The Water of Long Island Sound
The area of Long Island Sound with hypoxic conditions decreased in 2020.
The area of Long Island Sound with hypoxia, water with dissolved oxygen (DO) concentration less than 3.0 milligrams per liter (mg/l), decreased from 89 square miles in 2019 to 63 square miles in 2020. In addition, the duration of the hypoxic conditions decreased from 48 days in 2019 to 43 days in 2020. Both area and duration data for 2020 was less than the ten-year average. Most, if not all, of the hypoxic conditions are found in the western basin of the Sound. 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 dissolved nitrogen concentration at the bottom of the Sound was approximately 0.2 mg/l (.2 parts per million) in 2020; an increase from 2019, but less than the ten-year average of .21 mg/l.27 However, there were 25 percent less samples taken in 2020 compared to previous years.
Goal: The goal line on the top chart is an approximation of the maximum area of 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 2020 was lower than 2019.28
Connecticut has reduced nitrogen discharges by approximately 23 percent 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 and the area of hypoxia in the Sound (see above) has been reduced. 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 by 2014.29 Nitrogen discharges “upstream” of Connecticut (Massachusetts and Vermont) also contribute to the nitrogen loading in Long Island Sound.
27 DEEP, Long Island Sound Water Quality and Hypoxia Monitoring Program; portal.ct.gov/DEEP/Water/LIS-Monitoring/LIS-Water-Quality-Monitoring-Maps
28DEEP, Nitrogen Control Program for Long Island Sound; Personal communications from I. Raffa, March 12, 2021
29 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