A new red alert concerning our planet could be added to the already long list of its disturbances: the loss of oxygen in lakes, reservoirs and oceans. Scientists are now proposing to recognize this “aquatic deoxygenation” as the tenth “planetary boundary” that humanity can cross, in the same way as climate change or the erosion of biodiversity, explains an article in Popular Mechanics.
The concept of “planetary limits”, formulated in 2009, identifies nine ecological thresholds which must not be exceeded under penalty of seriously destabilizing ecosystems and human living conditions. Climate change, integrity of the biosphere, ocean acidification, disruption of the fresh water cycle and even depletion of the ozone layer make up this list. Six to seven of these nine guardrails have already been crossed.
Deoxygenation is a known phenomenon: certain areas of the planet are naturally poor in oxygen, such as certain basins of the Black Sea, the Baltic or the fjords. What is new is the rapid extension of this phenomenon to previously well-oxygenated water masses, on a global scale and at varied depths. It is no longer a geological curiosity, but a global process.
In forty-five years, lakes and reservoirs have lost respectively 5.5% and 18.6% of their dissolved oxygen, and the oceans around 2%. Compared to the volume of the oceans, this apparently modest decline becomes dizzying. Off the coast of California, midstream waters have seen their oxygen levels drop 40% since 1960, one of the most dramatic examples documented by a study published in Nature Ecology & Evolution.
The mechanisms are well identified. Warming water first reduces the solubility of oxygen: the warmer the water, the less it can hold. It also reinforces the stratification of water columns, a warmer and less salty surface layer “covering” the colder depths, which limits exchanges between layers rich and poor in oxygen. Added to this are the inputs of nutrients from the land (fertilizers, agricultural or urban pollution), which promote algae proliferation; as it decomposes at depth, this biomass consumes even more oxygen.
The vicious circle of climate change
For marine and freshwater life, the bill is salty. Fish, molluscs or crustaceans need well-oxygenated water to survive, feed and reproduce. When these “dead zones” expand, species desert the area and, in turn, entire food chains are destabilized. The consequences are not limited to ecosystems alone and affect fishing, aquaculture and coastal economies.
As if that were not enough, deoxygenation in turn fuels climate change. In oxygen-poor waters, microbial activity promotes the production of nitrous oxide and methane, two particularly powerful greenhouse gases. By losing oxygen, the hydrosphere releases compounds which heat the atmosphere even more.
“Dissolved oxygen regulates the role of marine and freshwater environments in modulating terrestrial climate”recalls Kevin Rose, researcher at Rensselaer Polytechnic Institute in Troy (New York) and lead author of the study. According to him, improving the situation requires dual action: tackling global warming and reducing runoff from artificial landscapes. Otherwise, he warns, deoxygenation will end up affecting “not only ecosystems, but also economic activity and society on a global scale.”
This “tenth threshold” makes visible an often underestimated dimension of the ecological crisis: the role of inland and marine waters as climate regulators. He reminds us that beyond the figures and models, a simple variable, oxygen, silently conditions the habitability of our planet.
The response proposed by the authors is nothing new: we must accelerate what science has been repeating for more than a century, namely the drastic reduction of greenhouse gas emissions as well as pollution from land.
