Red Tide

Introduction

Red tide is a type of algal bloom, which is a sudden, large increase in the number of microscopic organisms living in a body of water. These organisms are called dinoflagel-lates and are a type of phytoplankton that live in marine and freshwater habitats. Sudden large increases in their numbers are thought to be attributable to changes in salinity, temperature, and water depth, and sometimes to human activities such as the addition of nutrients to water from agricultural runoff.

When dinoflagellates that contain various natural neurotoxins—such as brevetoxin, domoic acid, and saxi-toxin—occur in large enough concentrations, they can cause heavy and murky spots of reddish, discolored surface waters. This reddish discoloration is the source of the term “red tide.” When these toxins are present in the dinoflagellates causing the “red tide,” it is termed a harmful algal bloom (HAB). In such cases, these HABs can be eaten by fish, birds, marine mammals, and humans, causing adverse symptoms and, sometimes, death. Toxins can become airborne, as well; when winds set inland during a large red-tide algal bloom off the coast of Florida, for example, asthma attacks send an increased number of people to hospital emergency rooms.

Historical Background and Scientific Foundations

Most species of phytoplankton are not harmful to humans. In fact, they are beneficial as a food source for larger fish and mammals. However, sometimes phytoplankton grow (bloom) so fast that they become visible on the surface of the water. When this happens, the term “algal bloom” is used to describe the various species of phytoplankton that are involved.

Because various phytoplankton species contain pigments of different colors—ranging from green to brown to red—they can turn water different colors depending on which types of phytoplankton have grown to large enough concentrations. The water can appear to be colored from white to nearly black, but usually the water looks reddish or brown, depending on the species. Because of this color variance, scientists usually prefer the term “algal bloom” to the term “red tide.” Red tide is used most frequently in the United States, while algal bloom is more often used in other parts of the world.

A particular species of dinoflagellate called Karenia brevis is often responsible for the red tides that occur in the waters of the eastern Gulf of Mexico. Red tides also occur in the northeastern part of the United States. The dinoflagellate species Alexandrium spp., which are especially common along the coasts of the North Atlantic Ocean and the Pacific Ocean of North America, are associated with red tides in these areas. These dinoflagellate species are eaten by a wide range of organisms, including mussels, soft-shell clams, sea scallops, oysters, lobsters, crabs, salmon, herring, mackerel, whales, sea birds, and sea otters.

Studies by researchers at the Woods Hole Oceanographic Institution and the U.S. National Oceanographic and Atmospheric Administration (NOAA) show that the frequency of algal blooms has increased along the coasts of the United States and other countries since the 1970s. Although the cause of the increased number of blooms is not absolutely certain, a general consensus among scientists is that the documented warming of the coastal oceans has made conditions more favorable for algal growth. If so, a consequence of global warming could be more algal blooms and more cases of marine toxin-related illness.

WORDS TO KNOW

ALGAL BLOOM: Sudden reproductive explosion of algae (single-celled aquatic green plants) in a large, natural body of water such as a lake or sea. Blooms near coasts are sometimes called red tides.

BREVETOXIN: Any of a class of neurotoxins produced by the algae that cause red tide (coastal algae blooms). Brevetoxins can be concentrated by shellfish and poisonous to humans who eat the shellfish.

DINOFLAGELLATE: Small organisms with both plant-like and animal-like characteristics, usually classified as algae (plants). They take their name from their twirling motion and their whip-like flagella.

DOMOIC ACID: A neurotoxin produced by the algae that cause red tide (coastal algae blooms). Domoic acid can be concentrated by shellfish and poisonous to humans who eat the shellfish.

NEUROTOXIN: A poison that interferes with nerve function, usually by affecting the flow of ions through the cell membrane.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

SALINITY: Measurement of the amount of sodium chloride in a given volume of water.

SAXITOXIN: A neurotoxin found in a variety of dinoflagel-lates. If ingested, it may cause respiratory failure and cardiac arrest.

