Water Quality

Rivers, Lakes, and Estuaries                 Water of Long Island Sound               Swimming               Clamming and Oystering                 Drinking Water

The Warming and Rising Waters of Long Island Sound

  Climate Change Indicator



Annual average bottom and surface water temperature increased from the previous year.

In 2023, the annual average surface water temperature for the Sound (60.3°F) was less than the average for the previous 30 years (61.53°F), while the annual average bottom water temperature for the Sound (57.8°F) was greater than the average for the previous 30 years (54.9°F).39 

Annual variations in water temperature and water levels in the Sound are less important than longer term trends. The trends for average annual bottom and surface temperature of the water in Long Island Sound have been rising, with the average bottom temperature rising at a faster rate than the surface water. While the long-term impact of warmer water in the Sound is unknown, species diversity and biomass remain high, although there has been a shift to more warm water tolerant species.


Water levels in the Sound are also increasing.**

The average monthly value for mean high water (MHW) for 2023 at Bridgeport was 3.86, which was higher than the average monthly MHW for the previous 50 years (3.41).40 As depicted in the chart, the trend for water levels at Bridgeport over the last almost 60 years is increasing. The natural "migration" of wetlands landward in response to sea level rise is prevented in many places by fill and development. As a result, shore birds that nest in coastal areas, such as the piping plover, might be displaced.

Goal: While there is no established goal for water temperature or sea level rise in Long Island Sound, it is assumed that a significant increase in temperature and water level is not a desired outcome.

Technical Note: *The vertical axis in the average temperature of Long Island Sound chart above has been shortened, beginning at 40.0°F rather than the customary zero. **The relative sea level trend is 3.33 millimeters/year (mm/yr) with a 95 percent confidence interval of +/- 0.38 mm/yr based on monthly mean sea level data from 1964 to 2023 which is equivalent to a change of 1.09 feet in 100 years. 41


39 DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, January 17, 2024.
National Oceanic and Atmospheric Administration (NOAA), Tides and Currents, Water Levels – 8467150 Bridgeport, accessed January 4, 2024; tidesandcurrents.noaa.gov/waterlevels.html?id=8467150&units=standard&bdate=19671001&edate=20230101&timezone=GMT&datum=MSL&interval=m&action=data.
NOAA, Tides and Currents, Relative Sea Level Trend 8467150 Bridgeport; tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=8467150.