Mahogany tide, a type of harmful algae that contributed to a fish kill last year in Peconic River, is probably blooming in Bellport Bay, turning the water a sort of rusty mustard color.

The algae, known as prorocentrum minimum, is typically confined to rivers and tributaries here and the discovery in Bellport Bay is unusual, said Christopher Gobler, a professor with Stony Brook University’s School of Marine and Atmospheric Sciences.

The state Department of Environmental Conservation, Suffolk County, and Gobler plan to sample parts of Great South Bay on Friday to get a better sense of what algae may be there.

“I’ve never seen it as widespread,” Gobler said. “In most cases it’s been confined to these tributaries. I can’t think of a time it was in the bay.”

DEC spokesman Sean Mahar said sampling last week in some bays appeared to be mahogany tide but the agency was “collecting additional water samples this week for further analysis and to delineate the extent of the bloom in the Great South Bay.”

Gobler said tests confirmed mahogany tide in the Peconic River, Georgica Pond in East Hampton and western Great South Bay.

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Mahogany tide has been present in New York waters since the 1980s and is fed by warm but not hot temperatures and the presence of nutrients. It can lead to low oxygen levels dangerous to fish and its mass can block out sunlight, harming sea grass. It’s not considered to be harmful to humans.

Friends of Bellport Bay co-chair Thomas V. Schultz told the DEC on Wednesday evening that a brownish sheen could be seen near the Bellport village dock. “I noticed this translucent, brown, yellowish slick,” he said.

He later took his boat out to see if it was more widespread. “There were bands of this substance in the greater bay as well,” Schultz said. “It’s like a mustard brown. It’s really gross.”

Just days ago, Schultz said he could see down to the bay bottom but that changed following heavy rainfall. “You can barely see three inches into the water.”

Runoff from fertilizers and rain can trigger a bloom, said Cynthia Heil, a senior research scientist and biological oceanographer with Bigelow Laboratory for Ocean Sciences in Maine.

“It’s very opportunistic,” Heil said. “It’s very fast-growing. It can tolerate a whole variety of salinities.”

Last year, three fish kills affecting Atlantic Menhaden happened in the tidal portion of the Peconic River. Prorocentrum minimum was involved during the first die off on May 16, according to a report from the county, state and Stony Brook. High water temperatures and limited tidal flushing also were factors.