Killer ferns starving Prospect Park lake
It’s getting harder to tell where the lake begins and the meadow ends in Prospect Park.
A thick green carpet of algae-like plants has blanketed the Prospect Park Lake since last fall. Known as azolla caroliana, they are an invasive and relatively rare species that, under ideal circumstances, can double in size every four or five days and reach thicknesses of up to a few feet.
These mats of azolla are currently thriving in our park’s water because, among other reasons, Prospect Park Lake is filled with nutritious city drinking water.
Park activists worry that the thick layer of azolla is literally starving the lake and harming the surrounding ecosystem. It thrives in the lake’s nutrient-rich water, and if left to spread, they say, can block sunlight from reaching underwater plants, keep essential oxygen out of the water by preventing photosynthesis and eventually kill the lake’s fish.
“Azolla mats are sucking the oxygen out of the water and preventing sunlight from penetrating the murky lake,” said Anne-Katrin Titze, a wildlife rehabilitator and park activist. “The fish and other aquatic life are suffocating from the de-oxygenation of the water caused by the pervasive invasion of the Azolla.”
Azolla caroliana, although native to the northeastern states, is extremely rare, according to Jan Schultz, an invasive plant specialist and botanist for the U.S. Forest Service. Given enough growth, Schultz said, overgrowth on a lake surface can harm plants and animals living below, as well as create a “goopy mess” on the surface layer.
The azolla first started covering the lake last fall, when the water course, coming straight from city taps, was turned off in anticipation of waters from Hurricane Irene and other large rainstorms. The invasive fern, which is grown in many parts of the world as fertilizer and animal feed, thrives in shallow and stagnant water, and also feeds on the phosphorus that is added to city drinking water to protect against leeching from lead and copper pipes.
Brett Branco, assistant professor of earth and environmental sciences at Brooklyn College, has worked with the Parks department to monitor the lake for the last four years.
“It is very easy to transfer invasive aquatic plants,” said Branco. He explained that invading fern could have been introduced a number of ways, from contaminated fishing gear to birds to pets in the lake.
“Because it’s such a fast-growing plant, there’s the aesthetic issue and it does block sunlight from reaching the water,” he explained, adding that without ample sunlight, many underwater plants are unable to live. This in turn can harm the lake fish that rely on these plants for the oxygen that is normally created from sunlight through photosynthesis.
Park activists were slightly more upset about the situation.
“The thick suffocating mats of azolla are taking over, encouraging birds, including Herons, Egrets and Cormorants to abandon these areas of the watercourse,” said Titze, who attributes the overgrowth in the park to what she calls “years of inaction and lack of commitment of meaningful resources by the Prospect Park Alliance,” referring to the city organization that manages the park.
A spokesperson for Prospect Park Alliance, Paul Nelson, said there is little that the alliance can do about the azolla overgrowth short of chemically treating the water.
“There is no other way to get rid of it except for using chemicals,” Nelson said, “and we absolutely will not do that.”
Nelson said the Alliance is monitoring nutrient and oxygen levels in the lake and hoping for colder temperatures this winter to kill off the invasive fern. In the meantime, he added, the Prospect Park Alliance has been harvesting the azolla with a floating weed-harvester, turning it into mulch and using it as a natural fertilizer throughout the park.
The azolla overgrowth is a relatively uncommon problem, Nelson said, echoing the Jan Schultz, the Forest Services botanist.
“One of the key things that aquatic plants need to grow rapidly is phosphorus,” he added. “It’s in our lake because it is fed by New York City drinking water — this doesn’t really happen in natural lakes.”
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