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 different species of such flora, dozens are hazardous toxic species capable of producing chemicals that can harm other forms of life, such as clams or even humans. Under conditions favorable for the “nuisance” seaweed species, many will rapidly bloom and accumulate into dense mats. They can either float (shadowing flora below), sink, or wash ashore. As they rot, they have the potential to change the chemical and biological composition of their surrounding environment. They prevent oxygen from reaching fish and other creatures, who as a result either flee, stress or die. However, anaerobic bacteria (bacteria that grows under oxygen deprived conditions) thrive. In sufficient quantity, a byproduct of their life cycle (hydrogen sulfide) can be toxic. Even small quantities look and smell terrible (like rotten eggs).

Popular names for such blooms - such as “red tide" or “brown tide” - are misleading. Although some species are colored, no singular color distinguishes the healthy species from the hazardous ones. Waters amid a harmful algal bloom (HAB) can look harmless but prove to be hazardous, while seemingly dangerous red or brown waters may be completely safe. In addition, no bloom may simply “come and go” with the tide. Proper identification requires special expertise.

HABs are being reported more frequently around the world, not excluding the United States. Better detection technology undoubtedly contributes to this occurrence, but pollution (nitrogen in saltwater, phosphorous in freshwater, etc.) appears to be the largest factor. Inadequately treated discharge from sewage 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, etc.) can become common and large enough to present problems. These problems may include a decline in water quality, lowered oxygen levels and the emission of repulsive Hydrogen Sulfide odors.

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