The Long Island Sound reached a big milestone in reducing pollution and becoming more suitable for marine life in 2025, according to local environmental groups.
During the summer of 1994, about a third of the 1,3000-square-mile tidal estuary was considered by scientists to be dead zones — waters with hypoxia, or very levels of dissolved oxygen — because of nitrogen pollution. But in 2025, the dead zones covered roughly 18 square miles, the lowest amount since state and federal agencies began monitoring the Sound in 1987.
“Back in the 1980s, dissolved oxygen levels were severely low, and fish and other organisms would literally suffocate in the water because there wasn’t enough oxygen,” said Peter Linderoth, director of healthy waters and lands for Save the Sound. “To see areas of hypoxia reduced to such a low tells a success story of a rebounding ecosystem.”
The hypoxia that has plagued the Sound for decades stems mainly from sewage plants. High concentrations of nitrogen lead to oxygen levels so low in the water that marine life cannot survive. Environmentalists hail the federal and state $2.5-billion, two-decade joint effort as the reason for the dramatic improvements.
The reduction in hypoxia is the result of controlling only one source of nitrogen pollution, sewage plants. Much of the funding went toward upgrading sewage plants to remove nitrogen. The most effective retrofits were made to New York City facilities around LaGuardia Airport and Rikers Island.
“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,” Environmental Protection Agency Region 2 administrator Michael Martucci wrote in a statement.
Despite the pollution reductions, Linderoth said there's still substantial work that must be done to continue improving the water quality in the Long Island Sound. Stormwater runoff and septic tanks are significant contributors for nitrogen pollution. About 75% of Suffolk County homes have septic systems, according to the county’s health department.
Nitrogen population is a common problem worldwide. According to the National Oceanic and Atmospheric Administration, the hypoxia in the Gulf of Mexico can be as large as the state of Massachusetts, and is primarily caused by fertilizer use, which is unregulated.
“In Long Island Sound, the main source has been a sewage treatment plants, and that's why the upgrade has made such a difference,” said James Ammerman, science coordinator for the Long Island Sound Partnership. “[Other] locations, like the Gulf of Mexico and the Chesapeake Bay, have major agricultural inputs of nitrogen as well, which is not regulated the way treatment plants are. Many of those places have not made much progress in decreasing their hypoxia.”
In 2001, the EPA, Connecticut Department of Energy and Environmental Protection and the New York State Department of Environmental Conservation agreed on a plan to reduce human-generated nitrogen by 58.5%. This was done primarily by upgrading the sewage plants.
“That tells us that the $2.5 billion that has been spent to upgrade major treatment plants to decrease the nitrogen has greatly reduced hypoxia, which means that the bottom waters are far more habitable for fish in other organisms,” Ammerman said.
The dead zones appear during the summer, especially in the bays where the water is shallow and near the western section, near New York City. Warm temperatures and calm waters are a perfect recipe for hypoxia. The dead zones can persist for a month or more.
According to the Long Island Sound partnership, the average dead zone lasted 58 days in 1987. This year, it dropped to 40 days. The longest recorded dead zone was 82 days in 1989. A 2024 Stone Brook University study mapped 36 dead zones in the Sound’s waters.
Excessive nitrogen pollution causes large algal blooms. When the algae dies and decomposes, bacteria deplete the oxygen in the water. As a result, mass fish die-offs are common in Long Island Sound. In 2022, there were around 50 reported mass die-offs of fish in Long Island Sound, a record high, according to Stony Brook University.
”We need to further reduce the nitrogen if we want to make further progress,” Ammerman said. “The other challenge is that as climate change warms the Sound, which it's clearly doing, warmer water holds less oxygen, so that will begin to undo our progress if we don't make further progress.”
