2019 CEQ Annual Report

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Water Quality

Warming and Rising Waters               Swimming               Rivers, Lakes, and Estuaries               Drinking Water

The Water of Long Island Sound

Climate Change Indicator

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The area of Long Island Sound with low levels of dissolved oxygen increased in 2019, but is on track to meet the 2035 goal.

The area of Long Island Sound with hypoxia, water with dissolved oxygen (DO) concentration less than 3.0 milligrams per liter (mg/l), increased in 2019 to 89 square miles from 52 square miles in 2018, but was less than the ten year average of 121 square miles. In addition, the number of days that Long Island Sound experienced hypoxic conditions increased in 2019 to 48 days from 35 days in 2018 and was higher than the 10 year average of 47 days. The primary cause of hypoxia is nutrient pollution, primarily nitrogen and phosphorus nutrients from runoff and wastewater treatment effluent that fuels the growth of phytoplankton in the Sound.

Goal: (updated) The goal line on the top chart, set at 149.8 square miles, is an approximation of the target adopted in the 2015 edition of the Long Island Sound Study's Comprehensive Conservation and Management Plan. That plan's goal calls for "measurably reducing the area of hypoxia in Long Island Sound from pre-2000 averages."


The amount of nitrogen discharged to the Sound was lower than 2018, but higher than the goal line.


Connecticut has reduced nitrogen discharges, and consequently the area of hypoxia in Long Island Sound, over the long term, by investing in nitrogen-removal technology at sewage treatment plants and has implemented a Nitrogen Control Program; however, more needs to be done.

Connecticut's Efforts to Reduce Nitrogen Discharges to Long Island Sound is Having Success

new data available graohic


Technical Note: New data for dissolved nitrogen for 2019 added July 6, 2020.