Red tide in a harbor, Japan
Red tide in a harbor, Japan

A red tide is a phenomenon of discoloration of sea surface. It is a common name for harmful algal blooms occurring along coastal regions, which result from large concentrations of aquatic microorganisms, such as protozoans and unicellular algae (e.g. dinoflagellates and diatoms).[1][2] Terrestrial runoff, containing fertilizer, sewage and livestock wastes, transports abundant nutrients to the seawater and stimulates bloom events. Natural causes, such as river floods or upwelling of nutrients from the sea floor, often following massive storms, provide nutrients and trigger bloom events as well. Increasing coastal developments and aquaculture also contribute to the occurrence of red tides.[1][2] Harmful algal blooms can occur worldwide, and natural cycles can vary regionally.[3]

The growth and persistence of an algal bloom depends on wind direction and strength, temperature, nutrients, and salinity.[3] Red tide species can be found in oceans, bays, and estuaries, but they cannot thrive in freshwater environments.[4][3] Certain species of phytoplankton, and dinoflagellates like Gonyaulax, are found in red tides and contain photosynthetic pigments that vary in color from brown to red. These organisms undergo such rapid multiplication that they make the sea appear red. When the algae are present in high concentrations, the water may appear to be discolored or murky. The most conspicuous effects of red tides are the associated wildlife mortalities and harmful human exposure. The production of natural toxins such as brevetoxins and ichthyotoxins are harmful to marine life.[5] Effects of red tides can worsen locally due to wind driven Langmuir circulation and their biological effects.

Harmful toxins produced by the red tide

Marine life exposure

Red tides occur naturally off coasts all over the world. Marine dinoflagellates produce ichthyotoxins. Not all red tides are harmful.[6][7] Where red tides occur, dead fish wash up on shore for up to two weeks after a red tide has been through the area. In addition to killing fish, the toxic algae contaminate shellfish. Some mollusks are not susceptible to the toxin, and store it in their fatty tissues. By consuming the organisms responsible for red tide, shellfish can accumulate and retain saxitoxin produced by these organisms. Saxitoxin blocks sodium channels and ingestion can cause paralysis within 30 minutes.[8] Other animals that eat the shellfish are susceptible to the neurotoxin, leading to neurotoxic shellfish poisoning[9] and sometimes even death. Most mollusks and clams filter feed, which results in higher concentrations of the toxin than just drinking the water.[3] Scaup, for example, are diving ducks whose diet mainly consists of mollusks. When scaup eat the filter-feeding shellfish that have accumulated high levels of the red tide toxin, their population becomes a prime target for poisoning. However, even birds that do not eat mollusks can be affected by simply eating dead fish on the beach or drinking the water.[10] The toxins released by the blooms can kill marine animals including dolphins, sea turtles, birds, and manatees.[6][11] Manatees can inhale brevetoxins, and, since brevetoxins can transfer through trophic levels of the food web, they can also ingest it. Manatees also have an immunoresponse to red tides and their toxins that can make them even more susceptible to other stressors. Due to this susceptibility, manatees can die from either the immediate, or the after effects of the red tide.[12] Fish such as Atlantic herring, American pollock, winter flounder, Atlantic salmon, and cod were dosed orally with these toxins in an experiment, and within minutes the subjects started to exhibit a loss of equilibrium and began to swim in an irregular, jerking pattern, followed by paralysis and shallow, arrhythmic breathing and eventually death, after about an hour.[13] Red tides have been shown to have a negative effect also in the memory functions of sea lions.[14]

Human exposure

Humans are affected by the red tide species by ingesting improperly harvested shellfish, breathing in aerosolized brevetoxins (i.e. PbTx or Ptychodiscus toxins) and in some cases skin contact.[5] The brevetoxins bind to voltage-gated sodium channels, important structures of cell membranes. Binding results in persistent activation of nerve cells, which interferes with neural transmission leading to health problems. These toxins are created within the unicellular organism, or as a metabolic product.[15] The two major types of brevetoxin compounds have similar but distinct backbone structures. PbTx-2 is the primary intracellular brevetoxin produced by K. brevis blooms. However, over time, the PbTx-2 brevetoxin can be converted to PbTx-3 through metabolic changes.[15] Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts over time into PbTx-3.[16]

In most cases like in the U.S., the seafood consumed by humans is tested regularly for toxins by the USDA to ensure safe consumption. However, improper harvesting of shellfish can cause paralytic shellfish poisoning and neurotoxic shellfish poisoning in humans.[9][8] Some symptoms include drowsiness, diarrhea, nausea, loss of motor control, tingling, numbing or aching of extremities, incoherence, and respiratory paralysis.[17] Reports of skin irritation after swimming in the ocean during a red tide are common, so people should try to avoid the red tide when it is in the area.[18]

