to the bottom.”
The bottom layer of most lakes often have more oxygen for fish, resulting in warmer water, EGLE’s Holden said. That oxygen causes organic material decomposition, which in turn impacts how nutrients and other parameters are released from the sediment. Lakes with higher nutrient levels and lower water clarity may not have much oxygen in the bottom lake layer. This can make it difficult, if not impossible, for fish to live in a lake’s bottom layer.
“When a lake stratifies and then mixes it distributes anything in the water throughout the entire lake. “(That) can add more nutrients to the top of the lake if the bottom of the lake was anoxic,” Holden says. A common outcome is nutrients moving to the top of the lake is the creation of algae blooms, she added. “The process allows for oxygen to be replenished and nutrients to be distributed throughout the lake.
On its own algae is good for inland lakes, as it is critical to the health of the Great Lakes ecosystem, according to GLISA, a collaboration between the University of Michigan and Michigan State University supported by the National Oceanic and Atmospheric Administration. GLISA also serves other midwestern U.S. states and the Canadian province of Ontario. Algae provides the main source of energy that sustains many species of marine life.
However, when algal blooms grow uncontrollably, they can create low-oxygen, hypoxic conditions, according to the GLISA website. The amount of viable habitat for fish decline without the replenishing of oxygen. These hypoxic or dead zones in the lake are generally only suitable for certain bacteria, according to the Clean Energy Alliance.
This is another reason why biannual lake turnover is ecologically important. It can both mitigate the negative impacts of low oxygen dead zones while providing the lake environment needed by fish and aquatic organisms to thrive. Having enough oxygen in lakes will thereby deliver oxygen and nutrients essential to aquatic life.
Based largely on temperature, this turnover allows aquatic life to successfully live in the lake as oxygen becomes more available. “Without this natural process, (Michigan’s lakes) would not be the vibrant ecosystem that it serves as today,” Kashian said.
While the stratification and turnover process is largely impacted by Mother Nature, humans can play a role as well. Kashian of Wayne State informally refers to how human can “mess up the process.” That refers to human actions like using fertilizer on lands that border lakes or having leaky septic fields. Ensuring a “healthy” stratification process means that the local ecosystem is being maintained in a way that is beneficial to the long-term viability of the lake, and its living organisms.
“Anything done on land can affect the water,” Kashian said.
There’s a widely known part of the Gulf of Mexico that is a “dead zone” for aquatic and plant life. It’s located in an area where much of the water from Mississippi land is discharged, Dearlove said. That discharge has historically included water runoff and pollution from manmade fertilizers and other harmful runoff.
“There’s so much “pollution” in that water that it causes bacteria decomposition to go crazy where the water enter the Gulf,” Dearlove said. “The decompensation makes the water devoid of oxygen, but it’s created by people. That’s not natural.”
This can happen locally as well, although on a smaller scale. “We over-fertilize our grass which causes too many nutrients to form,” he added. This can include phosphorus nitrogen, algae blooms or plant growth. Deeper lakes can also have dead zones, although they are much smaller than what has happened in the Gulf of Mexico.
Lake Mendoza in Madison, Wisconsin is impact by a significant amount of contaminated run off from farms and city streets, Dearlove said. When the lake stratifies in the summer, this deeper, cooler water gets sealed off from oxygen sources. It doesn’t mix up to the atmosphere. A bacterial respiration can form as a result at the bottom of the lake which depletes oxygen. Other lakes experience similar issues, depending on their location and the amount of fertilizers used by local landowners.
“Some anglers are fishing in oxygen deficient water,” Dearlove said. “If they don’t have experience fishing in that area or don’t see the signs, they may not know it.”
It’s not just property owners adjacent to the water that can damage the lake with manmade products. Dearlove says that anyone living in a lake’s watershed impact the health of that lake.
“We all live in a watershed. Small watersheds rest in the larger watersheds,” Dearlove said.
“What we do affects local waters and anything downstream. Our actions matter.”
Lutchko agrees. He believes landowners should confirm if their actions could have an adverse impact on lake health and stratification.
“You need to find out if you live within the watershed because you can be across the street from a lake but your land (leaks) into the lake,” Lutchko said. “Even if you’re not (directly located) on the lake you need to decide just how important it is to have that bright green lawn if you’re using (fertilizer) to treat it.”
The aftereffects of turnover can help to signify the lake’s overall water health. If large algae blooms form immediately after stratification, it could mean that the water is partially or completely void of oxygen, which signifies a natural and health turnover may not be possible, Dearlove said.
Outside of fertilizer, having soil fall into the lake can also adversely affect its health, and thereby stratification. “You want to maintain soil on land,” Dearlove said. Don’t let it leak or fall into the water.” That’s because nutrients in the soil may have a damaging effect.
Lutchko summarizes the impact humans have on both lake health in general and stratification specifically as the fact that what people do on land affects what happens to the water. As Kashian indicated, fertilizer is perhaps the most damaging substance to a lake. But Lutchko stressed that damage to septic systems is common and may be more prominent because septic system damage is often undiagnosed.
“It’s important to get your septic systems updated and inspected because unless that happens chances are you won’t know there’s a problem,” he says. Many households don’t regularly get their septic system inspected because there often aren’t signs of an issue until it’s too late.
Additionally, lakefront property is more likely to stay within a family for many years, so there’s no perceived need to get a formal septic system review. Furthermore, Michigan law doesn’t requirement landowners to conduct regular checks of septic fields unless a property is sold. An unnoticed septic issue may be go unchecked for several years or longer, causing ongoing damage to the water.
Another barrier to a healthy stratification process are seawalls. Depending on the type of construction, seawalls can have no or significant environmental impact. A natural seawall is typically is a complex ecosystem that sustains fish and wildlife and protects the entire lake. Native vegetation along the shore acts as a buffer against artificial nutrients, which can cause erosion and runoff. It can also protect plants from sedimentation.
The public perception is that native vegetation is less effective than something manmade but that’s simply not true, Kashian said.
On the other hand, “throwing a slab of sheet metal up and calling it a seawall” might protect a shoreline from erosion, but it can also harm the lake and it living organisms in other ways, Lutchko said. Eventually, most manmade seawalls will begin to erode, increasing runoff.
Studies such as one conducted in April 2021 by Nature Communications suggest that stratification periods will last longer in slightly warmer climates. But pronounced warmth or cold lead to “catastrophic changes” in some lakes and local ecosystems, disrupting the ecological lifeforms that rely on it, the study indicates.
Kashian agrees that moderate environments are best for healthy stratification. She says that delaying temperature cool downs in the fall or speeding warning in the spring by more than a few days could cause issues.
“The system has evolved around these nutrient inputs associated with turnover and spring in particular is a time of reproduction, hatching, and spawning,” she said. “If the nutrients (or food) is not available when species spawn or reproduce, there may be a mismatch and then the conditions are not optimal for reproduction or food availability for the young.”
The bottom line is the less humans interact with the lake, and the more moderate weather patterns are, the more likely most lakes in Michigan will experience the type of turnover needed to protect is health, its inhabitants and it’s future.
“These are complicated processes. When you mess with them, there can be (significant) consequences,” Kashian said.