Justine Braby and Reinhold Mangundu

To state the obvious, oxygen is fundamental to life. Carbon emissions (thats on us) have increased atmospheric carbon dioxide by more than 40% since pre-industrial times. While oxygen is decreasing (as a result of climate change, among other things) on land, this amount (so far) is negligible. The oceans are a different story. While atmospheric oxygen won’t plummet to dangerous levels anytime soon, the same cannot be said for undersea life that depends on dissolved oxygen, which is less abundant and more sensitive to change.

A recent article published in Science gives us a troubling account of depleting oxygen in our oceans. The oxygen content of the ocean constrains productivity, biodiversity, and biogeochemical cycles. Major extinction events in Earth’s history have been associated with warm climates and oxygen-deficient oceans. The open ocean lost an estimated 2% of its oxygen over the past 50 years. This might not sound like much, but this is significant. Open-ocean oxygen minimum zones have expanded by an area roughly the size of the European Union. The volume of water completely devoid of oxygen has more than quadrupled over the same period. Upwelling of oxygen-depleted water has intensified in severity and duration along some coasts, with serious biological consequences. Upwelling in the southern Benguela is predicted to increase in both duration and intensity by the end of the 21st century. Where the oxygen content of subsurface source waters declines, upwelling introduces water to the shelf that is both lower in oxygen and higher in carbon dioxide.

As oxygen depletion becomes more severe, persistent and wide-spread, a greater fraction of the ocean is losing its ability to support high-biomass, diverse animal assemblages and provide important ecosystem services.

Greenhouse gas-driven global warming is the likely ultimate cause of this ongoing deoxygenation in many parts of the open ocean. Warming is predicted to exacerbate oxygen depletion in many nutrient-enriched coastal systems through mechanisms similar to those of the open ocean.

Oxygen deoxygenation influences life processes from genes to emergent properties of ecosystems. Low oxygen levels can reduce the survival and growth of individual organisms. Brief, repeated exposure to low oxygen can alter immune responses and increase disease.

Deoxygenation is mechanistically linked to other ocean stressors, of course. Warming, acidification and deoxygenation are interlinked. Ocean warming is predicted to result in shifts in the distribution of fishes and invertebrates poleward by tens of hundreds of kilometers per decade. Models project that warming combined with even modest oxygen declines can cause declines in important fishery species that are sensitive to low oxygen levels. Physiological oxygen limitation in warming waters is also predicted to reduce the maximum sizes of many fish species, including some that support important fisheries.

What we need to take away from this important news urgently, is that we cannot think that simple once-off solutions will fix this. We need to put much more pressure on decision-making around climate change, carbon emissions, both at global and national level, but also make important and necessary changes at our own individual level.

Image Credit: Justine Braby