What is the most toxic Great Lake?

The Great Lakes region of North America contains five massive lakes that provide water for consumption, transportation, power, recreation and more to millions of people. However, decades of industrial pollution have contaminated these lakes with a toxic stew of chemicals and heavy metals. Determining which Great Lake is the “most toxic” depends on what pollutants are considered and how toxicity is measured.

What pollutants make the Great Lakes toxic?

There are many types of toxic contaminants found in the Great Lakes, but some of the major categories include:

  • PCBs (polychlorinated biphenyls) – Industrial chemicals used in electrical equipment that persist in the environment and accumulate in fish and wildlife.
  • Mercury – A heavy metal released from coal burning that bioaccumulates up the food chain.
  • Pesticides – Chemicals like DDT and dieldrin that enter the lakes from agricultural and residential runoff.
  • Dioxins – Highly toxic byproducts of industrial processes like bleaching and smelting.
  • PAHs (polycyclic aromatic hydrocarbons) – Compounds formed during combustion of organic matter like fossil fuels.

In addition, the Great Lakes suffer from eutrophication – excessive nutrient loading that causes algal blooms and dead zones low in oxygen. This is not a direct toxic contamination but still represents a major environmental problem.

How is toxicity measured in the Great Lakes?

There are a few ways that scientists measure the level of toxic contamination in the Great Lakes:

  • Chemical concentrations – Measuring the absolute amounts of particular chemicals like PCBs, mercury, pesticides, etc. in water and sediment samples from the lakes.
  • Fish advisories – States issue recommendations on how much fish is safe to eat from certain lakes based on chemical testing of fish tissue.
  • Toxicity testing – Exposing living organisms to lake water or sediment and measuring their biological responses such as survival, growth, or reproduction.
  • Wildlife monitoring – Documenting levels of toxins, developmental effects, and signs of disease in birds, fish, and other wildlife.

By combining these different measurements, researchers develop a portrait of the overall toxicity burden within each Great Lake.

Historical use patterns and regulation

Why are some Great Lakes more polluted than others? A lot depends on history…

The pattern of human settlement and industry around the different lakes left an enduring imprint on their contamination levels. Lake Erie and Lake Ontario, bordered by major urban and industrial centers, have historically suffered from more severe dumping of sewage and industrial waste.

However, pollution has also flowed downstream with currents that typically move from Lake Superior through Lakes Michigan, Huron and Erie before reaching Lake Ontario. So even the upper lakes show accumulation of toxins like PCBs transported from more heavily polluted sites farther south.

Starting in the 1970s, laws like the Clean Water Act and Great Lakes Water Quality Agreement forced industries and municipalities to reduce pollution discharges. So recent regulation has curbed contaminant inputs, especially from obvious “point sources” like industrial pipes. But legacy pollution trapped in sediments continues to be a problem, and “nonpoint” runoff from diffuse urban and agricultural sources is harder to control.

Toxic trouble spots in each Great Lake

Each Great Lake has its own hotspots of historic contamination that contribute to its overall pollution burden:

Lake Superior

  • Duluth, MN harbors and estuary – High levels of PCBs, mercury, dioxins from industrial waste discharges.
  • Torch Lake, MI – Extremely high copper levels from copper mining waste.

Lake Michigan

  • Green Bay, WI – PCBs, pesticides, heavy metals from paper mills and urban runoff.
  • Grand Calumet River, IN – Heavy metals, PAHs, PCBs from steel mills and refineries in Gary, IN.
  • Milwaukee Estuary, WI – PCBs, bacteria, viruses, nutrients from sewage discharges.

Lake Huron

  • Saginaw Bay, MI – PCBs, pesticides, mercury from agriculture and industry.
  • St. Mary’s River, MI/ON – Algal blooms caused by sewage inputs and urban runoff.

Lake Erie

  • Maumee River, OH – Agricultural fertilizer and soil runoff causes recurrent algal blooms and dead zones.
  • Detroit River, MI – Heavy industry contaminants like mercury, PCBs, VOCs.
  • Cuyahoga River, OH – Formerly heavily polluted by Cleveland industry, now recovering.

Lake Ontario

  • Toronto, ON – Wastewater discharges into Toronto Harbour.
  • Hamilton, ON – Chemical, oil, and steel industry pollution.
  • Rochester, NY – Kodak photography chemicals, PCBs, VOCs from industry.

Fish Consumption Advisories

One easy way to compare overall toxicity between the Great Lakes is by the number and severity of fish consumption advisories. All Great Lakes states and Ontario issue recommended safe eating guidelines for sport fish caught in the lakes, based on chemical testing of fish tissue.

More advisories and more restrictive recommended limits indicate greater contaminant levels in fish and therefore greater concern about the lake’s water quality. According to EPA data from 2019:

Lake Total fish advisories
Lake Superior 6
Lake Michigan 37
Lake Huron 13
Lake Erie 35
Lake Ontario 11

This shows that Lake Michigan and Lake Erie have the most individual advisories warning against consuming fish from contaminated waters. Lake Superior remains the “cleanest” of the Great Lakes with the fewest advisories.

We can also look at the restrictiveness of advisories – how much recommended fish consumption is limited:

  • Unrestricted – No limit on meals or serving size.
  • Limited – Limit of 1-14 meals per month.
  • Restricted – Less than 1 meal per month allowed.
  • Do not eat – No fish should be consumed, of any species.

The percentage of lake waters under each type of advisory is:

Lake Unrestricted Limited Restricted Do not eat
Superior 97% 3% 0% 0%
Michigan 71% 29% 0% 0%
Huron 79% 19% 2% 0%
Erie 60% 38% 2% 0%
Ontario 82% 12% 4% 2%

Lake Ontario stands out as having the highest percentage (2%) of its waters designated “do not eat” along with the largest portion (4%) listed as restricted consumption only. This aligns with its high level of industrial pollution.

