WEBSTER — The August sun is beating down and winds are slowing down, bringing blooming season for blue-green algae in lakes across eastern South Dakota.

Algae is common in most lakes in the eastern portion of the state, bringing a green coloring to the water. It’s safe to swim in or catch fish, but blue-green algae is unmistakably different and literally toxic.

When blue-green algae or cyanobacteria dies, it releases toxins into the water that can be harmful if ingested. The signs are obvious visually, textually and through scent. The South Dakota Game, Fish and Parks Department recently urged anyone who observes blue-green algae to contact their local GF&P office or the South Dakota Department of Energy and Natural Resources.

The color is not always blue or green, it can be brown, purplish red or even yellow. But blue-green algae can be distinguished through its thickness as it rests upon the surface of a lake or pond, bringing with it a sewer-like odor.

“If you encounter a really bad blue-green algae bloom, you’re going to know it,” said Mark Ermer, South Dakota Game, Fish and Parks Northeast Area Fisheries supervisor. “There’s going to be this thick layer — it almost looks like green paint — because it sits on top of the water and it may be an inch thick. Some of it will get washed up on the rocks of a beach and you’ll see it on any of the materials it gets on.”

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Algae resides in almost all of South Dakota’s lakes and ponds due to shallow, warm waters. It is a free-floating plant that flourishes with sun and nutrients. Blue-green algae, however, blooms in wetland areas where oxygen levels in the water get low due warmer temperatures.

Hot temperatures this summer — Mitchell averaged 85.9 degrees during June, with nine days above 90 degrees, according to the National Weather Service — have brought water temperatures into the 80s, rather than the typical temperatures in the 70s, per Ermer.

Blue-green algae typically blooms after a few days of high temperatures and little wind, which does not allow the algae to mix around lakes and ponds.

“Blue-green algae has a competitive advantage over green algae,” Ermer said. “In this low oxygen-level water, the blue-greens take over and they can do well. They can do well in this environment. They’re outcompeting green algae and they can build up in large numbers quickly.”

Bigger bodies of water like Lake Mitchell are not as susceptible to blue-green algae due to its openness, which allows wind mixing to eliminate stratification, where cooler water sits at the bottom and warmer water on the surface.

Instead, blue-green algae is much more common in wetland areas, where lakes or ponds have black bottoms, increasing water temperature from the sun. These bodies of water are also more shallow and protected from the wind due to cattails, creating more stagnant conditions.

“Most of the blue-green algae blooms in eastern South Dakota are happening in very secluded backwater wetland environments,” Ermer said. “There’s probably blue-green algae going somewhere in these wetlands all over South Dakota most days of the summer. It’s just a more rare situation in places like Lake Mitchell that are fairly open, fairly wind-swept.”

Locations of lakes with blue-green algae and the odor make ingestion of water by humans rare, but it is a cause for concern for dogs and other animals that may want to take a dip in the water on a warm day.

One drink of the toxic water can lead to death for dogs, while symptoms of algae poisoning can include lethargy, increased salivation, vomiting, diarrhea, seizures and loss of appetite.

While blue-green algae can be odorous and toxic, it does not last long in many cases in South Dakota. When the wind begins to hit the water surface again, it brings back oxygen to the water and allows the regular green algae to take control of the blue-green algae.

“It goes away on its own in South Dakota just as soon as the wind kicks up again,” Ermer said. “As the wind and wave action start mixing that water again, the oxygen levels come up, the blue-green algae can no longer outcompete green algae and the lake reverts back to its normal state of well-oxygenated water. … You’ve got to bring those oxygen levels back up and the bloom from one day earlier will be gone.”