Impacts and Issues

Scientists are unsure about the cause of red tides, and whether they are natural occurrences or due to human activities. Some parts of the world only experience red tides at certain times of the year, while other regions seem only to have red tides when human activities promote them.

Some scientific studies have found that any pollution that increases the temperature of the water can increase the frequency of red tides. Other studies show that the nitrates and phosphates found in agricultural runoff can cause red tides. Still other studies show an increase in iron concentration as the cause of red tides. However, currently no definitive link has been found between human activities and the incidence of red tides.

Organisms that consume HABs are harmed by the neurotoxin that the dinoflagellates contain. One such neurotoxin—saxitoxin—is found in shellfish that ingest the harmful dinoflagellates. This toxin can produce paralytic shellfish poisoning (PSP) in humans. Patients who eat shellfish containing this toxin experience symptoms—such as amnesia, diarrhea, disorientation, memory loss, nausea, paralysis, and vomiting—that can eventually worsen to severe problems with the respiratory, gastrointestinal, and neurological systems. Death usually comes from respiratory failure. According to the Woods Hole Oceanographic Institution, a person weighing 165 lb (75 kg) can die from digesting just 0.02 oz (0.5 g) of saxitoxin.

Brevetoxin, another neurotoxin, causes neurotoxic shellfish poisoning in humans, while domoic acid produces amnesic shellfish poisoning. Okadaic acid causes diarrhetic shellfish poisoning. Ciguatoxin—from the algae genus Gambierdiscus, which lives in tropical waters such as those around coral reefs—produces ciguatera fish poisoning.

The incidence of red tides has increased over the past 30 or so years, possibly implying that human activity is responsible. Scientists have suggested that climate change and increased pollution levels could make red tides more pronounced. The increased incidence of red tides could have an adverse effect on the availability of food sources for marine animals. However, at the present time, scientists do not know conclusively what causes red tides.

Why some dinoflagellate species produce harmful toxins while others do not also is unknown. Scientists continue to study red tides, especially in an attempt to identify the specific genes that are involved in producing red tides.

Scientists urge governments to use better and more frequent monitoring and detection systems for red tides. Although the annual incidence of death and serious illnesses in the United Sates due to fish and shellfish poisoning is relatively low, new toxins appear periodically that make it more difficult to protect the environment and human health.

The artificial creation of non-toxic algal blooms by adding powdered iron to the ocean surface has been proposed as a way of combating global climate change. The idea is that phytoplankton would absorb carbon dioxide from the atmosphere, incorporating its carbon into their tissues; when the phytoplankton died and sank, they would take the carbon with them to the bottom of the sea, sequestering it (isolating it for a long time) from the atmosphere. However, both the efficacy and safety of this technique, called iron fertilization, have been questioned by many scientists. Small-scale tests have seemed to show little carbon sequestration from artificially induced algal blooms.

In 2007, researchers announced that Karenia brevis not only produces a number of brevetoxins that can constrict bronchial passages and so interfere with breathing, but also produces an antidote to its own toxins, a substance called brevenal. Brevenal, first discovered in 2004, can be made synthetically and is being evaluated for

treatment of some of the symptoms of cystic fibrosis, a genetic disorder that afflicts about 30,000 people in the United States. Brevenal may also be used in Florida to save manatees (rare water-dwelling mammals found only in certain tropical coastal waters) that are threatened with poisoning by red tides.

See Also Algal Blooms; Coral Reefs and Corals; Iron Fertilization

BIBLIOGRAPHY

Books

Okaichi, Tomotoshi, ed. Red Tides. Dordrecht, Netherlands: Kluwer, 2004.

Subba Rao, D. V. Algal Cultures, Analogues of Blooms and Applications. Enfield, NH: Science Publishers, 2006.

Periodicals

Potera, Carol. “Florida Red Tide Brews Up Drug Lead for Cystic Fibrosis.” Science 316 (2007): 1561–1562.

Web Sites

Fathom (Columbia University) and Woods Hole Oceanographic Institution. “Toxic Blooms: Understanding Red Tides.” http://www.fathom.com/course/10701012/index.html (accessed March 29, 2008).

