Dissolved lake oxygen levels

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By Mike Scott

Inland lake property owners may not think about the impact of dissolved oxygen on their local environment often, but it plays a critical role in the health of their lake and the fish that live in it.

Dissolved oxygen is defined as the amount of oxygen mixed into the water. Fish, insects and bacteria all depend on it, much like humans depend on oxygen to breathe. Fish become stressed, and algae growth becomes extensive when oxygen levels drop too low.

Michigan’s seasonal climate has a big influence on oxygen levels. Inland lakes separate into layers during the warmer weather months, with warm surface water and colder, deeper water below. However, these layers don’t mix easily, so oxygen in the deeper parts of a lake can become depleted as the season progresses.

The winter months can further depress oxygen levels as ice and snow cover limit oxygen exchange with the air, especially in shallow lakes. This increases the instances of winterkill, which refers to when fish and other aquatic life die because oxygen levels in the water drop too low.

The University of Michigan School for Environment and Sustainability (SEAS) released a study in December that directly tracks changes in fish body size in Michigan’s inland lakes over many decades as temperatures have risen. The study considers data from long-term fisheries records dating back to 1945, drawn from almost 1,500 inland lakes in Michigan. These records were originally kept on handwritten index cards by professionals with the Michigan Department of Natural Resources (DNR). Once gathered, researchers digitized the records to create a unique multi-decade dataset, according to SEAS.

One of the most significant outcomes of the study was that inland lake fish are significantly smaller today than they were decades ago. SEAS researchers compared fish lengths across different species and age classes over time to reach this conclusion. The study included data on 13 common inland lake freshwater species.

Among other findings, the study revealed that 37 percent of fish age classes analyzed showed a statistically significant decrease in mean length over a 75-year timeframe. Additionally, 69 percent of age classes showed a qualitative downward trend in size, and the youngest and oldest fish showed the most statistically relevant decreases in size. For example, Northern pike showed significant size reductions across all age classes.

SEAS researchers highlighted two major reasons for the shrinking of Michigan lake fish species. First, warmer water accelerates metabolism, which requires fish to ingest enough food to maintain proper body condition. Limited food may adversely impact their size. Second, competition among other species and predators and density effects may intensify as reproduction changes with warming.

The office of the Oakland County Water Resources Commissioner actively manages water levels on 54 inland lakes to support environmental health and recreational use, according to communications director Trisha Bruzek. She says the office does not conduct countywide monitoring of lake water temperatures. However, data may be collected during specific third-party engineering or maintenance activities

“New technologies can support remote temperature monitoring, and we continue to stay informed on emerging tools and best practices through collaboration with regional partners and experts,” Bruzek said in an email. “Our role is to measure water levels if that makes sense.”

Volunteer testers often monitor temperatures during the open water season, according to Joe Nohner, an inland lake habitat specialist with the Michigan Department of Natural Resources (DNR). Testers may collect lake temperature data at any point during the open water season. This is important since lake temperatures directly affect dissolved oxygen levels.

The DNR conducts standard water quality monitoring, while the Michigan Department of Environment, Great Lakes, and Energy (EGLE) is responsible for some standardized monitoring activities. The federal government can also estimate lake surface temperatures using satellite data, Nohner said.

For example, a 2024 NASA Jet Propulsion Laboratory study used more than 25 years of satellite temperature data to analyze warming trends in lakes around the world. The satellites provide long-term temperature records by combining infrared and other remote sensing observations with ground data. Among other findings, the study revealed a general pattern of higher lake surface temperatures in the U.S.

Warmer inland lake temperatures significantly impact their surrounding ecosystems. Open water periods have gradually lengthened over the years, resulting in less winter ice and more time for aquatic plants to grow.

“We’re still learning what the impact of this is on aquatic plants,” Nohner said. However, longer growing seasons likely contribute to algae bloom formation growth, which can cause significant harm to lakes. Extended open water periods also affect plankton populations and fish communities.