Toxicity Testing

Scientists also directly measure the toxicity of Great Lakes water and sediment by exposing living organisms in laboratory tests. Species like water fleas (Daphnia magna), luminescent bacteria (Vibrio fischeri), fathead minnows (Pimephales promelas), and midge larvae (Chironomus dilutus) serve as model test organisms. Researchers can spike samples with these “biological sentinel” species and assess effects on survival, growth, metabolism, or reproduction as indicators of toxicity.

In the early 2010s, a team of Canadian government researchers conducted comprehensive toxicity testing on water and sediments from 17 tributaries flowing into the Great Lakes. They exposed bacteria, algae, invertebrates, and fish to the samples and found:

  • Water from Grand Calumet River (flowing into Lake Michigan) was most acutely toxic, causing 50-90% mortality in multiple test species. This was attributed to heavy metals and PCBs from nearby steel mills and refineries.
  • Sediments from Niagara River (connecting Lake Erie and Lake Ontario) were most chronically toxic, substantially impeding growth and reproduction of fish and invertebrates. Chemical culprits included PCBs, PAHs, pesticides, and heavy metals.
  • As expected, Lake Superior tributaries showed the lowest levels of both acute and chronic toxicity.

These bioassay studies lend support to Lake Michigan and Lake Ontario containing the most severely toxic hotspots, associated with proximity to major urban and industrial centers.

Wildlife Health

We can also look at assessments of chemical contamination and disease in Great Lakes fish and wildlife populations themselves. Species like bald eagles, mink, otter, reptiles, and common game fish serve as barometers of ecosystem health. Biologists examine tissue samples and living specimens for:

  • Chemical concentrations in fat, blood, eggs
  • Physical deformities like tumors or lesions
  • Reproductive problems – egg shell thinning, chick/offspring survival
  • Physiological measures like thyroid or immune function

A few noteworthy examples:

  • Lake Ontario herring gull eggs had some of the highest DDE/DDT levels ever recorded in the 1970s, along with severe eggshell thinning linked to dioxin exposure. These chemicals came from pesticide use and industrial sources like pulp mills.
  • Mink living along Lake Michigan tributaries show higher mercury levels and mortality than those inland. Mercury enters from upstream coal burning and accumulates in food chains.
  • Lake Huron lake trout display elevated PCB concentrations from sources like paper mills; their offspring show developmental abnormalities.

In general, wildlife studies have found impairments related to industrial chemicals most pronounced in Lakes Ontario, Michigan, and Erie – aligned with patterns from other measures like fish advisories. Lake Superior wildlife remains in the best shape, while Lake Huron is intermediate.

Current Pollution Levels

Regulations restricting industrial dumping since the 1970s have succeeded in reducing Great Lakes contamination from “point source” discharges like pipes and smokestacks. However, “nonpoint” runoff remains a problem, and legacy pollutants linger at the bottom of lakes and rivers.

According to EPA monitoring between 2008-2012, average toxic chemical levels were highest in these locations:

Pollutant Highest Location
PCBs Fox River and Green Bay, Lake Michigan
DDT Lake Michigan basin
Mercury Lake St. Clair (between Lakes Huron and Erie)
Dioxins/furans Lake Ontario basin

This data shows that on a basin-wide average, Lakes Michigan, Erie, and Ontario currently have the highest levels of key pollutant groups like PCBs, pesticides, and dioxins. Lake Superior remains less contaminated overall.

Fishing and Swimming Advisories

We can also examine where state environmental agencies recommend limiting fishing and swimming due to water pollution:

  • Lake Michigan – 19% of shoreline under advisory for swimming due to bacteria or viruses from sewage contamination.
  • Lake Erie – More swimming advisories than any other Great Lake, especially western end near Detroit River discharge.
  • Lake Ontario – At least 38% of New York shoreline under advisory for swimming due to untreated sewage discharges.

This matches expectations based on the urbanization and industrialization around these lakes.

Algal Blooms

Another way to compare Great Lakes pollution is the incidence of harmful algal blooms (HABs). Although not directly a measurement of toxicity, large algal blooms are fueled by fertilizer runoff and sewage discharges. They produce toxic cyanobacteria that threaten water supplies, aquatic life, and human health. According to EPA:

  • Lake Erie experiences the worst HABs of any Great Lake, including a massive bloom in 2011 affecting drinking water for Toledo, Ohio.
  • Lake Huron has recently seen expanding HABs in Saginaw Bay.
  • The other lakes have intermittent localized blooms near nutrient sources.

This points to Lake Erie as the most impaired by excess nutrients like phosphorus and nitrogen that drive eutrophication.


In summary, by the various metrics of industrial chemical contamination, toxins in fish and wildlife, fishing/swimming advisories, and algal blooms, the ranking of Great Lakes from most polluted to least is:

  1. Lake Erie
  2. Lake Ontario
  3. Lake Michigan
  4. Lake Huron
  5. Lake Superior

Lake Erie appears to deserve the title of “most toxic Great Lake” based on its long history of agricultural and urban pollution sources, its large fishing advisories and recurring algal blooms. Lake Ontario also consistently ranks near the top for industrial chemical contamination such as PCBs and dioxins. Meanwhile Lake Superior remains the cleanest and least impacted by human activity of the five.

However, the situation is complex as pollution problems like invasive species, habitat loss, and climate change impacts also factor into the lake health. And regulatory efforts to reduce industrial and municipal dumping since the 1970s have made substantial progress in cleaning up the Great Lakes from their most polluted era, although problems persist.

Ongoing monitoring, management, and public education are still needed to reduce legacy contamination, control runoff, and protect these precious freshwater ecosystems and drinking water sources for millions of people.

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