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 read the hypoxia section in our management plan.
The duration of hypoxia refers to the length of time during which hypoxia is observed in Long Island Sound. 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 Long Island Sound water quality monitoring program measures dissolved oxygen concentrations at multiple stations across the Sound every other week during summer months (June-September) and monthly thereafter.
The average duration of hypoxia in Long Island Sound from 1991 to 2012 is 55 days per year, with a range of 34 to 79 days. In 2012, the duration of hypoxia was 63 days. CTDEEP began operating the water quality monitoring program in 1991.