The Red Death: The Ebb and Flow of Red Tides

Red tides are a natural occurrence in nature, but recently the combination of changes in ocean temperature resulting from global warming coupled with anthropomorphic stressors (ex. pollution) has increased their occurrence. Not only has the occurrence of red tides increased, they are becoming more severe. Recently red tides have been in the news and linked to brain damage in sea lions where they lose their special abilities and cognitive function. Red tides are known to affect a wide array of organisms including mollusks, (ex. muscles and oysters), crustaceans (ex. shrimp and crabs), fish, and mammals (including humans) and are caused by small microorganisms called dinoflagellates.


Dinoflagellates are small protists that belong in the phytoplankton group and are predominately found in salt water, but some species can be found in freshwater. While they are not true algal organisms, Botanists treated them as a division of algae. Population dynamics and distribution are largely based on temperature, salinity, and depth. Most dinoflagellates are photosynthetic. They play important roles in the ocean such as forming endosymbiotic relationships with some marine animals and they are especially important to the health of coral reefs. However some populations reproduce out of control in the presence of an abundance of nutrients creating algal blooms, the basis of red tides. When this occurs there can be as many as one million cells in only a milliliter of seawater. The dangers do not come from the blooms themselves, but the reduction in oxygen availability and the toxins that dinoflagellates can produce.


The dinotoxins that dinoflagellates produce can have different effects on different animals. While as the name suggests some dinoflagelate such as Karenia brevis that occurs in the eastern Gulf of Mexico, Alexandrium fundyense that occur in the Northern Atlantic along the east coast of the US, or Alexandrium monilatum found in tropical and subtropical regions of the ocean (including the western Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, and the Eastern Pacific Ocean) that have a red pigment make the water appear red. Others that remain clear can be equally harmful such as Pfisteria blooms; one of the dinoflagellates responsible for fish kills. Despite the color, they are both considered to be red tides. Because these blooms are not truly defined by color, a better name for these occurrences is harmful algal blooms if they produce harmful dinotoxins and just algal blooms if they are not harmful. Not all dinoflageletes blooms are harmful. Some dinoflagellates produce blue or green fluorescent pigments often seen in bioluminescence at night.


Karenia brevis and the chemical structure of brevitoxin from

Red Tides are harmful because of the toxins released by the dinoflagellates. These toxins affect all marine life and even humans who consume seafood. Shellfish can cause nonfatal and fatal illnesses because they carry these toxins. One of these toxins is saxitoxin, produced by A. monilatum. Saxitoxin, a powerful neurotoxin that causes paralysis. It is found in filter-feeding shellfish such as bivalves, causing them to be poisonous if consumed. This includes marine mammals and humans. In Florida, Karenia brevis, the most common cause of red tides. It can cause respiratory irradiation in mammals (including humans) when it is present along the coast and winds blow its aerosolized brevetoxins inland. Swimming is normally allowed during this red tied, but swimming in areas of high K. brevis concentration can cause skin irritation. No human deaths have been attributed to K. brevis. The red tides caused by Gonyaulax spinifera are serious. G. spinifera produces yessotoxins, saxitoxin and gonyautoxins which accumulate in filter-feeders such shellfish. It can lead to paralytic shellfish poisoning (PSP)-like symptoms and PSP, which can be fatal. Alexandrium spp. and some puffer fish also produce saxitoxins. Nearly all toxins released during algal blooms are neurotoxins. Some bivalves can accumulate these toxins and store them from many weeks to up to two years. Ordinary cooking methods are ineffective in eliminating these neurotoxins, which is why even humans cannot avoid getting sick after consuming contaminated shellfish. Many marine mammals rely on shellfish as a source of food. While humans may avoid eating contaminated shellfish based on food testing, marine mammals such as sea lions and otters don’t know any different. Many sea lions and otters are dying from brain damage associated with red tides — see Manatee populations have also been drastically affected by red tides. In 2010, 766 manatees died, but that number was surpassed in 2013 when 803 manatees died. Manatees eat sea grasses contaminated with neurotoxins produced by the dinoflagellate species that is the casual agent of the algal bloom. The neurotoxins affect their nervous system and ultimately cause them to drown.



Notable Red Tide Occurrences 

Compiled from Wikipedia


  • 1793 – The first red tide case occurred in British Columbia, Canada
  • 1840 – Florida red tide resulted in respiratory irritation from aerosolized bervetoxin produced by Karenia brevis.
  • 1972 – A red tide was caused in New England by Alexandrium (Gonyaulax) tamarense, which produces saxitoxin and gonyautoxins.
  • 1976 – The first PSP case occurred in Sabah, Malaysian Borneo where 202 victims were reported became ill and 7 people died.
  • 2005 – A Canadian red tide was came further south than it has in years prior as far south as Long Island, NY resulting in the closure of the fishing and shellfish industry, which constitutes a significant portion of the island’s economy.
  • 2009 – Brittany, France experienced recurring algal blooms caused by the intensive pig farming where farmers released high amount of fertilizer by discharging in the sea. This also led to lethal gas emissions that have led to one case of human unconsciousness and three animal deaths.
  • 2010 – Dissolved the ash from the Eyjafjallajökull volcano triggered an algal bloom in the North Atlantic.
  • 2011 – An algal bloom occurred along the Northern California coast.
  • 2011 – An algal bloom occurred along the in the Gulf of Mexico
  • 2013 – A red tide occurred on the West Coast Sea of Sabah in the Malaysian Borneo where two fatalities from PSP after eating contaminated shellfish.
  • 2013 – A red tide bloom along Sarasota beach caused a fish kill and caused respiratory issues for beach-goers.
  • 2013 – An algal bloom was caused in Qingdao, China, by sea lettuce.
  • 2014 – Myrionecta rubra a protist that ingests cryptomonad algae, caused a bloom in southeastern coast of Brazil.
  • 2014 – Blue green algae caused a bloom in the western basin of Lake Erie that contaminated the Toledo, Ohio water system.
  • 2014 – A massive Florida red tide occurred that was 90 miles long and 60 miles wide.
  • 2015 – Toxins resulting form a red tide caused the hospitalization of 12 people.
  • 2015: A red Tide occurred in the Netherlands between Katwijk and Scheveningen where government officials dissuaded swimmers to go into the water.
  • 2015 – A red tide bloom occurred in the Gulf of Mexico along the coast of Texas affecting Padre Island National Seashore along North Padre Island and South Padre Island.


There are many contributing factors to red tides. They can occur naturally in the wild as a result of coastal upwelling — the seasonal changing of wind patterns that cause water to be pushed away from the coastline stirring the water column so that deep cold water rises to the surface of the water column. However, agricultural run-off, water pollution, large-scale oscillations in weather patterns such as El Niño, and changes in ocean temperature can exacerbate the blooms. These produce an abundance of nitrates and phosphates that allow dinoflagellates to thrive and flourish, resulting in algal blooms. Additionally, nitrate and phosphate dumping can lead to a less detectible algal bloom that does not give itself away with a distinct color. Pfisteria piscicida blooms are often results of farm run-off or dumping. Years back, there were issues with hog farms and chicken farms that were located near a body of water such as a river. Hog waste and Chicken waste entered the water and caused outbreaks of P.piscicida due to the rich nitrate environment produced by allowing hog and chicken waste to enter the waterway resulting in fish kills with distinct P.piscicida lesions. The release of pollutants into waterways, and subsequently the ocean, allows for similar algal blooms of various dinoflagellate species to occur that often results in a population decline in a number of species in the affected area. It is clear that algal blooms have become more frequent and more severe which can likely be correlated to pollution and a change in ocean temperatures.


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