Water Quality

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The Water of Long Island Sound

Climate Change Indicator

Summary Chart x x check 
The area of Long Island Sound with hypoxic conditions increased in 2023.

The maximum area of Long Island Sound with hypoxia, which is water with a dissolved oxygen (DO) concentration less than 3.0 milligrams per liter (mg/l), increased from 87 square miles in 2022 to approximately 127 square miles in 2023. The area of hypoxia for 2023 was also approximately 28 percent greater than the previous ten-year average. However, the duration of the hypoxic conditions decreased from 57 days in 2022 to 42 days in 2023.35 Most, if not all, of the hypoxic conditions are found in the western basin of the Sound, which is also affected by contributions from New York State. The primary cause of hypoxia is nutrient pollution, primarily nitrogen and phosphorus, from runoff and wastewater treatment effluent that fuels the growth of phytoplankton in the Sound. Several factors, such as the state’s Nitrogen Control Program, improvements to wastewater treatment facilities, and better controls for stormwater runoff have reduced the area of hypoxic conditions in the Sound since pre-2000 levels.

Goal: The goal line on the top chart is an approximation of the maximum area (~150 miles2) of the hypoxia target to "measurably reduce the area of hypoxia in Long Island Sound … by 2035, as measured by the five-year running average…” (2015).36

Nitrogen discharged in 2022 was lower than the previous ten-year average.

Connecticut has reduced nitrogen discharges over the last decade by investing in nitrogen-removal technology at sewage treatment plants and implementing a Nitrogen Control Program; however, reducing nitrogen discharges from non-point sources remains a challenge. The decline in nitrogen discharged in 2022 (most recent data available) is attributed to warmer weather, which is more conducive to nitrogen removal, and because 64 municipalities out of 78 participating in nitrogen credit exchange program upgraded wastewater treatment facilities that enhance nitrogen removal.37

Goal: Substantial reduction of nitrogen discharges is a goal that is shared by Connecticut and New York (2001). Connecticut established a goal of a maximum of 3,830 tons per year of nitrogen by 2014.38  Nitrogen discharges “upstream” of Connecticut also contribute to the nitrogen loading.


35 DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, November 29, 2023; portal.ct.gov/DEEP/Water/LIS-Monitoring/LIS-Water-Quality-Monitoring-Maps.
36 Long Island Sound Study Comprehensive Conservation and Management Plan, 2015; longislandsoundstudy.net/our-vision-and-plan/.

37 DEEP, Nitrogen Control Program for Long Island Sound; personal communications from I. Raffa, March 17 and 24, 2023.

38 DEEP, Nitrogen Control Program for Long Island Sound; The Long Island Sound TMDL Frequently Asked Questions; portal.ct.gov/-/media/DEEP/water/lis_water_quality/nitrogen_control_program/tmdlfaqpdf.pdf.