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Sustained high temperatures and continued drought this summer have led to below-normal streamflows in the Northeast, which can adversely affect water quality. Extreme fluctuations in dissolved oxygen levels were detected by three U.S. Geological Survey (USGS) continuous water-quality monitoring stations in the Norwalk River in Connecticut.

The sensors captured a pattern of high dissolved oxygen levels during the day and low dissolved oxygen levels at night on dates when air and water temperatures were above average.

“We have been seeing low dissolved oxygen levels less than 2 milligrams per liter in the upper Norwalk River, which can cause significant adverse ecological effects, according to the Long Island Sound Study,” said USGS Supervisory Hydrologic Technician Brittney Izbicki. The U.S. Environmental Protection Agency considers dissolved oxygen levels under 1 milligram per liter to be hypoxic, or low oxygen, and typically unable to support life. Most living organisms require oxygen to survive, and if dissolved oxygen levels remain abnormally low it can lead to fish kills.

Map of Norwalk Embayment Monitoring Sites
Locations of Norwalk embayment monitoring sites in Connecticut. Embayment monitoring is conducted to support nutrient management activities in Connecticut for the Long Island Sound.

The water-quality monitoring sites in the Norwalk River are situated at Ferry Point, the Maritime Aquarium at Norwalk, and Norwalk Cove Marina, where the water characteristics are more representative of Long Island Sound. State of Connecticut water quality standards for dissolved oxygen in this embayment is 5 milligrams per liter.

On multiple dates in July, the most upstream monitoring station in the river, at Ferry Point, presented dissolved oxygen levels of more than 20 milligrams per liter during the day and then dropped to near-zero levels at night at the water’s surface. Similarly, the Maritime Aquarium monitoring station at the water’s surface measured dissolved oxygen exceedances of more than 14 milligrams per liter during daylight hours, and then became hypoxic and anoxic, or having no oxygen, at night.

While this data is provisional and a study has not been completed, prior research has shown that long stretches of sunny, hot weather can boost algal growth, especially in water bodies enriched with nutrients. Algal blooms can trigger a chain-reaction of processes that lead to extreme changes in the dissolved oxygen levels in the water.

These USGS water-quality monitoring stations are a part of a larger project carried out in cooperation with Connecticut Department of Energy and Environmental Protection (CT DEEP). Water-quality and hydrologic data are being collected at four Connecticut coastal bays bordering Long Island Sound: Mystic, Norwalk, Saugatuck, and Sasco-Southport complex. USGS and CT DEEP will continue to monitor these sites over the next two to three years, amassing seasonal and spatial data to create models that will best define how nutrient pollution affects coastal embayments that drain into the Sound.

For more information on this project, click here: Embayment Monitoring to Support Nutrient Management Activities in Connecticut for the Long Island Sound | U.S. Geological Survey (

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