The organism, which turns bodies of water an opaque brown, starving plants and marine life, was reported to be appearing in Moriches, Quantuck and Shinnecock bays last week. A large plume also swept across the Great South Bay for a short time last spring.
Brown tide showed up abruptly in local waters in 1985, decimated scallop populations. The scallop fishery still has not recovered. It has not shown up in local bays since 1997.
However another harmful type of algae, known as red tide, has bloomed in local bay waters during the past three summers, including this one, experts said.
Unlike the brown tide algae, which disrupts feeding cycles, the red tide organism releases a lethal toxin, according to researcher Chris Gobler, an associate professor at the school of marine and atmospheric sciences at Stony Brook University. The toxin is lethal to fish and, because it accumulates in shellfish, it can be lethal to humans, too, Mr. Gobler said.
"Fortunately, it's been found in areas that are not open to shellfishing right now," he said.
Scientists believe the blooms are caused by the introduction of high quantities of nutrients, such as nitrogen, into the waterways, though no direct cause of the brown or red tides has been firmly established. The algae are "nitrogen-loving organisms," said Kevin McAllister, the executive director of the Peconic Baykeeper nonprofit group. "We've got to get a handle on nitrogen loading," he said, adding that lawn fertilizer is a major contributor.
Mr. McAllister said he would not expect brown tide to appear in the Peconics simply because blooms have been reported in South Shore waters, where environmental conditions may be quite different.
The brown tide bloom of 1985 virtually eliminated bay scallops in local waters. The scallop fishery had been a multimillion dollar fishery with landings in excess of 500,000 pounds annually, according to the Peconic Estuary Program.
Through efforts to restore the fishery, the scallop populations in local waters are growing again. Still, landings remain severely depleted, averaging about 3,200 pounds per year this decade.