Then in July, it surfaced across lakes in the Pacific Northwest and along the entire Mississippi seacoast, forcing the closure of the state’s beaches.
Now, the green-blue blooms of toxic algae have been found in lakes and ponds in three New York City parks, leading environmental officials to warn residents to keep pets and young children away from the affected water until the blooms clear.
The blooms contain cyanobacteria, single-celled organisms that can produce toxic substances when grown densely. The growth of the bacteria is fueled by warm, sunny days and nutrients from nearby surfaces that can wash into lakes and ponds during intense rain storms.
In Manhattan, the toxic algae has been found in Turtle Pond and Harlem Meer in Central Park, and in the pond in Morningside Park. Officials also found suspicious blooms in The Lake in Central Park earlier this summer, according to the New York State Department of Environmental Conservation website.
In Brooklyn, the toxic blooms have been detected in one part of the large pond in Prospect Park. The dedicated swimming area for dogs, the Prospect Park Dog Beach, remains open.
“It is important to try to avoid contact with any algae,” Meghan Lalor, a spokeswoman from the New York City Department of Parks and Recreation, said in a statement. “Keep pets on leashes and do not allow them to enter or drink from lakes and ponds, unless in areas specifically designated for such activities.”
The bacteria can cause skin rashes and neurological problems in animals, and can release toxins that can cause liver damage and respiratory paralysis. Dogs can collapse or die suddenly after swallowing contaminated water while swimming or licking toxic algae from their fur, according to the Centers for Disease Control and Prevention.
Presence of cyanobacteria was detected during routine testing earlier this month by the environmental conservation and city parks departments. As part of their biweekly sampling of suspicious sites, city officials will test sites on Monday to determine if the toxins are still present, and if any additional toxic blooms have grown.
Urban areas are particularly susceptible to the algae growth because of the variety of nutrients found on nearby roads, sidewalks and sewage pipes, as well as the relatively shallow lakes and ponds, which allow nutrients to mix more easily.
Though small blooms of the toxic algae have been consistently found in New York state lakes and ponds in recent years, scientists have said the intensity of recent rainfall, which has been linked to climate change, may be contributing to this year’s blooms.
“There is a relationship between a changing climate and increased intensity of storms,” said Marit Larson, the chief of natural resources in New York City’s Department of Parks and Recreation.
“Generally, in the last few years, we are seeing it occur in lakes around the city,” Ms. Larson added. “And because it’s something we know could become worse in the future, we are paying more attention to it.”
City park officials, for example, now sample suspicious algae growth for potential toxins at least every two weeks. The blooms will likely dissipate on their own when the chemical balance in the water is restored, officials said.
Until then, residents are advised to avoid contact with affected water, and to immediately wash skin that comes into contact with algae.
In June, New Jersey state officials discovered the toxic algae in Lake Hopatcong, forcing a swimming ban at the large lake that is usually bustling with activity in the summer. The discovery has sparked a heated debate among New Jersey politicians about how best to tackle the issue of toxic algae, including whether towns should charge property owners fees based on how much storm water they produce.
Dog owners have been on particularly high alert for the toxic algae since one woman in North Carolina reported the death of her three dogs after they swam in a pond.
On Sunday in Prospect Park, newly erected yellow signs warned, “Harmful algal blooms have been found in this water.”
But not every parkgoer was aware of the threat.
“I just had my dog on a walk right near the water,” said Marcin Nasuro, 24, who was with his poodle, Ted. “I wouldn’t come near the lake if I knew it was toxic. A dog’s like a kid. It’s like a part of your family.”
Toward the opposite end of the pond, away from the algae outgrowth, paddle boats continued to operate and parkgoers appeared less concerned.
“Well, I’m hoping we’ll just get on the boat, and get in the water, and it’ll be fine,” said Bethany Erskine, 41, as she waited in line for a paddle boat with her husband and three children.
“We’ve been canoeing in the Gowanus Canal so we’re possibly not the best judges.”
Will Dudding contributed reporting.