"Dead zone" is a more common term for hypoxia, which refers to a reduced level of oxygen in the water

Coast

Sometimes we can observe the factors leading to a dead zone such as a large surface algal bloom or the consequences such as a fish kill however, dead zones are actually invisible.

Less oxygen dissolved in the water is often referred to as a “dead zone” because most marine life either dies, or, if they are mobile such as fish, leave the area. Habitats that would normally be teeming with life become, essentially, biological deserts.

Hypoxic zones can occur naturally, but scientists are concerned about the areas created or enhanced by human activity. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those zones created by humans. Excess nutrients that run off land or are piped as wastewater into rivers and coasts can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life.

Dead zones occur in many areas of the country, particularly along the East Coast, the Gulf of Mexico, and the Great Lakes, but there is no part of the country or the world that is immune. The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.

For more information:
National Centers for Coastal Ocean Science
What is a dead zone? (audio podcast)
Hypoxia and Nutrient Pollution Overview
Hypoxia in the Northern Gulf of Mexico