Discover the Most Polluted Lake in Ohio (And What Lives In It)

Written by Ashley Day
Updated: August 11, 2023
© Aleksandrkozak/
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Key Points:

  • Grand Lake St. Mary’s in Ohio is the largest artificial lake in the state, spanning approximately 13,500 acres.
  • Despite its potential as a picturesque and biodiverse hub, it is unfortunately regarded as one of the most polluted lakes in the United States.
  • Regrettably, Grand Lake St. Mary’s is also recognized as one of the most polluted bodies of water globally.
Grand Lake St Marys is a man-made lake with a surface area of 13,500 acres.

The crown jewel of Ohio’s lake system, Grand Lake St. Mary’s, is one of the most polluted lakes in Ohio. Its sparkling waters, teeming with life and offering leisurely respite, have become a cause for concern. As the largest inland lake in Ohio, Grand Lake St. Mary’s has long been a vital ecosystem. It provides sanctuary to a plethora of wildlife and contributes to the well-being of the local community. But the troubling specter of pollution, primarily from agricultural runoff and antiquated sewage systems, has significantly degraded the lake’s water quality. And impacts the health of the local wildlife and the human communities surrounding it.

Environmental issues, especially those concerning water bodies, are complex and multifaceted. Our understanding of these issues necessitates delving into their root causes. This includes the effects they have on local wildlife, and the efforts made to restore and protect these precious ecosystems. Grand Lake St. Mary’s story is woven from elements of agriculture, urban development, ecology, and local heritage.

While heartbreaking in its stark reality, the tale also provides a blueprint for environmental restoration and a testament to our innate capacity to recognize and rectify our missteps. How did Grand Lake St. Mary’s become so polluted? Let’s connect the dots and explore the whole story.

Grand Lake St. Mary's dock at sunset
Grand Lake St. Mary’s is the largest man-made lake in Ohio.


Why Are There Such High Levels of Pollution?

Grand Lake St. Mary’s in Ohio, the largest artificial lake in Ohio with a surface area of around 13,500 acres, is a picturesque lake ringed by peaceful camping spots and leisure facilities and should be a thriving hub of biodiversity. Instead, it’s considered one of the most polluted lakes in the United States and one of the most polluted bodies of water in the world. Grand Lake St. Mary’s is battling high levels of pollution. This issue has become a major concern for residents, environmentalists, and government agencies. But why is this beautiful lake suffering so gravely? Let’s dive into the murky depths of this issue.

Human Causes

Firstly, it’s essential to recognize that this pollution issue is not a natural phenomenon. Grand Lake St. Mary’s pollution history is lengthy, with human activities in and around the lake having a significant role. As the largest inland lake in Ohio, Grand Lake St. Mary’s has, for many years, been a catchment area for various forms of runoff, particularly agricultural runoff. Like many agricultural regions, the areas surrounding the lake are heavily laden with fertilizers rich in phosphorus and nitrogen, which are fantastic for crops but disastrous for lakes.

When it rains, these nutrient-rich substances are washed into the lake, causing an overabundance of nutrients in the water. This phenomenon, known as eutrophication, creates the perfect breeding ground for harmful algal blooms, particularly cyanobacteria, which consume the oxygen in the water and produce toxins harmful to aquatic life and humans. As a result, it leads to oxygen-depleted “dead zones” and makes the lake unsafe for recreational activities.

Another significant source of pollution stems from antiquated sewage systems in communities surrounding the lake. Despite modernization efforts, many older homes still use outdated septic systems that can leak harmful substances into the water. And this can lead to increased levels of harmful bacteria and other pathogens, posing health risks for humans and animals that come into contact with the water.

The Bigger Picture

The issues plaguing Grand Lake St. Mary’s highlight a broader problem. Many other inland bodies of water worldwide face similar pollution challenges, typically stemming from agricultural runoff and inadequate waste management. Furthermore, this is a stark reminder of our collective responsibility to protect these valuable ecosystems. The stakes could not be higher for the aquatic life that calls these bodies of water home and for the human communities that rely on them.

Ultimately, cleaning up Grand Lake St. Mary’s requires a multi-pronged approach. And this includes upgrading the surrounding septic systems, enforcing agricultural best practices, and implementing various lake cleanups strategies, such as aeration and chemical treatments, to reverse eutrophication. While it’s a significant task, with collective action, there’s hope yet for this beautiful body of water.

