Cyanobacteria, also referred to as blue-green algae, are microscopic organisms that live primarily in fresh water and salt water, at the surface and below. They usually multiply and bloom when the water is warm, stagnant, and rich in nutrients (phosphorus and nitrogen) from sources such as fertilizer runoff or septic tank overflows.
An algal overgrowth is referred to as an “algal bloom.” Cyanobacterial blooms are usually blue-green in color, but algal blooms can vary in color, ranging all the way to red or brown. When a bloom occurs, scum (a layer of foul extraneous matter) might float on the water surface, resulting in a rotten plant-like odor. Blooms typically occur during late summer or early fall, but can occur anytime during the year.
Not all algal blooms are harmful. A cyanobacterial algal bloom can be harmful when the toxins (cyanotoxins) it produces in air and water reach concentrations that are dangerous to people, marine life, and the environment.
2022 Guidance for Local Boards of Health responding to Cyanohabs: mass.gov/doc/guidance-for-local-health-on-responding-to-cyanohabs/download?_ga=2.122715916.649401797.1655913663-289597129.1613746688