These days, Prospect Park lake looks a little less like a blue, shimmery body of water, and little more like the dull green of a public school cafeteria floor.
The culprit is azolla caroliniana, but whether it's a hazard to Prospect Park's wildlife has been the subject of great debate among the park's many protectors.
According to a report in The Brooklyn Paper, the gunk is an invasive species, possibly introduced by a contaminated boat or fishing gear.
In addition to being unsightly, Cornell University invasive species specialist Chuck O'Neill told the Paper that it also has the potential to be dangerous.
“It has a tendency to crowd out other plants and cause mortality in fish,” said O’Neill. “It’s a nuisance.”
Wildlife rehabilitater and longtime park watchdog Anne-Katrin Titze claims the fern is hazardous to a host of organisms, and called upon the Prospect Park Alliance to eradicate it.
"The inaction to maintain the only lake in Brooklyn as a valuable wildlife habitat is inexcusable," Titze wrote in an email to Patch. "A valuable wildlife habitat is in grim shape and getting worse."
But Paul Nelson, spokesman for the Prospect Park Alliance, begs to differ. He said this year's particularly aggressive crop of the fern—which he said is, in fact, native to the northeast—is due not to any boats, but is simply the product of an unusually temperate winter and prolonged spring.
He said the fern is not a threat to the park's wildlife, and even serves as food for the waterfowl.
"We're closely monitoring it," he said. "People obviously shouldn’t be drinking it, and there’s no swimming in the lake anyway, so we’re not worried about that."
Nevertheless, a boat is deployed into the lake once a week to skim the scum off of the surface, in order to ensure that the water continues to flow freely. As for a more permanent solution, Nelson said the arrival of winter will likely be the end of the fern for the year.
"It’s naturally occurring," he said. "We won’t do anything involving chemicals to kill it."