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Warming waters: Lakes signal climate change


By Hillary Brody Anchill


Coming across a body of water in Michigan doesn't take very long. Whether it’s the Great Lakes, which, according to the United States Environmental Protection Agency (EPA) comprises 84 percent of North America’s fresh surface water and approximately 21 percent of fresh surface water on the planet, or an inland lake, of which there are 1,200 in Oakland County alone, according to Oakland County Parks, water is an integral part of Michigan’s DNA. The United States Geological Survey counts 41.5 percent of the state as being covered in perennial water, categorized as surface water that flows continuously throughout the year, putting it on par with Hawaii, at 41.2 percent, and the most of any of the 50 states.


This unique access to fresh water is something often taken for granted by Michiganders. That is why it is all the more alarming that recent studies show that lake temperatures are increasing at a faster rate than also increasing air temperatures. In July 2020, Lake Michigan’s average surface water level was nearly 11 degrees higher than the average recorded temperature. That same month, Lake Ontario’s average temperature was nearly 10 degrees higher than its average surface temperature from the previous 25 years, and was the lake’s warmest surface water temperature ever recorded. Lake Superior, the largest, deepest, and coldest of the Great Lakes, is warming more rapidly than any of the other Great Lakes, all of which experienced similar warming patterns last summer.


In March 2021, the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory (NOAA GLERL) published results of a 30-year study tracking the temperature in Lake Michigan. NOAA and other government entities routinely track climate data, including temperatures and rainfall across the country, in order to provide foundational, baseline information. These data sets are imperative for researchers at labs across the country, whose job it is to interpret the numbers.


Lake Michigan covers 22,300 square miles, with a length of 307 miles that allows residents from Chicago to Traverse City to enjoy its fresh water and beaches. It is the second largest of the Great Lakes by volume and the third largest by surface area, and is the fifth largest lake in the world. In comparison, the largest of the Great Lakes, Lake Superior, is the world’s second largest lake by surface area, with a water surface area of 31,700 square miles and an average depth of 489 feet. Temperatures within the lakes can vary greatly depending on the water’s depth. According to the EPA, Lake Michigan’s average depth is 279 feet, with the depth typically increasing the more north one goes. Therefore, the 30-year study, which shows “Seasonal overturn and stratification changes drive deep-water warming in one of Earth’s largest lakes,” done in collaboration by NOAA GLERL, the University of Michigan School for Environment and Sustainability and the University of Toledo Department of Environmental Sciences, provides information that is monumental for assessing warming lake trends around the world.


Global lake surface water temperatures have warmed by an average of 0.21 degrees Celsius per decade. In Lake Michigan, average surface water temperatures from 1990-2019 have risen between 0.40 and 0.49 degrees Celsius a decade. According to Dr. Andrew Gronewold, professor at the University of Michigan’s School for Environment and Sustainability and co-author of the GLERL study, surface temperatures, like those record-setting numbers last summer, can change at a faster rate than deeper water due to water’s “climate memory.” That is why it is most notable that the 30-year study measured temperatures deep below the surface as well.


“So much research is done at the surface. That’s where humans, fish, and wildlife interact. It’s where the winds and the waves interact, where ice is formed. What we have been looking at is the overall temperature of the lake not just across the surface, but at its depth. A lot of that is borrowed from oceanographic research. There’s lots of research on the heat content on warming oceans, and the average temperature across oceans from top to bottom. The question in part was motivated by what’s the heat context of lakes and is that changing over time? We learned something pretty remarkable from directly measuring the subsurface,” Gronewold said.


The mooring station – the location from where the temperature was collected – was located in the southern part of Lake Michigan. The measurements were taken by a thermistor string, a vertical string of high tech thermometers with a heavy anchor on the bottom, measuring water temperatures at various depths. The anchor is decidedly low-tech, and may be a concrete block or an old train wheel – ”the key point is that they be heavy enough so that they stay on the bottom at a fixed place, and that they be findable so they’re available when you go to retrieve the data,” described Gronewold.


The thermistor string recorded water temperatures nearly every hour for the past three decades – the only consistent data of its kind.


In the deepest water, temperatures were rising approximately 0.06 degrees Celsius per decade, with temperatures on the surface rising at a much faster rate. While a fraction of a degree Celsius may not seem like much, according to Scott Tiegs, professor of biological sciences at Oakland University, it’s important to remember that, “when we talk about temperature, we often refer to it as the master variable. It’s one that governs so many ecosystem processes so even a degree or two can have profound implications for how those ecosystems function.”


