Red tide

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A red tide off the coast of La Jolla, San Diego, California.
Red tide in the harbor

Red tide is a common name for a worldwide phenomenon known as an algal bloom (large concentrations of aquatic microorganisms—protozoans or unicellular algae) when it is caused by species of dinoflagellates and other organisms.


Certain species of phytoplankton and dinoflagellates found in red tides contain photosynthetic pigments that vary in color from brown to red. When the algae are present in high concentrations, the water appears to be discolored or murky, varying in color from a rust color to pink to blood red. Specifically, red tide species can be found in oceans, bays and places where freshwater meets saltwater but they can not thrive in freshwater environments due to the lack of salinity. The growth of the algal bloom depends on wind, temperature, nutrients, and salinity.[1][2] Some red tide algal blooms are associated with fish kills. The production of natural toxins such as brevetoxins[3] and ichthyotoxins are harmful to marine life. Generally red tides are described as harmful algal blooms or HABs. The most conspicuous effects of these kinds of red tides are the associated wildlife mortalities as well as harmful human exposure.

List of common red tide genera[edit]

Harmful toxins produced by the red tide[edit]

Marine life exposure[edit]

Red tides occur naturally off coasts all over the world. Not all red tides have toxins or are harmful.[4] 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. Clams, mollusks and oysters tend to not be susceptible to the toxin and actually store it in their fatty tissues. Shellfish consume the organisms responsible for red tide and concentrate saxitoxin (produced from these organisms) in their tissues. Saxitoxin blocks sodium channels and ingestion can cause paralysis within 30 minutes.[5] The animals that eat the shellfish are susceptible to the neurotoxin leading to neurotoxic shellfish poisoning (NSP)[6] and sometimes even death. Most mollusks and clams filter feed which results in higher concentrations of the toxin than just drinking the water.[7] Scaup, for example, are diving ducks whose diet mainly consists of mollusks. When scaup eat the filter feeding shellfish that are concentrated with high levels of the red tide toxin, their population along with other types of diving ducks become a prime target for poisoning. However, even birds that don't eat mollusks can be affected by simply eating dead fish on the beach or drinking the water just like in the Peking duck experiment.[8][clarification needed] The toxins released by the blooms can kill marine animals including dolphins, sea turtles, birds and manatees just to name a few.[9][10] Marine dinoflagellates produce ichthyotoxins. Fish such as Atlantic herring, American pollock, winter flounder, Atlantic salmon and cod were dosed orally with these toxins in an experiment. Within minutes of receiving doses of the toxin, fish 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.[11] Scientists concluded that the toxic red tide had negative effects on fish that were exposed to it.

Human exposure[edit]

Humans are affected by the red tide species by ingesting illegally harvested shellfish, breathing in aerosolized brevetoxins (i.e. PbTx or Ptychodiscus toxins) and in some cases skin contact. The brevetoxins bind to voltage-gated sodium channels, an important structure 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.[12] There are two major types of brevetoxin compounds with 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.[12] Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts overtime into PbTx-3.[13] When the cells rupture it releases 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 things.[14] On a windy day, it is best to avoid contact with the aerosolized toxin. It has been shown that 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.[15] 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.[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, illegal harvesting of shellfish can cause Paralytic Shellfish Poisoning and Neurotoxic Shellfish Poisoning in humans. Some symptoms include: drowsiness, diarrhea, nausea, loss of motor control, tingling, numbing or aching of extremities, incoherence and respiratory paralysis just to name a few.[17] Lastly, reports of skin irritation after swimming in the ocean during a red tide are common so locals and tourists should try to avoid the red tide when it is in the area.

Red tide[edit]

The dinoflagellate labeled above is the microscopic algae Karenia brevis. It is the cause of red tide in the Gulf of Mexico. The algae propels itself by 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 or HABs. The term red tide specifically refers to blooms of a species of dinoflagellate.[18]

The term red tide is being phased out among researchers for the following reasons:

  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 favour of more precise terminology including the generic term harmful algal bloom for harmful species, and algal bloom for non-harmful species.

