Fact Sheet: Dissolved Oxygen Concentrations in Long Island Sound

Oxygen concentrations in Long Island Sound are improving

Click or tap the arrow to view the annual maximum extent of hypoxia in Long Island Sound from 19912023.

Like humans, fish and other aquatic life need oxygen to breathe. But around the world, coastal water bodies such as Long Island Sound face extreme low oxygen conditions known as hypoxia, or anoxia (no oxygen at all) that severely threatens wildlife. Humans help drive deoxygenation of coastal waters through eutrophication, a process in aquatic ecosystems where high nutrient concentrations stimulate blooms of algae, usually resulting in low dissolved oxygen levels in the water. Nutrients from the land, including from human waste, animal waste, and fertilizer, are prime sources of the nutrient concentrations.

Since the early 2000s, the Sound has seen a significant reduction in nitrogen, an essential nutrient for plant growth, thanks to a bi-state effort to upgrade wastewater treatment plants. The reduced nitrogen levels has been cited by scientists as an important reason why the Sound has experienced a decline in hypoxia. The animated maps in the video shows charts detailing the week each year when the maximum area of hypoxia was reported by the Long Island Sound Water Quality Monitoring program from 1991-2022. The year with the highest maximum area of hypoxia reported was in 1994 when 393 square miles, 37% of the Sound, had hypoxia. In 2022 the maximum area of hypoxia was 87 square miles, 8% of the Sound.

Hypoxia in Long Island Sound is defined as dissolved oxygen concentrations of less than three milligrams per liter of water. In the maps, blue is considered excellent water quality conditions. Orange is moderately hypoxic, red is moderately severe hypoxic, and black is severely hypoxic to anoxic conditions.

Source: charts produced by the Connecticut Department of Energy and Environmental Protection for the Long Island Sound Water Quality Monitoring Program. Animation by Abbie Winter, CT DEEP.

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