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Jul 25, 2012


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Rod McNeil
Planning, Prevention and Assistance


Be Alert for Toxic Algae Before You Swim in Ponds or Lakes


HELENA -- When the summer is hot and dry like this one, nothing seems more refreshing than a dip in a cool lake or pond. But before you hit the water you need to be aware of the dangers of potentially toxic blue-green algae.

Hot, dry weather triggers outbreaks, or blooms, of blue-green algae. The Montana Department of Environmental Quality (DEQ) is urging people to use common sense and avoid areas of ponds or lakes that have unsightly algae.

Blue-green algae, or cyanobacteria, reproduce rapidly in fresh water when the sunlight, temperature and nutrients are just right. Within a few days a clear lake, pond or ditch can become cloudy with algae growth. Potentially toxic blue-green algae can occur throughout Montana in any lake, reservoir, stock pond, or roadside ditch. Residential drinking water taken from a lake may also be affected. The algae is not normally are not found in rivers, streams, springs, irrigation canals, or wells.

A bloom often looks like bright green paint floating on the water; however it can also appear bluish, brownish or reddish green. It is made up of extremely small organisms that are very difficult to pick up or hold. They are usually suspended in water or formed into floating mats and can be several inches thick near the shoreline.

In most cases in Montana the algae is not toxic. Only certain species are capable of producing toxins, and even these species are harmless most of the time. But in their toxic form, blue-green algae can, and has, 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 large numbers of dead fish, water fowl or other animals, the sudden unexplained illness or death of a cat or dog, especially if it has algae on its mouth, legs or feet or a skin rash on humans after being in the water. There is no way to tell for sure if the water is toxic unless it is actually injected into an experimental laboratory animal. Harmless strains of potential toxin-producing algae look the same as deadly strains under a microscope.

If you suspect an algae bloom keep children, pets and livestock away from the area of contamination. The DEQ has posted a fact sheet about blue-green algae on its website at

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