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.
Some harmful algae species produce toxins that have devastating effects on fish and other marine life, both in wild and aquacultures. Several species can produce toxins which damage fish gills by hemolytic effects. This has resulted in extensive fish kills with major economic losses. Other algal species can kill fish, as a result of suffocation, when they decay and consume oxygen from the water (see Ecosystem Disruptive HABs).
Chloromorum toxicum, previously known as Chatonella verucullosa, is a potentially toxic species that has been associated with fish kills as near as the Delaware Bays and can be potentially harmful to humans when
Heterosigma produces an unidentified ichthyotoxin and is considered the causative organism involved in offshore fish farm kills in Washington State.
Fibrocapsa has had devastating impacts on mariculture
operations in Japan.
Karlodinium veneficum may cause water to become discolored a reddish-brown and form Mahogany tides. Mahogany tides may also severely reduce the amount of oxygen available to living resources at localized bloom sites.
Prymnesium parvuum, aka Golden Algae, releases unique toxins that affect gill-breathing aquatic organisms (no apparent effects on animals that do not breathe through gills). Under certain conditions this algae
outcompetes others to cause blooms with massive impacts.