When the red tide cells rupture, they release extracellular brevetoxins into the environment. Some of those stay in the ocean, while other particles get aerosolized. During onshore winds, brevetoxins can become aerosolized by bubble-mediated transport, causing respiratory irritation, bronchoconstriction, coughing, and wheezing, among other symptoms.[18] On a windy day, avoiding contact with the aerosolized toxin is recommended. These individuals report a decrease in respiratory function after only 1 hour of exposure to a K. brevis red-tide beach and these symptoms may last for days.[19] People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions. The National Oceanic and Atmospheric Administration's National Ocean Service provides a public conditions report identifying possible respiratory irritation impacts in areas affected by red tides.[20]

The ICD-10 Diagnosis Code as provided by the Center for Disease Control (CDC) is Z77.121.[21] It is applicable to the following:[22]


Not all red tides are produced by dinoflagellates. The mixotrophic ciliate Mesodinium rubrum produces non-toxic blooms coloured deep red by chloroplasts it has enslaved from the algae it eats [23]
Not all red tides are produced by dinoflagellates. The mixotrophic ciliate Mesodinium rubrum produces non-toxic blooms coloured deep red by chloroplasts it has enslaved from the algae it eats [23]
The dinoflagellate labeled above is the microscopic alga Karenia brevis. It is the cause of red tide in the Gulf of Mexico. The algae propel themselves using a longitudinal flagellum (A) and a transverse flagellum (B). The longitudinal flagellum lies in a groove-like structure called the cingulum (F). The dinoflagellate is separated into an upper portion called the epitheca (C) where the apical horn resides (E) and a lower portion called the hypotheca (D).
The dinoflagellate labeled above is the microscopic alga Karenia brevis. It is the cause of red tide in the Gulf of Mexico. The algae propel themselves using a longitudinal flagellum (A) and a transverse flagellum (B). The longitudinal flagellum lies in a groove-like structure called the cingulum (F). The dinoflagellate is separated into an upper portion called the epitheca (C) where the apical horn resides (E) and a lower portion called the hypotheca (D).

Red tide is a colloquial term used to refer to one of a variety of natural phenomena known as harmful algal blooms. The term specifically refers to blooms of a species of dinoflagellate.[24] It is being phased out by some researchers because:

  1. Red tides are not necessarily red and many have no discoloration at all.
  2. They are unrelated to movements of the tides.
  3. The term is imprecisely used to refer to a wide variety of algal species that are known as bloom-formers.

As a technical term, it is being replaced in favor of more precise terminology, including the generic term "harmful algal bloom" for harmful species, and "algal bloom" for benign species.

On the U.S. coasts

The term red tide is most often used in the US to refer to Karenia brevis blooms in the eastern Gulf of Mexico, also called the Florida red tide. K. brevis is one of many different species of the genus Karenia found in the world's oceans. [25] Major advances have occurred in the study of dinoflagellates and their genomics. Some include identification of the toxin-producing genes (PKS genes), exploration of environmental changes (temperature, light/dark, etc.) have on gene expression, as well as an appreciation of the complexity of the Karenia genome.[25] These blooms have been documented since the 1800s, and occur almost annually along Florida's coasts.[25] There was increased research activity of harmful algae blooms (HABs) in the 1980s and 1990s. This was primarily driven by media attention from the discovery of new HAB organisms and the potential adverse health effects of their exposure to animals and humans.[26][full citation needed] The Florida red tides have been observed to have spread as far as the eastern coast of Mexico.[25] The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolor the water a deep reddish-brown hue.

Red tide is also sometimes used to describe harmful algal blooms on the northeast coast of the United States, particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms of organisms cause severe disruptions in fisheries of these waters, as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin.[27] The related Alexandrium monilatum is found in subtropical or tropical shallow seas and estuaries in the western Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, and the eastern Pacific Ocean.

