Plant matter covering a lot of Tower Lakes, Paraparaumu, is a native plant called azolla, not algae, council biosecurity programme manager Rob Cross said.
"This plant doesn't pose a serious environmental threat and doesn't have adverse effects on wildlife.
"Accumulation like this on top of a pond is natural for a small closed body of water and tends to be seasonal, so it's likely to die back in winter.
"It's usually eaten by waterfowl but when it grows quickly they can't keep up, which is why it's more noticeable at the moment.
"It can be removed manually using a rake or moveable boom as it sits just on top of the water."