The Patchogue River in Patchogue on Friday.

The Patchogue River in Patchogue on Friday. Credit: Newsday/Steve Pfost

Scientific studies have found “an unprecedented mahogany tide” harmful to marine life in high densities in many South Shore tributaries, estuaries and bays this spring, Stony Brook University said.

The tides, which are not known to be harmful to humans, cause low oxygen levels in water and are known to cause large-scale fish kills on Long Island. Studies have found they also can be harmful to shellfish such as oysters and scallops, as well as hinder growth of sea grass.

Confirmed sightings of mahogany tides during the past week or so include Great South Bay from Amityville to Patchogue, as well as western Shinnecock Bay, Georgica Pond in East Hampton, and the Peconic estuary, according to a statement issued by the university.

Higher-than-normal densities have been found in: West Beach, Vanderbilt Wharf, Great River Dock, Timber Point West Marina and Timber Point Park in Great River; Amityville River, Ketcham’s Creek and the Patchogue River in Amityville and Patchogue; Deep Hole Creek, Flanders Bay East, Flanders Bay West and West Creek in the Peconic estuary; and Old Fort Pond and Quantuck Creek in Shinnecock Bay.

Mahogany tides are caused by algae that lead to severe turbidity — or lack of transparency — in water, as well as dangerously low oxygen levels harmful to marine life. Turbidity is a measure of suspended solids in water and, according to scientific research, is a key indicator of water quality.

Historically, scientists at Stony Brook said, mahogany tides have occurred in isolated tributaries on Long Island in spring.

Analysis of such tides by the University of Maryland has concluded they are associated with high  dissolved inorganic nitrogen and phosphorus “strongly influenced” by man-made sources — such as household cesspool runoff and farm runoff.

Stony Brook and other scientific observers have documented such areas in a recently completed study of Suffolk County subwatersheds, finding that “in all regions where mahogany tide is presently found,” the majority of the nitrogen entering surface waters is coming from wastewater.