PROVIDENCE, R.I. -- Guiding a 19-foot outboard across Narragansett Bay, John Torgan recalls how the deceptively clear water once harbored an aquatic jungle that could tangle the propeller of an unwary boater.
"As a little kid, I used to have to reverse the engine to kick the eelgrass out," says Torgan, 28, the baykeeper for Save the Bay, an environmental group here. "Now we search for it, squinting."
The eelgrass is gone -- all but a few scattered patches. And not coincidentally, so are the tasty scallops for which this bay used to be famous.
Stunted sea grasses and faded fisheries are just two of the symptoms of a subtle but growing environmental malady afflicting coastal areas around the nation and the world, discoloring once-sparkling waters and slowly robbing them of vitality.
From Narragansett to Chesapeake Bay and the Gulf of Mexico in this country, from the Baltic to the Black Sea in Europe, and along far-flung shores across Asia, waters are suffocating under a deluge of nutrients produced by human activities.
"It's an epidemic," says Donald Scavia, who oversees coastal ocean programs for the National Oceanic and Atmospheric Administration (NOAA). Preliminary results from a five-year survey find water quality woes related to nutrient pollution in more than half of U.S. bays and sounds, he says.
Scientists have a term for it: eutrophication. Derived from a Greek word meaning "well nourished," in this case it means overfed.
Nitrogen and phosphorus, the essential ingredients in plant fertilizer, are fueling runaway growths of algae in bays and poorly flushed coastal areas. These vast "blooms" of floating microscopic plants can turn the water green, brown or mahogany, and make it foul-smelling.
Algae overgrowth blocks out sunlight needed by aquatic vegetation like eelgrass. It also depletes oxygen in the water, contributing to declines in fish and shellfish. The NOAA survey found that about 30 percent of coastal areas sometimes have deep waters so devoid of oxygen they become virtual "dead zones" for shellfish and other bottom-dwelling marine animals.
As if that isn't bad enough, many researchers believe nutrients may be linked to outbreaks of toxic algae and microorganisms such as Pfiesteria piscicida, blamed for fish kills in the Chesapeake and elsewhere along the mid-Atlantic coast.
"The nutrient enrichment of the coastal oceans is happening everywhere," says Scott W. Nixon, an oceanographer at the University of Rhode Island here. He calls it "fertilization of the sea," and adds: "It looks like it's not going to stop."
The sources of the nutrients affecting coastal waters are the same, though they vary in importance and impact from place to place. Human and animal waste, commercial fertilizer and the byproducts of burning fossil fuels are rich in nitrogen or phosphorus, or both. All are getting in the water.
In Narragansett Bay, nutrient-laden wastewater is the chief cul- prit. It can turn the water brown with algae in spring and create pockets of oxygen-starved water by late summer.
Runoff of fertilizer and animal waste is the main problem in other areas like the Chesapeake Bay, the lower Neuse River in North Carolina and the massive 7,000-square-mile "dead zone" that forms in the spring in the Gulf of Mexico.
A century ago, ecologists first noted how nutrients were stimulating algae growth in the Baltic and North seas. But for decades scientists thought marine waters couldn't be overfertilized.
Only in the last 30 years have researchers focused on the harmful effects of nutrient enrichment of coastal waters. Over that same time, the amount of commercial fertilizer used on farms in the United States and the developed world has soared, as have emissions from coal-burning power plants.
Farm animal populations also have swelled, with chickens doubling worldwide since 1970, according to the World Resources Institute.
Those trends, which reflect increasing food production and a rising standard of living, are releasing more nitrogen and phosphorus into the environment, ecologists say. Human activities have essentially doubled the rate at which natural processes make nitrogen available for plant growth.
Even before Pfiesteria became a household word in Maryland this year, a panel of eight scientists with the Ecological Society of America issued a report warning of "serious environmental consequences" from the growing release of nitrogen -- more troubling than phosphorus because of its ability to travel long distances in the air and water.
"It's fairly clear we're losing biodiversity from heavily nitrogen-enriched areas," says Cornell University biologist Robert W. Howarth, one of the report's authors. So saturated are some parts of Europe that evergreens turn yellow and lose their needles, and the variety of plants dwindles on grasslands engorged with the nutrient.
"It's a wake-up call for what will happen here if we don't pay attention," Howarth says.
Experts say there are no easy remedies.