Along the sugar-sand beaches of Florida's Panhandle, the bodies of 115 dead dolphins wash up. In California's Monterey Bay, more than 400 sea lions die after agonizing seizures.
Maryland's Eastern Shore, people who find themselves surrounded by dying fish develop strange symptoms: rashes, breathing trouble and memory lapses.
These scary episodes can be traced to an emerging family of seaborne pests known to scientists as harmful algae blooms. Their outbreaks are in a worldwide upsurge. They force closings of shellfish beds; kill fish, seabirds and marine mammals; and bring on human sickness -- and, in rare cases, death.
In 1997, after a 450-mile-long algae bloom ruined summer along parts of the Baltic Sea coast from Poland to Finland, thousands of Finns signed petitions demanding a government promise that the outbreaks would never happen again. "People wanted to turn it off, like a faucet," says Tapani Kohonen, former director of an international commission that oversees Baltic water quality. "But of course no one can promise that."
Scientists don't know how to stop the blooms. In most cases, they're not sure how they start. And debate often rages over whether nutrient pollution plays a role in any individual outbreak.
But most top experts suspect that today's excesses of nitrogen and phosphorus in coastal waters play a key role in the worldwide toxic algae epidemic.
"There are few other causes, other than climate change, that could be responsible for such widespread increases," wrote a panel of scientists headed by University of Maryland ecologist Donald Boesch.
Algae need fertilizer to grow, so it makes sense that more nutrients would trigger more blooms, some of which could be harmful. But the natural world is seldom simple, and so far, "there are few definitive data that really allow us to say that there's a universal relationship between harmful algal blooms and increased nutrients," says Kevin Sellner of the National Oceanic and Atmospheric Administration.
The health threat posed by poisonous algae has been known since about A.D. 100, when the Japanese observed that people were dying from eating the flesh of the puffer fish, which accumulates toxic algae called fugu. The poison gave the fish its name in Japan, where it is prized as a potentially deadly delicacy.
For about 1,800 years, only three toxic algae were known to exist worldwide, according to marine toxicologist Daniel Baden, director of marine sciences at the University of North Carolina, Wilmington. But in the past 50 years, about 50 new species of dangerous algae have been discovered, with reports of toxic outbreaks doubling in the past 30 years.
The blooms now threaten virtually every coastal state, says Donald Anderson, a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts and the director of a nationwide initiative to track marine toxins. Between 1988 and 1997, toxic algae blooms cost the U.S. economy roughly $1 billion in lost seafood sales.
In Texas right now, oyster harvesting is banned in Galveston Bay because of a red tide spreading along the coast, the region's biggest such bloom since 1986.
The United States' most troublesome algae-borne illness is paralytic shellfish poisoning, caused by a group of algae found off Northern California, the Pacific Northwest and New England. The poison has forced Alaskan officials to forbid shellfish harvesting. Almost every year, some Alaskans ignore the ban and die. But these outbreaks happen in relatively clean waters, and there's no obvious link to nutrient pollution, Anderson says.
Sometimes the link between harmful algae blooms and nutrients is strong. Pfiesteria piscicida, the microorganism responsible for 1997's fish kills and human illnesses on some Lower Eastern Shore rivers, is a prime example. In lab tests, Pfiesteria piscicida multiplies rapidly in nutrient-rich water.
In other situations, the cause-and-effect relationship is indirect. Some algae blooms, such as Florida's red tide, might occur naturally but last longer or grow larger because of nutrient pollution from land.
Red tides have been reported occasionally along the southwestern Florida shore since the days of the Spanish explorers. In the past 50 years, they have appeared about 40 times, and some outbreaks last a year or more.
The algae release a nerve poison that kills fish, birds and marine mammals and can leave people gasping for breath. Florida was hard-hit last year with three outbreaks suspected of killing dolphins, sea turtles and manatees. And red tides have spread to the Carolinas and the entire Gulf of Mexico coast.
The blooms that crop up in the Baltic Sea turn its waters the opaque yellow-green of pea soup. After the disastrous blooms of 1997, when the algae grew so thick that sailors mistook large clumps for rocks, Finland's largest newspaper began including a toxic algae forecast in its summer weather reports.
The algae attack the liver and cause potentially fatal hemorrhaging. "It seems as if increasing nutrients give us more toxic strains, but we don't understand why," says Kirste Lahti, a microbiologist with the Finnish Environmental Institute.
Some scientists suspect that nutrients might merely set the stage for other human-generated changes. In the Bay of Bengal near Bangladesh, National Science Foundation Director Rita Colwell has tracked a clear correlation between higher-than-normal water temperatures and outbreaks of deadly cholera, a disease linked to algae blooms. Colwell believes high nutrient levels merely load the gun; it's warm water that pulls the trigger.
Ted Smayda, an oceanographer at the University of Rhode Island, says he's convinced that the link between nutrients and harmful algae is as strong as the smoking-and-cancer link. But Smayda says research into harmful algae blooms is in its infancy and proving the connection "is like looking for a needle in a haystack."