An oceanic dead zone is an area in the ocean where oxygen levels are so low that most marine life cannot survive. These zones are often caused by nutrient pollution, primarily from agricultural runoff, which leads to excessive growth of algae. When the algae die and decompose, the process consumes oxygen, creating a hypoxic environment. Dead zones can have significant impacts on marine ecosystems and local fisheries. They can lead to the decline of fish populations and disrupt the food chain. Some well-known dead zones include the Gulf of Mexico and the Chesapeake Bay.