• Miles of uncrowded white sandy beaches extend to the horizon, separating the clear blue ocean and undulating grass-covered dunes.

    Fire Island

    National Seashore New York

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Water Quality


Within the boundaries of Fire Island National Seashore there are almost 15,000 acres of open water.

More than three-quarters of Fire Island National Seashore is marine or estuarine habitat. Of the 19,580 acres within the park's designated boundaries, approximately 14,664 acres are open water. The Seashore boundary extends 1,000 feet into the Atlantic Ocean from Moriches Inlet to Robert Moses State Park, and up to 4,000 feet into the Great South Bay, and Bellport, Narrow and Moriches bays.

Aquatic ecosystems are highly sensitive to changes in water quality, so this is a resource that the National Park Service monitors and helps protect.


Water quality concerns for Fire Island National Seashore involve both surface- and groundwater resources, and marine, estuarine and fresh water. The park is within the Southern Long Island Watershed in New York.

A variety of factors influence water quality in the ocean and bays surrounding Fire Island, and this affects the habitat for the plants and animals that live here (including us!). Clean, safe water for recreation is an important resource. Drinking water for the south shore of Long Island comes from sources deep within the aquifers beneath the surface of Fire Island.

As a typical East Coast barrier island-lagoon system, circulation of higher salinity water from the Atlantic Ocean-flushing regularly through inlets on each end of Fire Island-is commingled in the estuary with fresh water from mainland Long Island and to a lesser degree from Fire Island.

Salinity and water temperature primarily affect the biology of the waters around Fire Island, but nutrients and turbidity also affect where plants and animals can survive. The chemicals and organisms in the water can also affect human health and safety.

A Fire Island National Seashore summer water quality monitoring program has been conducted through a cooperative agreement with Dowling College. Other federal, New York State, and Suffolk County agencies also conduct research and water quality monitoring programs within the boundaries of Fire Island National Seashore or adjacent waters.

Fire Island Water Quality Parameters Measured:

  • Salinity
  • Water Temperature
  • pH
  • Dissolved Oxygen (DO)
  • Depths
  • Total Coliform Counts (TCC)
  • Fecal Coliform Counts (FCC)
  • Clarity
  • Chlorophyll-a

The National Park Service (NPS) has created a protocol to monitor estuarine nutrient enrichment for its Vital Signs Monitoring Program of the Northeast Coastal and Barrier Network, using the following variables:

Estuarine Nutrient Enrichment: Water Quality

  • Estuarine Water Chemistry (temperature, dissolved oxygen, and salinity)
  • Estuarine Water Quality (chlorophyll a)
  • Estuarine Water Clarity (photosynthetically active radiation [PAR] light attenuation and turbidity)
  • Estuarine Sediment Organic Carbon (percent organic carbon of surficial sediments)
  • Seagrass Distribution (submerged aquatic vegetation [SAV] bed size, structure and location, percent cover, shoot density and biomass)
  • Seagrass Condition (within SAV-bed percent cover, shoot density and biomass)

Historic Quality of Shore Shore Waters:
Have conditions changed?

Fire Island National Seashore's Final Environmental Statement from 1977 provided the following description of water quality more than thirty years ago:

Atlantic Ocean

Water quality in the ocean adjacent to Fire Island is generally good. Salinity ranges from about 24 parts per thousand (ppt) to 33 ppt, or slightly less than the 35 to 36 ppt found in the open sea. The reduction is probably due to dilution by fresh water from the mainland and limited seepage of groundwater from the barrier island. Surface water temperatures range from about 37 degrees during late winter to 70 degrees in late summer, averaging 52 degrees. Comfortable temperatures for swimming occur from late June through October, although the water temperature may drop to the high 50s or 60s anytime during the summer. Dissolved oxygen level, turbidity, pH, nutrient levels, and coliform levels are all within normal limits.

Nearshore ocean waters in this area typically undergo major spring and autumn blooms of phytoplankton and minor midsummer blooms. Each bloom has its own assemblage of species. Productivity tends to increase nearshore, possibly due to the input of nutrients from the estuaries.

