Brown tide is spreading across the Great South Bay, endangering shellfish and eelgrass as concentrations of the algae are well above what is safe for some forms of marine life.
Brown tide is not toxic to humans.
Last seen in the Great South Bay in 2008, brown tide has shown up periodically along Long Island shores since 1985, when it destroyed a large bay scallop fishery in the Peconic Estuary, said Walter Dawydiak, acting director of environmental quality within Suffolk County's Department of Health Services.
"It's an eerie thing to see," said Dawydiak, who documented the algae when it first appeared locally. "It's almost like you have tinted glasses on and the whole universe is out of color."
Chris Gobler, a professor at Stony Brook University's School of Marine and Atmospheric Science, said samples taken near Heckscher State Park showed brown tide concentrations of 1.28 million cells per milliliter.
Concentrations of 50,000 to 100,000 cells per milliliter can hurt shellfish, slow clam recovery efforts and kill off eelgrass.
Off Bay Shore the concentration was more than 1.1 million and near Sayville 753,000 cells per milliliter were discovered. Near the new breach on Fire Island, concentrations were significantly lower at 97,000 cells.
Brown tide blooms generally turn waters coffee-colored.
"The water off of Captree is very dark, very brown," commercial fisherman Kenny Wolfe said. "It's been getting worse over the last few weeks."
The brown tide first began appearing near the end of June and intensified in the past week. Gobler points to drenching rains in June and nitrogen released by septic systems and wastewater treatment plants into ground-water that pushed into the bay.
"In western and central Great South Bay the bloom is getting pretty thick," Dawydiak said. "The concentrations are very high. The area that it occupies is very large."
Brown tide can be resilient and recur. In the mid-1980s, it bloomed beneath ice in the Peconic Estuary, Dawydiak said.
Temperatures in the high 70s tend to kill off the blooms. "With this high heat we're having, I'm hopeful that is going to back it up," Gobler said.
It's too soon to tell what the final impact may be on marine life. The Nature Conservancy of Long Island has a clam restoration and monitoring program on 21 square miles in the central part of the bay.
In the 2008 brown tide bloom there, 90 percent of the baby clams were lost, said Carl LoBue, the conservancy's senior marine scientist.
That bloom died off once in the summer, returned again and lasted until the fall, only to return that spring.
"If it persists for a long time, it's another setback for the hard clam restoration," LoBue said. "It's another setback for the eelgrass."