Factors that may contribute to a bloom

Red tide (NOAA)
Red tide (NOAA)

Red tides contain dense concentrations of organisms and appear as discolored water, often reddish-brown in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a red tide outbreak are not necessarily known.[4] However, three key factors are thought to play an important role in a bloom - salinity, temperature, and wind. Red tides cause economic harm, so outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission provides an up-to-date status report on red tides in Florida.[28] The Texas Parks and Wildlife Department also provides a status report.[29] While no particular cause of red tides has been found, many different factors can contribute to their presence. These factors can include water pollution, which originates from sources such as human sewage and agricultural runoff.[30]

The occurrence of red tides in some locations appears to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents)[31][32] while in others they appear to be a result of increased nutrient pollution from human activities.[33] The growth of marine phytoplankton is generally limited by the availability of nitrates and phosphates, which can be abundant in agricultural run-off as well as coastal upwelling zones. Coastal water pollution produced by humans and systematic increase in seawater temperature have also been implicated as contributing factors in red tides.[citation needed] Other factors such as iron-rich dust influx from large desert areas such as the Sahara Desert are thought to play a major role in causing red tides.[34] Some algal blooms on the Pacific Coast have also been linked to occurrences of large-scale climatic oscillations such as El Niño events. While red tides in the Gulf of Mexico have been occurring since the time of early explorers such as Cabeza de Vaca,[35] what initiates these blooms and how large a role anthropogenic and natural factors play in their development is unclear. Whether the apparent increase in frequency and severity of algal blooms in various parts of the world is in fact a real increase or is due to increased observation effort and advances in species identification methods is also debated.[36][17]

A multi-partner project funded by the federal EcoHab program (NOAA) and published by the Mote Marine Laboratory shows a list of what feeds red tides.[37] A study from the Florida FWC shows the Karenia brevis algae red tide found in Florida is fed and worsened by nitrogen (N) and phosphorus (P).[38]

