Water Quality

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

  The Warming and Rising Waters of Long Island Sound

Annual average bottom and surface water temperature increased over the last 29 years.

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 2020, the average annual surface water temperature (62.4°F) for the Sound was above the average for the previous 29 years (61.2°F); the trend indicates an increase of approximately eight percent over that same period. Likewise, the average annual bottom water temperature for the Sound (59.6°F) was above the average for the previous 29 years (54.4°F); the trend indicates an increase of approximately 15 percent over that same period.³⁰  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 may 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. 

<sub>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 chart above has been shortened, beginning at 40.0 rather than the customary zero. Year to year variations in water temperature and water levels in the Sound are less important than trends.</sub>

_{—————}

³⁰ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program; personal communication from K. O’Brien-Clayton, January 5, 2021.}
³¹ _{NOAA, Tides and Currents; tidesandcurrents.noaa.gov/map/index.html?region=Connecticut}