This image taken Sunday, July 30, 2017, shows the waters...

This image taken Sunday, July 30, 2017, shows the waters of Bellport Bay clearing up as brown tide is receding. Credit: Charlie Flagg

Brown tide, caused by harmful algal blooms that endanger shellfish and eelgrass, has cleared from the Great South Bay — for now — environmental authorities said Tuesday.

“This is some good news,” said Christopher Gobler, a professor at Stony Brook University’s School of Marine and Atmospheric Sciences. “This is one of the longest brown tides there have ever been.”

The discolored water, which resembles coffee mixed with skim milk and does not pose a threat to humans, first appeared in May, — about two months earlier than expected. Gobler said that in 2013, for example, the brown tide only lasted about two weeks, from late June to early July.

Brown tide had not been seen with such intensity in the Great South Bay since 2008, though it has sporadically surfaced on Long Island shores since 1985 and destroyed a large bay scallop fishery in the Peconic Estuary in the mid-1980s.

At their peak, this year’s blooms covered from the Jones Inlet to the Shinnecock Inlet, Gobler said. “They died off to the west and east in June, but intensified in the Great South Bay into July. In early June was the highest — 2.3 million cells per milliliter.”

The severity of an algal bloom is measured by its cell density, harmful to marine life — particularly clams — at 50,000 cells per milliliter, Gobler said. He said blooms had also been found this year in the Bay Shore region and in South Oyster Bay.

“It’s odd for this to occur there,” Gobler said of Oyster Bay. “That was a bad sign. The last time that happened was 2008 when it spread everywhere and lasted a really long time, and that’s exactly how it played out this year.”

He added, “The first cells were seen in early May and they were still in some locations until last Monday.”

Gobler said the high levels of nitrogen “going from land to sea” were a “big contributor” to the blooms forming, and that it was organic, or recycled, nitrogen that fed the bloom. He said the nitrogen came from septic systems and, to a lesser extent, fertilizers.

“In early June it went from Southampton well into Nassau County, so it covered the whole South Shore, but that only lasted a week or two, and in South Oyster Bay it lasted a few weeks,” Gobler said of the brown tide. “It’s been really persistent in the Great South Bay.”

Gobler said the blooms die off once extended hot weather sets in.

“When the water temperatures get consistently in the high 70s they lose their ability to dominate and die off.”

“The expectation was when it got really hot this would happen,” Carl LoBue, the New York Oceans Program director based in the Cold Spring Harbor office of the Nature Conservancy, said of the clearing of the blooms. “When the water gets really warm it ticks back.”

Gobler said the bloom situation was so intense this year that, “It definitely caused the die-off of sea grass and there was definitely a loss of early life-stage clams — early life-stage clams are not able to live under those conditions.”

And the blooms could return, authorities say.

LoBue also said the problem resembled the brown tide situation of 2000 and 2008, when the damaging blooms killed shellfish and eelgrass.

“When the temperatures go back down in September or October then we get it back again. It can die in the winter and then come back in the spring when it gets this bad.”

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