List of common red tide genera

Notable occurrences

See also


  1. ^ a b Anderson, Donald M.; Glibert, Patricia M.; Burkholder, Joann M. (August 2002). "Harmful algal blooms and eutrophication: Nutrient sources, composition, and consequences". Estuaries. 25 (4): 704–726. doi:10.1007/BF02804901. S2CID 44207554. Retrieved 7 April 2021.
  2. ^ a b Hall, Danielle. "What Exactly Is a Red Tide?". Smithsonian. Retrieved 7 April 2021.
  3. ^ a b c d Brand et al., Larry E., Lisa Campbell, Eileen Bresnan. "Karenia: The biology and ecology of a toxic genus." Harmful Algae 14 (2012): 156–178. 6 March 2018.
  4. ^ a b "Red Tide FAQ". Austin, TX: Texas Parks and Wildlife Department. Retrieved 2018-08-15.
  5. ^ a b Backer, Lorraine C; Fleming, Lora E; Rowan, Alan; Cheng, Yung-Sung; Benson, Janet; Pierce, Richard H; Zaias, Julia; Bean, Judy; Bossart, Gregory D (March 2003). "Recreational exposure to aerosolized brevetoxins during Florida red tide events". Harmful Algae. 2 (1): 19–28. doi:10.1016/s1568-9883(03)00005-2. ISSN 1568-9883.
  6. ^ a b "Red Tide & Red Algae Effects". 2015.
  7. ^ Gregg W. Langlois, Pamela D. Tom. "Red Tides: Questions and Answers". U.S. Government. Retrieved 2009-08-23.
  8. ^ a b "Red Tide FAQ – Is it safe to eat oysters during a red tide?". Retrieved 2009-08-23.
  9. ^ a b Watkins, Sharon M.; Reich, Andrew; Fleming, Lora E.; Hammond, Roberta (2008). "Neurotoxic Shellfish Poisoning". Marine Drugs. 6 (3): 431–455. doi:10.3390/md20080021. PMC 2579735. PMID 19005578.
  10. ^ Forrester et al., Donald J., Jack M. Gaskin, Franklin H. White. "AN EPIZOOTIC OF WATERFOWL IN FLORIDA." Journal of Wildlife Diseases 13 (1997): 160–167.
  11. ^ "Top 10 Red Tide Facts" (PDF). Florida Department of Health. 2016.
  12. ^ Landsberg, J.H.; Flewelling, L.J.; Naar, J. (March 2009). "Karenia brevis red tides, brevetoxins in the food web, and impacts on natural resources: Decadal advancements". Harmful Algae. 8 (4): 598–607. doi:10.1016/j.hal.2008.11.010. ISSN 1568-9883.
  13. ^ White, A. W. "Sensitivity of Marine Fishes to Toxins from the Red-Tide Dinoflagellate Gonyaulax excavata and Implications for Fish Kills." Marine Biology 65 (1981): 255–260. 6 March 2018.
  14. ^ Cook, P.F.; Reichmuth, C. (2015). "Algal toxin impairs sea lion memory and hippocampal connectivity, with implications for strandings". Science. 350 (6267): 1545–1547. Bibcode:2015Sci...350.1545C. doi:10.1126/science.aac5675. PMID 26668068. S2CID 22981507.
  15. ^ a b Pierce, R. H.; Henry, M. S. (2008). "Harmful algal toxins of the Florida red tide (Karenia brevis): Natural chemical stressors in South Florida coastal ecosystems". Ecotoxicology. 17 (7): 623–631. doi:10.1007/s10646-008-0241-x. PMC 2683401. PMID 18758951.
  16. ^ Pierce, R.H., M. S. Henry. "Harmful algal toxins of the Florida red tide (Karenia brevis): natural chemical stressors in South Florida coastal ecosystems." Ecotoxicology 2008; 623–631.
  17. ^ a b Van Dolah, F. M. (2000). "Marine algal toxins: Origins, health effects, and their increased occurrence". Environmental Health Perspectives. 108 (Suppl 1): 133–141. doi:10.1289/ehp.00108s1133. JSTOR 3454638. PMC 1637787. PMID 10698729.
  18. ^ a b Backer et al., Lorraine C., Laura E. Flemming, Alan Rowan. "Recreational exposure to aerosolized brevetoxins during Florida red tide events." Harmful Algae 2 (2003): 19–28. 6 March 2018.
  19. ^ Fleming LE, Kirkpatrick B, Backer LC, et al. Initial evaluation of the effects of aerosolized Florida red tide toxins (brevetoxins) in persons with asthma. Environ Health Perspect. 2005;113:650–657.
  20. ^ "Harmful Algal Bloom Operational Forecast System". Retrieved 2012-02-14.
  21. ^ "Search Results | CDC". 2018-09-24. Retrieved 2018-12-06.
  22. ^ "2018/2019 ICD-10-CM Diagnosis Code Z77.121: Contact with and (suspected) exposure to harmful algae and algae toxins". Retrieved 2018-12-06.
  23. ^ Dierssen, Heidi; McManus, George B.; Chlus, Adam; Qiu, Dajun; Gao, Bo-Cai; Lin, Senjie (2015). "Space station image captures a red tide ciliate bloom at high spectral and spatial resolution". Proceedings of the National Academy of Sciences. 112 (48): 14783–14787. Bibcode:2015PNAS..11214783D. doi:10.1073/pnas.1512538112. PMC 4672822. PMID 26627232.
  24. ^ "Harmful Algal Blooms (HABs): Red Tide". U.S. Centers for Disease Control and Prevention. Retrieved 2 Oct 2011.
  25. ^ a b c d Fleming, L.E.; Kirkpatrick, B.; Backer, L.C.; Walsh, C.J.; Nierenberg, K.; Clark, J.; et al. (2011). "Review of Florida red tide and human health effects". Harmful Algae. 10 (2): 224–233. doi:10.1016/j.hal.2010.08.006. PMC 3014608. PMID 21218152.
  26. ^ Abraham and Baden, 2006; Backer et al., 2003a, 2005a; Backer and Fleming, 2008; Fleming et al., 2001; Fleming et al., 2004; Okamoto and Fleming, 2005; Twiner et al., 2008; Zaias et al., 2010.
  27. ^ "Red Tide (Paralytic Shellfish Poisoning)" (PDF). Boston, MA: Massachusetts Department of Public Health. 2015.
