Anoxia

Anoxia is the complete absence of dissolved oxygen in water or sediment, the end point of the oxygen-depletion gradient that runs from normoxia through hypoxia. Hypoxia is conventionally set at dissolved oxygen below 2 mg/L; anoxia is 0 mg/L, at which aerobic life cannot persist and sediments turn to sulfate reduction, releasing hydrogen sulfide. Anoxia in coastal systems is driven by the eutrophication cascade: nutrient loading fuels an algal bloom, the bloom sinks and decays, and bacterial respiration strips oxygen from stratified bottom water. Persistent seasonal anoxia produces dead zones such as the northern Gulf of Mexico and the Baltic. Benthic kills and fish kills follow.