Water pollution by blooming blue-green algae - Cyanobacteria is world environmental problem. Water bodies, rivers and lakes with harmful algal blooms. Ecology concept of polluted nature.
Algae blooms, which are a common occurrence in this lake, are toxic to wildlife.


The Wildlife Found in Grand Lake St. Mary’s and How the Pollution Affects Them

Grand Lake St. Mary’s, the largest inland lake in Ohio, once boasted an array of wildlife. A unique mix of aquatic and terrestrial species has made this beautiful area their home, creating a veritable mosaic of life around the lake. Yet, now that it is one of the most polluted lakes in Ohio, the area’s flourishing biodiversity is under threat due to the pollution woes that the lake is grappling with. And it is affecting wildlife in unimaginable ways.

Native Wildlife

In the lake’s shimmering waters, a variety of fish species abound, such as the Bluegill, Largemouth Bass, Crappie, and Channel Catfish. These fish play a critical role in the aquatic ecosystem, with their presence indicating the health of the lake’s water. In the sky above, one can observe majestic birds like the Bald Eagle, Red-Tailed Hawk, and a host of waterfowl, including the Mallard and Canadian Geese. These birds add beauty to the landscape and contribute to its ecological balance, aiding in pest control and seed dispersal.

In the shallower waters and marshy regions, you can find the tireless American Bullfrog alongside various turtles, such as the Snapping Turtle and Painted Turtle. Frogs and turtles play a pivotal role in the ecosystem by controlling insect populations and providing food for other species, maintaining a healthy equilibrium. The riparian zones, or areas around the lake, are home to mammals like White-Tailed Deer, Raccoons, and Red Foxes, contributing to the lake’s vibrant wildlife diversity.

The Challenges They Face From Pollution

However, the pollution of Grand Lake St. Mary’s is severely impacting this idyllic wildlife paradise. Nutrient pollution from agricultural runoff triggers harmful algal blooms, mainly of cyanobacteria. These algal blooms create hypoxic conditions, leading to a deficiency of dissolved oxygen in the water, which can result in fish kills. Seeing fish gasping at the water’s surface is a heart-wrenching testament to this environmental tragedy. The algal blooms also produce toxins harmful to fish, causing damage to their liver and nervous system, and can lead to massive die-offs.

The pollution also has a cascading effect on the entire food chain. With fish populations declining, birds that rely on them for food must seek sustenance elsewhere or face starvation. The toxins from the algal blooms can also accumulate in the tissues of fish-eating birds, leading to illness or death.

Similarly, amphibians such as frogs and turtles are susceptible to water pollution due to their semi-aquatic lifestyles and permeable skins. Pollution exposure can lead to physiological stress, developmental abnormalities, and even mortality in these species. Mammalian species like raccoons and foxes that rely on the lake for drinking water or prey can also suffer from ingesting toxins.

The Ecological Importance of Native Wildlife and Their Survival

Ecological services like pollination, pest control, and seed dispersal that these animals provide are crucial for the health of the environment and the well-being of human societies. Furthermore, protecting Grand Lake St. Mary’s wildlife has cultural and recreational significance. These creatures play a central role in the region’s cultural fabric and the enjoyment of its natural spaces.

As such, restoring the health of one of the most polluted lakes in Ohio is a conservation imperative and a recognition of the intricate and invaluable relationships that link humans, wildlife, and the environment. Indeed, in the story of Grand Lake St. Mary’s and its wildlife, we are reminded of the deep interdependencies that bind us to the natural world and our collective responsibility to care for it.

The wildlife of Grand Lake St. Mary’s, from its diverse fish populations to its sky-patrolling avians, water-loving amphibians, and lakeside mammals, presents a fascinating picture of the diversity and resilience of life. However, the pollution that plagues the lake casts a dark shadow over this vibrant ecosystem. And poses severe threats to the health and survival of these creatures. Despite this, Grand Lake St. Mary’s story remains a testament to nature’s resilience. And a clarion call for stronger environmental stewardship. Efforts are underway to restore the lake’s health. But these efforts are not just for the lake but for the multitude of creatures that call it home.

Dead fish swim in the water of the reservoir. Environmental pollution. Earth Day. Purification, care of lakes, rivers. The problem of human ecology. Ecosystem disruption.
Pollution in Grand Lake St. Mary’s can be deadly to wildlife.