Woods Hole Oceanographic Institution. “Harmful Algae and Red Tides.” http://www.whoi.edu/page.do?pid=11913 (accessed April 2, 2008).

William Arthur Atkins

Red Tide

Introduction

Red tide is a specific term for one type of an aquatic phenomenon generally called an algal bloom—an unexplained increase in the number of microscopic organisms. These organisms are called dinoflagellates, and are a type of phytoplankton that live in marine and freshwater habitats. The increases in their numbers are thought to be attributable to changes in salinity, temperature, and water depth, and possibly to human intervention.

When dinoflagellates that contain various natural neurotoxins—such as brevetoxin, domoic acid, and saxitoxin—occur in large enough concentrations, they can cause heavy and murky spots of reddish, discolored surface waters. This reddish discoloration is the source of the term “red tide.” When these toxins are present in the dinoflagellates causing the “red tide,” it is termed a harmful algal blooms (HABs). In such cases, these HABs might be eaten by fish, birds, marine mammals, and humans, causing adverse symptoms and, sometimes, death.

Historical Background and Scientific Foundations

Most species of phytoplankton are not harmful to humans. In fact, they are beneficial as a food source for larger fish and mammals. However, sometimes phytoplankton grow (bloom) so fast that they become visible on the surface of thewater. Whenthis happens,the term “algal bloom” is used to describe the various species of phytoplankton that are involved.

Because various phytoplankton species contain pigments of various colors—ranging from green to brown to red—they can turn water different colors depending on which types of phytoplankton have grown to large enough concentrations. The water can appear to be colored from white to nearly black, but usually the water looks reddish or brown, depending on the species. Because of this color variance, scientists usually prefer the term “algal bloom” to the term “red tide.” Red tide is used most frequently in the United States, while algal bloom is more often used in other parts of the world.

A particular species of dinoflagellate called Karenia brevis is often responsible for the red tides that occur in the waters of the eastern Gulf of Mexico. Red tides also occur in the northeastern part of the United States. The dino-flagellate species Alexandrium spp., which are especially common along the coasts of the North Atlantic Ocean and the Pacific Ocean of North America, are associated with red tides in these areas. These dinoflagellate species are eaten by a wide range of organisms, including mussels, soft-shell clams, sea scallops, oysters, lobsters, crabs, salmon, herring, mackerel, whales, sea birds, and sea otters.

Impacts and Issues

Scientists are unsure about the cause of red tides, and whether they are natural occurrences or due to human activities. Some parts of the world only experience red tides at certain times of the year, while other regions seem only to have red tides when human activities promote them.

Some scientific studies have found that any pollution that increases the temperature of the water can increase the frequency of red tides. Other studies show that the nitrates and phosphates found in agricultural runoff can cause red tides. Still other studies show an increase in iron concentration as the cause of red tides. However, currently no definitive link has been found between human activities and the incidence of red tides.

Organisms that consume HABs are harmed by the neurotoxin that the dinoflagellates contain. One such neurotoxin—saxitoxin—is found in shellfish that ingest the harmful dinoflagellates. This toxin can produce paralytic shellfish poisoning (PSP) in humans. Patients who eat shellfish containing this toxin experience symptoms—such as amnesia, diarrhea, disorientation, memory loss, nausea, paralysis, and vomiting—that can eventually worsen to severe problems with the respiratory, gastrointestinal, and neurological systems. Death usually comes from respiratory failure. According to the Woods Hole Oceano-graphic Institution, a person weighing 165 lb (75 kg) can die from digesting just 0.02 oz (0.5 g) of saxitoxin.

Brevetoxin, another neurotoxin, causes neurotoxic shellfish poisoning (NSP) in humans, while domoic acid produces amnesic shellfish poisoning (ASP). Okadaic acid causes diarrhetic shellfish poisoning (DSP). Ciquatoxin— from the algae genus Gambierdiscus, which lives in tropical waters such as those around coral reefs—produces ciguatera fish poisoning (CFP).

