Hypoxia

In 2025, the Long Island Sound had an all-time low of 18 square miles with an average (from 2021-2025) of 83 square miles – a 60 percent reduction from the baseline. The 2025 hypoxic event lasted an estimated 40 days, beginning on July 14 and ending August 22.  The duration was below the average of 51 days for the entire time series. The Long Island Sound last experienced anoxia (no oxygen), on a large scale, in 2021, when the western Sound had 8 square miles of anoxic conditions. While anoxia is relatively uncommon in Long Island Sound (no detection in the Sound during 16 of the last 31 years), it is important to note that some anoxic areas have been as large as 62 square miles (in 2003), and even larger before the Long Island Sound Partnership intervened in 2001 and established  the Total Maximum Daily Load (TMDL). The TMDL initiated upgrades to wastewater treatment plants  designed to reduce nitrogen loading and thereby reduce the hypoxic area and its duration in the Sound.

Hypoxia conditions that occur in bodies of water when dissolved oxygen concentrations decrease to levels where organisms become physically stressed and ultimately cannot survive. The Long Island Sound Partnership defines “low” for Long Island Sound to occur when less than 3 milligrams of oxygen are dissolved in each liter of water, or 3 mg/L. Anoxia, occurs when oxygen is completely absent (i.e., 0 mg/L), however, Long Island Sound Partnership defines anoxia as oxygen concentrations below 1 mg/L because this is the threshold below which most marine animals cannot survive even for short durations .

While hypoxia in estuarine waters does not harm humans directly, it does harm the communities of aquatic organisms that live in Long Island Sound. Without enough oxygen in Long Island Sound, fish and other organisms suffocate. Hypoxia is often an invisible killer, affecting organisms that are vulnerable, small, and typically unseen. Prolonged hypoxic conditions result in severe die-offs of animals that are unable to move out of hypoxic waters, mass migrations of mobile animals, changes in water chemistry and other adverse ecological effects. Furthering the impact of hypoxia on aquatic communities, when larger organisms like fish avoid areas with hypoxia, it leads to a less productive and less ecologically diverse ecosystem.

Hypoxia typically occurs during summer when warming surface waters form a distinct temperature and density layer that “floats” over water in the bottom layer. This bottom layer is colder, and often saltier, and therefore denser than the surface water. This layering, which is called “stratification”, prevents oxygen in the surface layer from mixing into  bottom layer. Simultaneously, excessive inputs of nitrogen and phosphorus, collectively referred to as nutrients, trigger increased algae growth. As the algae die and sink into the bottom layer, bacteria decompose the algae, consuming oxygen as they do in a process called “respiration”, the same as human respiration. This bacterial respiration is the largest cause of low oxygen in the bottom layer. Aquatic oxygen concentrations also decrease with increases in temperature. As water temperatures rise, research shows that additional nitrogen reductions are needed to maintain past achievements in reducing the hypoxic area (Whitney and Vlahos, 2021).

Since hypoxic conditions are driven by nutrient loading, this data serves as an indicator for the Nutrient Objective. By focusing on implementing nutrient reduction actions in Long Island Sound, hypoxia in the Sound will continue to decrease in extent, duration, and severity.