CONNECTICUT AND NEW YORK (May 20, 2026)—The Long Island Sound Partnership has released its 2026 forecast for hypoxia in Long Island Sound, projecting another year of relatively limited levels of low-oxygen conditions in the Sound’s bottom waters.
Hypoxia, in Long Island Sound defined as dissolved oxygen levels below 3 milligrams per Liter, is expected to affect a maximum of 15 square miles during the peak summer period. The highest extent is likely to occur around August 2, with a projected range between 8 and 23 square miles.
If realized, the 2026 forecast would represent a modest improvement over 2025 conditions, when hypoxia covered approximately 18 square miles at its peak, the smallest extent recorded since monitoring began in 1987.
“This year’s hypoxia forecast shows Long Island Sound’s water quality is on the right track and that the years-long trend of improvement will continue, building on last year’s historic progress,” said EPA Region 2 Administrator Michael Martucci. “Better oxygen levels mean a healthier Sound, and a healthy Sound benefits people and drives local economies all along its shores.”
“We are seeing improvements in the health of Long Island Sound over time, and these forecasts help us track how the Sound is responding to our ongoing joint efforts,” said EPA New England Regional Administrator Mark Sanborn. “Together with our partners, we are continuing to make progress in improving the health of the Sound and the economies and communities that rely on it.”
The 2026 forecasted hypoxic area is 89 percent smaller than the average extent observed between 1993 and 2009, reflecting long-term reductions in nitrogen loading to the Sound. Excess nitrogen fuels the growth of algae, and oxygen in the water is consumed as the algae die and decompose. Over the past several decades, improvements to wastewater treatment facilities and septic systems have reduced nitrogen loading to Long Island Sound by 65 percent compared to the baseline established in 2000, helping decrease the severity and extent of hypoxia.
Despite this progress, hypoxia levels can vary significantly from year to year, with observed conditions often fluctuating by as much as plus or minus 50 percent of the predicted extent based on environmental conditions.
“Year to year variation is expected when it comes to hypoxia in Long Island Sound,” said James Ammerman, environmental analysist with NEIWPCC and Long Island Sound Partnership Science Coordinator. “While reducing nitrogen is key, and something we can directly manage, other factors such as water temperature, rainfall, wind, and storm activity can also influence how hypoxia develops each year. Even with that variability, long-term trends show how the Sound is getting healthier, and reducing nutrient inputs remains the most effective way to lower the risk of more severe hypoxia over time.”
A key question for 2026 is whether hypoxia will be as limited as it was in 2025. Based on long-term observations and model results, scientists who worked on the forecast suggest conditions could be similar or even slightly improved this year. However, after several years of declining hypoxia, a modest rebound toward slightly more hypoxia is also possible.
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