Scientists Call for Including Oxygen Loss in Bodies of Water as New Planetary Boundary

Oxygen concentrations in the waters of our planet are rapidly decreasing, threatening ecosystems, as well as the livelihoods of many.

In a new study, an international team of researchers are calling for oxygen loss in bodies of water to be recognized as a planetary boundary so that it can be more effectively addressed, a press release from the Helmholtz Association of German Research Centres said.

“It’s important that aquatic deoxygenation be added to the list of planetary boundaries,” said Dr. Kevin C. Rose, lead author of the study and a professor at New York’s Rensselaer Polytechnic Institute, in the press release. “This will help support and focus global monitoring, research, and policy efforts to help our aquatic ecosystems and, in turn, society at large.”

As oxygen is a fundamental requirement of all life on Earth, oxygen loss in water — deoxygenation — threatens all living creatures.

If critical thresholds of global scale processes, known as “planetary boundaries,” are crossed, the risk of abrupt, large-scale or irreversible changes to the environment — “tipping points” — increases, jeopardizing our planet’s resilience.

The study’s authors argue that aquatic deoxygenation responds to and regulates the processes of the nine established planetary boundaries, which include climate change, biodiversity loss and land use change.

“Planetary boundaries represent thresholds in major Earth system processes that are sensitive to human activity and control global-scale habitability and stability. These processes are interconnected such that movement of one planetary boundary process can alter the likelihood of crossing other boundaries,” the authors of the study wrote.

The study, “Aquatic deoxygenation as a planetary boundary and key regulator of Earth system stability,” was published in the journal Nature Ecology & Evolution.

In recent decades, concentrations of dissolved oxygen have substantially and rapidly declined across all aquatic ecosystems — rivers and streams, lakes, reservoirs, estuaries and ponds.

Since 1980, reservoirs and lakes have had oxygen losses of 18.6 and 5.5 percent, respectively. The ocean has experienced roughly a two percent oxygen loss since 1960, which represents an extensive amount due to its enormous volume.

There has been substantial variability in the oxygen depletion of marine ecosystems. In the past few decades, Central California’s midwaters have experienced oxygen depletion of 40 percent. The amount of aquatic ecosystems being affected by deoxygenation has increased substantially across all types.

“The causes of aquatic oxygen loss are global warming due to greenhouse gas emissions and the input of nutrients as a result of land use,” said co-author of the study Dr. Andreas Oschlies, a marine biogeochemical modeling professor at GEOMAR Helmholtz Centre for Ocean Research Kiel, in the press release. “If water temperatures rise, the solubility of oxygen in the water decreases. In addition, global warming enhances stratification of the water column, because warmer, low-salinity water with a lower density lies on top of the colder, saltier deep water below. This hinders the exchange of the oxygen-poor deep layers with the oxygen-rich surface water. In addition, nutrient inputs from land support algal blooms, which lead to more oxygen being consumed as more organic material sinks and is decomposed by microbes at depth.”

Parts of the ocean where oxygen levels are so low that fish, crustaceans and mussels cannot survive threaten marine life as well as the ecosystem services — such as aquaculture, fisheries, cultural practices and tourism — that they provide.

“Aquatic deoxygenation trends represent a clear warning and call to action that should inspire changes to slow or even mitigate this planetary boundary,” the press release said.

Microbiotic processes in regions that have become oxygen depleted also produce more greenhouse gases like methane and nitrous oxide, which can lead to increased global heating. This in turn contributes to further oxygen depletion.

The authors of the study warned that the planet is approaching critical aquatic deoxygenation thresholds that will eventually impact several other planetary boundaries.

“Dissolved oxygen regulates the role of marine and freshwater in modulating Earth’s climate. Improving oxygen concentrations depends on addressing the root causes, including climate warming and runoff from developed landscapes,” Rose said in the press release. “Failure to address aquatic deoxygenation will, ultimately, not only affect ecosystems but also economic activity, and society at a global level.”