Red tide

A "red tide" off the coast of La Jolla, California.

Red tide is a common name for a phenomenon known as an algal bloom, an event in which estuarine or marine algae accumulate rapidly in the water column, or "bloom". These algae, more correctly termed phytoplankton, are microscopic, single-celled, plant-like organisms that can form dense, visible patches near the water's surface. Certain species of phytoplankton contain photosynthetic pigments that vary in color from green to brown to red, and when the algae are present in high concentrations, the water appears to be colored red. Not all algal blooms are dense enough to cause water discoloration, and not all discolored waters associated with algal blooms are red. Additionally, red tides are not typically associated with tidal movement of water, hence the preference among scientists to use the term algal bloom.

The term "red tide" is most often used to describe a particular type of algal bloom common to the eastern Gulf of Mexico, and is also called "Florida red tide". This type of bloom is caused by a species of dinoflagellate known as Karenia brevis, and these blooms occur almost annually along Florida waters. The density of these organisms during a bloom can exceed tens of millions of cells per liter of seawater, and often discolor the water a deep reddish-brown hue.

The most conspicuous effects of red tides are the associated wildlife mortalities among marine and coastal species of fish, birds, marine mammals and other organisms. In the case of Florida red tides, these mortalities are caused by exposure to a potent neurotoxin produced naturally by Karenia brevis, called brevetoxin.

(In some regions, such as the Northwest United States, the term "red tide" is casually but inaccurately used to describe blooms of phytoplankton that cause paralytic shellfish poisoning. These blooms typically do not discolor the water.)

It is unclear what causes red tides, but the frequency and severity of algal blooms in many parts of the world have been linked to 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. 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. Algal blooms in many parts of the world cannot be reasonably linked to human influence, and are generally accepted as a natural occurrence. 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 effectiveness of monitoring programs and species identification ability .

Some red tide organisms produce large quantities of toxins, such as saxitoxin, ciguatoxin, and brevetoxin, which disrupt the proper function of ion channels in neurons. Domoic acid, a toxin produced by diatoms of the genus Pseudonitzschia, has been linked to neurological damage in certain marine mammals, and is frequently found in algal blooms on the U.S. West Coast. Some red tide toxins can become highly concentrated in various marine organisms that have the ability to filter and consume large quantities of toxic plankton directly from seawater. These include shellfish, finfish, baleen whales, and benthic crustaceans. Frequently, shellfish collected in areas affected by algal blooms can be potentially dangerous for human consumption, leading to closures of shellfish beds for harvesting. Initial signs of shellfish poisoning from red tide toxins such as domoic acid is tingling in the lips followed by a reduction of motor abilities and difficulty breathing and can be fatal if consumed in sufficient amounts. If these symptoms occur after eating shellfish, seek immediate medical treatment. Standard medical treatment is to give victims oxygen, or to hook them up to a breather. There exists no antidote, and treatment consists of keeping the patient alive until the toxin has passed from the system. Note that in some cases, the brevetoxin in Florida red tides can become aerosolized, causing respiratory irritation to beachgoers.

Toxins commonly associated with red tide are brevetoxins, saxitoxins, domoic acid, okadaic acid, azaspiracid, and ciguatoxin.

See also

• algal bloom (see "toxic chemicals" in paragraph 2)
• ciguatera
• dinoflagellate (see "neurotoxins" and "red tide" under Ecology and fossils and see "phosphate" under Cautions)
• domoic acid
• The Marine Mammal Center
• paralytic shellfish poisoning (PSP)
• phytoplankton

External links

• Harmful Algae and Red Tide Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution
• Toxic Blooms: Understanding Red Tides, a seminar by the Woods Hole Oceanographic Institution
• Gulf of Mexico Dead Zone and Red Tides
• Red Tide updates for the Gulf Coast of Florida provided by Mote Marine Laboratory in Sarasota, FL
• California Program for Regional Enhanced Monitoring for PhycoToxins, California Department of Health Services and the University of California, Santa Cruz
• Red Tide FAQ, Florida's Fish and Wildlife Research Institute
• Red Tide Report A Compilation of citizen based, media and official reports of the locations and severity of current Red Tide Blooms.
• Florida Red Tide Coalition A citizen based group dedicated to raising awareness of Red Tide, debunking myths, educating the public and taking action to promote or oceans health and help stop red tide.
• NIEHS study of airborne impacts of Florida red tide
• Washington State Shellfish Biotoxin Program