WORDS TO KNOW

ALGAL BLOOM: Sudden reproductive explosion of algae (single-celled aquatic green plants) in a large, natural body of water such as a lake or sea. Blooms near coasts are sometimes called red tides.

BREVETOXIN: Any of a class of neurotoxins produced by the algae that cause red tide (coastal algae blooms). Brevetoxins and other toxins from algal blooms can be concentrated by shellfish and poison humans who eat the shellfish.

DINOFLAGELLATE: Small organisms with both plant-like and animal-like characteristics, usually classified as algae (plants). They take their name from their twirling motion and their whip-like flagella.

DOMOIC ACID: A neurotoxin produced by the algae that cause red tide (coastal algae blooms). Domoic acid and other toxins from algal blooms, such as brevetoxin, can be concentrated by shellfish and poisonous to humans who eat the shellfish.

NEUROTOXIN: Poison that interferes with nerve function, usually by affecting the flow of ions through the cell membrane.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

SALINITY: The degree of salt in water. The rise in sea level due to global warming would result in increased salinity of rivers, bays, and aquifers. This would affect drinking water, agriculture, and wildlife.

SAXITOXIN: Neurotoxin found in a variety of dinoflagellates. If ingested, it may cause respiratory failure and cardiac arrest.

The incidence of red tides has increased over the past 30 or so years, possibly implying that human activity is responsible. Scientists have suggested that climate change and increased pollution levels could make red tides more pronounced. The increased incidence of red tides could have an adverse effect on the availability of food sources formarine animals. However, at the present time, scientists do not know conclusively what causes red tides.

Why some dinoflagellate species produce harmful toxins, while others do not is also unknown. Scientists continue to study red tides, especially in an attempt to identify the specific genes that are involved in producing red tides. Scientists also are interested in determining if red tides are a natural phenomenon or if human activities are responsible, or both.

IN CONTEXT: MARINE TOXINS

In the United States, about 30 people are poisoned by the toxins in seafood each year. The consequences of this poisoning can range from a short-term and inconvenient illness to permanent damage, memory loss, and death.

Because coastal areas often attract tourists and tourists often want to sample the local seafood, an outbreak of poisoning by a marine toxin can affect the local economy. For example, in 1987 there was an outbreak of amnesic shellfish poisoning on Prince Edward Island, Canada, which sickened more than 100 people and caused several deaths. In the years following the outbreak, fear over consumption of seafood and a lingering perception that the coast of the province was dangerous caused a marked drop in visitors. This adversely affected the island's economy, which heavily relies on summer tourism.

Periodic outbreaks involving larger numbers of people also occur. In fact, studies by the Woods Hole Oceanographic Institution and the U.S. National Oceanographic and Atmospheric Administration indicate that the frequency of algal blooms has been increasing along the coasts of the United States and other countries since the 1970s. Although the cause of the increased number of blooms is not absolutely certain, a general consensus among scientists is that the documented warming of the coastal oceans has made conditions more favorable for algal growth. If so, a consequence of global warming could be more algal blooms and more cases of marine toxin-related illness.

Scientists urge governments to use better and more frequent monitoring and detection systems for red tides. Although the annual incidence of death and serious illnesses in the United Sates due to fish and shellfish poisoning is relatively low, new toxins appear periodically that make it more difficult to protect the environment and human health.

See Also Coral Reefs and Corals; Oceans and Seas.

BIBLIOGRAPHY

Books

Okaichi, Tomotoshi, ed. Red Tides. Dordrecht, Netherlands: Kluwer, 2004.

Subba Rao, D.V. Algal Cultures, Analogues of Blooms and Applications. Enfield, NH: Science Publishers, 2006.

Web Sites

“Harmful Algae and Red Tides.” Woods Hole Oceanographic Institution, July 5, 2007. <http://www.whoi.edu/page.do?pid=11913> (accessed November 1, 2007).

