A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. Although they are a small percent of the algal species, toxic algae blooms have a large impact.
A broad classification of HABs distinguishes two groups of organisms: the toxin producers, which can contaminate seafood or kill fish, and the high-biomass producers, which can cause anoxia and indiscriminate kills of marine life after reaching dense concentrations. Some HABs have characteristics of both.
Harmful algae can have large and varied impacts on marine ecosystems. High biomass blooms cause negative impacts through the sheer abundances of cells. This can lead to impacts to other organisms via poor food quality, gill irritation/clogging, decreased photosynthesis of underwater vegetation, production of allelochemicals, habitat alteration and anoxia/hypoxia following the decay of large blooms. Economic impacts of high biomass blooms range from increased treatment costs to water treatment facilities, dead fish/shellfish and decreased tourism.
Aureococcus anafagefferens, aka Brown Tide, bloom in such densities that the water turns dark brown and cause harm to many estuarine organisms like scallops and clams because it is a poor food source (some varieties also produce a toxin) and blocks light needed for submerged aquatic vegetation (SAV).
Didymosphenia gemin, aka Rock Snot, is an invasive algae that impacts our fisheries. When introduced to a new habitat, plants, parasites and animals negatively impact our natural resources.
Prorocentrum minimum, aka Mahogany tide, is a high biomass bloom species that is harmful because it shades underwater grasses and impacts fish and shellfish habitat (when the large biomass dies and decays consuming oxygen from the water).
Macroalgae, aka seaweed, can produce large accumulations which lead
to shading of SAV and low oxygen when mats decay. This grouping also
includes benthic cyanobacteria mats of Lyngbya in the upper Bay and
Isothrix (Planktothrix) in the non-tidal Potomac which can become so
dense they shade seagrasses, impact fish habitat and decrease oxygen.
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