Severely Hypoxic and Anoxic Areas

WHAT IS HYPOXIA?

Hypoxia is a condition that occurs in bodies of water as dissolved oxygen concentrations decrease to levels where organisms become physically stressed and ultimately cannot survive. Prolonged hypoxic conditions result in severe die-offs of animals that are unable to move out of hypoxic waters, mass migrations of mobile animals, changes in water chemistry and other adverse ecological effects. The Long Island Sound Study defines hypoxia as waters with dissolved oxygen concentrations less than 3 mg/L.

For more information on hypoxia and efforts to reduce its occurrence in Long Island Sound visit: http://longislandsoundstudy.net/about/our-mission/management-plan/hypoxia/

WHAT IS severely HYPOXIc?

Moderate to severe hypoxia is a condition in which dissolved oxygen concentrations in a body of water are less than 2 mg/L.

WHAT IS ANOXIA?

Anoxia is typically defined as the complete lack of oxygen. LISS, however, defines anoxia as oxygen concentrations below 1 mg/L because this is the threshold below which most fish cannot survive even for a short period of time.

WHAT DOES THIS INDICATE?

The area of hypoxia in Long Island Sound refers to the number of square miles in which dissolved oxygen concentrations were less than 3 mg/L over the course of a single year. Hypoxia is most common during summer months, when waters are stratified (preventing mixing of oxygen from the surface to the bottom) and temperatures are higher (so less oxygen stays in the dissolved form in the water). The area of hypoxia is calculated from measurements taken every other week during summer months as part of the Long Island Sound Study Water Quality monitoring program.

STATUS

The maximum area of hypoxia averaged 180 square miles from 1987 to 2017.  The summers of 2012 and 2016  were relatively severe years, while 2013-2015 had comparatively very mild hypoxia. While there is a general trend of improvement over the last decade, the stark difference between 2012 and 2016 and 2013-2015 and 2017 highlights the high amount of interannual variability in hypoxia caused by factors such as temperature, wind, and precipitation.

Five  square miles of the Sound were affected by moderate to severe hypoxia in 2016, relative to the 1991-2017 average of 51 square miles.

Anoxia has not been detected during the summer since 2012 when it affected 18 square miles. While the overall average area of anoxia is 12 square miles, the variability in area of anoxia from year to year can be quite high. No anoxia was detected in the Sound during 14 of the last 27 years, but the area affected by anoxia has been as high as 61.7 square miles (in 2003).

For reference, the entire area of Long Island Sound is about 1,300 square miles.