The bright, blue-green film piling up in the canal in Jason Pim’s backyard is the first thing he wakes up thinking about and the last thing before his head hits the pillow.
The pungent odor of the algae, which has traveled dozens of miles from Lake Okeechobee, Florida’s largest freshwater lake, is hard to describe. But Mr. Pim, 37, said the smell is like opening a bag of moldy bread.
“To just watch it in the water and kind of just lap up and down with the water is a pretty eerie sight,” he said on Sunday. “It’s kind of mesmerizing.”
An unusually large algae bloom has filled the lake with a pea soup-like mixture that has built up because of rain, hot weather and a heavy concentration of phosphorus and nitrogen from fertilizer, Richard P. Stumpf, an oceanographer with the National Oceanic and Atmospheric Administration, said.
The bloom has covered about 90 percent of the lake, which has a surface area of 730 square miles, an extent that some officials said they’ve never seen before and that raises concerns about health risks of exposure and the impact on the economy.
Mr. Stumpf warned that if people come in contact with the algae, they should wash it off to avoid irritation to their skin.
The algae in Lake Okeechobee, while it has no effect on fish, can be toxic if consumed, he said. It can also be fatal for dogs that swim in or drink the scum, the foamlike substance that forms near the surface, he said.
The scene at Lake Okeechobee, which is used mainly for commercial and sport fishing, is one of several across the United States that scientists are monitoring.
There’s a perception that algae blooms are becoming more frequent and severe but federal agencies, including NOAA and the United States Geological Survey, are doing research to determine whether that is the case, Mr. Stumpf said.
The Finger Lakes in New York and Utah Lake in Utah, both of which are used for recreational purposes, experienced an unusual number of blooms in the past couple of years, Mr. Stumpf said. An algae bloom in 2014 in Lake Erie in Ohio left 500,000 people without drinking water for three days.
Sharon Anderson, environmental team leader at the Cornell Cooperative Extension office in Ithaca, N.Y., said there was significant concern over harmful algae blooms in the Finger Lakes last year.
One of them, Owasco Lake, was plagued by algae blooms last year, and this year “suspicious” blooms have been spotted in Cayuga Lake, she said. Residents and business owners are worried about how the algae could affect tourism, she said.
Mr. Stumpf said Lake Okeechobee, which is about 95 miles northwest of Miami and draws about six million visitors a year, experiences algae blooms annually. The degree to which the bloom has spread this year was unusual.
In a satellite image take on June 12, only 1 percent of the lake was covered in algae. By July 2, 90 percent of the lake was covered.
Lake Okeechobee flows into the 67-mile-long Caloosahatchee River and both play a large role in Florida’s tourism economy.
Florida lawmakers have asked Gov. Rick Scott to declare a state of emergency in Lee County, which the Caloosahatchee River runs through. The governor was scheduled to tour the Caloosahatchee River by boat on Monday morning, his office said.
John Cassani, the head of the nonprofit Calusa Waterkeeper, who has worked on the river for about 40 years, said the bloom was “historic.”
The boatbuilding industry is hurt when water quality is degraded, a point he said he makes when speaking to the community as head of the local chapter of the Waterkeeper Alliance, a global organization focused on clean water. The Centers for Disease Control and Prevention estimatesthat monitoring and managing harmful algae blooms costs states millions of dollars each year.
“I’ve never seen anything like this,” Mr. Cassani said. He has avoided spending too much time near the river because of the increased possibility of respiratory irritation.
Mr. Pim, who volunteers for the Calusa Waterkeeper, said he’s avoided exposing his two daughters, who are 10 and 7, to the water. He pays a friend to take video by drone of the algae and posts the footage on social media to alert his neighbors of the health risks.
He’s grown angrier over the expanding algae and what it could mean for the future of Cape Coral, where he’s spent most of his life.
He said posting videos and photos online could harm the real estate market in the city but he wants to raise awareness about the water quality and persuade residents to contact their elected officials to take action.
“I’ve watched the water quality devolve in my lifetime and I don’t want to stand here and watch it continue in my children’s lifetime,” he said.