“Toxic Blooms: Understanding Red Tides.” Fathom (Columbia University) and Woods Hole Oceanographic Institution. <http://www.fathom.com/course/10701012/index.html> (accessed November 1, 2007)

William Arthur Atkins

Red tide

Red tide is the common name for phenomenon created when toxic algal blooms turn seawater red, killing marine life and making water unsuitable for human or animal use. Red tides are caused by several species of dinoflagellates and diatoms, microscopic unicellular phytoplankton that live in cold and warm seas. A red pigment, called peridinin, which collects light during photosynthesis , colors the water red when large numbers of the plankton populate an area.

Red tides are a danger because the toxins released by large numbers of these plankton can paralyze fish and bioaccumulate in the tissues of shellfish and filter-feeding mollusks. Predators of the shellfish, including humans, consume the toxins causing paralysis and death.

Species of dinoflagellates that cause red tides generally belong to the "red tide genera," Gymnodinium and Gonyaulax. Though several are known to cause red tides, a single red tide is nearly always tied to a specific species. This may be the result of three factors. First, the conditions responsible for the bloom may also cause the red tide species to reproduce more rapidly than other phytoplankton, thus outcompeting them for available nutrients. Second, the toxins excreted may prevent the growth of other species. Or, behavioral differences may give them competitive advantages.

Red tides have occurred periodically through recorded history in seas around the world. The Red Sea is thought to have been named for algal blooms back in biblical times. However, in recent decades red tides have been more frequent in areas where algal blooms have never been a problem.

The coast of Chile has reported red tides since the beginning of the nineteenth century, and the Peruvian coast has recorded red tides, which they named aguajes, since 1828. They appear more frequently in the summer months when the water becomes warmer, and they also seem to be associated with the El Ni ño current. Coastal up-welling events are also known to be essential for the occurrence of Gonyaulax tamarensis blooms in the Gulf of Maine.

Red tides have been plaguing the east and west coasts of Florida on a nearly annual basis since at least 1947. Blooms on the west Florida shelf are initiated by the Loop Current, an annual intrusion of oceanic waters into the Gulf of Mexico. Here, the toxic dinoflagellate Gymnodinium breve produces rather predictable blooms. The red tide is transported inshore by winds, tides, and other currents where it may be sustained if adequate nutrients are available.

In 1987 and 1988, red tide spread northward from the Gulf Coast of Florida as far as North Carolina, where it caused a loss of $25 million to the shellfish industry due to brevetoxin contamination. This was the first time that G. breve blooms were reported so far north, and it raises the question of whether blooms will occur annually in this region. Also during the 1987–88 blooms, 700 bottlenose dolphins washed ashore along the United States Atlantic coast. Red tides commonly occur along the northwest coast of British Columbia, Canada, and as far north as Alaska and along the Russian coast of the Bering Sea.

Researchers blame the recent spread of red tides and the occurrence of algal blooms, which ordinarily occur in harmless low concentrations, on the increasing amount of coastal water pollution . Sewage and agricultural runoff increase concentrations of nutrients, such as nitrogen and phosphorus , in coastal waters. This increase in nutrients may create favorable conditions for the growth of red tide organisms.

In April 1992, two fishermen suffered paralytic shellfish poisoning (PSP) within several minutes of eating a few butter clams from Kingcome Inlet, British Columbia. In August 1992, saxitoxin, another dinoflagellate toxin associated with PSP, was found in the digestive systems of dungeness crabs caught in Alaskan waters. On Prince Edward Island, Canada, in 1987, three people died and more than 100 others became ill from domoic acid-contaminated mussels. Some of those who became ill from this toxin developed amnesic shellfish poisoning, which still causes short-term memory loss.

