TOLEDO, Ohio — For those who live and play on the shores of Lake Erie, the spring rains that will begin falling here soon are less a blessing than a portent. They could threaten the very future of the lake itself.

Lake Erie is sick. A thick and growing coat of toxic algae 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. The private forecaster Accuweather predicts a wetter than usual March and April throughout the region.

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 then, thanks to a multibillion-dollar cleanup by the United States and Canada that became a legendary environmental success story.