Oxygen (O₂) is considered an effective indicator of ocean health and climate change. Its level, distribution and variability from sub-seasonal to multidecadal scales provide relevant information on the physical and biogeochemical functioning of the ocean and often act to constrain life in the ocean.

Current evidence indicates that the coastal (i.e., most directly influenced by land) and open-ocean areas have been losing O₂ since the middle of the last century, with consequences for living organisms and biogeochemical cycles that are not yet fully understood. In the open ocean the O₂ inventory has decreased by a few percent (i.e., 0.5–3%), and oxygen minimum zones (OMZs) are expanding, which is primarily attributed to global warming, although a quantitative understanding is still lacking. The number of hypoxic coastal sites has increased, predominantly in response to worldwide eutrophication, yet trends in deoxygenation in the global coastal zone remain ill-defined.

The development and extension of low-O₂ concentration areas degrade the living conditions and contract the metabolically viable habitat for a large number of pelagic, mesopelagic, and benthic organisms. The effects on individuals exposed to low O₂ can result in altered foodweb structures.

Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance and also through altered spatial distributions of the harvested species causing changes in fishing activity. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data, leading to misinformed management advice.

Model simulations for this century project a decrease in oxygen under both high- and low-CO₂ emission scenarios, while the projections of the coastal ocean at the land–ocean interface indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify deoxygenation in coastal areas influenced by eutrophication by strengthening and extending stratification.

This special issue will investigate new developments and insights related to low-oxygen environments and deoxygenation in open and coastal waters.