“It’s complicated because you want lakes to provide enough habitat for fish while also keeping the water as clear as possible,” Nohner said. “Reduced ice coverage has a major impact on a local lake environment.”

Some inland lakes have experienced an increase in summertime fish kills, which refer to either natural deaths or unnatural deaths caused by environmental factors. Cool or cold-water species such as walleye, lake trout, and cisco may be especially at risk. Bluegill, largemouth bass, and yellow perch are among the types of warm-water fish that are prevalent in shallow, inland lakes.

“We expect to see (cold-water) fish struggle more in the future if water temperatures continue to rise,” Nohner said. “That’s one reason so many lakes stock walleye. Climate change is having a real impact on walleye and other cold-water fish. It’s a concerning pattern that will be difficult to reverse.”

Without the cold water they require, walleye struggle to spawn successfully enough to maintain stable populations in Michigan’s inland lakes amid increasingly challenging environmental conditions. “You don’t want to lose the ability to provide cooler refuge for those cold-water species,” Nohner said.

Grand Rapids-based design, architecture and consulting firm Progressive Companies regularly samples inland lakes in Oakland County and throughout Michigan for clients that include such quasi-governmental agencies and organizations as lake improvement boards, neighborhood lake associations and municipalities. The firm’s work is largely funded by residents in lake communities and/or living on inland lakes, according to its Water Resources Practice Leader Paul Hausler.

He indicates some inland lake macro trends related to dissolved oxygen levels and lake temperatures have remained consistent over the 35+ years that Hausler has been in the business. Levels of dissolved oxygen have fallen across the board, often to less than half a part per million, especially at deeper levels. That causes higher nutrient levels that can adversely affect aquatic species, shrinking their available habitat areas.

Yet higher lake levels can also experience lower levels of oxygen. Hausler indicated a client project on Belleville Lake in Wayne County uncovered lower levels of dissolved oxygen near the surface last year.

“We definitely have seen an increase in anoxic conditions,” he said.

Progressive collects watershed, plant and species data — along with other relevant lake information — for many of its clients. Its professionals also gather input from residents, stakeholders, and local municipalities to capture both objective and subjective data. In addition, Progressive helps clients pursue financing when possible, often through special assessments or grants from state agencies such as EGLE, provided there is a demonstrated watershed benefit. Implementing a full management plan developed by the consulting firm can cost upward of $100,000.

“One of the strategies we may address is how to reduce the levels of phosphorus in a lake,” Hausler said. Phosphorus can trigger algae blooms, lower water clarity, and reduced oxygen levels. His firm often uses an aluminum sulfate base to reduce phosphorus levels, which can help reduce the severity of algal blooms. Grants are rarely available for inland lake chemical treatment strategies.

Progressive also regularly surveys lakes to produce detailed contour maps used in invasive species management. Based on those findings, the firm identifies herbicide or mechanical harvesting options, which—if approved—are carried out by a trusted third party.

“We need to create a plan with detailed maps to do that work effectively so we aren’t wasting money,” said Hausler. “This process is very science-driven.” Along with its contractors, Progressive uses geo-referenced technologies to help ensure the work is done correctly.

As Oakland County Parks and Recreation’s Freshwater Program Coordinator, Eric Diesing and his colleagues oversee fisheries and stormwater management for lakes within the county’s parks. The department collects a wide range of data, including water temperature, dissolved oxygen, pH, turbidity (a measure of water clarity), nutrients, total dissolved solids, and more.

“Much of what we do relates to habitat management work,” Diesing said. “The amount of testing we can do depends on timing and funding constraints.”

The department typically collects data from up to seven lakes per year on a rotating basis. Under Diesing’s leadership, the team has been actively collecting data since 2024, when he joined the department.

Historically, data collection relied on “grab samples,” which required staff to visit sites in person. Today, the department has installed real-time water quality sensors that record data every 15 minutes and upload it to a database every four hours. The sensors are used during warm-weather months and were first deployed as part of a pilot program in 2025.

