Red tides are 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 unicellularalgae (e.g. dinoflagellates and diatoms).
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. Harmful algal blooms can occur worldwide, and natural cycles can vary regionally.
The growth and persistence of an algal bloom depends on wind direction and strength, temperature, nutrients, and salinity. Red tide species can be found in oceans, bays, and estuaries, but they cannot thrive in freshwater environments. 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. 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, but not all red tides are harmful. 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. Other animals that eat the shellfish are susceptible to the neurotoxin, leading to neurotoxic shellfish poisoning and sometimes even death. Most mollusks and clams filter feed, which results in higher concentrations of the toxin than just drinking the water.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. The toxins released by the blooms can kill marine animals including dolphins, sea turtles, birds, and manatees. 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. 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. Red tides have been shown to have a negative effect also in the memory functions of sea lions.
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. 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. 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. Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts over time into PbTx-3.
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. Some symptoms include drowsiness, diarrhea, nausea, loss of motor control, tingling, numbing or aching of extremities, incoherence, and respiratory paralysis. 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.
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. 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. 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.
The ICD-10 Diagnosis Code as provided by the Center for Disease Control (CDC) is Z77.121. It is applicable to the following:
Contact with and (suspected) exposure to (harmful) algae bloom NOS
Contact with and (suspected) exposure to blue-green algae bloom
Contact with and (suspected) exposure to brown tide
Contact with and (suspected) exposure to Florida red tide
Contact with and (suspected) exposure to pfiesteria piscicida
Contact with and (suspected) exposure to red tide
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 
The dinoflagellate labeled above is the microscopic algaKarenia 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. It is being phased out by some researchers because:
Red tides are not necessarily red and many have no discoloration at all.
They are unrelated to movements of the tides.
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.
 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. These blooms have been documented since the 1800s, and occur almost annually along Florida's coasts. 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.[full ] The Florida red tides have been observed to have spread as far as the eastern coast of Mexico. 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. 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 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. 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. The Texas Parks and Wildlife Department also provides a status report. 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.
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) while in others they appear to be a result of increased nutrient pollution from human activities. 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. 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. 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, 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.
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. 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).
1530: First alleged case off the Florida Gulf Coast is without foundation. According to Marine Lab at University of Miami, the first possible 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.
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.
1844: First possible 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.
1916: Massive fish kill along SW Florida coast. Noxious air thought to be seismic underwater explosion releasing chlorine gas.
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.
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.
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 and alerting authorities as far south as Montauk (Long Island, NY) to check their beds. 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.
2013: In 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.
2014: In August, massive 'Florida red tide' 90 miles (140 km) long and 60 miles (97 km) wide.
2015: June, 12 persons hospitalized in the Philippine province of Bohol for red tide poisoning.
2015: August, several beaches in the Netherlands between Katwijk and Scheveningen were plagued. Government institutions dissuaded swimmers from entering the water.
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."
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. Peaked in the summer of 2018. Toxic harmful algae bloom red tide in Southwest Florida. A rare harmful algal bloom along Florida's east coast of Palm Beach County occurred the weekend of September 30, 2018.
2021: In July, a large red tide occurred on the Gulf Coast of Florida in and around Tampa Bay. The event has caused the death of millions of pounds of fish, and led to the National Weather Service declaring a Beach Hazard.
Milky seas effect - A phenomenon in which disturbed red algae dinoflagellates will make the water glow blue, at night