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Harmful Algal Blooms

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Harmful Algal Blooms (HABs)

A HAB can harm people, animals, or the local ecosystem. A HAB can cause harm by producing toxins that can poison humans, fish, seabirds, aquatic animals, livestock, wildlife, and household pets (such as dogs) that are near the water, consume the water, or swim in the water. 

Where are toxic blue-green algae found? 

Cyanobacteria are naturally-occurring photosynthetic bacteria found in many habitats (including recreational waters). Potentially toxic blue-green algae occur throughout Montana in any standing bodies of water exposed to the sun, e.g., lakes, reservoirs, stock ponds, and roadside ditches.  They normally are not found in rivers, streams, springs, irrigation canals, or wells.

What does a bloom look like? 

A bloom of potentially toxic blue-green algae appears as "pea soup," "grass clippings," or "green latex paint."  The algae usually are suspended in the water column or aggregated into floating mats; they do not grow from the bottom as do mosses or "water weeds."

The following photos show visual signs of cyanobacterial blooms, including surface water discolorations, scum or mat-like accumulations along the surface, and a fish kill.

Harmful Algae Bloom and Fish Kill

Report a Harmful Algal Bloom

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For more information on HABs click on one of the following links: