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High Tide for Red Tide
A toxic strain of red tide in the Strait of Magellan, located off the coast of Punta Arenas, Chile, has led to 15 deaths and hundreds of hospitalizations, and has virtually shut down the once booming commercial fishing industry and the unrestricted harvesting of shellfish in the region. According to scientists, the Chilean experience is far from unique. Over the last two decades, red tides have been increasing worldwide. The phenomenon, can devastate a country's fishing and tourism industries.
Red tides result from a massive buildup of certain species of the microscopic sea organisms known as phytoplankton. Some of these tiny plankton organisms produce compounds that are toxic to fish that feed on them. In addition, the toxins accumulate in filter-feeding shellfish, such as clams, mussels, and oysters. The shellfish themselves are rarely affected by the toxins, but the poison can persist in their tissue for years and can be passed along to people who eat the contaminated shellfish. Of the more than one thousand species of phytoplankton, only about two dozen are toxic. Heavy concentrations of both toxic and nontoxic phytoplankton blooms can lend a reddish-brown tint to the surrounding water, but contamination can occur even without visible discoloration.
Different species of phytoplankton produce different types and combinations of toxins that build up in shellfish. People who eat the contaminated seafood can fall victim to various kinds of shellfish poisoning, depending on which poisons have accumulated. The most common types of shellfish poisoning are diarrhetic, paralytic, and neurotoxic. Diarrhetic poisoning affects the digestive system, causing diarrhea, vomiting, and nausea. Paralytic and neurotoxic types of poisoning attack the nervous system, causing a tingling or numbing sensation around the lips and tongue. At high levels, the paralytic toxins can lead to respiratory failure and death. The casualties of Chile's red tide suffered acute cases of paralytic shellfish poisoning.
Red tides, which usually bloom in spring and summer and can last from a few hours to several months, have been known for centuries. In recent years, however, the phenomenon, especially the toxic kind, has spread rapidly to harbors around the world. Since the 1970s, toxic red tides have been reported off the coasts of the United States, Sweden, Norway, Spain, Japan, and China. The tides are showing up in places where previously they had never been reported, and new types of organisms are being identified. Scientists do not know exactly what causes red tides, but they think rising pollution levels, natural changes in the environment, and climate may play a role in the increased incidence. Some scientists, however, caution against assuming the existence of a global epidemic of red tides. These scientists note that the apparent increase could be the result of using technologically advanced monitoring equipment and increased scientific awareness.
Scientists do know that red tide blooms occur in nutrient-rich waters that receive abundant sunlight. Under these conditions, phytoplankton rapidly multiply through a process of asexual reproduction. Many of the toxic species of phytoplankton depend on the nutrients nitrogen and phosphorous for growth. Wastewater runoff and other forms of coastal pollution contain an abundance of both elements and other nutrients favorable to plant growth. Although scientists have not proved conclusively that the increasing number of red tides worldwide is directly linked to rising pollution levels, studies strongly suggest a relationship.
In the absence of sunlight and nutrients, some species of phytoplankton are able to become thick-walled, dormant cells, known as cysts, that can survive for years on the ocean floor. When favorable growth conditions return, these dormant cells become swimming cells that can take advantage of the new conditions to produce a bloom. According to scientists, cysts account for the recurring and often perennial nature of red tides. A huge toxic red tide that stretched from the coast of Maine to Massachusetts appeared for the first time following a 1972 hurricane. Every year since, red tides have reappeared in the region.
Scientists believe that cysts, stirred up and swept along by ocean currents, are one of the ways by which red tides can appear in previously unaffected waters. In addition, as blooms of phytoplankton move with currents, they can leave a trail of cysts that will eventually bloom under favorable conditions. Ships carrying phytoplankton or cysts in their ballast water can introduce the organisms to a new region when they empty their ballast tanks at ports.
The amount of destruction caused by toxic red tides varies. In industrialized countries, where monitoring systems are more advanced, it is rare for human beings to become sick or die from shellfish poisoning. Instances of people suffering from illness and death as a result of toxic blooms are much higher in developing countries, especially those with long coastlines where seafood is a dietary staple.
Industrialized countries, however, are not immune to the consequences of red tides. Piles of fish killed by a toxic strain of red tide in the Gulf of Mexico have been washing up on Florida's shores for more than a century. The massive fish kills have caused millions of dollars worth of damage to the state's tourism industry. In 1987, 14 dead humpback whales washed ashore in Cape Cod Bay, Massachusetts. Scientists discovered that the whales had died as a result of feeding on mackerel that had ingested marine organisms that had, in turn, eaten a toxic strain of phytoplankton. The toxins became more concentrated at each step up the food chain, until they reached lethal levels in the humpbacks.
While industrialized countries are adept at monitoring red tides, they have not yet discovered a way to neutralize the harmful effects or to eradicate the toxic algae blooms. In addition, many scientists view the blooms as a natural part of the ocean environment and are reluctant to interfere with a phenomenon whose role in the ecosystem remains unclear. Countries around the world are forced to wait for the blooms to disappear and hope that, in the meantime, people heed official warnings against harvesting shellfish in affected areas.
Source: Encarta Yearbook, August 1995.
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