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/
Moderate to severe hypoxia is a condition in which dissolved oxygen concentrations in a body of water are less than 2 mg/L.
Anoxia (or severe hypoxia) is an extreme form of hypoxia in which dissolved oxygen concentrations in a body of water are less than 1 mg/L.
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.
The maximum area of hypoxia averaged 192.18 square miles from 1987 to 2011. The summer of 2011 was the seventh least severe year with hypoxic conditions at 130,3 square miles. The entire area of Long Island Sound is about 1,300 square miles.
Thirty-five square miles of the Sound were affected by moderate to severe hypoxia in 2011, relative to the 1991-2011 average of 57.7 square miles.
Anoxia affected an average area of 14 square miles between from 1991 to 2011. While the overall average area is 13.9 square miles, the variability in area of anoxia from year to year can be quite high. No anoxia was detected in the Sound during 9 of the last 21 years, but the area affected by anoxia has been as high as 61.7 square miles (in 2003).