"TOLEDO, Ohio — It took a serendipitous slug of toxins and the loss of drinking water for a half-million residents to bring home what scientists and government officials in this part of the country have been saying for years: Lake Erie is in trouble, and getting worse by the year.

Flooded by tides of phosphorus washed from fertilized farms, cattle feedlots and leaky septic systems, the most intensely developed of the Great Lakes is increasingly being choked each summer by thick mats of algae, much of it poisonous. What plagues Toledo and, experts say, potentially all 11 million lakeside residents, is increasingly a serious problem across the United States.

But while there is talk of action — and particularly in Ohio, real action — there also is widespread agreement that efforts to address the problem have fallen woefully short. And the troubles are not restricted to the Great Lakes. Poisonous algae are found in polluted inland lakes from Minnesota to Nebraska to California, and even in the glacial-era kettle ponds of Cape Cod in Massachusetts.

When Mayor D. Michael Collins told Toledo residents on Monday that it was again safe to use the city’s water, he was only replaying a scene from years past. Carroll Township, another lakefront Ohio community of 2,000 residents, suspended water use last September amid the second-largest algae bloom ever measured; the largest, which stretched 120 miles from Toledo to Cleveland, was in 2011. Summertime bans on swimming and other recreational activities are so routine that the Ohio Environmental Protection Agency maintains a website on harmful algae bloom.

Five years ago this month, the federal Environmental Protection Agency and state water authorities issued a joint report on pollution of the nation’s waterways by phosphorus and other nutrients titled “An Urgent Call to Action.”

“Unfortunately, very little action has come from that,” said Jon Devine, the senior lawyer for the water program at the Natural Resources Defense Council in Washington.

“When we bring this subject up for conversation with the regulators, everyone sort of walks out of the room,” Donald Moline, the Toledo commissioner of public utilities, said in an interview on Monday. “The whole drinking-water community has been raising these issues, and so far we haven’t seen a viable response.”

Lake Erie’s travails — and now, Toledo’s — are but the most visible manifestation of a pollution problem that has grown as easily as it has defied solution. Once the shining success of the environmental movement — Lake Erie was mocked as dead in the 1960s, then revived by clean-water rules — it has sunk into crisis again as urbanization and industrial agriculture have spawned new and potent sources of phosphorus runoff."