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 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. Karenia brevis produces brevetoxins. Brevetoxins are neurotoxins that bind to voltage-gated sodium channels in nerve cells, leading to disruption of normal neurological processes and causing the illness clinically described as neurotoxic shellfish poisoning. The term "Red tide" is also commonly used on the northern east coast of the United States, and particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. Dinoflagellates are unicellular protists which exhibit a great diversity of form. These creatures often have a big impact on the environment around them. Many are photosynthetic, manufacturing their own food using the energy from sunlight, and providing a food source for other organisms. Some species are capable of producing their own light through bioluminescence, which also makes fireflies glow. There are some dinoflagellates which are parasites on fish or on other protists.
Red tide is a phenomenon caused by algal blooms during which algae become so numerous that they discolor coastal waters. The algal bloom may also deplete oxygen in the waters and/or release toxins that may cause illness in humans and other animals. A red tide occurs when the population of certain kind of algae known as dinoflagellates explodes creating algal blooms. Red tides are sometimes called as harmful algal blooms (HABs). An algal bloom is a rapid increase in the population of algae in a water system. Algal blooms result when water temperatures are warm and when nutrients, such as nitrogen and phosphorus, are present in the water. Major factors influencing red tide events include warm ocean surface temperatures, low salinity, high nutrient content, calm seas, and rain followed by sunny days during the summer months. In addition, algae related to red tide can spread or be carried long distances by winds, currents, storms, or ships.
The most dramatic effect of dinoflagellates on life around them comes from the coastal marine species which "bloom" during the warm months of summer. These species reproduce in such great numbers that the water may appear golden or red, producing a "red tide". When this happens many kinds of marine life suffer, for the dinoflagellates produce a neurotoxin which affects muscle function in susceptible organisms. Humans may also be affected by eating fish or shellfish containing the toxins. The resulting diseases include ciguatera (from eating affected fish) and paralytic shellfish poisoning, or PSP (from eating affected shellfish, such as clams, mussels, and oysters); they can be serious but are not usually fatal. Mortality events attributed to HABs have been documented for fish, manatee, dolphins, and seabirds.
These blooms of organisms cause severe disruptions in fisheries of these waters as the toxins in this organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin. 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.
Not all algal blooms are harmful. However, blooms can be harmful when they are so thick that they block sunlight that other organisms need to live. When bloom organisms die and decompose, they deplete the oxygen in the water and starve fish and plants, causing fish kills and damaging local ecology. Some algae produce toxins and release them into the water. During a bloom, the amount of toxin present in the water can poison people, wild animals, and pets that go near the water, consume the water or swim in the water.
Many dinoflagellates are photosynthetic and bioluminescent. Red tides are natural occurrences; the plankton community becomes dominated by one or a few species at extraordinarily high concentrations. The net growth rate of the cells is higher than their net loss rate. The growth can be fueled by nutrients brought to the surface by upwelling. There is some evidence that development and growth of red tides can be facilitated by discharges of treated sewage from municipal sewage treatment plants containing nitrates, urea, and other nutrients, and also from ‘urban runoff’ that contains fertilizers washed off of landscaped areas.
Although most of these species of phytoplankton and cyanobacteria are harmless, there is a few dozen of that create potent toxins under 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. Red tide is also potentially harmful to human health. Humans can become seriously ill from eating oysters and other shellfish contaminated with red tide toxin. Karenia brevis blooms can potentially cause eye and respiratory irritation (coughing, sneezing, tear production, and itching) to beachgoers, boaters, and coastal residents. People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions.
Red tide is a global phenomenon. However, since the 1980s harmful red tide events have become more frequent and widespread. Detection of a spread is thought to be influenced by a higher awareness of red tide, better equipment for detecting and analyzing red tide, and nutrient loading from farming and industrial runoff. Countries affected by red tide events include: Argentina, Australia, Brazil, Canada, Chile, Denmark, England, France, Guatemala, Hong Kong, India, Ireland, Italy, Japan, the Netherlands, New Zealand, Norway, New Guinea, Peru, the Philippines, Romania, Russia, Scotland, Spain, Sweden, Thailand, the United States, and Venezuela.
Red tide algae make potent natural toxins. It is unknown why these toxins are created, but some can be hazardous to larger organisms through the processes of biomagnification and bioaccumulation. Grazers such as fish and krill are unaffected by the toxins, so as they eat the algae the toxins are concentrated and accumulate to a level that is poisonous eat to organisms that feed on them. Large fish kills and several mammalian diseases and deaths have been attributed to consumption of shellfish during red tide algal blooms. Diseases that may affect humans include:
Paralytic Shellfish Poisoning (PSP) - This disease is caused by the production of saxitoxin by the Alexandrium species. It is common along the Atlantic and Pacific coasts in the US and Canada. Poisoning occurs when one ingests shellfish contaminated with PSP toxins causing disruption of nerve function and paralysis. Extreme cases may result in death by asphyxiation by respiratory paralysis.
Diarrhetic Shellfish Poisoning (DSP) - This disease is caused by the Dinophysis species. It generally occurs in Japan and Europe, but it has also been found in other countries such as Canada, the US, Chile, New Zealand, and Thailand. Symptoms of DSP include diarrhea, nausea, vomiting, abdominal pain, and cramps. DSP is generally not lethal.