“This technology is especially useful at lake inlets and outlets,” Diesing said. “It allows us to measure incoming and outgoing water temperatures and better understand watershed impacts at a broad scale.”

Diesing also plans to install up to three permanent, unmanned weather stations at county park locations by 2027. These stations will collect remote data on surface water temperature, nutrient concentrations, and other parameters, which can be correlated with information from other water quality centers.

“These remote technologies have strong implications for water management strategies,” Diesing said. “They also support analysis of high-water events. Our staff checks the data every couple of weeks. It’s an innovative, efficient and comprehensive method of collecting information.”

Compared with traditional grab sampling, these sensors provide more accurate readings and allow monitoring at the sub-watershed scale.

The department will analyze how these data affect flow, discharge and the “first flush” phenomenon, which is the initial portion of runoff during a storm. That runoff carries the highest concentration of pollutants into lakes, rivers or streams. This first flush transports substances, bacteria and other materials, both beneficial and harmful, into the lake.

Nohner said that fish body temperatures depend entirely on their surroundings. Changes in water temperature, the length of winter ice coverage, and the duration of warm-weather open-water seasons affect all fish species. “It’s really affecting cold-water fish,” Nohner said. “When surface water temperatures rise later in the season, their only protection is to move deeper. The problem is that oxygen levels are lower at those depths.”

Various data points show that some Michigan inland lakes have experienced a gradual decline in ice coverage over several decades, Nohner said. For example, Fair Lake in Barry County northwest of Battle Creek has lost 37 days of ice coverage over the past 70 years. Gull Lake in nearby Kalamazoo County has lost 26 days of ice coverage since 1924. Data from Grand Traverse Bay extend even further back, showing the bay has lost 42 days of annual ice coverage since the 1850s.

Trends in ice fishing tournaments further support this decline. One notable example is the frequent cancellation of the Black Lake sturgeon ice fishing season in northern Michigan in recent years due to insufficient ice coverage. Tip Up Town USA, an annual winter festival in Houghton Lake, has postponed its fishing tournaments multiple times because of a lack of ice coverage, most notably in 2024. The Oakland County Sheriff’s Department indicates that ice should be at a minimum of five inches in thickness for ice fishing, skating, and foot traffic

Reduced ice coverage creates serious environmental hazards and safety risks. Inland lakes across the state now have fewer days safe for fishing or walking on ice, forcing local residents to monitor ice depth throughout the winter. This trend also increases demands on local law enforcement and rescue teams, including the Oakland County Sheriff’s Office Marine Unit, which consists of part-time emergency-response marine deputies and full-time deputies trained in search-and-rescue diving.

“We do, in fact, have less ice coverage here, and we can expect that trend to continue,” Nohner said.

Hausler and his team also conduct water quality monitoring services that provide detailed assessments of the lake’s plant community, utilizing the latest in GIS mapping.

“We really look to limit the impact to native plants, so it is a detailed process,” Hausler said. “A lot of our clients want to make sure their lake can function like a swimming pool, but we go much deeper in looking at the best interests of the lake environment.”

Invasive plans generally enter the inland lake’s environment using boats and recreational equipment that have been in other lakes, waterfowl and wildlife, bait bucket releases, and more, according to the DNR.

Progressive’s water testing and research is generally conducted from late March, about two weeks after ice coverage has melted, through October or early November when air temperature cools. The firm works with an average of 120 inland lakes around Michigan each year, roughly 25 percent of which are in Oakland County.

With a longer open water season and reduced ice coverage period, the end of Progressive’s typical testing season has been starting later in the year because of higher air temperatures from September through November. Climate summaries indicate that Michigan’s average annual temperatures have increased by about two-three degrees Faranheight over the past century, according to the Michigan Department of Health and Human Services. This overall warming includes seasonal effects that can lengthen the warmer part of the year

“It hasn’t affected the end of the (ice coverage) season as much, which generally remains sometime in March, but we used to end our testing in September,” Hausler. Now, it’s often deep into November as we’ve experienced global warming.”