It is difficult to accurately predict when and where red tides may occur, and how seriously they will contaminate shellfish. Monitoring levels of toxins in shellfish meat and population sizes of the dinoflagellates is important in preventing widespread poisoning of humans. Any detectable level of brevetoxin in 3.5 oz (100 g) of shellfish meat is potentially harmful to humans. The U.S. Food and Drug Administration (FDA) does not inspect shellfish growing areas regularly, but relies on state agencies to monitor toxin levels. In selected states, surveillance programs have been established that randomly examine shellfish that have been harvested in natural shellfish beds. Florida is the only state that has a constant monitoring and research program for both shellfish poisoning and dinoflagellate blooms that cause poisoning. Monitoring for shellfish poisoning is costly, but the expense may increase if red tides become more frequent due to deteriorating coastal water quality . In April and May of 1996, 158 manatees in southwest Florida died from an unusual type of red tide. In China from 1997 until 1999, there were 45 red tides which wiped out 75% of the entire stock of Hong Kong's fish farms. This was an estimated $240 million loss to Hong Kong's ecomonic system.

See also Agricultural pollution; Bioaccumulation; Competition; Marine pollution; Sewage treatment

[William G. Ambrose and Paul E. Renaud ]

RESOURCES

BOOKS

Taylor, D., and H. Seliger, eds. Toxic Dinoflagellate Blooms. Vol. 1. New York: Elsevier North Holland, 1979.

PERIODICALS

Culotta, E. "Red Menace in the World's Oceans." Science 257 (1992): 1476–1477.

Konovalova, G. V. "Harmful Dinoflagellate Blooms Along the Eastern Coast of Kamchatka." Harmful Algae News 4 (1993): 2.

Taylor, F. J. R. "Artificial Respiration Saves Two From Fatal PSP in Canada." Harmful Algae News 3 (1992): 1.

Red Tide

Marine toxins and their effects

Resources

A red tides is a phenomenon in which water is colored red, brown, or yellowish because of the temporary abundance of a particular species of phytoplankton. In the majority of cases red tides are caused by dinoflagellates, single-celled algae of the class Dinophyceae that move using tail-like structures called flagella. These organisms photosynthesize accessory photosynthetic pigments that absorb all but the red and yellow wavelengths of light discolor the water during blooms. Dinoflagellates are common and widespread. Under appropriate environmental conditions, certain species of dinoflagellates can grow very rapidly, or bloom, causing red tides. Red tides occur in temperate and tropical marine and estuarine waters.

The environmental conditions that cause red tides are a combination of nutrient availability and water temperature. Red tides have existed for thousands of years and were likely documented in the Bible. However, it is suspected that human activities that affect nutrient concentrations in water influence an increase in the occurrence of red tides in some areas. In particular, the levels of nitrogen, phosphorous, and other nutrients in coastal waters due to runoff from fertilizers and animal waste have an impact of the frequency of red tides. Global changes in climate also may affect red tides. Water used as ballast in oceangoing ships may introduce red tide causing phytoplankton to new habitats.

Some of the phytoplankton involved with red tides synthesize toxic chemicals. Genera that are commonly associated with harmful red tides include Alexandrium, Dinophysis, and Ptychodiscus. The algal toxins can accumulate in marine organisms that feed by filtering large volumes of water, such as clams, oysters, and mussels. If these shellfish are collected while contaminated by red-tide toxins, they can poison the human beings who eat them. Marine toxins can also affect local ecosystems by poisoning animals. Some toxins, such as that from Ptychodiscus brevis, the organism that causes Florida red tides, are airborne and can cause throat and nose irritations.

Red tides can cause ecological damage when the algal bloom collapses. Under some conditions, so much oxygen is consumed to support the decomposition of dead algal biomass that anoxic conditions develop. This can harm or kill animals that are intolerant of low-oxygen conditions. Some red-tide algae can also clog or irritate the gills of fish.

Marine toxins and their effects

Saxitoxin is a natural but potent neurotoxin that is synthesized by certain species of marine dinoflagellates. Saxitoxin causes paralytic shellfish poisoning, a toxic syndrome that affects humans who consume contaminated shellfish. Other biochemicals synthesized by dinoflagellates are responsible for diarrhetic shellfish poisoning. Some red tide dinoflagellates produce reactive forms of oxygen—superoxide, hydrogen peroxide, and hydroxyl radical—which may be toxic. Diatoms in the genus Nitzchia synthesize domoic acid, a chemical responsible for amnesic shellfish poisoning in humans.

