Water Quality

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

The Warming and Rising Waters of Long Island Sound

  Quick Summary -  check x dashClimate Change Indicator



Annual average bottom and surface water temperature decreased from the previous year, but remains higher than the 10 and 30-year average.


The average bottom and surface temperature of the water in Long Island Sound has been rising, with the average bottom temperature rising faster than the surface water.

In 2021, the average annual surface water temperature (62°F) for the Sound was above the average for the previous 30 years (61.24°F. Likewise, the average annual bottom water temperature for the Sound (59.1°F) in 2021 was also above the average for the previous 30 years (54.6°F).38  The average monthly value for mean high water (MHW) for 2021 was unchanged from 2020 and was less than the average for the last ten years. However, the trend for monthly average for MHW data from 2000 to the present for the monitoring station near New Haven indicates that water levels have increased over the prior 21-year period.39

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 warm water tolerant species. Conversely, the decline in lobster population in the Sound might be the result of warmer water.

As the Sound rises, more tidal wetlands will be flooded. The natural "migration" of wetlands landward in response to sea level rise is prevented in many places by fill and development. In addition, 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 an increase in both 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. Year to year variations in water temperature and water levels in the Sound are less important than longer term trends.


38 DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, January 5, 2022.
39 National Oceanic and Atmospheric Administration (NOAA), Tides and Currents, Station - 8465705 New Haven, CT, Datum - MSL; tidesandcurrents.noaa.gov/waterlevels.html?id=8465705.