  28. ^ "Red Tide Current Status Statewide Information". Florida Fish and Wildlife Research Institute. Archived from the original on 2009-08-22. Retrieved 2009-08-23.
  29. ^ "Red Tide Index". Texas Parks and Wildlife Department. Retrieved 2018-08-15.
  30. ^ West, L. (2016). "Red Tide: Causes and Effects". About News.
  31. ^ Trainer, VL; Adams, NG; Bill, BD; Stehr, CM; Wekell, JC; Moeller, P; Busman, M; Woodruff, D (2000). "Domoic acid production near California coastal upwelling zones, June (1998)". Limnol Oceanogr. 45 (8): 1818–1833. Bibcode:2000LimOc..45.1818T. doi:10.4319/lo.2000.45.8.1818. S2CID 54007265.
  32. ^ Adams, NG; Lesoing, M; Trainer, VL (2000). "Environmental conditions associated with domoic acid in razor clams on the Washington coast". J Shellfish Res. 19: 1007–1015.
  33. ^ Lam CWY, Ho KC (1989) Red tides in Tolo Harbor, Hong Kong. In: Okaichi T, Anderson DM, Nemoto T (eds) Red tides. Biology, environmental science and toxicology. Elsevier, New York, pp 49–52.
  34. ^ Walsh; et al. (2006). "Red tides in the Gulf of Mexico: Where, when, and why?". Journal of Geophysical Research. 111 (C11003): 1–46. Bibcode:2006JGRC..11111003W. doi:10.1029/2004JC002813. PMC 2856968. PMID 20411040.
  35. ^ Cabeza de Vaca, Álvar Núnez. La Relación (1542). Translated by Martin A. dunsworth and José B. Fernández. Arte Público Press, Houston, Texas (1993)
  36. ^ Sellner, K.G.; Doucette G.J.; Kirkpatrick G.J. (2003). "Harmful Algal blooms: causes, impacts and detection". Journal of Industrial Microbiology and Biotechnology. 30 (7): 383–406. doi:10.1007/s10295-003-0074-9. PMID 12898390. S2CID 6454310.
  37. ^ "Nutrients That Feed Red Tide "Under the Microscope" in Major Study". News & Press. Sarasota, FL: Mote Marine Laboratory. Retrieved 2018-07-05.
  38. ^ "What forms of nutrients can Karenia brevis use to grow and bloom?". HAB Research. Tallahassee, FL: Florida Fish and Wildlife Conservation Commission. Retrieved 2018-08-15.
  39. ^ A historical assessment of Karenia brevis in the western Gulf of Mexico (PDF), 2018-08-16
  40. ^ a b "Log In or Sign Up to View" (PDF). Retrieved 2018-07-21.
  41. ^ a b "PARALYTIC SHELLFISH POISONING (PSP)". Sabah Fish Retrieved 2013-01-11.
  42. ^ "Marine & Natural Resources – Red Tide & Fish Kill Resources – Taylor County Extension Office". Retrieved 18 October 2016.
  43. ^ "Punta Gorda Herald, Dec. 7 1916".
  44. ^ HAB 2000 Archived 2008-12-11 at the Wayback Machine
  45. ^ MacLean, J.L. (February 1974). "Shellfish Poisoning in the South Pacific" (PDF). South Pacific Commission.
  46. ^ a b c "Red tide warning". New Straits Times. 2013-01-06. Archived from the original on 2013-01-07. Retrieved 2013-01-07.
  47. ^ a b "2 Red Tide deaths in Sabah". Daily Express. 2013-01-06. Retrieved 2013-01-11.
  48. ^ "Red Tides" (PDF). Retrieved October 3, 2020.
  49. ^ "R/V Oceanus Archived Information". Retrieved 18 October 2016.
  50. ^ Moore, Kirk. "Northeast Oysters: The bigger danger, growers assert, would be the label of endangered". National Fisherman. Archived from the original on 2007-08-08. Retrieved 2008-07-31.
  51. ^ Fimrite, Peter (2011-09-17). "Red tide killing abalone off California". The San Francisco Chronicle.
  52. ^ "Texas Gulf Coast Sees Largest Algae Bloom in Over A Decade". Huffington Post. 2011-10-18.
  53. ^ a b MUGUNTAN VANAR (2013-01-07). "Sabah issues red tide alert". The Star Online. Archived from the original on 2013-01-08. Retrieved 2013-01-07.
  54. ^ McSwane, J. David. "UPDATE: Red tide, fish kill reported at Sarasota beaches". Retrieved 18 October 2016.
  55. ^ Netburn, Deborah (11 August 2014). "Massive 'Florida red tide' is now 90 miles long and 60 miles wide". Orlando Sentinel. Retrieved 30 September 2015.
  56. ^ Israel, Dale G. (June 25, 2015). "12 persons hospitalized in Bohol for red tide poisoning".
  57. ^ "Rijkswaterstaat, do not swim between Katwijk and Scheveningen". Dutch Public Broadcasting, NOS. 3 August 2015. Retrieved 30 September 2015.
  58. ^ "Red Tide in Texas, Current Status". Texas Parks and Wildlife. 15 September 2015. Retrieved 30 September 2015.
  59. ^ Nora Salinas (2016-09-15). "Red tide arrives at South Padre Island".
  60. ^ "Algae, red tide impacting SWFL water quality". WINK NEWS. 2018-07-04. Retrieved 2018-07-05.
  61. ^ Glenn, Julie. "Toxic Algae Blooms, Red Tide, and the Need for a Permanent Solution". Retrieved 2018-07-05.
  62. ^ "What forms of nutrients can Karenia brevis use to grow and bloom?". Retrieved 2018-08-14.
  63. ^ "Red tide confirmed off Palm Beach in rare outbreak for Florida's east coast". Miami Herald.
  64. ^ "Pinellas County already matches 2018 fishkill by cleaning up over 3 million pounds of dead fish". WFTS. 2021-07-23. Retrieved 2021-07-27.
  65. ^ "National Weather Service issues beach hazard statement over red tide concerns". Retrieved 2021-07-27.