Definitions

gonyaulax catanella

Red tide

"Red tide" is a common name for a phenomenon known as an algal bloom, an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column, or "bloom". These algae, more specifically phytoplankton, are microscopic, single-celled protists, plant-like organisms that can form dense, visible patches near the water's surface. Certain species of phytoplankton contain photosynthetic pigments that vary in colour from green to brown to red, and when the algae are present in high concentrations, the water appears to be discoloured or murky, varying in colour from purple to almost pink, normally being red or green. Not all algal blooms are dense enough to cause water discolouration, and not all discoloured 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.

Some red tides are associated with the production of natural toxins, depletion of dissolved oxygen or other harmful impacts, and are generally described as harmful algal blooms. 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 called brevetoxin which is produced naturally by the marine algae Karenia brevis.

Word usage

"Red tide" is a colloquial term used to refer to a natural phenomenon known as a "harmful algal bloom" or "HAB." Since 1) a wide variety of algal species can cause a red tide, 2) red tides are not necessarily red, and many have no discolouration at all, 3) are unrelated to movements of the tides, and 4) a wide variety of algal species are known bloom-formers, the term "red tide" is being phased out among researchers in favour of the more appropriate "harmful algal bloom" for harmful species, or simply "algal bloom" for non-harmful species.

The term "red tide" is often used in the United States of America 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 litre of seawater, and often discolour the water a deep reddish-brown hue.

The term "red tide" is also commonly 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.

Causes of red tide

It is unclear what causes red tides; their occurrence in some locations appears to be entirely natural, while in others they appear to be a result of human activities The frequency and severity of algal blooms in some parts of the world have been linked to increased nutrient loading from human activities. In other areas, algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents. 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 . The dinoflagella is a symbiont with many coral species. Biologist Tyler Campbell hypothesizes that they may be "moving", or leaving their mutualistic host coral population, either seasonally, or for more favorable living conditions.

Associated illnesses

Marine and fresh waters teem with life, much of it microscopic, and most of it harmless; in fact, it is this microscopic life on which all aquatic life ultimately depends for food. While most of these species of phytoplankton and cyanobacteria are harmless, there are a few dozen that create potent toxins given the right conditions. Harmful algal blooms may cause harm through the production of toxins or by their accumulated biomass, which can affect co-occurring organisms and alter food-web dynamics. Impacts include human illness and mortality following consumption of or indirect exposure to HAB toxins, substantial economic losses to coastal communities and commercial fisheries, and HAB-associated fish, bird and mammal mortalities. To the human eye, blooms can appear greenish, brown, and even reddish- orange depending upon the algal species, the aquatic ecosystem, and the concentration of the organisms.

Notable occurrences

  • 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 toxic aerosol onshore. Swimming is usually safe, but skin irritation and burning is possible in areas of high concentration of red tide.
  • In 1972 a red tide was caused in New England by a toxic dinoflagellate Alexandrium (Gonyaulax) tamarense.
  • In 2005 the Canadian red tide was discovered to have come further south than it has in years prior by a ship called The 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 coming years, halting the area's fishing and shellfish industry, which has endured for hundreds of years, and threatening the tourist trade, which constitutes a significant portion of the 118-mile-long island's economy.

See also

References

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