Amnesic Shellfish Poisoning (ASP) - This disease, which has been found along the eastern Canadian coast, is caused by domoic acid producing planktonic and benthic algae, including Pseudo-nitzschia pungens, Pseudo-nitzschia multiseries and Amphora coffaeformis. It can also be found in soft shell clams and blue mussels infected by Pseudo-nitzschia delicatissima. Gastric and neurological symptoms include dizziness, disorientation, and memory loss.
The term “red tide” was coined and officially recognized as a marine occurrence in 1904. Later on, it has occurred throughout the year and appears to be increasing in recent years. Among others, red tide hit Tampa Bay in Florida in 1947, the Seto Inland Sea of Japan in 1957, the Northumberland coast of Britain in 1968, North America’s eastern seaboard and Papua New Guinea in 1972, Spain and the Netherlands in 1978, the east coast of India in 1981, New Zealand in 1984, and Canada, Argentina and Thailand in 1985. In 1987 alone, contaminated shellfish were reported from Tasmania, Taiwan, Korea, Hong Kong, Guatemala, and Venezuela. As well as evidence of an expanding habitat, there are also indications that the numbers and regularity of red tides in on the increase.
However, in Korea, the number of red tide outbreaks has been recorded since 1981. The outbreak number of red tides in Korean coastal waters from 1981 to 2001 has been are tremendous.
Based on the annual total number of red tide outbreaks recorded in the harmful algal blooms (HABs) monitoring program, there were 8 events in 1981, including 3 harmful dinoflagellate blooms. Now the dinoflagellate blooms affect most of the coastal areas. Diatoms were most common bloom species until the first half of the 1970s. Other species of dinoflagellate such as Prorocentrum micans, P. minimum, Heterosigma akashiwo, Gymnodinium mikimotoi, G. sanguineum and Cochlodinium ploykrikodes were found to be major blooms. Some cyst-forming dinoflagellate species make bloom repeatedly at the same place and same time of the year.
From 1996 to 1998, the Cochlodinium bloom occurred every August and persisted for about one month. After C. ploykrikoides blooms caused severe economic loss in 1995, dynamic modeling has been used to study the initiation and subsequent development of the blooms. C. ploykrikoides grows well in eutrophic water similar to a chemical oxygen demand (COD) of 1 ppm. When C. ploykrikoides bloom is fully developed, it gradually forms a plume-like patch and grows. Bloom movement and distribution are dependent on the wind direction and tidal currents. The bloom approaches the coast during flood currents when SW winds prevail, and vice versa during neap currents with an NW wind.
The Korean fisheries economy depends on heavily upon the coastal zone for marine products and so it is especially sensitive to constraints from HABs. Resources must be used in the most efficient way and there is a need for sustainable, multiple-use management schemes. It is very necessary to forecast the outbreaks of blooms in order to take appropriate measures. Some fish killing dinoflagellates can devastate the farms. Most of the coastal fish farm cannot avoid the HABs due to fixed aquaculture facilities. For the monitoring of shellfish poisoning, toxicological tests are run for PSP and DSP using bioassays and HPLC, and ASP using HPLC. The annual budget for the 1995 national red tide monitoring was about US$250,000 exclusive of the personnel expenses and instrument purchase.
The mitigation strategies for HABs include cage movement to the outside of the affected area, diminution of feed supply, and early harvesting if the bloom develops to fatal levels. Another most important mitigation strategy is the scattering clay. Clay has the capability to scavenge particles and carry them to bottom sediments. The removal efficacy was up to 80% using clay concentrations of 10 g l-1. In the field, dissolved inorganic nitrogen, chemical oxygen demand, and chloroplyll-α concentration decreased slightly after dispersion. However, scattering clays into coastal farms has some impact on marine animals, especially on abalone, with no large effects on flatfish.
The biological control of red tides by using copepods and bivalves such as oysters has been examined, but the results were minimal because of the huge scale of red tides. The viruses, parasites, and harmless phytoplankton such as diatoms, had not been investigated for the application to biological control of red tides except for a few studies on bacteria. The possible mitigation strategies are the prevention of red tides by diatoms, algicidal bacteria, and algicidal viruses may apply. Other prevention and mitigation of HABs may include, Determine the relationship between HABs and land-use practices, Assess the role of altered hydrology in causing HABs and develop policies to reduce impact, Evaluate the potential for establishing or enhancing shellfish communities to restore natural phytoplankton populations and reduce dominance of HAB organisms, and Assist managers in controlling nutrient loading in coastal areas susceptible to HABs by providing decision support tools, including predictive models, GIS applications and other methods
Technological advancements such as satellite imagery have allowed scientists to better track and monitor harmful algal blooms. Tracking and monitoring red tide algae helps reduce harmful effects of the algae by providing warnings against eating infected shellfish and against swimming in infected waters. For example, the Sarasota Operations Coastal Oceans Observation Lab has developed instruments that can test for red tide algae in coastal waters. Finally, researchers are attempting to develop an antidote to the red tide toxins.
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