Great South Bay and Moriches Bay

Great South Bay and Moriches Bay are part of an interconnected series of shallow estuaries that lie between the barrier beach and Long Island from Southampton on the east to Atlantique Beach on the west, a distance of about 75 miles. Water quality in these estuaries is strongly influenced by land-use patterns and development on Long Island and, to a lesser degree, on the barrier islands.

The estuaries are shallow throughout the study area; mean low-water depths range from about 7 feet near the Robert Moses Causeway to about 4 feet in Moriches Bay. Navigation channels have been dredged to facilitate east-west boat traffic along the intracoastal waterway and for access between Long Island and the barrier islands. Most such channels average 6 to 10 feet deep, but some reach 28 feet in places. The bays are used primarily for navigation, recreation, and commercial shellfishing, all of which are essential to the regional economy.

The water in the bays is derived primarily from groundwater seepage from Long Island and tidal inflow through the inlets; surface flow from Long Island streams is relatively insignificant. Tidal influx is largely restricted to the inlets, where the average tidal range is about 4 feet (data for Fire Island Inlet). Near the center of the bay, tidal range is only about 1 foot, decreasing to about 8 inches along the Long Island shore. Tidal currents during flood tides generally flow eastward from Fire Island Inlet and westward from Moriches Inlet, reversing direction on the ebb. Areas of Fire Island nearest the inlet therefore receive relatively efficient flushing of pollutants due to tidal action. In general, however, directional water movement in the bays is slow, and the flushing time is long (a 1972 Corps of Engineers estimate is 48 days for the Great South Bay System). Because of the slow flushing rate, accumulation of pollutants is a potentially more serious problem in this system than in many other estuaries.

Throughout most of the bay, circulation is strongly influenced by wind. In Moriches Bay, the wind is capable of preventing the normal reversal of tidal currents between ebb and flood tides. Circulation patterns, and therefore the dispersion of pollution from known sources, are difficult to predict because of the great variability in wind-induced water movement.

During the last 20 years, there have been many studies of the chemical and biological characteristics of the south-shore estuary system (Lackey et al. 1950; Guillard et al. 1960; Redfield 1951, 1952; Ryther et al. 1956, 1957, 1958).

Additional water quality studies have been conducted over the past three decades.


A series of Science Synthesis Papers was published in 2005 to support the preparation of a General Management Plan for Fire Island National Seashore, and includes the following report:


The U.S. Geological Survey (USGS) National Water-Quality Assessment Program (NAWQA) provides an understanding of water-quality conditions and how those conditions may vary locally, regionally, and nationally; whether conditions are getting better or worse over time; and how natural features and human activities affect those conditions. USGS Water Resources Divsion has conducted a number of studies on Fire Island and in the Great South Bay.

Most of the drinking water on Fire Island is now provided through Suffolk County Water Authority (SCWA), which supplies water to the majority of people in Suffolk County. Their drinking water supplies are tested to ensure that federal, state and local water quality standards are met.

A detailed annual report is available on the SCWA's web site.

The U. S. Environmental Protection Agency (EPA) and New York State Health Department prescribe regulations that limit the amount of certain contaminants in drinking water provided in public water systems.

Suffolk County Department of Health Services (SCDHS) Office of Ecology also conducts water quality testing within the boundaries of Fire Island National Seashore, and monitors public bathing beaches to ensure that bacterial levels do not exceed State criteria.


Learn More

Suffolk County Water Authority's Education Page includes a link to its "Water Cycle on Long Island" Demo.

For More Information

National Park Service Water Resources
Water Quality Program

NPS Northeast Coastal Barrier Network (NCBN)
Inventory & Monitoring Program:
Monitoring Nutrient Enrichment

United States Geological Survey (USGS)
Inventory of Data Sources Used for Watershed Condition Assessments of Fire Island National Seashore, Gateway National Recreation Area, and Sagamore Hill National Historic Site, New York and New Jersey

New York State South Shore Estuary Reserve

Environmental Protection Agency Office of Water
Water Quality Standards
Drinking Water Standards

Did You Know?

Close view of sand showing bands of colors: white, dark grey, and reddish purple.

New York's state gem—the garnet—may be found among the sands that comprise Fire Island's beaches. Due to differences in size and weight of the grains of sand, you may sometimes see ribbons of garnet and magnatite among the white quartz, as the sand settles on the beach. More...