©Julia Pavaliuk/

Conservation Efforts to Improve Pollution in Grand Lake St. Mary’s

In recent years, the troubling state of Grand Lake St. Mary’s as one of the most polluted lakes in Ohio has garnered attention, resulting in numerous efforts to rehabilitate this precious ecosystem. Despite the complex issues and the severe pollution levels, local communities, governmental agencies, and environmental organizations have not lost hope. All are taking multiple measures to breathe new life into the lake. This story is a tale of resilience and commitment. And a testament to our collective ability to correct our mistakes and give nature a fighting chance.

What is Being Done to Fix the Causes of Pollution

One of the primary steps to restore the lake’s water quality involves addressing the nutrient pollution at its source. Which primarily originates from agricultural runoff that carries phosphorus and nitrogen from fertilizers into the lake. Conservation practices have been adopted in farmlands around the lake to mitigate this issue. These include using buffer strips, cover crops, and controlled drainage systems to minimize nutrient runoff.

The Ohio Department of Agriculture has also implemented nutrient management plans, mandating farmers to properly manage and apply fertilizers to reduce the nutrient loads entering the lake. They are also encouraged to use precision farming techniques, applying just the right amount of fertilizer at the right time, reducing both wastage and the chance of runoff.

Moreover, action to upgrade the outdated septic systems in homes around the lake is underway. Faulty septic systems can leak harmful substances into the lake, exacerbating the pollution problem. Therefore, improving these systems is a significant step towards safeguarding the lake’s water quality.

Efforts are also underway to physically remove harmful algal blooms and nutrient-rich sediment from the lake. Techniques such as sediment dredging, algaecide application, and aeration combat the harmful effects of eutrophication. These methods help improve the water quality and restore the lake’s natural biodiversity by creating a more hospitable environment for aquatic life.

Education and Monitoring of Pollution

Education and public involvement play a crucial role in the lake’s rehabilitation process. Local communities, schools, and volunteer groups are actively involved in cleanup events, tree-planting initiatives, and education campaigns to raise awareness about the lake’s health. These grassroots efforts directly aid in the lake’s recovery. And instill a sense of environmental stewardship among community members, fostering long-term commitment to the lake’s well-being.

In parallel with these efforts, ongoing monitoring and research efforts are essential. Regular water testing and biodiversity assessments allow for tracking the progress of restoration efforts and adjusting strategies as necessary. It also helps in identifying new issues promptly, allowing for swift intervention.

Overall, the approach to restoring Grand Lake St. Mary’s is multifaceted, combining practical remediation efforts with education and research. It exemplifies a collective commitment to reviving a cherished ecosystem. This mission reflects the intricate connection between humans and the natural world. While the challenge is daunting, the progress made thus far offers hope. It is a journey of transformation. In addition to reclaiming an ecological treasure and reaffirming our role as guardians of our planet’s precious resources.

Early morning clouds reflected in a newly-dredged rhyne (or drainage ditch) on West Sedgemoor, part of the Somerset Levels.
To prevent agriculture pollution, buffer strips, cover crops, and controlled drainage systems are used to minimize nutrient runoff.

©Dick Kenny/

Why This Lake Is an Example of the Importance of Conservation

As mentioned, re-eutrophication (recurring nutrient overloads) severely disrupts the lake’s natural ecosystem and health. It threatens aquatic life and the local communities that depend on the lake for recreation, fishing, and other activities. Over time, the effects of such a degraded ecosystem may also impact local economies that hinge on the lake’s health and vitality.

A Symbol of a Broader Problem

Importantly, we must view the pollution of Grand Lake St. Mary’s as something other than an isolated event. Instead, it stands as a potent symbol of the broader environmental challenges our world faces today. The lake’s degradation is a stark reminder of the profound impacts of human activity on the natural ecosystem as well as the urgency with which we need to address such issues. Tackling pollution in one of the most polluted lakes in Ohio and bodies of water like it is crucial in our broader fight against biodiversity loss and environmental degradation.

Solving this crisis requires local and international cooperation. Upgrading local septic systems, enforcing responsible agricultural practices, and implementing active lake cleanup measures are all part of the solution. In tandem with educational initiatives about the importance of protecting natural water bodies, these steps can go a long way toward restoring Grand Lake St. Mary’s to its former glory. In doing so, we save a single lake and contribute to the broader effort of preserving our planet’s precious freshwater resources.