Coming across a body of water in Michigan doesn't take very long. Whether it’s the Great Lakes, which, according to the United States Environmental Protection Agency (EPA) comprises 84 percent of North America’s fresh surface water and approximately 21 percent of fresh surface water on the planet, or an inland lake, of which there are 1,200 in Oakland County alone, according to Oakland County Parks, water is an integral part of Michigan’s DNA. The United States Geological Survey counts 41.5 percent of the state as being covered in perennial water, categorized as surface water that flows continuously throughout the year, putting it on par with Hawaii, at 41.2 percent, and the most of any of the 50 states.


This unique access to fresh water is something often taken for granted by Michiganders. That is why it is all the more alarming that recent studies show that lake temperatures are increasing at a faster rate than also increasing air temperatures. In July 2020, Lake Michigan’s average surface water level was nearly 11 degrees higher than the average recorded temperature. That same month, Lake Ontario’s average temperature was nearly 10 degrees higher than its average surface temperature from the previous 25 years, and was the lake’s warmest surface water temperature ever recorded. Lake Superior, the largest, deepest, and coldest of the Great Lakes, is warming more rapidly than any of the other Great Lakes, all of which experienced similar warming patterns last summer.


In March 2021, the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory (NOAA GLERL) published results of a 30-year study tracking the temperature in Lake Michigan. NOAA and other government entities routinely track climate data, including temperatures and rainfall across the country, in order to provide foundational, baseline information. These data sets are imperative for researchers at labs across the country, whose job it is to interpret the numbers.


Lake Michigan covers 22,300 square miles, with a length of 307 miles that allows residents from Chicago to Traverse City to enjoy its fresh water and beaches. It is the second largest of the Great Lakes by volume and the third largest by surface area, and is the fifth largest lake in the world. In comparison, the largest of the Great Lakes, Lake Superior, is the world’s second largest lake by surface area, with a water surface area of 31,700 square miles and an average depth of 489 feet. Temperatures within the lakes can vary greatly depending on the water’s depth. According to the EPA, Lake Michigan’s average depth is 279 feet, with the depth typically increasing the more north one goes. Therefore, the 30-year study, which shows “Seasonal overturn and stratification changes drive deep-water warming in one of Earth’s largest lakes,” done in collaboration by NOAA GLERL, the University of Michigan School for Environment and Sustainability and the University of Toledo Department of Environmental Sciences, provides information that is monumental for assessing warming lake trends around the world.


Global lake surface water temperatures have warmed by an average of 0.21 degrees Celsius per decade. In Lake Michigan, average surface water temperatures from 1990-2019 have risen between 0.40 and 0.49 degrees Celsius a decade. According to Dr. Andrew Gronewold, professor at the University of Michigan’s School for Environment and Sustainability and co-author of the GLERL study, surface temperatures, like those record-setting numbers last summer, can change at a faster rate than deeper water due to water’s “climate memory.” That is why it is most notable that the 30-year study measured temperatures deep below the surface as well.


“So much research is done at the surface. That’s where humans, fish, and wildlife interact. It’s where the winds and the waves interact, where ice is formed. What we have been looking at is the overall temperature of the lake not just across the surface, but at its depth. A lot of that is borrowed from oceanographic research. There’s lots of research on the heat content on warming oceans, and the average temperature across oceans from top to bottom. The question in part was motivated by what’s the heat context of lakes and is that changing over time? We learned something pretty remarkable from directly measuring the subsurface,” Gronewold said.


The mooring station – the location from where the temperature was collected – was located in the southern part of Lake Michigan. The measurements were taken by a thermistor string, a vertical string of high tech thermometers with a heavy anchor on the bottom, measuring water temperatures at various depths. The anchor is decidedly low-tech, and may be a concrete block or an old train wheel – ”the key point is that they be heavy enough so that they stay on the bottom at a fixed place, and that they be findable so they’re available when you go to retrieve the data,” described Gronewold.


The thermistor string recorded water temperatures nearly every hour for the past three decades – the only consistent data of its kind.


In the deepest water, temperatures were rising approximately 0.06 degrees Celsius per decade, with temperatures on the surface rising at a much faster rate. While a fraction of a degree Celsius may not seem like much, according to Scott Tiegs, professor of biological sciences at Oakland University, it’s important to remember that, “when we talk about temperature, we often refer to it as the master variable. It’s one that governs so many ecosystem processes so even a degree or two can have profound implications for how those ecosystems function.”


