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 the red tide 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. Some red tide algal blooms are associated with fish kills. The production of natural toxins such as brevotoxins 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.
Red tides occur naturally off coasts all over the world. Not all red tides have toxins or 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 contaminates 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. The animals that eat the shellfish are susceptible to the neurotoxin leading to neurotoxic shellfish poisoning (NSP) and sometimes even death. Most mollusks and clams filter feed which results in higher concentrations of the toxin than just drinking the water. Scaup 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. The toxins released by the blooms can kill marine animals including dolphins, sea turtles, birds and manatees just to name a few. 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. Scientists concluded that the toxic red tide had negative effects on fish that were exposed to it.
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. 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. Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts overtime into PbTx-3. 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. 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. 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.
In most cases like in the U.S., the seafood consumed by humans is tested regularly for toxins by the USDA to insure 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. 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 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.
The term red tide is being phased out among researchers for the following reasons:
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.
The term red tide is most often used in the United States of America 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. 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 impacts 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 waters. 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 impacts of their exposure to animals and humans. Recently, 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.
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. 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.
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, it has been said that three key factors play an important roll 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 provides an up-to-date status report on the red tide in Florida. Texas also provides a current status report as well as other regions. 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 a water and coastal pollution which originates from sources such as human sewage and agricultural runoff. 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. Red tide algal blooms tend to be more abundant during the summer months because of the warm temperatures. Lately, these blooms have been occurring more frequently during the winter months because of the changes in climate.
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 loading 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 sea water 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 Saharan 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, 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.
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. 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.  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.