Learning About Hypoxia Through the Hypoxia Communications Toolkit

What Is Hypoxia?

Hypoxia in an estuary can be defined as a low level of dissolved oxygen in the water. The Long Island Sound Partnership defines “low” levels for Long Island Sound as less than 3 milligrams of dissolved oxygen in each liter of water, or 3 mg/L.

While hypoxia does not harm humans directly, it does harm the communities of aquatic organisms that live in Long Island Sound. In the same way that humans need oxygen to live, fish and other aquatic life need to obtain oxygen from the water. Without enough oxygen in Long Island Sound, fish and other organisms can be harmed or killed, leading to degraded aquatic communities.

How does hypoxia occur? During summer, the surface water of the Sound heats up and forms a distinct layer “floating” over the cooler bottom water. This reduces mixing of oxygen from the surface layer to the bottom layer, making hypoxia more likely. Excessive inputs of nutrients can amplify hypoxic conditions by triggering increased algae growth. Zooplankton, microscopic animals that eat algae, also may increase in abundance and biomass. When algae or the organisms that eat algae die, the organic matter sinks to the bottom. Bacteria and other microbes then feed on the decomposing matter, consuming oxygen in the process. The depleted oxygen leaves fish and shellfish stressed or displaced and, in severe cases, causes die-offs.

Long Island Sound has had widespread hypoxia in late summer for many years but significant improvements have been achieved through targeted nitrogen reduction and management. In 2025, the Sound had an all-time low of 18 square miles with a running five-year average of 83 square miles (2021-2025). In contrast, in 1994, when efforts to improve water quality were just beginning, over one-third of the Sound, 394 square miles, experienced hypoxia.

While the extent of hypoxia has decreased greatly in the Sound, there is still much more to be done to reduce the impact of low oxygen. The Long Island Sound Partnership remains dedicated to supporting continuous monitoring and modeling, informing science-driven management, and informing and engaging with the public to further reduce nutrients and hypoxia in the Sound.

What Is the LIS Partnership Doing About It?

Since 2020, the Partnership has invested over $65 million towards reducing hypoxia and plans ongoing investments in water quality improvements. Some of these projects include, but are not limited to, implementing nutrient reduction actions, developing site-specific plans, and educating the public. While significant improvements have resulted from prior actions, there is still a need for researchers to investigate the extent of hypoxia, its ongoing causes, and its impacts to better restore and protect aquatic life. Using over 30 years of water quality monitoring data, scientists have developed a hypoxia forecast model to predict where and when hypoxia will occur in Long Island Sound this summer. You can learn about the forecast model in the ArcGIS StoryMap we created.

---

Check Out Our Hypoxia Toolkit!

The Long Island Sound Partnership developed a Hypoxia Communications Toolkit because it is important to communicate clearly and effectively on the impact of low concentrations of oxygen to Long Island Sound. Communicating about hypoxia is challenging because water that lacks oxygen may look the same as water with an abundance of oxygen. But hypoxia is often an invisible killer, affecting aquatic life that are vulnerable, small, and typically unseen. Check out the toolkit and find an array of resources to educate peers, friends and family on how to reduce hypoxia and care for the Sound.