WHOI  WHOI People  

Harmful Algae & Red Tides

Hamrful Algae and Red Tideicon-caption
A “red tide” of blooming algae (the dinoflagellate Noctiluca scintillans) stretched more than 20 miles along the coast near LaJolla, California, in spring 1995. Such massive blooms can harm human health, coastal economies, and marine ecosystems. Algal blooms occur naturally, but have become much more common in recent years, some due to human activities that put excess nutrients into the water. (Photo courtesy of Peter Franks, Scripps Institution of Oceanography)

What are Harmful Algae & Red Tides?

Marine and fresh waters teem with life, much of it microscopic, and most of it harmless. In fact, it is this microscopic life on which all aquatic life ultimately depends for food.

Although most of these species of phytoplankton and cyanobacteria are harmless, there are a few dozen that create potent toxins under the right conditions. Harmful algal blooms may cause harm through the production of toxins or by their accumulated biomass, which can affect co-occurring organisms and alter food-web dynamics.

Impacts include human illness and mortality following consumption of or indirect exposure to HAB toxins, economic losses to coastal communities and commercial fisheries, and HAB-associated fish, bird, and mammal deaths.

To the human eye, blooms can appear greenish, brown, and even reddish-orange depending upon the algal species, the aquatic ecosystem, and the concentration of the organisms.

Harmful Algal Blooms or Red Tide?
These outbreaks are commonly called red tides, but scientists prefer the term "harmful algal blooms," or HABs. The term red tide often and mistakenly refers to many organisms that discolor the water, but that cause no harm. It also excludes blooms of highly toxic cells that cause problems at low, and essentially invisible, concentrations. Therefore, harmful algal bloom is a more appropriate term.