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Avoid swimming in stagnant water

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News from the Montana Department of Environmental Quality

HELENA — The Montana Department of Environmental Quality advises people to avoid swimming in ponds, lakes, or reservoirs that seem stagnated and harbor large quantities of algae. Mid-summer hot weather can trigger outbreaks, or blooms, of potentially toxic blue-green algae, or cyanobacteria, that can reproduce rapidly in standing fresh water when sunlight, temperature, and nutrient levels are high. 

Potentially toxic blue-green algae can occur throughout Montana in lakes, reservoirs, stock ponds, or roadside ditches. These types of blooms are not normally found in moving waters, such as streams, irrigation canals, and springs. Because they require sunlight, the blooms do not occur in wells.

A bloom may appear similar to bright green latex paint floating on the water. However, it can also appear bluish, brownish, or reddish-green, or even look like grass clippings in the water. 

In most cases in Montana the algae is not toxic. Only certain species are capable of producing toxins, and even these remain harmless most of the time. Nonetheless, certain blue-green algae can, and have, sickened or killed pets, waterfowl, and other animals, including livestock. They can also cause serious illness in humans.

Signs of a toxic bloom may include the presence of dead fish, waterfowl or other animals, or a sudden unexplained illness or death of a pet, especially if it has algae on its mouth, legs or feet. 

If you suspect a bloom in standing waters, keep children, pets, and livestock away. The DEQ has posted a fact sheet about blue-green algae on its website at:


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