Water

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The Warming and Rising Waters of Long Island Sound

  

 

 

 

In 2024, the annual average surface water temperature for Long Island Sound was 59.2 degrees Fahrenheit (°F) and the annual average bottom water temperature for the Sound was 56.1°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 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, and there has been a shift to more warm water tolerant species.

Water Levels**

The average monthly value for mean high water (MHW) for 2024 at Bridgeport was 3.96 feet.⁴⁰   The trend for water levels at Bridgeport over the last 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.

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 3.44 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 2024 which is equivalent to a change of 1.13 feet in 100 years, which is an increase from last year’s estimate that was 1.09 feet in 100 years.⁴¹}

 

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³⁹ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, December 13, 2024.}
⁴⁰ _{National Oceanic and Atmospheric Administration (NOAA), Tides and Currents, Water Levels – 8467150 Bridgeport, accessed January 16, 2025; tidesandcurrents.noaa.gov/waterlevels.html?id=8467150&units=standard&bdate=19600101&edate=20240101&timezone=GMT&datum=MSL&interval=m&action=data.}
⁴¹ _{NOAA, Tides and Currents, Relative Sea Level Trend 8467150 Bridgeport; tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=8467150.}