On the U.S. coasts[edit]

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. In the past decade, it has been elucidated that K. brevis is only one of many different species of the genus Karenia that is found in the world's oceans. [19] There have been major advances 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.[19] These blooms have been documented since the 1800s and occur almost annually along Florida waters.[19] There was increased research activity of 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.[20][full citation needed] Recently, the Florida red tides have been observed to have spread as far as the eastern coast of Mexico.[19] 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.

The term red tide is also sometimes used to describe harmful algal blooms on the northern east 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.[21] 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[edit]

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.[22] However, it has been said that three key factors play an important role in a bloom. These factors are salinity, temperature and wind. Red tide causes economic harm and for this reason red tide outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission (FWC) provides an up-to-date status report on the red tide in Florida.[23] The Texas Parks and Wildlife Department also provides a status report.[24] While there is no one particular cause of red tides, there are many different factors that contribute to its presence. These factors can include things such as water pollution which originates from sources such as human sewage and agricultural runoff.[25] There are other factors that have been associated with the increase in red tides, such as weather, climate change and tidal patterns, although the correlation isn't always quite clear.[25] Red tide algal blooms tend to be more abundant during the summer months because of the warm temperatures.

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)[26][27] while in others they appear to be a result of increased nutrient pollution from human activities.[28] 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 sea water 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 Saharan desert are thought to play a major role in causing red tides.[29] 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,[30] it is unclear what initiates these blooms and how large a role anthropogenic and natural factors play in their development. It is also debated 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.[31][32]

While the human contribution to the long-term increase in red tides is apparent, some researchers propose that climate change is also a factor, with more research still needed to be done in order to claim it as a definitive causal relationship. Increasing temperature, enhanced surface stratification, alteration of ocean currents, intensification or weakening of local nutrient upwelling, stimulation of photosynthesis by elevated CO2, reduced calcification through ocean acidification, and heavy precipitation and storm events causing changes in land runoff and micronutrient availability may all produce contradictory species- or even strain-specific responses.[33] In terms of harmful algal blooms (HABs), we can expect: (i) range expansion of warm-water species at the expense of cold-water species, which are driven poleward; (ii) species-specific changes in the abundance and seasonal window of growth of HAB taxa; (iii) earlier timing of peak production of some phytoplankton; and (iv) secondary effects for marine food webs, notably when individual zooplankton and fish grazers are differentially impacted by climate change.[33] However, the potential consequences of these changes for HABs have received relatively little attention and are not well understood. Substantial research is needed to evaluate the direct and indirect associations between HABs, climate change, ocean acidification, and human health.[34]

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.[35] 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).[36]

Notable occurrences[edit]

