26 Harmful Algal Blooms

Ever been told to only eat shellfish during the months that have the letter “R”, (September-April)? Well this rule is actually pretty important for keeping the health of people safe and to allow for many species of shellfish to repopulate. But why are the other months of the year not safe for people to eat shellfish? In short its because of the algae that grow during this time of year and as ocean temperatures rise. During these specific months of warmer weather, billions upon billions of these microorganisms start to take over our oceans and can have many consequences for us.


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“Multicellular Green Algae” by Frank Fox [CC by 2.0]

Before going into what red tide is or how the populations of these microorganisms seem to be increasing significantly as oceans warm up, lets take a closer look at algae. Most species of algae are single-celled organisms but some species can be multi-cellular as seen in the photo above. Algae are autotrophs, meaning they use photosynthesis as their means of producing energy for themselves. Though similar to plants in the way they are both producers, algae have no stems or leaves and are more closely related to other groups of protists. Habitats for algae include any bodies of water including fresh and salt water, or have extreme external environment factors. There are few cases where they have been found on land such as rocks, trees, hot springs, etc… Species of algae have been well documented to be able to survive many harsh environments and have been on earth far longer than most living organisms to this day. They contributed to the Earth being able to house life by producing oxygen through photosynthesis. Overall Algae species are very tough and can survive in a wide range of environments, which can be seen as both a positive and negative situation.


The red tide occurs when the algae from algal blooms becomes so numerous that it discolors the water. It is also sometime referred to as a Harmful Algal Bloom or “HAB”. This is where the name “red tide”comes from. Some key factors involved in red tides forming are warm ocean surface temperatures, low salinity, high nutrient content, calm seas, and rain followed by sunny days during the summer months. Some effects of the red tide are that it could deplete the oxygen in the water and/or release toxins into the water. The toxins in the water could have negative effects on the health of humans and animals exposed to them. There are three types of algae that can release these harmful toxins, they are Alexandrium fundyense, Alexandrium catenella and Karenia brevis.


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“Red Tides in Isahaya Bay, Japan / Algal Bloom in Pelee Island, Ontario” by Marufish and Tom Archer [CC by 2.0]

What is important to recognize about “Red tides” and Algal blooms is that it isn’t always obvious that algae growth is there. They are not always a red color. The photos above show two examples of Algal blooms from two very different parts of the world, yet both species are considered “Red Tide” and harmful to some shellfish and animals that eat the shellfish.  Algae alone is not an issue and even during the time of the year where there seems to be an excess amount of growth, this is a natural occurrence. What becomes a problem or what classifies as a “Red Tide” are the algae that release toxins in the air and water when they grow. Very few algae species can produce this toxin but when a large enough group forms on shores it can have a negative effect on both the marine environment and humans. The toxins produced can often affect the respiratory and nervous systems of all life forms. Thus when smaller marine animals feed on the algae, the trophic level above them can become poisoned as well. Paralytic Shellfish Poisoning is typically found along the Pacific and Atlantic coasts of the United States and Canada. It can cause paralysis and in extreme cases death. Some of the toxins that cause Paralytic Shellfish Poisoning can be 1,000 times more potent than cyanide. Diarrhetic Shellfish Poisoning is another example of a harmful effect from eating contaminated shellfish. It is caused by Okadaic acid, which is produced by several species of dinoflagellates, and is usually non-deadly to humans. Small amounts of the okadaic acid usually do not have any harmful effects and only become an issue when large amounts are consumed. Amnesic Shellfish Poisoning is the third common poisoning that humans will get from eating contaminated shellfish. It can be life threatening and cause both gastrointestinal and neurological disorders. These disorders are caused by domoic acid. After an incident in Canada in 1987 where 4 people died from Amnesic Shellfish Poisoning, the levels of domoic acid in shellfish are now being monitored.


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“Toxic Algae Bloom in Lake Erie” by NASA Earth Observatory Under Public Domain [CC by 2.0].

Algal Blooms can have serious effects on corals. Red algae, brown algae, and green algae are a few examples of macro-algae that can have a very negative effect on corals. They do this by outcompeting, overgrowing and eventually replacing sea-grasses and coral reef habitats. According to some research that is being done, harmful tropical algal blooms are increasing in frequency and intensity. This can have a significant impact on coral reefs.

Notable Red Tides:

1844: First recorded case off the Florida Gulf Coast.

1972: Red tides killed 3 children and hospitalized 20 adults in Papua New Guinea.

2005: The Canadian red tide was discovered to have come further south than it has in years prior by the ship (R/V) Oceanus, closing shellfish beds in Maine and Massachusetts. Authorities were also alerted as far south as Montauk to check their beds. The experts who discovered the reproductive cysts in the seabed warned of a possible spread to Long Island in the future. This halted the area’s fishing and shellfish industry.

2013: In January, a red tide occurred on the West Coast Sea of Sabah in the Malaysian Borneo. There were two fatalities reported after they consumed a shellfish that had been contaminated with the red tide toxin.

2015: In September, a red tide bloom occurred in the Gulf of Mexico, affecting Padre Island National Seashore along North Padre Island and South Padre Island in Texas.


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“Map of Padre Island National Seashore” By U.S. National Park Service Under Public Domain [CC 0]

Scientists have been able to help control the spread of the effects of harmful algal blooms by developing new technology to help track them better. Tracking these harmful algal blooms could help prevent people from eating contaminated shellfish and knowing which areas will be most effected by them. Some examples of the technology that can help with monitoring them are better and more advanced satellite imagery. Also the development of an antidote to the toxins produced is another way to reduce the harmful effects. Even though they are a natural occurrence, what is alarming some scientists is that they may start to last longer and occur more often during the year as ocean temperatures and CO2 levels rise.

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“Red Tide in Naples” [CC by 2.0]

The information in this chapter in thanks to content contributions from Emily Michaeles and Alana Olendorf


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A Student's Guide to Tropical Marine Biology by by Keene State College Students, BIO 381 Tropical Marine Biology is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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