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Harmful Algal Blooms (HABs)

Through the food chain, all living things ultimately depend on vegetation in the sea: single-celled plants (algae), tiny organisms (phytoplankton), and seaweed (macro algae). Like plants on land, they convert sunlight, carbon dioxide, and minerals into a basic food.

Among the thousands of species of such flora, a couple of dozen ("hazardous" or "toxic" species) also produce chemicals that can harm other forms of life, such as clams or people. Under favorable conditions (for them!), some seaweed ("nuisance" species) will rapidly bloom and accumulate into dense mats. They float, shadowing flora below, sink, or wash ashore. As they rot, they change the chemistry and biology of their environment. They divert oxygen from fish and other creatures that then flee, stress, or die. Anaerobic bacteria, though, thrive. In sufficient quantity, a byproduct of their life cycle (hydrogen sulfide) can be toxic. Even small quantities look and smell nasty, like rotten eggs.

Popular names for such blooms - such as "red (or brown) tide" - are misleading. Although some species are colored, no single color distinguishes healthy from hazardous ones. Waters in the midst of a harmful algal bloom (HAB) can look harmless, while red or brown waters may be fine. And none of these blooms comes and goes with the tide. Proper identification requires special expertise.

HABs are being reported more frequently around the world, including the U.S. Better detection technology is, no doubt, a factor. But pollution (nitrogen in salt water; phosphorous in fresh) seems to be the major one. Inadequately treated discharge from sewerage plants, erosion, agricultural run-off, and waste from birds and mammals, including pets, provide nutrients that feed blooms, especially in sheltered waters.

To date, Rhode Island has not detected significant blooms of toxic HABs, and even nuisance blooms seem to be restricted to the late summer, when the Bay warms. Nevertheless, these nuisance blooms (such as Ulva, "sea lettuce") can become common and large enough to present a problem. The most serious environment local consequence has been through their effect on water quality, particularly dissolved oxygen, and release of hydrogen sulfide.

(See also "Hypoxia and Anoxia")


  • If there is a bloom and the species does not itself produce toxins, there is no reason to be alarmed. Some algae are actually prime food for sea life. Little can be done except letting nature run its course.
  • Check for updates and advisories on the Bay Water Quality Report or from Bay Line: 222-8888 (May 15 to Oct. 15).
  • In the meantime, help prevent HABs by supporting efforts to improve water quality in the Bay. (See "Introduction to Narragansett Bay")