Ecosystem Targets and Supporting Indicators
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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/
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.
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.
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.
The five-year rolling average (2017-2021) of hypoxia is 83 square miles compared to an average of 208 square miles from 1987-1999, a 61 percent reduction (see extent of Hypoxia ecosystem target). 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 2021 severe hypoxia covered 42 square miles of the Sound. The average for the entire time series from 1991-2021 is 47 square miles.
In 2021, Anoxia covered eight square miles. The average for the entire times from 1991-2021 is 10 squre miles. The variability in area of anoxia from year to year can be quite high. No anoxia was detected in the Sound during 16 of the last 31 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.
Here are links to find data on dissolved oxygen levels and the extent of hypoxia in Long Island Sound:
2019 Long Island Sound CT DEEP and IEC Hypoxia Review Report
Frequency of Hypoxia, 1994-2019
Hypoxia is more frequent in the western Sound.