Global Ocean Oxygen Network

1. Oxygen is critical to the health of the ocean. It structures aquatic ecosystems and is a fundamental requirement for marine life from the intertidal zone to the greatest depths of the ocean. 

2. Oxygen is declining in the ocean. Since the 1960s, the area of low oxygen water in the open ocean has increased by 4.5 million km2, and over 500 low oxygen sites have been identified in estuaries and other coastal water bodies.

3. Human activities are a major cause of oxygen decline in both the open ocean and coastal waters. Burning of fossil fuels and discharges from agriculture and human waste, which result in climate change and increased nitrogen and phosphorus inputs, are the primary causes.

4. Deoxygenation (a decline in oxygen) occurs when oxygen in water is used up at a faster rate than it is replenished. Both warming and nutrients increase microbial consumption of oxygen. Warming also reduces the supply of oxygen to the open ocean and coastal waters by increasing stratification and decreasing the solubility of oxygen in water.

5. Dense aquaculture can contribute to deoxygenation by increasing oxygen used for respiration by both the farmed animals and by microbes that decompose their excess food and faeces.

6. Insufficient oxygen reduces growth, increases disease, alters behaviour and increases mortality of marine animals, including finfish and shellfish. The quality and quantity of habitat for economically and ecologically important species is reduced as oxygen declines.

7. Finfish and crustacean aquaculture can be particularly susceptible to deoxygenation because animals are constrained in nets or other structures and cannot escape to highly-oxygenated water masses.

8. Deoxygenation affects marine biogeochemical cycles; in particular phosphorus availability, hydrogen sulphide production and micronutrients. 

9. Deoxygenation may also contribute to climate change through its effects on the nitrogen cycle. When oxygen is insufficient for aerobic respiration, microbes conduct denitrification to obtain energy, a process that produces N2O – a powerful greenhouse gas.

10. The problem of deoxygenation is predicted to increase in the coming years. Continued greenhouse gas emissions are expected to enhance global warming and in consequence deoxygenation. The global discharge of nitrogen and phosphorus to coastal waters may increase in many regions of the world as human populations and economies grow. It is expected that many areas will experience more severe and prolonged hypoxia (substantially reduced oxygen concentrations) than at present. 

11. Slowing and reversing deoxygenation will require reducing greenhouse gas emissions globally and decreasing nutrient discharges that reach coastal waters. Concerted international efforts can reduce carbon emissions.

12. The decline in oxygen in the ocean is not happening in isolation. At the same time, food webs are disturbed due to overfishing and physical destruction of habitats, and waters are getting warmer, more acidic, and experiencing higher nutrient loads. Management of marine resources will be most effective if the cumulative effects of human activities on marine ecosystems are considered. 

13. More accurate predictions of ocean deoxygenation, as well as improved understanding of its causes, consequences and solutions, require expanding ocean oxygen observation, long-term and multi-stressor experimental studies, numerical modelling and historical & paleo reconstructions.

14. Publicly available, accurate oxygen measurements with appropriate temporal resolution and spatial coverage in the marine environment are needed to document the current status of our ocean, to track changing conditions, to build and validate models that can project future oxygen levels, and to develop strategies to slow and reverse deoxygenation.

15. Steps required to slow or reverse ocean deoxygenation can directly benefit marine biodiversity and human health and well-being. Sewage treatment and limiting global warming, for example, will improve marine ecosystem health with societal benefits that extend beyond improving oxygen in our ocean and coastal waters.

Adapted from Global Ocean Oxygen Network, Breitburg, D., M. Gregoire, K. Isensee (eds.) 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp. (IOC/2018/TS/137)