Photos of the Long Island Sound

Research & Monitoring

Status and Trends: LISS Environmental Indicators

Duration of Hypoxia

LISS Water Quality Monitoring Program, CTDEEP

Duration of Hypoxic Conditions
Start Date Duration End Date
1987 58
1988 73
1989 82
1990 49
1991 7/19 41 8/28
1992 7/7 55 8/30
1993 7/9 64 9/10
1994 7/1 68 9/6
1995 7/12 35 8/15
1996 8/10 34 9/12
1997 7/27 48 9/12
1998 7/5 73 9/15
1999 7/2 51 8/21
2000 7/2 35 8/6
2001 7/10 66 9/14
2002 6/25 65 8/28
2003 7/5 61 9/3
2004 7/20 55 9/12
2005 7/14 69 9/20
2006 7/6 53 8/27
2007 7/16 58 9/11
2008 7/3 79 9/19
2009 7/19 45 9/1
2010 7/5 40 8/13
2011 7/6 54 8/28
2012 7/10 63 9/10
2013 7/8 62 9/7
2014 7/24 35 9/9
2015 7/16 57 9/10

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 average duration of hypoxia in Long Island Sound from 1991 to 2013 was 55 days per year, with a range of 34 to 79 days. In 2014, the duration of hypoxia was only 35 days, which is well below average.  Although the start and end dates were relatively typical (mid-July to early September), usually hypoxia persists throughout this period, while in 2014 a large wind and rain event caused mixing which ventillated the water and alleviated hypoxia during much of August.

CTDEEP began operating the water quality monitoring program in 1991.

*In 2014 there was a clear period of 14 days where the DO concentration rose above the 3.0 mg/L threshold in the middle of August before dipping again during late August and early September.

 

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