Hausler has observed that the reduction in ice coverage days is more dramatic on a macro level than the decrease in lake temperatures. He identified increased algal growth and higher decomposition rates, caused by lower dissolved oxygen levels, as major issues.

Other common trends include an increase in major stormwater events caused by intense rainfall. Such heavy rain can trigger nutrient runoff, carry more sediment, and fuel algal blooms. It can also increase the seepage of chemicals and other harmful substances into lakes.

To address these issues, Hausler recommends implementing green infrastructure. Examples include buffer strips, which are vegetated areas along inland lakes. and rain gardens. Both help reduce runoff and protect water quality.

Algae growth also continues to increase at alarming levels around many inland lakes, Hausler said. This can be caused by more invasive species that, over time, become increasingly tolerant of their new environments if not properly removed.

“We’re also seeing chlorine levels rise from increased road salt and runoff from water softeners,” Hausler said. More intense winter storms distribute larger amounts of road salt, considered another effect of global warming. “Across the state, we’re seeing more intense lake-effect snowfall in areas surrounding the Great Lakes,” Hausler added.

The National Oceanic and Atmospheric Administration and regional climate assessments show that the frequency of extreme precipitation, defined as two-inch or greater events, is among the highest on record for recent multi-year periods, according to the Midwestern Regional Climate Center.

Diesing is cautious about citing definitive trends since the Oakland County Parks and Recreation has been collecting data only since 2023.

“We need to engage in an adaptive management process,” Diesing said. “The lake can’t talk to me, but it can communicate. To understand the ecosystem, I first need to collect data.” For example, if data reveal that dissolved oxygen levels fall below preferred thresholds or that forage and fish populations are low, Diesing and his colleagues can make informed habitat restoration decisions, such as adjusting breeding strategies for feeder fish.

Diesing is confident that because every lake in Oakland County is interconnected, data from county park lakes can provide insights relevant to other local inland lakes. By pooling resources with partners such as the Oakland County Water Resources Commissioner’s Office and the DNR, the county’s parks and recreation team can identify emerging trends.

“The DNR may send staff to join us in the field for data collection,” Diesing said.

Higher lake temperatures, for instance, might lead the department to recommend tree planting along inland lakes to lower mid-summer water temperatures and support fish species. While habitat restoration projects may not directly affect neighboring lakes, they contribute to effective county-wide watershed management. Other strategies can also enhance species management and stormwater best practices.

“What we do at county park lakes can have broader impacts if we manage stormwater to maintain higher oxygen levels,” Diesing said.

He highlighted the presence of freshwater mussels in some Oakland County lakes. Freshwater mussels are considered to be one of the world’s most threatened species. Maintaining mussel populations in Michigan and nationally has been a multi-decade effort, as mussels often rely on fish gills for movement.

“We have a great natural resources team focused on freshwater programs and other areas beyond lakes,” Diesing said. “Our goal is to capture the view of the whole ecosystem so we can make the right decisions.”

The SEAS study produced by the University of Michigan also shows that longer open water periods lead to extended periods of stratification, another name for the separation of lake into lawyers in the spring and fall. That process occurs when warm water mixes with cold water. The mixing process replenishes dissolved oxygen levels at different layers in the water column, which in turn supports plant and biological life, water supply quality, and more. Longer stratification reduces oxygen levels in inland lakes, particularly in colder bottom waters.

Many lakefront property owners are aware of the risks associated with nutrient runoff, which can further reduce dissolved oxygen levels. Nutrient runoff may include compounds that stimulate plant and algae growth, such as nitrogen and phosphorus. Chemical treatments for aquatic plants and algae blooms can also contribute to oxygen depletion.

