Hypoxia

Hypoxia is a dissolved-oxygen level too low to support most marine animals, conventionally below about 2 milligrams per liter. It develops where stratification cuts off resupply and respiration of sinking organic matter (often from eutrophication) draws oxygen down, killing or driving off bottom life and creating seasonal dead zones. Coastal hypoxic zones have spread worldwide since the 1960s, and the mid-water oxygen-minimum zones are expanding with ocean warming.