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.

Gobler’s assessment came amid a generally mixed review for the state of the Great South Bay at a conference by the Save the Great South Bay conservation group in Oakdale Friday. 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.

"Water close to Fire Island Inlet is in pretty good shape," he said. Same for the far eastern part of the bay, helped by a breach at Bellport Bay. But in other areas "where the water is not moving as much, there’s a lot of water-quality impairment." The trend is being exacerbated by warming waters following 2020s status as the warmest year on record. "When it gets warmer the water holds less oxygen," he said.

In the case of the red seaweed, he said, his studies confirm its growth is the result of "wastewater-derived nitrogen." In nitrogen surveys across the region's waterways, Gobler said he’s found 17 sites not meeting state standard for nitrogen. "Only a handful are hitting the mark," he said.

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.

Dissolved oxygen has shown a steady increase at both the top and bottom levels of the bay over the past five years, according to the center’s research, including the highest level of upper-level dissolved oxygen, at 8.07 milligrams per liter in 2020. The level stood at 4.26 mg/liter in 2015, according to the report.

Gobler earlier this year discounted those claims, pointing to what he said were "multiple signs that the data being presented was not collected in a manner that would produce high quality, reproducible or accurate results and that the study was not designed with a high degree of scientific rigor and care."

Tanacredi dismissed Gobler’s criticism, noting that he’s been collecting the data using standard scientific methods for two decades and that it’s never been questioned.

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 alga 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.

But Gobler said for the range of problems that have impacted the Great South Bay and other local waterways, "the only lever is mitigating the nitrogen. We need to take tackle this at the local level."

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