The first things to know. Red tides are caused by Harmful Algal Blooms (HABs). They are dangerous to animals and fish, and also to humans.
Many of us have been disturbed to smell and see green algae blooming in local lakes in huge numbers, but HABs can result in growing widespread devastation. Green algae, while it is worse than unsightly, is not as toxic.
Red tides are common in nature, but like so many things, are exacerbated by climate change and pollution. In Tampa Bay Florida in 2021 a tear in a waste water pool lining resulted in millions of gallons of wastewater being released into the ocean, rather than flood the community of Piney Point. Tampa Bay Waterkeeper has reported that after the discharge ceased on April 9, localized red tide events were reported in different parts of the bay on April 15, April 20, June 23, June 27, and July 8–as well as many general events in the middle of the bay–between May and June.
Another disturbing event occurred in the Florida Keys, 80 species of fish, sharks, and rays were seen spinning in constant circles and dying. National Geographic reported that “the experts concluded the symptomatic fish died from combined exposure to multiple toxins, possibly originating from multiple species of dinoflagellates, a type of microscopic algae.” Dying animals included the endangered smalltooth sawfish ray.
In an article in Nature in August 2003, experts reported more than 10 humpback whales on the West Coast died from HABs, while Argentina has found it is common for whales to die from toxic algae blooms—especially baby whales.
Types of HABs
Two species of toxic HAB algae, Alexandrium catanella and Alexandrium fundyense, found in the Gulf of Maine and California, have been responsible for paralytic shellfish poisoning in people. Other forms of toxic algae include Karenia brevis, Pyrodinium bahamense, Pseudo-nitzschia, Gambierdiscus, and diatoms.
We also can eat animals, and fish that have been poisoned, because they eat contaminated seafoods. These poisonous algae affect fish, birds, turtles, dolphins, sea lions, whales, manatees, and otters that are have been found washed up dead on coastal shores, rivers, lakes, and estuaries.
The term hypoxia is also used to describe how red tides cause the life-giving oxygen in waters to become depleted, suffocating marine life. NOAA, (the National Oceanic and Atmospheric Administration), has explained that “since oxygen is vital for marine life, these combined red tide-hypoxia events can be devastating to marine ecosystems by causing mass die offs of marine life.”
Causes of HABs
Other ways humans feed this destructive cycle include pollution from agricultural and urban runoff. Chemicals pinpointed as culprits, are silicas, iron, ammonia, carbon, nitrate ions, and phosphorus. HABs can result in boil water warnings on shore, as well as the closure of fisheries.
According to Wikipedia, many cities along the Mediterranean Sea discharge untreated sewage. Red tide algae growth is also caused by rising water temperatures, and heavy rainfall from climate change. The explosive growth in the toxic microbes themselves can also result in rising marine temperatures, because they absorb sunlight.
The US government released a report in 2008 called the Harmful Algal Bloom Management and Response: Assessment and Plan 2 that stated: “it is generally believed that the frequency and distribution of HABS and their impacts have increased considerably in recent years in the United States and globally.”
Sources
What is a red tide? A “red tide” is a common term used for a harmful algal bloom. (2024, June 16). NOAA-National Oceanic and Atmospheric Administration. https://oceanservice.noaa.gov/facts/redtide.html
Scientists Confirm Link Between Red Tides and Low Oxygen Areas. (2022, April 27). NOAA-National Oceanic and Atmospheric Administration. https://www.fisheries.noaa.gov/feature-story/scientists-confirm-link-between-red-tides-and-low-oxygen-areas
Climate Change and Freshwater Harmful Algal Blooms. (2025, February 26). Environmental Protection Agency. https://www.epa.gov/habs/climate-change-and-freshwater-harmful-algal-blooms
Red Tide / Piney Point Crisis. (2021). Tampa Bay Waterkeeper. https://www.tampabaywaterkeeper.org/piney-point-red-tide
Harmful Algal Bloom Management and Response: Assessment and Plan-Interagency Working Group on Harmful Algal Blooms, Hypoxia, and Human Health. (2008, September). US Office of Science and Technology Policy. https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://obamawhitehouse.archives.gov/sites/default/files/microsites/ostp/jsost_hab0908.pdf&ved=2ahUKEwiBwP6nobaOAxX0v4kEHbGQNBcQFnoECBgQAQ&usg=AOvVaw1o-mXiHjYuQcnQBAPVsJ-E
Toxic Algae suspected in whale death. (2003, August 4). Nature. https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.nature.com/articles/news030804-1&ved=2ahUKEwjhiLKWoraOAxVrnokEHdMVEqcQFnoECBYQAw&usg=AOvVaw1TIJW-WGIY1VhyyhA8ZB4r
What exactly is a red tide—and how does it affect humans? (2023, March 16). National Geographic. https://www.nationalgeographic.com/environment/article/red-tides
Hazardous spill in Florida highlights environmental threat decades in the making. (2021, April 7). National Geographic. https://www.nationalgeographic.com/environment/article/hazardous-spill-in-florida-highlights-environmental-threat-decades-in-the-making
We finally know what caused Florida fish to spin in circles until they died. (2021, September 6). National Geographic. https://www.nationalgeographic.com/animals/article/why-fish-are-spinning-to-death-florida
Harmful algal bloom. (2025, 10 July). Wikipedia. https://en.wikipedia.org/wiki/Harmful_algal_bloom
UNESCO report on Harmful Algal Blooms (HABs). (2021). UNESCO. https://www.phosphorusplatform.eu/scope-in-print/spotlight/41-previous-spotlight/2084-unesco-report-on-harmful-algal-blooms-habs
Earth in the Future: Ocean Acidification, Red Tides, and Monster Jellyfish – Red Tides. (2025). College of Earth and Mineral Science, The Pennsylvania State University. https://www.e-education.psu.edu/earth103/node/689
Toxic Algae Could Be Killing Dozens of Whales: Experts say climate change is likely to blame for the “red tides” of toxic algal blooms. (2015, September 16). Inverse.com. https://www.inverse.com/article/6223-toxic-algae-blooms-could-be-causing-whale-deaths