Anyone who’s ever brought a goldfish home from a carnival recognizes how susceptible fish are to changes in water temperature. Each species of fish needs water at a set temperature to not only live, but to reproduce. According to the Great Lakes Fishery Commission, the Great Lakes support 139 native species, including many which are regularly featured in grocery stores and restaurant menus, such as whitefish, walleye, lake trout, brook trout, and large and smallmouth bass. Some non-native fish species also now call the Great Lakes their home, finding a compatible ecosystem after entering the water via migration and human intervention. As Kevin Wherly, research biologist in the Michigan Department of Natural Resources (DNR) Fisheries Division explains, “the species that are here migrated into the region after the last glacial event; the whole region was covered with ice until about six to 10,000 years ago. In their [fish] ancestry, they’re a migratory lot. However, the rate of warming is much faster than we’ve ever seen, so there are concerns that not all fish species will be able to migrate to suitable habitats due to the pace of projected warming.”


The rising lake temperatures are quickly impacting these fish species, as well as those which rely on them for sustenance, income, and recreation. Cold water fish, like walleye and yellow perch, are finding their current homes inhospitable, while warmer weather fish like bass are thriving. As a reaction to the warming waters, cooler water fish have already begun migrating northward.


“The Great Lakes are massive bodies of water,” reminds Nicholas J. Schroek, director of the environmental law clinic at the University of Detroit Mercy School of Law. “Lake Michigan and Lake Huron are connected lakes. Fish can migrate all along Michigan’s Lower Peninsula to find a suitable environment. As water temperatures are changing, they’re going to move.” He notes that, while Lake Superior’s temperatures are also increasing, since its “baseline starting point” is much colder than the other lakes, cold water fish like whitefish are suffering less immediate impacts than in other lakes. Over time, however, it is likely that there will be “a change in the environmental or ecological character of the lake.”


Water depth plays a significant role when it comes to the rising lake temperatures, as the warmer surface water mixes with deeper, cooler water. These distinct bands of temperature, known as stratification, are more pronounced the deeper the water is. According to Wherly, “a naturally deep lake will stratify in the summertime, becoming thermally stratified. As the surface starts to warm, ice melts and the water is initially pretty uniform from top to bottom. As the wind blows across the surface of the water, the lake can mix from top to bottom. Wind is the main driver of lake mixing that occurs in the spring. As we warm up in the springtime, the surface water warms up faster because it absorbs most of the heat from the sun and the lower depths don’t warm up as much. As the surface waters warm more and more, they become lighter and less dense. The bottom water is more dense. … Eventually, as the temperature gradient increases in the summer, you get stratification, a warm layer on top that mixes, and a cold, more dense layer on the bottom that doesn’t mix with the upper waters. That cold water is locked away from the warmer surface waters and that’s what allows cold water to persist in those deeper lakes. Those deep lakes will still maintain some cold water habitat even as we experience climate warming because of that physical process of stratification. Shallower lakes won’t have that buffer. They’re going to mix and be warmer from top to bottom.”


Notably, according to the GLERL study, “the greatest surface warming rate occurs in October. … Monthly trends … confirm a surface warming trend in September and October with upticks occurring after 2010, though winter trends at the surface are relatively flat.” Warmer fall weather has led to a longer period of stratification.


Ice levels on Lake Michigan have also changed, with the lake no longer freezing every year. In January 2021, the Great Lakes hit record low levels, with only 3.9 percent of the Great Lakes covered in ice. For Lake Michigan, January 2021 was the second lowest ice level on record. According to NOAA, in mid-February, they predicted that at most, 38 percent of the Great Lakes would be covered with ice in 2021. By comparison, the average annual maximum ice coverage is 53.3 percent. It was expected that approximately 27 percent of Lake Michigan would have ice coverage this year, compared to its long-term average of 40 percent. In 2014, 93.1 percent of Lake Michigan was covered in ice. Lake Superior, which as recently as 1996 had 100 percent ice coverage, was expected to have 46 percent coverage in 2021. It’s most recent long-term average is 61.5 percent. The lakes rely on these cooler temperatures to regulate both their water temperatures and oxygen levels throughout the year.


When the lakes stratify, all of the oxygen in the deep, cooler layers is “locked away,” according to Wherly. “No other sources of oxygen get into that water in the summer.” The fish living in those deep waters consume the oxygen, and “you can have considerable loss of oxygen in the bottom waters and that starts to creep upward into the water column as the summer progresses.”


He goes on, “you might have good cold water temperatures in the bottom of the lake to support cold water species, but they can’t use the entire bottom waters because there isn’t enough oxygen to support them.” In spring and fall, there is turnover and the water column mixes, replenishing the lost oxygen. When excess nutrients enter the water, that loss of oxygen may be accelerated. “The main thing with climate change is, especially in these cold water lakes, they’re warming from the top and seeing a reduction of oxygen in the bottom, which creates a squeeze. The fish can’t go all the way down because of the lack of oxygen.”


