The Forge River, here seen from Montauk Highway in Mastic,...

The Forge River, here seen from Montauk Highway in Mastic, was one of three Long Island waterways rated as "poor" in the Long Island Water Quality Report. Credit: Newsday/John Paraskevas

It was another week of mixed-bag water-quality findings for Long Island bays, harbors, rivers and inlets, based on samplings  at 29 locations.

Of  those, 15 got “good” ratings, meaning clear water, no or low levels of algae and/or bacteria from human or animal waste, and hospitable conditions for fish and shellfish. Eleven were rated fair, and three poor.

That’s according to the Long Island Water Quality Report, a scorecard issued weekly from Memorial Day to Labor Day. It’s created by Chris Gobler, professor of marine science at Stony Brook University and director of the New York State Center for Clean Water Technology, and his team of more than 20 students and scientists.

Reiterating last week’s sentiment, “Water quality is mixed across Long Island,” he said.

Conditions found at the poor sites — the Peconic River, Forge River and central Great South Bay — were harmful algal blooms, low levels of oxygen and water clarity and elevated levels of fecal bacteria.

Temperatures in South Shore bays were found to have risen to the high 70s to low 80s, the warmest of the season so far, he said. Water quality conditions in those bays were good, with the exception of that central area of the Great South Bay, where the harmful bloom chattonella “was occurring, along with near-zero oxygen levels at night,” the report said.

Seawater at such high temperatures “naturally holds less oxygen,” and it takes just “a slight disturbance in the natural balance of the ecosystem for oxygen levels to get dangerously low at night,” Gobler said.

Chattonella, rarely found around Long Island, has potential to be a fish-killer and, given the densities found through the sampling, could be having a negative impact on “resident fish populations.” It’s not toxic or any danger to humans.

Since the water quality project was launched in 2014, the team has picked up on how water quality tends to morph as the summer progresses.

Over the years, “we've learned that oxygen levels are high and good in June, but declined through July and August, sometimes to levels dangerously low for marine life,” Gobler said.

Three factors are at play, he said. As waters warm up, we get “the cumulative growth of algae over the summer,” warmer water holds less oxygen, and, finally, shorter days mean less oxygen is produced, Gobler said.

The aim of the weekly water-sampling project is to provide regular snapshots of ecosystem health, with an eye to how well locations are supporting — or not supporting — robust fishing and shellfishing activity.

While the beach crowd may find some basics useful — water temperature and clarity assessments — this is not the place to go to see what locations are up to snuff for swimming and splashing around. Look, instead, to official calls from county health departments on where to swim or not, Gobler said. And, for any curtailment of shellfishing, the state Department of Environmental Conservation is the go-to official source.

Sampling is done on Mondays, as six teams “fan out across Long Island, collecting water samples, making measurements, and downloading data from logging devices, like oxygen meters,” he said.

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