Water Quality

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The Water of Long Island Sound

 

 

The area of Long Island Sound with hypoxic conditions increased in 2021.

 

The maximum area of Long Island Sound with hypoxia, water with dissolved oxygen (DO) concentration less than 3.0 milligrams per liter (mg/l), increased from 63 square miles in 2020 to 142 square miles in 2021. In addition, the duration of the hypoxic conditions increased from 43 days in 2020 to 47 days in 2021.³⁴ The area of hypoxic conditions for 2021 was approximately 30 percent higher than the ten-year average. 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. The average annual dissolved nitrogen concentration at the bottom of the Sound was approximately 0.18 mg/l (.18 parts per million) in 2021; a decrease from 2020 and less than the ten-year average of .21 mg/l.³⁵ However, there were 25 percent and 33 percent less samples taken in 2020 and 2021, respectively compared to previous years.

_{Goal: The goal line on the top chart is an approximation of the maximum area of the hypoxia target adopted in the 2015 edition of the Long Island Sound Study's Comprehensive Conservation and Management Plan to "Measurably reduce the area of hypoxia in Long Island Sound … by 2035, as measured by the five-year running average size of the zone."} 

The amount of nitrogen discharged to the Sound in 2021 was lower than in 2020.³⁶

 

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, nitrogen discharges from point sources have been reduced; however, reducing nitrogen discharges from non-point sources remains a challenge.

_{Goal: Substantial reduction of nitrogen discharges to the Sound is a goal that is shared by Connecticut and New York. Connecticut established a reduction goal of about 6,670 tons annually by 2014, which is the result of a 63.5 percent reduction from the point source baseline of 10,500 tons per year. Therefore, Connecticut’s goal was established as a maximum of 3,830 tons per year.³⁷  Nitrogen discharges “upstream” of Connecticut (Massachusetts, New Hampshire, and Vermont) also contribute to the nitrogen loading in Long Island Sound.}

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³⁴ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, November 28, 2021.}
³⁵ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, March 4, 2022.}
³⁶ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communications from I. Raffa, April 6, 2022.} 
³⁷ _{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.}