What Caused This Famous U.S. Lake to Turn an Unusual Green This Summer?

Thanks to satellite images, the National Oceanic and Atmospheric Administration (NOAA) noticed that during the summer, Lake Erie, the fourth largest lake in North America, had turned emerald green. While the sight of such a color must surely be very beautiful, it is more synonymous with the presence of particular organisms that can be toxic to aquatic and human life.

Lake Erie, a symbol

In the United States and more broadly in North America, the American-Canadian border is particularly marked, in the East, by the presence of the Great Lakes. There are five of them:

• Lake Superior
• Lake Michigan
• Lake Huron
• Lake Erie
• Lake Ontario

Together, these gigantic bodies of fresh water form what is more commonly known as the “Great Lakes region”. Although Lake Erie is only the fourth largest in area, covering 25,655 km2, it is strategically located for tourists and landscape lovers. For comparison, Lake Superior, the largest body of fresh water in the world, covers 82,097 km2.

Indeed, a few hours by car from New York and bordering Cleveland, it is a natural border between the United States and Canada. But more than that, Lake Erie is particularly known for being downstream from the famous Niagara Falls.

And in the summer of 2024, tourists and locals noticed that the fourth largest body of fresh water in North America had partially turned a strange green color.

Cyanobacteria at work

If Lake Erie has taken on this particular color, it is not because of a celebration of St. Patrick's Day that was too late, or too early. Quite the opposite. If the usually blue water has turned green, it is because of particular algae: cyanobacteria.

According to theEnvironmental Protection Agency (EPA), a US government organization, it is a type of algae that occurs naturally in all freshwater ecosystems.“But then, if this algae is present in a completely normal way in the water, why is it so surprising to see the water turn green?

Well, the EPA says it's not its presence that's a problem, but its excessive proliferation.These blooms can be identified as floating mats of gelatinous, smelly, decaying scum in lakes and form carpets on the bottom of streams..” we can read on their page dedicated to this algae and its consequences.

But that's not what's most worrying. No, if the algae were just present in large quantities and the smell was unpleasant, it would be possible to ignore it, as long as it remains reasonable of course. However, these are not the only consequences linked to the presence of this algae. No, there is a real risk for the aquatic life of Lake Erie, but also for the inhabitants nearby.

Indeed, the cyanobacteria that occupied Lake Erie during the month of August 2024, did not only cover a surface of water, between 830 and 1700 km2, with an emerald green. These algae are also responsible for the release of toxins, cyanotoxins into the water.

What can be the consequences of cyanotoxins on humans?

“Cyanobacteria can release algal toxins that can be harmful to aquatic and human life.” concluded the EPA's definition of cyanotoxins. But to what extent does this pose a danger to humans?

Here is a non-exhaustive list of the different symptoms that can be contracted following exposure to cyanotoxins according to the Institut national de santé publique du Québec (INSPQ):

• Diarrhea
• Vomiting
• Gastroenteritis
• Gastrointestinal bleeding
• Dermatitis
• Skin and eye irritations
• Renal failure
• Liver hemorrhages

The INSPQ also specifies that in certain cases and following “chronic exposure“cyanotoxins ingested by the body can have a carcinogenic effect, particularly in the event of exposure to Microcystiswhich have been found in Lake Erie. Thus, some of the symptoms listed above appear with prolonged exposure. On the other hand, others do not need much time to appear.

It is in this sense that cyanobacteria, and the resulting cyanotoxins, in Lake Erie can represent a danger. All the more so when we know that the place is considered a very good fishing spot by fishing enthusiasts. Indeed, it is not only direct contact that is likely to represent a danger. Indeed, according to the National Agency for Food, Environmental and Occupational Health Safety (ANSES), the consumption of contaminated fish can expose you to the contraction of the less serious symptoms mentioned above. Lake Erie therefore represented a danger in general.

What caused these algae blooms to appear on Lake Erie?

As we have previously explained, the presence of cyanobacteria in freshwater streams, or large areas, is something completely normal. However, their proliferation is not at all normal and often results from external causes.

IFLScience, which is relaying this information, specifies that Lake Erie experienced, in April 2024, a record episode of rain which was followed by a very intense heat wave. Enough to allow the explosion of the presence of cyanobacteria during the summer.

Currently, although the levels have dropped in the Lake, the concentration of cyanobacteria remains very high at the end of the summer period according to NOAA.

According to a 2019 study published in the journal Naturethe presence of these algae and their significant proliferation from year to year is one of the side effects of climate change. Indeed, the fact that temperatures are increasing means that bodies of water, whether large or small, have more difficulty returning to their normal state. This therefore gives cyanobacteria the opportunity to settle more easily and more sustainably.

Source: IFLScience / NOAA / ANSES / INSPQ