What a Marine "Dead Zone" Is

A “dead zone” is an area of hypoxia—so little dissolved oxygen that most familiar aquatic animals cannot live there normally. Mobile fish often leave, while less mobile shellfish may die; this does not mean no life exists at all, since microorganisms can persist. Excess nitrogen and phosphorus from fertilizer runoff, wastewater, and other sources often cause or intensify the problem: nutrients fuel algal blooms, and when the organic material dies and decomposes, microbes consume oxygen. Water stratification can prevent oxygen from being replenished near the bottom. Hypoxia can also occur naturally.