Ecosystem Targets and Supporting Indicators

Duration of 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. Duration of hypoxia refers to the number of days in a year during which hypoxia was observed in Long Island Sound

Show/Hide Table Data

Duration (days)
YearNumber of DaysFive-year Moving Average
198758
198873
198982
199049
19914161
19925560
19936458
19946855
19953553
19963451
19974850
19987352
19995148
20003548
20016655
20026558
20036156
20045556
20056963
20065361
20075859
20087963
20094561
20104055
20115455
20126356
20136253
20143551
20155754
20165154
20172646
20183541
20194843

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 read the hypoxia section in our management plan.

WHAT DOES THIS INDICATE?

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.

STATUS

The 2019 hypoxic event lasted an estimated 48 days, beginning on July 12 and ending Aug. 28. Compared to the previous 31 years, 2019 was below the average of 55 days.

CTDEEP began operating the water quality monitoring program in 1991.

DATA NOTE

*In 2014-2018 there were periods when dissolved oxygen concentrations rose above the hypoxia threshold of 3 mg/L in the early/middle part of August before dipping again during late August and early September. For example, dissolved oxygen levels were greater than 3mg/L for seven days in the middle of August in 2018. Those days are not counted in calculating the duration of hypoxia.

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