Blue-green algae, also called cyanobacteria, are microscopic organisms normally present in aquatic ecosystems, including lakes and ponds. Scientists have identified thousands of species of blue-green algae; at least 80 are known to produce toxins that can cause illness and death in animals as well as humans.

Heavy growth of these toxin-producing algae (“blooms”) can cause high concentrations of toxins in the water. In North America, Anabaena, Aphanizomenon, Oscillatoria, and Microcystis are the blue-green algae species most commonly associated with poisoning.

In Central Kentucky, blooms are most common in late summer and early fall, during hot, sunny weather. Contamination of water with excess nutrients, particularly nitrogen and phosphorus, further encourages algal growth. Common sources of excess nutrients include fertilizer runoff from fields, lawns, and gardens and direct manure and urine contamination from livestock.

Blooms can produce a blue-green sheen on the water surface, or they can be pea-green and thick, like spilled paint. Blooms can also be brown or white. They can form scums, slimes, or mats. It is impossible to tell if a bloom is toxic just by its appearance; consider all blooms potentially toxic