They are great gardens of the sea, gorgeous and rich with life. Algal blooms can be green or brown or golden and even luminescent. Composed of tiny plants called algae, which can provide an important food source to marine life, they erupt mysteriously into enormous plumes that can stretch for hundreds of miles along coastal or estuarine waters.

They can also be deadly - deadly to marine life and to humans.Sometimes called red tides, these harmful algal blooms contain single-celled plants called phytoplankton ) some species of which are virulently toxic.

While humans have been aware of red tides since biblical times - one may have been the plague of blood mentioned in Exodus - they were relatively rare phenomena. In recent years, however, they have been proliferating so rapidly that some scientists warn that an ''epidemic'' of algal blooms now threatens marine ecosystems on a global scale.

Dr. Paul Epstein of the Harvard Medical School, who has extensively studied the phenomenon, notes that the intensity, duration and extent of harmful algal blooms is increasing and so is their toxicity. Unless they can be prevented, these toxic sea-gardens pose a growing threat to marine life, the economy of coastal communities, and the health and well-being of people around the world.

The poisons released by the tiny organisms that make up red tides are lethal to a wide range of species, from the smallest to the biggest. In 1987, for example, 14 humpback whales died in Cape Cod Bay shortly after ingesting large amounts of toxic algae.

That same year there were two episodes of large scale poisoning of people in North Carolina and Canada who were directly exposed to the toxic plants or had eaten shellfish contaminated by them.

Over the past decade, the poisons from these algal blooms have caused sickness and death to thousands of people throughout the world. And there is a growing body of evidence to suggest that neurotoxins released by some strains of the phytoplankton have resulted in temporary loss of memory and other cognitive disabilities, including diminution of speech and reading levels, to those who have been exposed to them.

Even when they do not contain toxic organisms, particularly large algal blooms can kill thousands or even millions of fish by using up the available oxygen in the water.

Like many other ''natural'' disasters, red tides are exacerbated by human activity. Increased nutrient levels in coastal waters resulting from sewage and agricultural runoff create a fertile environment for the rapid reproduction of the tiny plants.

Chemical pollution can alter the balance of marine ecosystems in ways that create ecological openings for the spread of the blooms. Acid rain from auto and truck exhaust and the burning of coal and oil by utilities and industry deposit huge amounts of nitrogen and other nutrients into ocean waters that are favorable to the growth of algae.

The massive loss of wetland habitat to development, industrial activity and pollution is also encouraging the growth of the ocean flora. Eelgrass beds, salt marshes, mangroves and other coastal habitats filter out pollutants and nutrients such as nitrogen and phosphorus.

These habitats also provide fish with nursery areas, feeding grounds and refuge from predators. The loss of these habitats leave fish and other marine life more vulnerable to the blooms.

Finally, there is growing consensus among scientists that global warming caused by human activity is encouraging the spread of algal blooms by raising the temperature of the oceans and thereby creating an environment more favorable to plant reproduction and growth. Dr. Epstein and others have noted that the increase of algal blooms may be the first biological evidence that climate change is under way.

Stronger steps must be taken by the United States and other nations to prevent the further spread of toxic algal blooms, along with other pollutants that represent a threat to the marine environment and human health. Policies to control the flow of sewage, agricultural runoff and other contaminants into coastal waters must be strengthened and enforced. More aggressive air emission standards are needed to further reduce levels of acid deposition, and far stronger policies should be enacted to preserve our dwindling wetlands and reduce overfishing in both domestic and international waters.

Finally, the international community needs to take decisive action to address the causes of global warming.

These changes will not be easy, and are likely to require years of dedicated effort. In the meantime, an early warning system needs to be established that includes increased satellite surveillance of the spread of toxic algal blooms and a rapid public health response to protect fishermen, coastal residents and seafood eaters from their dangers.

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