Central Park Birding Report, Part 2: Why Is the Water Green?

Text and Photographs by Boysenberry45

How green is that pond? Very green, in the case of the Central Park Pool, near the west side entrance to the park at 100th Street, because it’s covered by duckweed (aka water lentils).

A high-protein source of food for waterfowl, these simple plants have no stems or leaves – they are merely a few cells around an air pocket which enables them to float effortlessly atop fresh water.

They are spread by waterfowl and proliferate when water levels are low, reducing water evaporation while also providing shade for frogs and turtles. Think again if you are not in favor of this bright green blanket on the water – it absorbs excess nitrogen and phosphates, prevents algae from growing (which would be odorific), and slows mosquito breeding. Amazingly, Mother Nature has perfectly timed this neon-green water bloom to coincide with summer molting for many ducks.

Did you ever wonder where all of the beautiful mallard ducks went during the summer? They are still here, but in “eclipse plumage” during their summer molt, a labor-intensive process during which worn-out feathers are replaced. Much as the sun is eclipsed by the moon, the brightly colored plumage is eclipsed by dull brown replacement feathers. Beginning with their vibrant wing feathers, ducks lose all of their feathers at once, rendering them flightless for about one month each summer. The first feathers to return are dully colored and provide protective cover for the flightless ducks to swim incognito, making it difficult to distinguish between males and females, too.

The duck weed is a high protein food (higher in protein than soybeans) that helps fuel the regrowth of the ducks’ feathers. It’s messy for the pond’s residents, sticking to everything and coating them like an oil slick, but not dangerous. In Southeast Asia, resourceful humans also enjoy this delicacy.