The VanderGast sisters were admiring the turtles paddling through the Lake in Central Park under a blazing sun the other afternoon.
“If one swam up to me, I was going to wade in and try to hold it,” said Zoe, 15. Her sister, Anna, 13, took one look at the strange greenish tint — a color reminiscent of green antifreeze — and grimaced. “The water looks pretty nasty,” Anna said.
Summer is here, and the lakes and ponds that dot the city’s parks are awash in algae. Much of it is just unsightly. But some types, like the blue-green algae in the Lake in the southern half of Central Park, can be harmful, causing rashes on people and posing a lethal risk to dogs.
Blue-green algae can be hard to see. Its hallmark is a uniform green hue, sometimes with large swirls — as if someone spilled pale green paint on the darker green surface.
There is no simple fix for blue-green algae, also known as cyanobacteria, which is caused by an excess of nutrients like phosphorous and nitrogen in the water. In most parts of the country, those nutrients result from storm water and agricultural runoff, fertilizers, dog waste and nearby septic tanks.
In some New York City parks, however, their presence may be traced to the city’s municipal water supply, which feeds a number of water bodies. Since the early 1990s, the Environmental Protection Agency has required cities to add orthophosphate to drinking water to reduce the incidence of lead poisoning from old pipes.
As soon as tests reveal the presence of cyanobacteria, park officials post signs, like the ones seen around the Lake in Central Park this week under the heading, “Algae Bloom Advisory.” It cautions visitors not to drink, wade or fish in the water and urges that animals and children be kept away.
The New York State Department of Environmental Conservation has tracked the presence of so-called harmful algal blooms since 2012, and this year, the season is off to a robust start. Each week, the department’s websiteupdates a list of sites statewide with suspicious or confirmed blooms. It now includes 30 lakes and ponds. “By August, we usually have 70 to 80 ponds and lakes on the list,” said Rebecca Gorney, a research scientist for the department, which has no oversight of New York City parks.
Only the Lake in Central Park and the Prospect Park Lake in Brooklyn are listed as “confirmed with high toxins,” meaning that water samples collected there have toxins in large enough quantities to cause health effects in people and animals.
Officials at the state agency have noted a rise in the number of sites in recent years, but it is unclear whether that is because of more awareness and reporting, or an actual increase in blue-green algae. Reducing the sources of nutrients, such as runoff, along with chemical treatment and aerators, are some of the tools that lake managers are trying around the state.
The presence of phosphate in the water source of the lakes in New York City parks has limited the options for addressing blue-green algae. But in Prospect Park, where the 60-acre artificial lake is fed by a mile-long water course, including four man-made waterfalls, park officials are poised to try something new. The source of the nutrients bedeviling the lake is a 12-inch-wide pipe connected to the municipal water supply at the uppermost waterfall.
The Prospect Park Alliance, a nonprofit group that helps manage the park, just received a $390,000 grant from the state to pursue a pilot project that will filter out phosphate from the headwaters of the water course. The project, now in the initial planning stages, will use plants rooted in underwater containers adjacent to one of the top falls.
It’s like putting the water through a giant Brita filter but with plant material instead,” said Christian Zimmerman, vice president of capital and landscape management for the Prospect Park Alliance, which received the grant from the state’s Office of Parks, Recreation and Historic Preservation. “It draws up phosphates from the water and after a certain amount of time, a baffle opens and the water gets released back into the upper pool.”
Mr. Zimmerman said that he hoped the pilot project, if successful, could be replicated in urban parks nationwide. “Everyone has a problem of nutrients in the lake,” he said. “This is national. Nobody wants a green lake.”
The greatest risk from blue-green algae is to dogs, because they are more likely to drink the water and the toxins can affect the animal’s liver and neurological system if ingested. Licking fur that has come in contact with cyanobacteria can be dangerous as well.
Dr. Vanessa Hammer, a veterinarian with the Gotham Veterinary Center on Manhattan’s Upper West Side, said that a dog owner brought in his pet two weeks ago, frantically worried that he was exposed to blue-green algae. The dog had swum in another of Central Park’s ponds, but only the Lake currently contains the harmful algae in bloom quantities.
“The dog was fine,” Dr. Hammer said. “But in general, the toxins in blue-green algae can cause severe neurological symptoms that can lead to death within a couple of hours.”
The Central Park Conservancy, the nonprofit group that manages the park for the city, collects weekly samples from the Harlem Meer, the Pool, Turtle Pond and the Lake. The samples are sent to a lab and depending on the results, the city’s parks department then makes recommendations about signage.
In Central Park on Thursday, Myung-Hi Kim was walking her dog, Nika, along the southern shore of the Lake. “She tries to go near it, but I always pull her away,” she said. “I’m a true west sider so I know about the dangers of algae blooms.”