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

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Show/Hide Table Data

Duration (days)
Year	Number of Days	Five-year Moving Average
1987	58
1988	73
1989	82
1990	49
1991	41	61
1992	55	60
1993	64	58
1994	68	55
1995	35	53
1996	34	51
1997	48	50
1998	73	52
1999	51	48
2000	35	48
2001	66	55
2002	65	58
2003	61	56
2004	55	56
2005	69	63
2006	53	61
2007	58	59
2008	79	63
2009	45	61
2010	40	55
2011	54	55
2012	63	56
2013	62	53
2014	35	51
2015	57	54
2016	51	54
2017	26	46
2018	35	41
2019	48	43
2020	43	41
2021	47	40
2022	57	46
2023	42	47
2024	38	45

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 2023 hypoxic event lasted an estimated 42 days, beginning on July 12 and ending August 22.  The duration was below the average of 54 days for the entire time series.

CT DEEP began operating the water quality monitoring program in 1991.