Each inland lake responds differently to foreign substances and changes in air and water temperatures. One potential reaction involves the sediment cap at the lake bottom, which can undergo chemical changes that prevent nutrients from being released, Nohner said. While nutrients are often viewed positively, given an assumption of nutritious benefits, nutrient imbalances in freshwater systems can be harmful.

“When that cap breaks down, sediments can release nutrients like phosphorus, which degrades lake health,” Nohner said. “In highly impacted lakes, you may see more algae blooms, even extending into boundary waters at higher levels.”

Additional nutrients released into bottom waters can also delay fall turnover. “Changes in stratification patterns are influenced by the release of nutrients from bottom waters,” Nohner said. “That leads to nutrient pollution in lakes.”

Nutrient runoff in fact acts like fertilizer once it reaches a lake, which in turn can trigger algal blooms and excessive aquatic plant growth. When algae die, they decompose in a process that uses up oxygen, Nohner said. This can lead to lower dissolved oxygen levels, more winter fish kills, and stress or loss of aquatic species, a scenario known as eutrophication.

Hausler referenced successful efforts by the state of Minnesota in developing smart salting protocols for the winter, which can prevent salt runoff that impacts lake temperatures and dissolved oxygen levels.

The Minnesota Pollution Control Agency (MPCA) leads the Smart Salting initiative, a training and certification program that teaches winter maintenance crews, property managers, and others how to apply the right amount of salt efficiently, at the right time, and with the right equipment. According to the MPCA, the goal is to reduce environmental impacts without compromising safety. Michigan communities are increasingly adopting and applying these strategies.

Progressive Companies has developed educational materials for customers outlining that more than two million tons of road salt are applied to ice roads by public agencies statewide, but this causes several problems for inland lakes. EGLE has established water quality values for chloride to describe the impact to aquatic life at different values.

Hausler points to recent data from the Huron River that chlorine levels have risen by five-six times over the last 50 years, from 15-20 ppm to more than 100. The chronic toxicity standard is 150. “This can have an intense impact on aquatic species,” said Hausler, whose firm has measured chronic toxicity levels in a few waterbodies in Oakland County.

Because water density decreases with salinity, chloride can accumulate in a lake’s bottom waters, reaching levels significantly higher than at the surface. Elevated chloride levels can create a chemocline, a chemical gradient between surface and deep water. This gradient can block a lake from fully turning over in the spring, further depleting oxygen, Hausler said. Cold-water fish, such as trout and whitefish, largely cannot survive without sufficiently oxygenated deep water.

There are no easy solutions for the impact of these environmental trends, but lakefront residents can do their part, Hausler said. In addition to increasing buffer strips and natural habitat along their own shorelines and reducing runoff, residents can partner with such organizations as the Michigan Natural Shoreline Partnership or the Michigan Inland Lake Partnership. Both organizations collaborate with lake associations and inland lake stakeholders to create best practices for neighborhood property owners.

“Just because you move to a lake doesn’t mean you can incorporate the same type of lawn maintenance strategies you could (previously),” Hausler said. “We will get some clients who blame what is happening upstream, and while there are some things you can’t control, we can all do our part. Education is key.”

Nohner offers several suggestions for lakefront property owners who want to maintain healthy oxygen levels. First, he emphasizes the importance of knowledge. Residents and lake associations should conduct qualitative water monitoring to better understand lake temperatures and their effects on aquatic life.

“Know what’s happening in your lake,” Nohner said. “The best time to start water monitoring is right now.”

Second, residents should take responsibility for managing their shorelines and nearshore habitats. Maintaining a vegetated buffer is critical because it provides natural habitat for frogs, butterflies and other species that support overall lake health. Homeowners and lake associations should also reduce the amount of nutrients that enter the water, helping preserve the integrity of natural shorelines.

“Broadly, it can feel overwhelming to think about the impacts of climate change on local lakes, including water temperatures and oxygen levels,” Nohner said. “But lake associations can work together to properly manage aquatic plant treatments and reduce nutrient pollution. Every action makes a difference.”­ ­­­­