Environmental advocates are hailing new findings that show that hypoxia in the Long Island Sound is at its lowest level in nearly four decades, according to the Long Island Sound Partnership.
Hypoxia is an environmental condition where low dissolved oxygen levels are found in the water. The primary cause of hypoxia is nitrogen pollution, which comes from a variety of sources including sewage treatment plants, stormwater runoff and atmospheric conditions. According to officials, hypoxia has impacted up to half of the Long Island Sound’s waters each summer.
Results from the Connecticut Department of Energy and Environmental Protection’s 2025 Long Island Sound Water Quality Monitoring Program showed that hypoxia covered a maximum area of 18.34 square miles this year between July 29 and July 31. Hypoxia conditions also lasted for just 40 days, from July 14 through Aug. 22, a shorter duration than in previous years.
Over the past decade, there were only three other years where the duration of hypoxia was 40 days or less.
Officials said the five-year rolling average for 2021 through 2025 showed 83 square miles of hypoxia in the Long Island Sound compared to an average of 208 square miles from 1987 through 1999, a 60% reduction. This decline in hypoxia in the open waters of the Long Island Sound is being hailed as a massive environmental victory.
“This year’s historic decrease in hypoxia illustrates 40 years of amazing progress through the Long Island Sound Partnership,” said EPA Region 2 administrator Michael Martucci. “Dedicated efforts and investments by EPA, Connecticut, New York and local governments have drastically reduced the amount of nitrogen pollution entering the Sound, resulting in smaller affected areas and fewer days of low oxygen.”
Advocates say this is welcome news for protecting wildlife in the Long Island Sound. Hypoxia is one of the main contributors to mass fish die-offs and can have devastating impacts on fish and shellfish that rely on high levels of oxygen saturation in the water. Save the Sound, an environmental advocacy group, said these results are the reward after decades of improvements in water quality.
“This is a great success story for a rebounding ecosystem,” said Peter Linderoth with Save the Sound.
“In the 1980s, dissolved oxygen leaves were very low and fish die-offs were very common and organisms were dying, literally suffocating in the water because there wasn’t enough oxygen,” Linderoth said. “Hypoxia has plagued the Sound for a very long time, so to see the area of hypoxia to hit such a low last year, it’s just such an amazing success.”
A view of New Haven Harbor and Long Island Sound.
(Peter Marteka/Hartford Courant)
Linderoth noted that Connecticut and New York teamed up in 2000 to implement a plan to limit nitrogen seeping into the Long Island Sound. That plan, he said, made both states take measurable action steps, including fixing old wastewater infrastructure, to ensure less nitrogen gets into the Sound each year. This year also marked the first test of a new Long Island Sound Hypoxia Forecasting Model.
“Developed with scientists from the U.S. EPA, the model projected that up to 31 square miles of bottom waters could experience hypoxia, peaking around mid-August. The actual results showed a smaller and earlier event, roughly two weeks ahead of the forecast but within the predicted range,” according to officials.
While the data shows a big improvement in the Long Island Sound’s water quality, Linderoth said there is still work to be done.
The findings mostly show areas on the open waters of the eastern side of the Long Island Sound and not the coastline, bays or beaches. Linderoth said those areas, often more impacted by humans and stormwater runoff, continue to show signs of hypoxia each year.
The western side of the Long Island Sound, which is closer to large population centers like New York City, is still impacted by hypoxia, Linderoth said.
“As encouraging as this is, there is still hypoxia in the western Sound,” Linderoth said. “Then we have to calibrate for climate change. Long Island Sound is warming up, and warmer waters can’t hold as much oxygen. So as the waters warm up, there might be a need to reassess nitrogen unloading into the Sound at current levels and try to ratchet that down a little more as the Sound continues to warm.
“This also doesn’t take into account some of the hyper-local pollution that goes into our bays and harbors that leads to adverse ecological impacts,” Linderoth added. “Stormwater runoff and local pollution sources can be harder to pin down. That’s the next big thing that the EPA will be addressing in their Long Island Sound plan.”
Save the Sound offers a yearly Long Island Sound Report Card, which grades water quality in five open water regions of the Sound and 57 bay segments along coastlines and estuaries. The advocacy group also releases a yearly Beach Report that scores nearly 200 public and private beaches along the Sound for water quality. More information on the Long Island Sound can be found at savethesound.org.
Stephen Underwood can be reached at [email protected].