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Hypoxia in Long Island Sound hits lowest level in nearly 40 years, officials say
By
Austin Mirmina,Staff WriterUpdated Dec 8, 2025 8:50 a.m.
A fisherman at Burying Hill Beach, in Westport, Conn. in 2023. Hypoxia in Long Island Sound fell to a 30-year low in 2025, highlighting the success of ongoing cleanup efforts, officials said.
Ned Gerard/Hearst Connecticut Media
Hypoxia levels in Long Island Sound fell to their lowest point in nearly 40 years – a milestone officials say proves that
efforts to restore the East Coast's second-largest estuary are
paying off.
New state water quality data for 2025 shows that the Sound's
oxygen-starved "dead zones" shrank to their smallest extent since monitoring began in the late-1980s. The zones were also shorter-lived than in most recent years, improving conditions for fish, shellfish and other marine life, as well as for shoreline communities that depend on the Sound.
The Sound's hypoxic area peaked at 18.34 square miles between July 29-31 – down from about 43 square miles last year, according to results from the state Department of Energy and Environmental Protection’s Long Island Sound Water Quality Monitoring Program.
In 2025, hypoxic conditions, defined as dissolved oxygen levels below 3 mg/L, lasted for 40 days, from July 14 to August 22, data shows. That's just the fourth time in the past decade that hypoxia persisted for 40 days or less, according to the
Long Island Sound Partnership (LISP), which tracks hypoxia in the Sound using DEEP data. Hypoxia is more common during warmer months and at lower depths.
Between 2021-25, the average hypoxic area in the Sound was 83 square miles – nine fewer than the previous five-year period and a dramatic 60% drop from the baseline average of 208 square miles recorded between 1987-99, the LISP said. The group uses a five-year rolling average because year-to-year conditions can vary widely with extreme heat or rainfall.
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Scientists attributed the water quality gains to decades of work by federal, state and local agencies following a
2000 joint agreement to reduce nitrogen pollution, a main driver of hypoxia.
“This year’s historic decrease in hypoxia illustrates 40 years of amazing progress through the Long Island Sound Partnership,” Michael Martucci, a Region 2 administrator with the Environmental Protection Agency, said in a statement. “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.”
The LISP said this summer's dry weather likely helped keep hypoxia in check. When it rains, nutrients from wastewater and stormwater wash into the Sound, fueling algae growth. The algae eventually decays and consumes oxygen, leaving too little for fish and shellfish.
If the dissolved oxygen concentration in water falls to hypoxic levels (typically 2-3 mg/L), organisms will migrate out of the area, according to the
EPA. But less mobile and immobile animals that can't move to more oxygenated waters, like mussels and crabs, are often killed during hypoxic events, sometimes causing die-offs.
Hypoxia in the Sound reached a breaking point in the late 1980s and early 1990s, when a steady increase in population and poorly treated sewage led to harbors full of dying fish and shellfish, dirty beaches and waters almost devoid of oxygen. Water quality west of Bridgeport was especially bad, with
dead fish, garbage and wiped-out shellfish beds.
“I grew up in Connecticut and remember all the times the beaches were shut down because it was unsafe to swim,” New Haven Mayor Justin Elicker said in 2018, then the executive director of the city's Land Trust. “But now, even with all the challenges, you see how far we’ve come.”
In 2000, the EPA, along with Connecticut and New York, agreed to a plan designed to drastically cut nitrogen pollution that was fueling the Sound's chronic hypoxia problem. The plan required wastewater treatment plants in the western Sound to reduce nitrogen runoff by 60%, a target that was reached in 2016.
New York and Connecticut have spent $2.5 billion upgrading wastewater treatment plant, cutting nitrogen discharges by 49 million pounds annually compared to the early 1990s, LISP science coordinator James Ammerman wrote in a
2024 article.
Other efforts include routine hypoxia monitoring, conducted by the University of Connecticut from 1987-91 and by DEEP since then. Two other monitoring programs, run by the Interstate Environmental Commission and the Long Island Sound Integrated Coastal Observing System, provide additional data. Together, these three programs give a comprehensive view of both the area and duration of hypoxia across the Sound.
The
water quality improvements have come even as a warming climate puts coastal and marine ecosystems at greater risk of hypoxia. According to the EPA, stronger storms and warmer waters can cause layers to form in the water, bring in more nutrients and reduce the amount of available oxygen for marine life.
The LISP said that while the decline of hypoxia in the Sound over the past five years is a "major achievement," further reductions are needed to "fully attain water quality standards and achieve the ecosystem target goal" by 2035.
Residents can help limit excess nutrient runoff by reducing fertilizer use, cleaning up after pets and keeping storm drains clear.
Correction: This story and headline have been corrected to reflect that hypoxia levels are at their lowest in nearly 40 years.
Dec 8, 2025|Updated Dec 8, 2025 8:50 a.m.
Austin Mirmina
Reporter
Austin Mirmina is a reporter with CT Insider where he covers the Long Island Sound, among other topics. He joined Hearst Connecticut Media Group in 2022 and previously worked for the Journal Inquirer.
