Harmful Algae & Red Tides

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.

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News & Insights

Hearing on The Harmful Algal Blooms and Hypoxia Research and Control Amendments Act of 2011

before the Subcommittee on Energy and Environment Committee on Science, Space and Technology U.S. House of Representatives June 1, 2011…

Hearing on “Harmful Algal Blooms and Hypoxia: Formulating an Action Plan”

Dr. Donald M. Anderson Senior Scientist, Biology Department, Woods Hole Oceanographic Institution Director, U.S. National Office for Harmful Algal Blooms…

Hearing on “Harmful Algal Blooms: The Challenges on the Nation’s Coastlines”

Dr. Donald M. Anderson Senior Scientist, Biology Department Woods Hole Oceanographic Institution Director, U.S. National Office for Marine Biotoxins and…

Briefing to the Senate Commerce Committee on Harmful Algae Bloom

October 9, 2007 Don Anderson briefed the Senate Commerce Committee on Harmful Algae Blooms in general, and ) about a…

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From Oceanus Magazine

The Recipe for a Harmful Algal Bloom

Harmful algal blooms can produce toxins that accumulate in shellfish and cause health problems and economic losses. They have increased in strength and frequency worldwide. Can we get advance warnings of when and where they will occur?

As Bay Warms, Harmful Algae Bloom

Warming coastal waters off southern Massachusetts are worsening the effects of pollution from septic systems, wastewater treatment plants, and fertilizer…

Not Just Another Lovely Summer Day on the Water

It looks like nice summer day on the water, but Alexis Fischer (right) and Alice Alpert, graduate students in the…

Setting a Watchman for Harmful Algal Blooms

As harmful algal blooms are becoming more frequent and severe worldwide, researchers in the lab of WHOI biologist Don Anderson are testing an array of new instruments that can be used in early-warning monitoring systems for coastal waters.

Dropping a Laboratory into the Sea

Scientists at WHOI deploy moored robotic laboratories in the Gulf of Maine for long-term monitoring of red tide algae


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