All forms of algae may develop into nuisance blooms in waters that are rich in nutrients, especially phosphorus, and when conditions of light and temperature are favorable. But not all algae blooms are "harmful."
Blue-green algae (cyanobacteria) are technically not an algae, but a bacteria that can form nuisance and harmful blooms. Many blue-green algae species have internal "flotation devices" that allow them to remain within the lake's surface layer and form large floating mats or scums. Decaying algae may accumulate along the shoreline and smell like sewage. Algae mats also may harbor E. coli or other bacteria and pathogens.
Problems associated with blue-green algae include discolored water, reduced water clarity and light penetration, musty drinking water taste and odor, reduced oxygen levels in the water due to the decay of large amounts of algae, and fish kills. Some blue-green algae can produce neurotoxins, liver toxins, or cell toxins and all blue-green algae can produce skin irritants. The release of toxins can be harmful to fish, other aquatic life, wildlife, pets, livestock and even humans. Not all blue-green algae produce toxins and some produce toxins only under certain conditions.
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