Paralytic, diarrhetic, and amnesic shellfish poisoning can make large numbers of people ill and can cause death in cases of extreme exposure or sensitivity. Because of the risks of poisoning associated with eating marine shellfish at time of the year when harmful algal blooms are common, many countries routinely monitor the toxicity of these foods.

Marine animals can also be poisoned by toxic chemicals synthesized during blooms. For example, in 1991 a bloom in Monterey Bay, California, of the diatom Nitzchia occidentalis resulted in the accumulation of domoic acid in filter-feeding zooplankton. These small animals were eaten by small fish, which also accumulated the toxic chemical and then

KEY TERMS

Bloom— An event in which a species of phytoplankton grow quickly and reach high concentrations so that the water is distinctly colored by the algal pigments.

poisoned fish-eating cormorants and pelicans that died in large numbers. In addition, some humans who ate shellfish contaminated by domoic acid were made ill.

In another case, a 1988 bloom of the planktonic alga Chrysochromulina polylepis in the Baltic Sea caused extensive mortalities of various species of seaweeds, invertebrates, and fish. A bloom in 1991 of a closely related species of alga in Norwegian waters killed large numbers of salmon that were kept in aquaculture cages.

In 1996, a red tide killed 149 endangered manatees (Trichechus manatus latirostris) in the coastal waters of Florida.

In 1985, 14 humpback whales (Megaptera novaeangliae) died in Cape Cod Bay, Massachusetts, during a five-week period. This unusual mortality was caused by the whales eating mackerel (Scomber scombrus) that were contaminated by saxitoxin synthesized during a dinoflagellate bloom. In one observed death, a whale was seen to be behaving in an apparently normal fashion, but only 90 minutes later it had died. The symptoms of the whale deaths were typical of the mammalian neurotoxicity that is associated with saxitoxin, and fish collected in the area had high concentrations of this poisonous chemical in their bodies.

See also Plankton; Poisons and toxins.

Resources

BOOKS

Okaichi, Tomotoshi. Red Tides (Ocean Sciences Research). Tokyo, Japan: Terra Scientific Publishing Company, 2004.

PERIODICALS

Fleming, Lora E., Lorraine C. Backer, and Daniel G. Baden. “Overview of aerosolized Florida red tide toxins: exposures and effects.” Environmental Health Perspectives. 113 (2005): 618-620.

OTHER

Woods Hole Oceanographic Institute. “The Harmful Algae Page.” August 8, 2006. <http://www.whoi.edu/redtide/> (accessed October 30, 2006).

Bill Freedman

Red tide

Red tides are a marine phenomenon in which water is stained a red, brown, or yellowish color because of the temporary abundance of a particular species of pigmented dinoflagellates (these events are known as "blooms"). Also called phytoplankton , or planktonic algae , these single-celled organisms of the class Dinophyceae move using a tail-like structure called a flagellum. They also photosynthesize and it is their photosynthetic pigments that can tint the water during blooms. Dinoflagellates are common and widespread. Under appropriate environmental conditions, various species can grow very rapidly, causing red tides. Red tides occur in all marine regions with a temperate or warmer climate.

The environmental conditions that cause red tides to develop are not yet understood. However, they are likely related to some combination of nutrient availability, nutrient ratios, and water temperature . Red tides are ancient phenomena and were, for example, recorded in the Bible. However, it is suspected that human activities that affect nutrient concentrations in seawater may be having an important influence on the increasingly more frequent occurrences of red tides in some areas. In particular, the levels of nitrogen , phosphorous, and other nutrients in coastal waters are increasing due to runoff from fertilizers and animal waste. Complex global changes in climate also may be affecting red tides. Water used as ballast in ocean-going ships may be introducing dinoflagellates to new waters.

