An algae bloom causing brown tide is again spreading across the Great South Bay, 30 years after the first was detected on Long Island.

This is the third straight year algae has bloomed in the bay, a potential risk to the health of adult clams at a time when they reproduce and juvenile clams are vulnerable, said Christopher Gobler, a professor at Stony Brook University's School of Marine and Atmospheric Sciences.

"The young ones, they need a lot of food and a lot of good conditions," Gobler said.

VideoBrown tide found in South Shore waters More coverageWater quality on Long Island

Algae blooms flourish in shallow, poorly flushed and nitrogen-rich areas. High concentrations can cloud the water and deplete oxygen levels.

Densities of more than 50,000 cells per millimeter can be harmful to marine life. Gobler's lab last week found levels as high as 273,000 in Patchogue Bay. Western Shinnecock, Eastern Moriches, Quantuck and central Great South bays also had high concentrations.

The algae is not harmful to humans.

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Last year, Gobler's lab, with support from the National Park Service, seeded clams in central Great South Bay, near the Fire Island Inlet and where superstorm Sandy formed a breach, allowing for more water circulation. Clams near the inlets fared well, but those in the more stagnant central area did not.

"As soon as the brown tide hit, they stopped growing entirely, and we had some dieback," Gobler said.

Adult clams may also be affected because brown tides hit in the fall and subsequent spring -- two big feeding times when they eat to grow and make energy to produce eggs.

"They don't spawn as well as they do when they are healthy and fat," said Carl LoBue, senior marine scientist with The Nature Conservancy of Long Island, which has been working on clam restoration in Great South Bay since 2003.

Recently, the organization began focusing restoration efforts near the breach because of better water conditions. Reducing nitrogen from fertilizers, wastewater and septic systems is also a priority.

Bill Zeller, owner of Captree Clam Co. Inc. in West Babylon, sources clams from a number of locations, so he does not believe business will be hurt.

"Great South Bay is in a low, as it has been for some years," he said. "We are cognizant of the ups and downs in science and the gives and takes of nature."