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Status & Trends

LISS Ecosystem Targets and Supporting Indicators

Severely Hypoxic and Anoxic Areas

( Click labels in legend to hide data and adjust scale )
Area of Hypoxia Severity (sq. miles)
  Hypoxia Severe Hypoxia Anoxia
1987 309 no data no data
1988 251 no data no data
1989 328 no data no data
1990 174 no data no data
1991 122   36  0
1992 80    7  0
1993 202    9  1
1994 393 212 28
1995 305   71 18
1996 220   27 4
1997 30   2 0
1998 168  43 7
1999 121  52 7
2000 173  52 0
2001 133  32 0
2002 130  61 42
2003 345 186 62
2004 202   62 38
2005 177   95 28
2006 199   61 18
2007 162   30   0
2008 180   71 40
2009 169   19   0
2010 101   47   0
2011 130   35   0
2012 289   67 18
2013 81   33   0
2014 87   20   0
2015 38    0   0
2016 197  40   0
2017 70   5   0
2018 52  0  0
2019 89  21  1

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 visit: http://longislandsoundstudy.net/about/our-mission/management-plan/hypoxia/

WHAT IS severe HYPOXIA?

The Long Island Sound Study defines dissolved oxygen concentrations of less that 2 mg/L as severely hypoxic. In most other ecosystems with similar oxygen depletion problems, like the Chesapeake Bay and the northern Gulf of Mexico, 2 mg/L of dissolved oxygen is the upper limit for hypoxia.

WHAT IS ANOXIA?

Anoxia is typically defined as the complete lack of oxygen or often less than 0.2 mg/L. The Long Island Sound Study, however, defines anoxia as oxygen concentrations below 1 mg/L because this is the threshold below which most marine animals cannot survive even for a short period of time.

WHAT DOES THIS INDICATE?

The area of hypoxia in Long Island Sound refers to the number of square miles in which dissolved oxygen concentrations were less than 3 mg/L over the course of a single year. 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 can be dissolved in the water). The area of hypoxia is calculated from measurements taken every other week during summer months as part of the Long Island Sound Study Water Quality monitoring program by the Connecticut Department of Energy and Environmental Protection.

STATUS

The five-year rolling average (2015-2019) of hypoxia is 89  square miles compared to an average of 208 square miles from 1987-2000, a 57 percent reduction (see extent of Hypoxia ecoystem target). The hypoxic area also has declined by 63 percent from the peak five-year period (1992-1996), which was 240 square miles.  Based on the last 20 years of interannual variability, a 28 percent reduction is necessary to achieve a measurable reduction (see data note). Further reductions in the area of hypoxia are needed in order to fully attain water quality standards for dissolved oxygen.

In 2019 severe hypoxia covered 21 square miles of the Sound, relative to the 1991-2019 average of 48 square miles.

Anoxia covering 1 square mile was detected in the Long Island Sound water quality monitoring program in the western Sound for the first time since 2012, when it affected 18 square miles. While the overall average area of anoxia is 11 square miles from 1991-2019, the variability in area of anoxia from year to year can be quite high. No anoxia was detected in the Sound during 14 of the last 27 years, but the area affected by anoxia has been as high as 61.7 square miles (in 2003). Other monitoring programs by the Interstate Environmental Commission and the Long Island Sound Integrated Coastal Observing System that monitor western Long Island Sound more intensively have found more frequent anoxia in this area.  

For reference, the entire area of Long Island Sound is about 1,300 square miles.

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