Ecosystem Targets and Supporting Indicators

Water Quality Index

The water quality index is a calculation that combines several water quality measurements to rate overall water quality in Long Island Sound on an annual basis.

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Show/Hide Table Data

The Narrows
	Good	Fair	Poor
2002	12.00	45.60	42.40
2003	5.30	54.40	40.30
2004	0.00	47.40	52.60
2005	4.10	52.00	43.90
2006	0.80	70.60	28.60
2007	3.60	71.20	25.20
2008	4.80	70.60	25.20
2009	14.30	64.30	21.40
2010	3.03	86.36	10.61
2011	19.30	63.16	17.54

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	39.98	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
2011	22.22	65.74	12.04

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	37.50	0.00
2010	50.00	50.00	0.00
2011	46.81	48.94	4.26

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
2011	53.85	46.15	0.00

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 (click “Show/Hide Table Data”  to view) 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 2011 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 2011 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.