Brown tide algae has returned to Long Island's South Shore at a time of year that helps set next spring's hard-clam reproductive season, according to a report from the Marine Science Research Center at Stony Brook University.

The algae blooms -- toxic to marine life but not humans -- "has intensified this month to nearly 1,000,000 cells per milliliter in central Great South Bay," center officials reported. "Densities exceeding 200,000 cells per milliliter were also present in western Great South Bay, Moriches Bay, Quantuck Bay, and Shinnecock Bay."

Concentrations of more than 50,000 cells per milliliter can be harmful to shellfish, especially clams, according to the center.

"The occurrence of a fall brown tide is not uncommon, particularly after a summer with a dense and widespread brown tide," said Christopher Gobler, professor of marine biology at Stony Brook University's Southampton campus. "We knew that the summer brown tide would end when the bays heated up above 75 degrees. We also knew it could return once the bays cooled down in the fall."

Brown tides hit the same areas in May, June and July. Such algal blooms have been forming in South Shore bays for at least the past 25 years, Gobler said. The growths largely result from residential cesspools and septic tanks draining into groundwater, which eventually makes its way into the Great South Bay.

Algal concentrations are at safe levels in ocean inlets, including the breach cut through Fire Island by superstorm Sandy a year ago. The breach is flushing Bellport Bay with ocean water and has kept brown tide densities below 20,000 cells per milliliter, according to a news release issued by the center Tuesday.

Suffolk County Senior Public Health Sanitarian Michael Jensen said the last two "substantial" brown tides in the Great South Bay occurred in 2008 and 2011. The 2011 algal bloom started in the fall and ended in November. "Data collected at this point suggests that this year's fall bloom mimics that of the 2011 fall bloom," Jensen said in a statement.

When he checked two locations this week, Jensen said, he saw dense blooms about 2 feet deep. When he visited the same spots on Sept. 24, he saw no significant brown tide.The blooms diminish when water temperatures reach the mid-70s, Gobler said, with the possibility of a return in the fall when temperatures cool down.

"This is disappointing but not surprising," said Carl LoBue of The Nature Conservancy, which is working to restore the Great South Bay hard-clam population. Earlier experience shows "that back-to-back brown tide blooms not only impacted survival and growth of young clams, it also impacted spawning of adult clams the following season."

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