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Status and Trends: LISS Environmental Indicators

Type of Indicators: Health/Condition Response/Performance Socio-Economic Historical/Background

Water Quality Index

  • Averages (1991-2010)
  • The Narrows
  • Western Basin
  • Central Basin
  • Eastern Basin

Source: EPA National Coastal Assessment (NCA), LISS Water Quality Monitoring Program, CTDEEP, Interstate Environmental Commission (IEC)

Western

Good Fair Poor
1991 0.00 91.67 8.33
1992 33.33 66.67 0.00
1993 33.33 61.11 5.56
1994 33.33 66.67 0.00
1995 38.98 59.32 1.69
1996 35.00 63.33 1.67
1997 24.07 74.07 1.85
1998 41.38 53.45 5.17
1999 40.98 49.18 9.84
2000 20.22 73.03 6.74
2001 4.48 86.57 8.96
2002 6.67 88.33 5.00
2003 20.31 76.56 3.13
2004 13.43 77.61 8.96
2005 18.75 58.75 22.50
2006 23.81 65.08 11.11
2007 26.00 68.00 6.00
2008 26.67 73.33 0.00
2009 33.33 66.67 0.00
2010  23.33 73.33 3.33
Central
Good Fair Poor
1991 50.00 50.00 0.00
1992 66.67 33.33 0.00
1993 66.67 33.33 0.00
1994 66.67 33.33 0.00
1995 75.00 25.00 0.00
1996 75.00 25.00 0.00
1997 69.57 30.43 0.00
1998 79.17 16.67 4.17
1999 75.00 25.00 0.00
2000 41.18 58.82 0.00
2001 32.00 68.00 0.00
2002 37.50 62.50 0.00
2003 40.00 60.00 0.00
2004 51.85 48.15 0.00
2005 37.50 59.38 3.13
2006 48.28 51.72 0.00
2007 60.00 40.00 0.00
2008 56.52 43.48 0.00
2009 62.50 37.5 0.00
2010  50.00 50.00 0.00
Eastern
Good Fair Poor
1991 100.00 0.00 0.00
1992 83.33 16.67 0.00
1993 100.00 0.00 0.00
1994 83.33 16.67 0.00
1995 88.89 11.11 0.00
1996 94.44 5.56 0.00
1997 75.00 25.00 0.00
1998 100.00 0.00 0.00
1999 100.00 0.00 0.00
2000 95.00 5.00 0.00
2001 68.75 31.25 0.00
2002 72.22 27.78 0.00
2003 75.00 25.00 0.00
2004 84.21 15.79 0.00
2005 60.00 40.00 0.00
2006 54.55 45.45 0.00
2007 86.67 13.33 0.00
2008 88.24 11.76 0.00
2009 86.67 13.33 0.00
2010  80.00 20.00 0.00
The Narrows
Good Fair Poor
2002 12 45.6 42.4
2003 5.3 54.4 40.3
2004 0 47.4 52.6
2005 4.1 52 43.9
2006 0.8 70.6 28.6
2007 3.6 71.2 25.2
2008 4.8 70.6 24.6
2009 14.3 64.3 21.4
2010 3.03 86.36 10.61

What is the Water Quality Index?

The EPA’s National Coastal Assessment (NCA) index  has been used to evaluate water quality trends in Long Island Sound over the last two decades.  The NCA index is based on five chemical and biological measures:

  • Nitrogen (Dissolved inorganic nitrogen in surface waters)
  • Phosphorus (Phosphate, or PO4, in surface waters)
  • Chlorophyll a (in surface waters)
  • Dissolved Oxygen (in bottom waters)
  • Water Clarity (Secchi disk depth)

Good water quality is defined here as water containing low concentrations of nitrogen, phosphorus and chlorophyll a, high concentrations of dissolved oxygen and high water clarity.  The NCA Index Thresholds (below) rate each measurement as good, fair or poor based on the following thresholds:

DIN Surface PO4 Surface Chl a Surface Oxy Bottom Secchi Depth
Units mg/L mg/L ug/L mg/L meter
Threshold 1 0.1 0.01 5 2 0.7
Threshold 2 0.5 0.05 20 5 1.1

How is the Water Quality Index calculated in long island sound?

Monthly data from May to October were collected from 1991 to 2009 from the Long Island Sound Study water quality monitoring program.  Stations in the Long Island Sound Study monitoring program were grouped into Eastern, Central and Western basins (designations based on Kaputa and Olsen 2002, see reference below).

Monthly data from June through September were collected from 2001 to 2009 from the Interstate Environmental Commission Ambient Water Quality monitoring program in the far western Long Island Sound.  This monitoring program does not collect nitrogen or phosphorus data, so the water quality index is based solely on chlorophyll a concentration, dissolved oxygen concentration and secchi disk depth.  The results for this part of Long Island Sound are reported below as The Narrows.

What does this indicate?

The water quality index is a calculation that combines several water quality measurements to rate overall water quality in Long Island Sound. The index incorporates physical, chemical and biological components. Nitrogen and phosphorus are included because of their contributions to eutrophication in the Sound.  Chlorophyll a concentration is included because it is a measurement of phytoplankton in the water, and too much phytoplankton has been linked to hypoxia.  Dissolved oxygen concentration is included because of the harmful effects of hypoxia on living marine resources.  Secchi depth is included because it is a measurement of water clarity.

Water quality within the Sound can be influenced both directly and indirectly by many factors.  Pollution, population density, development, runoff and other factors all contribute to changes in water quality.

Status

The western basin consistently had a lower water quality index, with a rating of “fair” the majority of the time over the last 20 years. The water quality index was generally higher in the central basin, and in the eastern basin water quality was most often rated “good.” The gradient in improving water quality from the west to the east is a reflection of decreasing population, development and other anthropogenic influences along the same distance.

Data Notes

The water quality index is calculated based on spatial as well as temporal data. The data reported above are based on an average that includes several stations in each basin as well as several time points in which these stations were sampled every year. The units used above, “percentage of station visits,” represents the average of all stations visited from April through October each year.

Basin designations can be found in Kaputa, Nicholas P., and Christine B. Olsen. (2000) Long Island Sound summer hypoxia monitoring survey
1991-1998 data review. CTDEEP Bureau of Water Management, Planning and Standards Division, 79 Elm Street, Hartford, CT 06106-5127, 45 p.

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Long Island Sound with Basins
 
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