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

Area in Square Miles
YearArea of HypoxiaFive-year Rolling Average
1987309
1988251
1989328
1990174
1991122237
199280191
1993202181
1994393194
1995305220
1996220240
199730230
1998168223
1999121169
2000173142
2001133125
2002130145
2003345180
2004202197
2005177197
2006199211
2007162217
2008180184
2009169177
2010101162
2011130148
2012288174
201380154
201487137
201538125
2016197138
20177095
20185289
20198989

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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