Rapidly spreading plant forms thick mat on Peconic River’s surface, could pose danger to river’s fish and plants

Scientists have identified a prolific plant growth overspreading a portion of the Peconic River as mosquito fern, an invasive, rapidly reproducing aquatic plant that thickly covers the surface of a waterbody.

Mosquito fern, known also by its scientific name of Azolla, thrives in waters that are rich in nutrients.

The rapidly reproducing plant can double its biomass in three to 10 days. It has spread rapidly across the freshwater portion of the Peconic River over the past two weeks, from Peconic Avenue to approximately Forge Pond.

Mosquito fern can cover an entire body of water. It derives its name from its thickness, which can prevent mosquito larvae breeding in stagnant water from penetrating the plant to reach the surface, killing them and acting as a sort of natural larvicide.

The plant is not dangerous to humans – some species are actually used as biofertilizers for food crops.

But an overgrowth of Azolla can be damaging to other native plants and fish, leading to low oxygen as it slows water motion, according to Stony Brook University marine researcher Christopher Gobler.

The plant cannot survive the winter, so when it dies off and decomposes, it will likely produce even more nutrients in the already nutrient-heavy Peconic River. An excess of nitrogen was blamed for the historic series of fish kills last summer, when high nitrogen levels led to dangerously low levels of dissolved oxygen and essentially asphyxiated hundreds of thousands of bunker fish.

Removal of the plants would require a freshwater wetlands permit, according to Department of Environmental Conservation spokesperson Aphrodite Montalvo.

When reviewing permit applications for mosquito fern removal, she added, the Department of Environmental Conservation will take into account “any potentially negative effects [the plant] may have on the river ecosystem.”

Department of Environmental Conservation researchers first spotted the plant about a month ago upstream in the Peconic River, Montalvo said.

“It is native to New York, but can form a dense mat over the surface of the water when conditions are right, which appears to be the case in the lower Peconic now,” she said.