  • 1530: First alleged case off the Florida Gulf Coast is without foundation.[37] According to Marine Lab at University of Miami, the first Red Tide in Florida was in 1844. Earlier "signs" were from boats sorting fish on their way to home port dumping trash fish overboard. Thus "dead fish" reports along the coast were not Red Tide.[38]
  • 1793: The first recorded case occurring in British Columbia, Canada.[39]
  • 1840: No deaths of humans have been attributed to Florida red tide, but people may experience respiratory irritation (coughing, sneezing, and tearing) when the red tide organism (Karenia brevis) is present along a coast and winds blow its aerosolized toxins. Swimming is usually safe, but skin irritation and burning is possible in areas of high concentration of red tide.[40]
  • 1844: First verified case off the Florida Gulf Coast according to Marine Lab University of Miami, probably by ships off shore, no known inhabitants of the coast reporting.[41]
  • 1916: Massive fish kill along SW Florida coast. Noxious air thought to be seismic underwater explosion releasing chlorine gas.[42]
  • 1947: Southwest Florida
  • 1972: A red tide was caused in New England by a toxic dinoflagellate Alexandrium (Gonyaulax) tamarense. The red tides caused by the dinoflagellate Gonyaulax are serious because this organism produces saxitoxin and gonyautoxins which accumulate in shellfish and if ingested may lead to paralytic shellfish poisoning (PSP) and can lead to death.[43]
  • 1972 and 1973: Red tides killed two villagers west of Port Moresby. In March 1973 a red tide invaded Port Moresby Harbour and destroyed a Japanese pearl farm.[44]
  • 1976: The first PSP case in Sabah, Malaysian Borneo where 202 victims were reported to be suffering and 7 deaths.[39][45][46]
  • 2005: The Canadian red tide was discovered to have come further south than it has in years prior by the ship (R/V) Oceanus,[47] closing shellfish beds in Maine and Massachusetts and alerting authorities as far south as Montauk (Long Island, NY) to check their beds.[48] Experts who discovered the reproductive cysts in the seabed warn of a possible spread to Long Island in the future, halting the area's fishing and shellfish industry and threatening the tourist trade, which constitutes a significant portion of the island's economy.
  • 2005-2006: Southwest Florida Karenia brevis
  • 2011: Northern California[49]
  • 2011: Gulf of Mexico[50]
  • 2013: On January, a red tide occurred again on the West Coast Sea of Sabah in the Malaysian Borneo.[45][51] Two human fatalities were reported after they consumed shellfish contaminated with the red tide toxin.[45][46][51]
  • 2013: January, a red tide bloom appeared at Sarasota beach – mainly Siesta Key, Florida causing a fish kill that had a negative impact on tourists, and caused respiratory issues for beach-goers.[52]
  • 2014: August, massive 'Florida red tide' 90 miles long and 60 miles wide.[53]
  • 2015: June, 12 persons hospitalized in the Philippine province of Bohol for red tide poisoning.[54]
  • 2015: August, several beaches in the Netherlands between Katwijk and Scheveningen were plagued. Government institutions dissuaded swimmers from entering the water.[55]
  • 2015: 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.[56]
  • 2016: September, Texas Parks and Wildlife report red tide in the Lower Laguna Madre. "High to moderate concentrations of red tide have been found from Beach Access 6 to the Brazos Santiago jetties. Moderate cell concentrations have been found at the Isla Blanca Park boat ramp."[57]
  • 2017 and 2018: K. brevis red tide algae with warnings not to swim, state of emergency declared, dead dolphin and manatee, worsened by Caloosahatchee River. Toxic harmful algae bloom red tide in Southwest Florida.[58][59][60]

See also[edit]


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  8. ^ 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.
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  32. ^ Van Dolah, F.M. (2000). "Marine Algal Toxins: Origins, Health Effects, and Their Increased Occurrence". Environmental Health Perspectives. 108 (suppl.1): 133–141. doi:10.2307/3454638. JSTOR 3454638. PMC 1637787Freely accessible. PMID 10698729. Archived from the original on 2009-01-20. 
  33. ^ a b Hallegraeff, Gustaaf M. (2010-04-01). "Ocean Climate Change, Phytoplankton Community Responses, and Harmful Algal Blooms: A Formidable Predictive Challenge1". Journal of Phycology. 46 (2): 220–235. doi:10.1111/j.1529-8817.2010.00815.x. ISSN 1529-8817. 
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  37. ^ A historical assessment of Karenia brevis in the western Gulf of Mexico (PDF), 2018-08-16 
  38. ^ "Log In or Sign Up to View" (PDF). Retrieved 2018-07-21. 
  39. ^ a b "PARALYTIC SHELLFISH POISONING (PSP)". Sabah Fish Retrieved 2013-01-11. 
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  41. ^ "Log In or Sign Up to View" (PDF). Retrieved 2018-07-21. 
  42. ^ "Punta Gorda Herald, Dec. 7 1916". 
  43. ^ HAB 2000 Archived 2008-12-11 at the Wayback Machine.
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  47. ^ "R/V Oceanus Archived Information". Retrieved 18 October 2016. 
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  53. ^ Netburn, Deborah (11 August 2014). "Massive 'Florida red tide' is now 90 miles long and 60 miles wide". Orlando Sentinel. Retrieved 30 September 2015. 
  54. ^ Israel, Dale G. (June 25, 2015). "12 persons hospitalized in Bohol for red tide poisoning". 
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  57. ^ Nora Salinas. "Red tide arrives at South Padre Island". 
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External links[edit]