Microbial Life > Topics of Interest > Red Tide

Red Tide - A Harmful Algal Bloom


Created by Monica Bruckner, Montana State University

Red tide in Penn Cove, Washington
The water in Penn Cove (Coupeville, WA) turned orange on 5/8/04 due to a "red tide." Photo courtesy of Mary Jo Adams (copyright 2004).

What is Red Tide?


Red tide is a phenomenon caused by algal blooms (Wikipedia definition) during which algae become so numerous that they discolor coastal waters (hence the name "red tide"). The algal bloom may also deplete oxygen in the waters and/or release toxins that may cause illness in humans and other animals. Species in the United States that release these harmful toxins include:

What Causes Red Tide?


Major factors influencing red tide events include warm ocean surface temperatures, low salinity, high nutrient content, calm seas, and rain followed by sunny days during the summer months (NOAA). In addition, algae related to red tide can spread or be carried long distances by winds, currents, storms, or ships.

Where Are Red Tides Found?


Red tide is a global phenomenon. However, since the 1980s harmful red tide events have become more frequent and widespread. Detection of a spread is thought to be influenced by higher awareness of red tide, better equipment for detecting and analyzing red tide, and nutrient loading from farming and industrial runoff. Countries affected by red tide events include: Argentina, Australia, Brazil, Canada, Chile, Denmark, England, France, Guatemala, Hong Kong, India, Ireland, Italy, Japan, the Netherlands, New Zealand, Norway, New Guinea, Peru, the Philippines, Romania, Russia, Scotland, Spain, Sweden, Thailand, the United States, and Venezuela (WHOI, 2007, CDC, 2012).

How Are Red Tides Harmful?


Red tide algae make potent natural toxins. It is unknown why these toxins are created, but some can be hazardous to larger organisms throught the processes of biomagnification and bioaccumulation. Grazers such as fish and krill are unaffected by the toxins, so as they eat the algae the toxins are concentrated and accumulate to a level that is poisonous eat to organisms that feed on them. Large fish kills and several mammalian diseases and deaths have been attributed to consumption of shellfish during red tide algal blooms. Diseases that may affect humans include:

This Alexandrium species can cause paralytic shellfish poisoning. Image taken by David Patterson and Bob Andersen, provided courtesy of the microscope website.


This Dinophysis species can cause diarrhetic shellfish poisoning. Image taken by Mona Hoppenrath, courtesy of the microscope website.

What is Being Done About Red Tide?

Technological advancements such as satellite imagery have allowed scientists to better track and monitor harmful algal blooms. Tracking and monitoring red tide algae helps reduce harmful effects of the algae by providing warnings against eating infected shellfish and against swimming in infected waters. For example, the Sarasota Operations Coastal Oceans Observation Lab (SO COOL (more info) ) has developed instruments that can test for red tide algae in coastal waters. Finally, researchers are attempting to develop an antidote to the red tide toxins. Interestingly, while developing such anti-toxins, researchers have found a possible cystic fibrosis (more info) treatment.



Red Tide Collections


Photo by Miriam Godfrey.

General Collection Resources such as news articles, web sites, and reference pages provide a comprehensive array of information about red tide and other harmful algal blooms.


Advanced Collection: Compiled for professionals and advanced learners, this red tide collection includes resources such as journal articles, academic reviews, and surveys.


For Educators: This red tide collection includes activities, assignments, and reading materials created specifically for educators.



Additional Resources


For additional resources about red tide, harmful algal blooms, and related issues search the Microbial Life collection.

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