Ocean deoxygenation

Dissolved oxygen enters the ocean through exchange with the atmosphere at the surface and through photosynthesis by aquatic plants and algae. It is consumed by respiration and other biological processes. Deoxygenation occurs when oxygen is used faster than it is replenished. Oxygen concentrations naturally vary by depth and place; oxygen-minimum zones are persistent low-oxygen layers in the open ocean, while hypoxia describes oxygen concentrations low enough to stress organisms. A natural low-oxygen zone is not automatically evidence of a new pollution event, although human-driven oxygen loss can intensify or expand such conditions.

Climate warming contributes in two related ways: warmer water holds less dissolved oxygen, and warming at the surface can strengthen stratification, reducing vertical mixing that carries oxygen into deeper waters. In coastal regions, excess nutrients from sources such as agricultural runoff and wastewater can stimulate algal growth. When this organic matter dies and decomposes, microbes consume oxygen. The relative importance of these processes differs among regions; UNESCO's Global Ocean Oxygen Network identifies warming and human nutrient inputs as major drivers.

The Global Ocean Oxygen Network reports that since the 1960s the area of low-oxygen water in the open ocean has increased by about 4.5 million square kilometres, alongside more than 500 identified low-oxygen sites in estuaries and other coastal waters. Scripps Institution of Oceanography summarizes estimates of roughly a 2% decline in global ocean oxygen content since the 1960s, while emphasizing that trends vary regionally. Lower oxygen can compress suitable habitat, alter species' distribution and behaviour, reduce growth or reproductive success, and increase stress or mortality. These effects can affect fisheries and aquaculture as well as marine ecosystems; organisms confined in aquaculture pens may have limited ability to move away from poor-oxygen water.

Scientists monitor oxygen with sensors deployed on ships, moorings, profiling instruments, floats and other platforms, then compare observations over time and across regions. Responses include reducing greenhouse-gas emissions to limit warming and curbing nutrient discharges that reach coastal waters through improved wastewater management and nutrient control. Long-term oxygen observations and regional forecasting are important because deoxygenation is uneven and interacts with other pressures, including warming, acidification and habitat disturbance.