Sometimes the dinoflagellates involved with red tides synthesize toxic chemicals. Genera that are commonly associated with poisonous red tides are Alexandrium, Dinophysis, and Ptychodiscus. The algal poisons can accumulate in marine organisms that feed by filtering large volumes of water, for example, shellfish such as clams, oysters, and mussels. If these shellfish are collected while they are significantly contaminated by red-tide toxins, they can poison the human beings who eat them. Marine toxins can also affect local ecosystems by poisoning animals. Some toxins, such as that from Ptychodiscus brevis, the organism that causes Florida red tides, are airborne and can cause throat and nose irritations.

Red tides can cause ecological damage when the algal bloom collapses. Under some conditions, so much oxygen is consumed to support the decomposition of dead algal biomass that anoxic conditions develop. This can cause severe stress or mortality in a wide range of organisms that are intolerant of low-oxygen conditions. Some red-tide algae can also clog or irritate the gills of fish and can cause stress or mortality by this physical effect.

Marine toxins and their effects

Saxitoxin is a natural but potent neurotoxin that is synthesized by certain species of marine dinoflagellates. Saxitoxin causes paralytic shellfish poisoning, a toxic syndrome that affects humans who consume contaminated shellfish. Other biochemicals synthesized by dinoflagellates are responsible for diarrhetic shellfish poisoning, another toxic syndrome. Some red tide dinoflagellates produce reactive forms of oxygen—superoxide, hydrogen peroxide , and hydroxyl radical—which may be responsible for toxic effects. A few other types of marine algae also produce toxic chemicals. Diatoms in the genus Nitzchia synthesize domoic acid, a chemical responsible for amnesic shellfish poisoning in humans.

Paralytic, diarrhetic, and amnesic shellfish poisoning all have the capability of making large numbers of people ill and can cause death in cases of extreme exposure or sensitivity. Because of the risks of poisoning associated with eating marine shellfish, many countries routinely monitor the toxicity of these foods using various sorts of assays. One commonly used bioassay involves the injection of laboratory mice with an extract of shellfish. If the mice develop diagnostic symptoms of poisoning, this is an indication of contamination of the shellfish by a marine toxin. However, the mouse bioassay is increasingly being replaced by more accurate methods of determining the presence and concentration of marine toxins using analytical biochemistry . The analytical methods are generally more reliable and are much kinder to mice.

Marine animals can also be poisoned by toxic chemicals synthesized during blooms. For example, in 1991 a bloom in Monterey Bay, California, of the diatom Nitzchia occidentalis resulted in the accumulation of domoic acid in filter-feeding zooplankton . These small animals were eaten by small fish, which also accumulated the toxic chemical and then poisoned fish-eating cormorants and pelicans that died in large numbers. In addition, some humans who ate shellfish contaminated by domoic acid were made ill.

In another case, a 1988 bloom of the planktonic alga Chrysochromulina polylepis in the Baltic Sea caused extensive mortalities of various species of seaweeds, invertebrates , and fish. A bloom in 1991 of a closely related species of alga in Norwegian waters killed large numbers of salmon that were kept in aquaculture cages. In 1996, a red tide killed 149 endangered manatees (Trichechus manatus latirostris) in the coastal waters of Florida.

Even large whales can be poisoned by algal toxins. In 1985, 14 humpback whales (Megaptera novaeangliae) died in Cape Cod Bay, Massachusetts, during a five-week period. This unusual mortality was caused by the whales eating mackerel (Scomber scombrus) that were contaminated by saxitoxin synthesized during a dinoflagellate bloom. In one observed death, a whale was seen to be behaving in an apparently normal fashion, but only 90 minutes later it had died. The symptoms of the whale deaths were typical of the mammalian neurotoxicity that is associated with saxitoxin, and fish collected in the area had large concentrations of this very poisonous chemical in their bodies.

See also Plankton; Poisons and toxins.

Resources

books

Freedman, B. Environmental Ecology. 2nd ed. New York: Academic Press, 1994.

Okaichi, T., D. M. Anderson, and T. Nemoto, eds. Red Tides:Biology, Environmental Science, and Toxicology. New York: Elsevier, 1989.

Bill Freedman

KEY TERMS

Bloom

—An event of great abundance of phytoplankton, to the degree that the water is distinctly colored by the algal pigments.