Ecosystem Targets and Supporting Indicators

Severely Hypoxic and Anoxic Areas

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. 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. Severely hypoxic areas are less than 2 mg/L and anoxic areas are less than 1 mg/L.

Show/Hide Table Data

Area of Hypoxia Severity (sq. miles)
HypoxiaSeverly HypoxicAnoxia
1987309no datano data
1988251no datano data
1989328no datano data
1990174no datano data
1991122360
19928070
199320291
199439321228
19952057118
1996220274
19973020
1998168437
1999121527
2000173520
2001133320
20021306142
200334518662
20042026238
20051779528
20061996118
2007162300
2008180.17140
2009169.1190
2010101.1470
2011130.3350
2012288.56718
201380.7330
201487200
201538.300
2016197.4540.20
20177050
20185200
201989211

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 severe HYPOXIA?

The Long Island Sound Study defines dissolved oxygen concentrations of less than 2 mg/L as severely hypoxic. In most other ecosystems with similar oxygen depletion problems, like the Chesapeake Bay and the northern Gulf of Mexico, 2 mg/L of dissolved oxygen is the upper limit for hypoxia.

WHAT IS ANOXIA?

Anoxia is typically defined as the complete lack of oxygen or often less than 0.2 mg/L. The Long Island Sound Study, however, defines anoxia as oxygen concentrations below 1 mg/L because this is the threshold below which most marine animals 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 can be dissolved 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 by the Connecticut Department of Energy and Environmental Protection.

STATUS

The five-year rolling average (2015-2019) of hypoxia is 89  square miles compared to an average of 208 square miles from 1987-2000, a 57 percent reduction (see extent of Hypoxia ecosystem target). The hypoxic area also has declined by 63 percent from the peak five-year period (1992-1996), which was 240 square miles.  Based on the last 20 years of interannual variability, a 28 percent reduction is necessary to achieve a measurable reduction (see data note). Further reductions in the area of hypoxia are needed in order to fully attain water quality standards for dissolved oxygen.

In 2019 severe hypoxia covered 21 square miles of the Sound, relative to the 1991-2019 average of 48 square miles.

Anoxia covering 1 square mile was detected in the Long Island Sound water quality monitoring program in the western Sound for the first time since 2012 when it affected 18 square miles. While the overall average area of anoxia is 11 square miles from 1991-2019, 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). Other monitoring programs by the Interstate Environmental Commission and the Long Island Sound Integrated Coastal Observing System that monitor western Long Island Sound more intensively have found more frequent anoxia in this area.  

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

Please complete your newsletter signup.