TOLEDO, Ohio - Lake Erie is sick. A thick and growing coat of scum appears each summer, so vast that in 2011 it covered a sixth of its waters, contributing to an expanding dead zone on its bottom, reducing fish populations, fouling beaches and crippling a tourism industry that generates more than $10 billion in revenue annually.

The spring rains reliably predict how serious the summer algae bloom will be: The more frequent and heavy the downpours, the worse the outbreak. And this year the National Weather Service says there is a higher probability than elsewhere of above-normal spring rains along the lake's west end, where the algae first appear.

It is perhaps the greatest peril the lake has faced since the 1960s, when relentless and unregulated dumping of sewage and industrial pollutants spawned similar algae blooms and earned it the nickname "North America's Dead Sea." Erie recovered, thanks to a multibillion-dollar cleanup by the United States and Canada.

But while the sewage and pollutants are vastly reduced, the blooms have returned, bigger than ever.

A PHOSPHATE DIET

Once, fisheries and sports anglers pulled 5 million walleye from the rejuvenated lake every year. Today the catch is roughly one-fifth that, the Environmental Protection Agency says. Commercial fisheries' smelt catch is three-fifths of past levels. The number of charter fishing companies has dropped 40 percent. Sport fish like walleye and yellow perch are deserting the lake's center and moving shoreward in search of oxygen and food.

The algae are fed by phosphorus, the same chemical that U.S. and Canadian authorities spent billions to reduce in the 1970s and '80s. This time, new farming techniques, climate change and even a change in Lake Erie's ecosystem make phosphorus pollution more intractable.

Like plants, algae thrive on a phosphorus diet. Decades ago, some 64 million pounds of phosphorus flowed into Lake Erie each year from industrial and sewer outfalls, leaky septic tanks and runoff from fertilized lawns and farms.

The U.S. and Canadian governments responded by capping household detergent phosphates, reining in factory pollutants and spending $8 billion to upgrade sewage plants. Phosphorus level plunged by two thirds, and the algae subsided. But in the mid-1990s, it began creeping back.

"2002 was the last year that we didn't have much of a bloom," said Thomas Bridgeman, a professor at the Lake Erie Center at the University of Toledo. "2008, '09 and '10 were really bad years for algal blooms.

"And then we got 2011."

2011 was the wettest spring on record. That summer's algae bloom, mostly poisonous blue-green algae called Microcystis, produced lake-water concentrations of microcystin, a liver toxin, that were 1,200 times World Health Organization limits, tainting the drinking water for 2.8 million consumers.

THE DEAD ZONE

Dead algae sink to the lake bed, where bacteria that decompose the algae consume most of the oxygen. A dead zone now covers a large portion of the lake bottom in bad years.

"Fish that like to live in cold bottom waters have to move up in the thermocline, where it's too warm for them," said Peter Richards, a senior research scientist. "They get eaten, and that tends to decrease the growth rates of a lot of the fish."

Last spring, the rains arrived amid a record drought, and the algae retreated to waters near Toledo.

But no one hopes for a drought. To cut phosphorus levels this time, scientists say, the habits - and the expensive equipment - of 70,000 farmers near the Erie shore must change, for most of the phosphorus that feeds algae these days comes from farmland.

Much of the phosphorus originates near Toledo, where the Maumee River completes a 137-mile journey and empties into the lake's western basin.

The Maumee watershed is Ohio's breadbasket, two-thirds farmland, mostly corn and soybeans. Farming there is changing radically, said Steve Davis, a watershed specialist with the U.S. Agriculture Department's Natural Resource Conservation Service.

Plowing is declining; 55 percent of farmland is planted using anti-erosion methods promoted by the Resource Conservation Service, like no-till farming, in which seeds are inserted into small holes in unplowed ground. Fertilizing is now contracted to companies that cast pellets onto the bare ground from trucks or to "factory farms" that spray their liquefied animal waste on cropland.