Long Island Sound reached a major milestone in water quality this year, with hypoxia, or low oxygen in bottom waters, at its lowest since the Long Island Sound Partnership began tracking the levels in 1987. That means fewer areas of the Sound were uninhabitable for fish, shellfish, and other aquatic animals.
According to results from the Connecticut Department of Energy and Environmental Protection’s 2025 Long Island Sound Water Quality Monitoring Program, hypoxia covered a maximum area of 18.34 square miles (47.5 square kilometers) for the year between July 29 and July 31. Hypoxia, or low-oxygen, conditions persisted for 40 days, from July 14 through August 22, a shorter duration than in many previous years. Over the past decade there were only three other years where the duration of hypoxia was 40 days or less.
Scientists say these results reflect ongoing improvements in water quality, supported by decades of work by federal, state, and local partners to reduce nitrogen pollution through the Long Island Sound Partnership. The Partnership is comprised of the Connecticut and New York Sea Grant programs, the EPA and other state agencies and non-profits with an interest in the Sound.
“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.”
Improved oxygen conditions benefit the Sound’s ecosystems and the communities that depend on healthy waters for fishing, recreation, and coastal livelihoods.
Hypoxia happens when excess nutrients, especially nitrogen from wastewater and stormwater runoff, fuel algae growth. When algae or the organisms that eat algae die, they sink to the bottom where bacteria and other microbes feed on them, using up oxygen in the process. This leaves fish and shellfish stressed or displaced and, in severe cases, causes die-offs.
This year also marked the first test of a new Long Island Sound Hypoxia Forecasting Model. Developed with scientists from the US 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.
Tools like the Hypoxia Forecasting Model and the new Hypoxia Toolkit, a collection of educational resources, maps, and animations, help scientists, managers, and residents better understand what’s happening beneath the surface and what actions make a difference.
Residents in the Long Island Sound watershed can help limit excess nutrient runoff with simple actions such as reducing fertilizer use, cleaning up after pets, and keeping storm drains clear.
Learn more about hypoxia in Long Island Sound and explore the Hypoxia Toolkit at https://lispartnership.org/our-vision-and-plan/clean-waters-and-healthy-watersheds/reduce-nutrients/hypoxia-toolkit/.