Invasive Seaweed Spreading in Great South Bay

An invasive seaweed from Japan first detected in Long Island waterways three years ago has spread to the point of being “everywhere,” a marine researcher said Friday in a report that presented mixed results for the health of the Great South Bay.

The seaweed, a macroalgae known as Dasysiphonia japonica, was first detected on Long Island in 2018 and is sometimes evident in a deep red coloration of summer water and winter ice.

“This thing is everywhere,” said Christopher Gobler, a professor and research at Stony Brook University’s School of Marine and Atmospheric Sciences, noting that field studies as recently as Friday found it on the shore of Hecksher State Park. High levels of nitrogen and CO2 can fuel growth of the seaweed, Gobler said. “We think that combination could be driving the invasion of this seaweed.”

The seaweed “becomes a human health issue when it decays,” Gobler said, noting it can release hydrogen sulfide gas. It can also release a “suite of compounds” that can be harmful to marine life, including larval fish and bivalves, Gobler said.

Speaker Series Highlights State of the Bay

Save The Great South Bay recently hosted the quarterly Speaker Series featuring the second annual State of the Bay presentation with Gobler.  Gobler gave a generally mixed review for the state of the Great South Bay. The group plans to begin releasing an annual report card on the state of the bay, similar to one now released on Long Island Sound, to more carefully track its health, said executive director Robyn Silvestri.

Gobler said some areas of the bay that are near homes with municipal sewer systems, or closest to tidal flushing by ocean inlets, have seen either modest increases in quality or stabilization. But the central part of the Great South Bay remains under pressure by nitrogen that he said is chiefly the result of homes that use traditional septic systems.

Gobler’s studies confirm that growth of the Japanese seaweed is the result of nitrogen in the water along with higher than normal water temperatures.

Another Point of View

But one researcher who has studied the Great South Bay for 21 years says there’s also cause for optimism.

John Tanacredi, who directs the Center for Environmental Research and Coastal Oceans Monitoring for Molloy College, reported in his final 2020 analysis of water quality in the Great South Bay that several critical measures showed improvements.

“We’ve also seen a dramatic uptick in the natural resources supporting a host of organisms along the coastline,” with increased sightings of humpback whales, menhaden, striped bass, bluefish and winter flounder, he said.

Many Contributing Factors

Tanacredi, who emphasizes his views on the data are his own and not Molloy College’s, said ecologists should be focusing on the range of factors that contribute to toxins in the waterways, not just nitrogen from older septic systems, which are in some cases miles away from the life-supporting bays he’s studying.

“There needs to be more attention to other sources and types of contaminants or things that may contribute” to harmful algal blooms and brown tides. He cited an increase in ground-level ozone from CO2 and particulate-spewing cars, trucks and buses, a “dramatic” increase in precipitation, and lawn-based fertilizers flowing into street sewers as other major contributors.