Brown tide -- an overgrowth of algae that clouds the water and harms shellfish -- is back in Long Island's waters for the eighth year in a row, according to Stony Brook University researchers, who also discovered a swath of unidentified seaweed in the Great South Bay.
In addition, county officials said Monday that toxic blue-green algal blooms have now hit four Suffolk County water bodies.
Christopher Gobler, a professor at the university's School of Marine and Atmospheric Sciences, said the number of algae that cause brown tide have been edging up in Quantuck, western Shinnecock and eastern Moriches bays since May.
"The timing's right about where we'd expect it," he said.
Brown tide turns the water cloudy brown, blocking out light. In addition to being toxic to shellfish, brown tide is also harmful to sea grass, which roots underwater and provides critical habitat for some shellfish and fin fish.
Meanwhile, researchers discovered a swath of an unidentified seaweed in northern Great South Bay between Bellport and Islip.
Gobler said he took samples of the seaweed, also known as macroalgae, for DNA sequencing to determine what it is. But the seaweed thrives in clear water that is also high in nitrogen, he said. The high nitrogen levels are caused largely by septic systems that discharge wastewater that ends up in the bay.
"Because there's so much nitrogen coming into the system, that allows the macroalgae to come in," Gobler said.
The seaweed grows quickly and can choke out more beneficial sea grass, said Carl LoBue, senior marine scientist with The Nature Conservancy on Long Island in Cold Spring Harbor.
LoBue said the seaweed "isn't necessarily a bad thing. It's when a lot of it grows, fueled by nitrogen."
In addition, Gobler said when the seaweed eventually dies, the resulting sudden release of nitrogen from the decaying plants could trigger another harmful algal bloom.
While the brown tide is expected to last until early July, the area's waters were largely spared this spring from the worst effects of red tide, a harmful algal bloom that makes an appearance in spring.
But another harmful algal bloom has hit a fourth water body in Suffolk County, according to the state Department of Environmental Conservation and the Suffolk County Department of Health.
Mill Pond in Southampton has a confirmed bloom of blue-green algae, also known as cyanobacteria, officials said. The algae can be dangerous to humans and animals, and the county advised residents not to touch the water or allow their pets and children near the area.
Cyanobacteria blooms were also found earlier this month in Big Reed Pond in Montauk, Lake Agawam in Southampton, and Marratooka Lake in Mattituck.
CORRECTION: An earlier version of this story gave an incorrect location for Marratooka Lake.