Aquatic Deoxygenation is overall decline in the oxygen content of oceanic and coastal waters (happens when oxygen consumption is greater than oxygen replenishment). 

Status of AD

• Ocean: About 2% of oxygen content in ocean has been lost since 1960s.
  • More than 500 low-oxygen sites have been identified in coastal waters.
• Other water bodies: Lakes and reservoirs have experienced oxygen losses of 5.5 and 18.6 % respectively since 1980. 

Reasons underscoring AD

• Global Warming caused by GHG: Rise in temperatures decreases solubility of oxygen in water.
  • Also, warm surface layers in ocean prevent oxygen from mixing deeper into ocean (leading to low oxygen level in deep sea waters)
• Eutrophication: Nutrient over enrichment from anthropogenic sources (e.g. agriculture) leads to algal blooms and increased consumption of oxygen.

Impact on Ecosystems

• Occurrences of dead zones and  ocean hypoxia effect 
• Habitat compression (reducing quality and quantity of suitable habitat) for fisheries leads to reduced biomass, species loss.
• Affects regulation and modulation of Earth's climate (due to production of GHGs by microbiotic processes in such regions)
• Changes in marine food web due to increased risk of predation, and other factors e.g. Ocean acidification