Long Island Sound Records Third Smallest ‘Dead Zone’ Since Monitoring Began in 1987

Water Quality Conditions in Long Island Sound Continue to Improve

FOR IMMEDIATE RELEASE

Anya Grondalski, Science Communicator

agrondalski@longislandsoundstudy.net        

[STAMFORD, CT] ­— Scientists from the Long Island Sound Study (LISS) report the third smallest area of hypoxia—zones with low dissolved oxygen—since monitoring began in 1987. During the 2024 summer hypoxia monitoring season, the affected area measured 43.4 square miles, roughly the size of the Bronx, one of New York City’s five boroughs.

The duration of hypoxia measured 38 days, a decrease from 42 days reported for the summer of 2023. The minimum dissolved oxygen level observed during Connecticut Department of Energy and Environmental Protection (CT DEEP) open-water cruises reached 2.25 mg/L. In Long Island Sound, levels below 3 mg/L of oxygen have been considered the threshold for measuring hypoxic area. The minimum dissolved oxygen concentration observed during Interstate Environmental Commission (IEC) western Sound cruises was 0.76 mg/L in Manhasset Bay, New York, observed on August 6. Oxygen levels less than 1 mg/L are considered severely hypoxic and under these conditions, most marine life cannot survive even for short periods. Low levels of dissolved oxygen sometimes occur in bays due to reduced water circulation and high nutrient loads from runoff and wastewater, both associated with urban and suburban development.

The small area of hypoxia this summer has been attributed to weather conditions.

“Most of the summer through August was hot and wet, and repeated wind events mixed oxygen to deep waters which limited the area of hypoxia,” said Jim Ammerman, PhD, the Long Island Sound Study’s Science Coordinator.

Hypoxic areas, commonly referred to as dead zones, are places where water does not have enough dissolved oxygen to support aquatic life and are usually caused by nutrient pollution. Excess nitrogen, a nutrient that promotes algae growth, is a major contributor to hypoxia. When algae, fueled by these excess nutrients, die and bacteria decompose, any oxygen left in the water is used up. Excess nutrients can also increase the occurrence and severity of harmful algal blooms that can be toxic to humans and animals. A lack of oxygen in the water can lead mobile animals, like fish and crabs, to leave the area in search of healthier water. But species that can’t escape, like shellfish and worms, are either harmed significantly or die from suffocation.

Hypoxia can impact coastal waters, rivers, streams, lakes and estuaries like Long Island Sound, and happens most often in the summer when waters are naturally layered, or stratified, due to higher temperatures. Heat from the sun warms the top layer of the Sound, which floats on denser, cooler and saltier water beneath. When water stratifies, mixing of oxygen from the surface to bottom waters is less frequent. Hypoxia usually ends in September when temperatures cool and weather events, such as increased wind, mix water layers and redistribute nutrients and oxygen.

“Physical conditions in the estuary’s waters, such as stratification, create the potential for oxygen to decrease in deeper waters,” said Jim Hagy, PhD, an expert on coastal hypoxia with the US Environmental Protection Agency’s Office of Research and Development. “However, hypoxia is primarily caused by excess nutrients, which increases the amount of organic matter that eventually sinks and decomposes, consuming oxygen faster than it can be replaced.”

Environmental analysts with CT DEEP lead annual water quality monitoring in the Sound through the Long island Sound Water Quality Monitoring program, which is funded by LISS. Dissolved oxygen levels are measured at multiple stations across the estuary bi-weekly from June-September aboard the research vessel John Dempsey. You can find annual WQMP reports here.

Hypoxia is measured in the western Long Island Sound by the Interstate Environmental Commission (IEC), which monitors 22 stations weekly during the summer. 

How Hypoxia is Being Reduced

LISS tracks both the extent (size) and duration (number of days) of hypoxia during summer months. Reducing hypoxia in Long Island Sound is a key priority for LISS and its partners.  To address this issue, a Total Maximum Daily Load (TMDL) agreement was established in 2000 between the EPA and the states of New York and Connecticut to limit nitrogen inputs into the Sound. LISS met its TMDL goal in 2016 and has continued efforts to reduce nitrogen loads by investing in wastewater treatment plant upgrades and addressing non-point source pollution from septic systems, stormwater, and agricultural runoff.

A 2021 research article by University of Connecticut scientists Michael Whitney and Penny Vlahos titled Reducing Hypoxia in an Urban Estuary Despite Climate Warming showed that decreasing trends in the size and volume of dead zones in the Sound coincide with reduced nitrogen loads. Water quality improvements have kept roughly 50 million pounds of nitrogen from polluting the Sound each year.

Predicting Dead Zones

In 2022, EPA Region 2, in collaboration with the EPA’s Office of Research and Development, launched the development of a Long Island Sound Hypoxia Forecast Tool, designed to predict the extent and duration of hypoxia in the Sound and its bays each summer. The tool, set to be released in spring 2025, will combine the work of scientists, researchers and communicators and will act as an engagement opportunity, increasing public awareness of hypoxia and its impact on water quality, habitat and wildlife. Stay tuned for more information about the tool’s release at longislandsoundstudy.net.

“Through consistent investments in research and environmental monitoring, the Long Island Sound science and management community has learned a great deal about hypoxia in Long Island Sound,” said Hagy. “The Long Island Sound Hypoxia Forecast will provide an opportunity to share and highlight that understanding with the public, thus increasing awareness of hypoxia and other nutrient-related issues in the Sound.  While substantial progress has been made in reducing hypoxia in recent decades, there’s more work to do.”