Chapter 02 · Nature, land & waterWater, Waste & Pollution
Dead zone
Definition
A dead zone is a hypoxic — low-oxygen — area of an ocean or lake in which few organisms can survive. Dead zones form through eutrophication: excess nutrients (nitrogen and phosphorus) from agricultural runoff, sewage and atmospheric deposition fuel algal blooms; when the algae die, bacterial decomposition consumes dissolved oxygen, and waters that cannot be reoxygenated become uninhabitable for most aquatic life.
References
definition and eutrophication mechanism
nutrient sources and oxygen depletion process
scale and impacts
Overview
What it means
The downstream, aquatic endpoint of nutrient pollution — visible in systems such as the Gulf of Mexico and historically the Baltic Sea.
How it is used
The term is used in water-quality science, fisheries management and nutrient-policy debates; WRI identifies eutrophication and hypoxia as among the leading causes of water-quality impairment over the past 50 years.
Why it matters
Dead zones are a planetary-scale symptom of the nitrogen and phosphorus boundary being exceeded, linking farm policy directly to marine ecosystem collapse.