The main cause of these excess nutrients entering the water is from chemical contamination, whether from runoff from fertilizers or fossil fuels. And water temperature plays a role in the chemical reactions that take place in the water, according to Donna Kashian, director of Environmental Sciences at Wayne State University and visiting scientist at NOAA GLERL. That includes an increase in phosphorus levels, which leads to algal blooms which deplete oxygen levels, as well as an increase in mercury levels. Mercury levels are higher in larger, longer-lived fish, details Schroek. “Larger fish like catfish and lake trout, because they eat smaller fish, they are accumulating that mercury, and then we accumulate it in our body.”


For those who rely on Great Lakes fish as part of their diet, and who may assume that eating local is the best option, this increase in mercury levels is particularly concerning. While there are consumption advisories for many of these fish that recommend limiting intake depending on age (children and women of childbearing age being most susceptible to the contaminants), many communities – in particular disadvantaged communities and those of color – rely on these fish as a regular part of their diet. This environmental justice concern is one that both Kashian and Schroek have, and continue, worked to tackle.


Kashian described the River Walkers program, a collaboration with the Michigan Department of Health and Human Services (MDHHS) and funded in part by the Erb Family Foundation and the Michigan Sea Grant, which is an educational campaign along the Detroit River indicating which fish are better to eat – for example, “yellow perch are really good, but step away from some of the larger, fattier fish.” For those fishing at many locations along the Detroit River, much of the fish consumption is cultural, so “if it’s part of your culture to do these big fish fries of carp and catfish, are there ways to change how you cut and clean the contaminant load? You can’t remove mercury from the fish, but you can remove some other contaminants.” This concern for the food chain is enhanced because “temperature change makes the pollutants more prevalent.”


Additionally, the warming water temperatures affect the reproduction levels of the fish, according to the DNR’s Wherly, so “we see a reduction in the number of those species that prefer cool and cold water.”


Historically, “as an agency,” Wherly said, they “might be stocking lakes with cool and cold water species, but if they become unsuitable, we’ll have to change our management approach. Stocking is a longstanding practice in the state to create fishing opportunities. There are a lot of lakes that are suitable for species like walleye that never had walleye in them; they likely just didn’t have the access in that postglacial colonization. But many of those places may not be suitable in the future, so we’d have to adjust our management on the waters and the public will have to adjust their expectations. We can’t put fish in places that are not suitable.”


The Great Lakes are not the only waterways in Michigan experiencing the impact of warming temperatures. The thousands of inland lakes throughout Michigan are also experiencing warmer temperatures and the repercussions that come with. However, there is less consistency as to how much temperatures have increased. Said Oakland University's Tiegs, “both types of lakes are susceptible to warming. There’s a lot more variation in temperatures in those smaller lakes both among lakes, a to b, and then through time. Those inland lakes undergo a lot of changes thermally, near freezing, and then they might get close to 30 degrees Celsius. The Great Lakes don’t fluctuate as much and are more thermally stable.”


Since the depth of many of the inland lakes, particularly those in Oakland County, are not nearly as deep as the Great Lakes, that layer of deeper, cooler water which occurs during stratification is less likely to occur. As such, an entire lake may mix and warm. Adds Kashian, “water contains heat energy really well. A massive body of water won’t heat up as fast as inland lakes. A smaller body of water will warm up faster and retain the heat.”


That can have deleterious consequences for the fish that live in those lakes.


As fish look to migrate to more hospitable environments, those that are unable to migrate will die off. The migratory pattern of the fish that live in inland lakes is less direct than those in the Great Lakes system. While some of the inland lakes may be connected to other bodies of water by rivers and streams, creating migratory opportunities, others are completely insular, leaving nowhere for the fish to go. Wherly said that the DNR is “trying to identify where we could improve connectivity.” One option he mentions, although he says “we’re not there yet,” is removing dams in order to “help fish populations in the face of climate change.”


Fortunately, many of Oakland County’s lakes are connected.


Right now, according to Schroek of University of Detroit Mercy School of Law, “bass is able to live in warmer temps, so they’re becoming more prevalent in inland lakes, whereas walleye and other larger fish are having a harder time surviving in our inland lakes. As that trend continues, we’ll see more shifting of these species – both those that were previously unable to survive that now are able to and some iconic fish species that will unfortunately not be able to live in these lakes if these trends continue.”

Oakland County has more lakes than any other county in Michigan, contributing to the allure of the area. With many homes – and their lawns – abutting the lakes, much of the runoff from these fertilized lawns ends up in the lakes. Schroek notes that as “those chemicals get into the lakes, it can lead to increased algae and seaweed growth. That plant growth depletes the oxygen levels, which over time affects the fish.” Additionally, he points out that it will be “aesthetically less pleasing if you have what had been a lake with open water that is now covered with weeds. It’s not as nice for boating and water skiing. We shouldn’t minimize that. Part of why property values are the way they are around Oakl