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.

Show/Hide Table Data

The Narrows
GoodFairPoor
200212.0045.6042.40
20035.3054.4040.30
20040.0047.4052.60
20054.1052.0043.90
20060.8070.6028.60
20073.6071.2025.20
20084.8070.6025.20
200914.3064.3021.40
20103.0386.3610.61
201119.3063.1617.54
Western
GoodFairPoor
19910.0091.678.33
199233.3366.670.00
199333.3361.115.56
199433.3366.670.00
199539.9859.321.69
199635.0063.331.67
199724.0774.071.85
199841.3853.455.17
199940.9849.189.84
200020.2273.036.74
20014.4886.578.96
20026.6788.335.00
200320.3176.563.13
200413.4377.618.96
200518.7558.7522.50
200623.8165.0811.11
200726.0068.006.00
200826.6773.330.00
200933.3366.670.00
201023.3373.333.33
201122.2265.7412.04
Central
GoodFairPoor
199150.0050.000.00
199266.6733.330.00
199366.6733.330.00
199466.6733.330.00
199575.0025.000.00
199675.0025.000.00
199769.5730.430.00
199879.1716.674.17
199975.0025.000.00
200041.1858.820.00
200132.0068.000.00
200237.5062.500.00
200340.0060.000.00
200451.8548.150.00
200537.5059.383.13
200648.2851.720.00
200760.0040.000.00
200856.5243.480.00
200962.5037.500.00
201050.0050.000.00
201146.8148.944.26
Eastern
GoodFairPoor
1991100.000.000.00
199283.3316.670.00
1993100.000.000.00
199483.3316.670.00
199588.8911.110.00
199694.445.560.00
199775.0025.000.00
1998100.000.000.00
1999100.000.000.00
200095.005.000.00
200168.7531.250.00
200272.2227.780.00
200375.0025.000.00
200484.2115.790.00
200560.0040.000.00
200654.5545.450.00
200786.6713.330.00
200888.2411.760.00
200986.6713.330.00
201080.0020.000.00
201153.8546.150.00

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 SurfacePO4 SurfaceChl a SurfaceOxy BottomSecchi Depth
Unitsmg/Lmg/Lug/Lmg/Lmeter
Threshold 10.10.01520.7
Threshold 20.50.052051.1

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.

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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