From an ecological perspective, every body of water has an intrinsic value that extends beyond its utilitarian function for human beings. Our water bodies are part of an interconnected global ecosystem that supports a vast array of life. As such, Grand Lake St. Mary’s plight offers an important lesson in ecological responsibility and stewardship. It reminds us that in caring for our environment, we ultimately care for ourselves. As well as ensure a healthier, more sustainable future for all species that share our planet.

Atwood Lake
Bodies of water around the world face environmental challenges.

©Karl Zhong/

In Conclusion

Located amid a highly agricultural region, the lake’s contamination is significantly due to nutrient runoff from surrounding farms. Fertilizers rich in phosphorus and nitrogen, while beneficial to crops, have spelled disaster for this inland body of water. Heavy rains wash these substances into the lake, triggering an overabundance of nutrients, known as eutrophication. This results in the proliferation of harmful algal blooms, particularly cyanobacteria, that consume vast quantities of oxygen and release toxins. The result is a cascade of adverse effects, from oxygen-depleted “dead zones” to health risks for humans and animals in contact with the water.

Meanwhile, outdated sewage systems in neighboring communities have compounded the pollution problem, increasing harmful bacteria levels in the lake. Despite modernization attempts, numerous older homes still employ archaic septic systems prone to leaks, contributing significantly to the water’s contamination.

Human Community Impact

As one of the most polluted lakes in Ohio, Grand Lake St. Mary’s plight reflects a larger, global issue. Various bodies of water worldwide face similar environmental challenges, chiefly from agricultural runoff and poor waste management. It paints a sobering picture of our collective responsibility for protecting these invaluable ecosystems. The stakes for the myriad of lifeforms inhabiting these water bodies and the human communities reliant on them are immense.

Simultaneously, the pollution has had a catastrophic impact on the diverse wildlife calling Grand Lake St. Mary’s home. The lake, once brimming with species such as Bluegill, Largemouth Bass, Crappie, and Channel Catfish, has seen a significant decline in aquatic life. Birds, including the Bald Eagle, Red-Tailed Hawk, Mallard, and Canadian Geese, also face a food shortage due to the diminished fish populations. Amphibians like the American Bullfrog and turtles, such as the Snapping Turtle and Painted Turtle, which rely on clean water for survival, are also under threat. The riparian zones, home to mammals like White-Tailed Deer, Raccoons, and Red Foxes, have not been spared either, with pollution affecting the health of these animals.

A Collective Responsibility for Conservation

Grand Lake St. Mary’s also underscores the importance of conservation from a socio-economic perspective. Local economies often depend heavily on such bodies of water for tourism, fishing, and other industries. Compromised due to pollution, it can lead to economic loss, affecting livelihoods and well-being. Therefore, investing in conservation is also a way of investing in the stability and prosperity of our communities.

Preserving natural bodies of water such as Grand Lake St. Mary’s also holds cultural and recreational significance. They are integral to local cultures and traditions, often serving as central gathering spots for recreational activities like boating, fishing, and bird-watching. They create shared experiences that bind communities together, fostering a sense of identity and belonging. When we protect these places, we are conserving biodiversity and ensuring ecological bal as well as preserving our cultural heritage and the spaces that bring joy and connection to our lives.

At the heart of Grand Lake St. Mary’s story, one of the most polluted lakes in Ohio is a call to action. It is an invitation to each of us to step up and take responsibility. And additionally, actively participate in caring for our natural world. Whether participating in cleanup drives, educating others about the importance of conservation, advocating for policy changes, or adopting more sustainable lifestyles, each action, big or small, makes a difference. The lake’s story urges us to transform our relationship with nature, shifting from exploitation to respect, care, and mutual thriving.

Where is Grand Lake St. Mary Located on a Map?

Grand Lake St. Marys State Park is a public recreation area located on the expansive Grand Lake, spanning 13,500 acres in Mercer and Auglaize counties, Ohio. As the largest inland lake in Ohio in terms of area size, Grand Lake St. Marys boasts a significant size.

However, it is characterized by its shallow nature, with an average depth ranging between 5 to 7 feet.

Here is Grand Lake St. Mary on a map:

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About the Author

Ashley is a freelance writer and wildlife photographer. Insatiably curious and drawn to knowledge, she has a passion for conservation and sharing the wonder of the natural world with others.

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