Oxygen loss in the ocean and its alarming consequences
On 7 December 2019, the International Union for Conservation of Nature (IUCN) presented a new report on the state of the oceans at COP25 in Madrid, this time focusing on an underestimated but pernicious phenomenon: deoxygenation of the oceans.
The report, based on the work of 67 researchers worldwide, including Marilaure Grégoire, Research Director for Fund for Scientific Research – FNRS at the Research Unit MAST-FOCUS, Department of Astrophysics, Geophysics and Oceanography of the University of Liège (ULiège, Belgium), states that global reserves of ocean oxygen have decreased by 2% over a period of only 50 years (from 1960 to 2010). This loss of oxygen is a growing threat to marine ecosystems and the human populations that depend on them, particularly through fishing. Fish such as tuna, marlin or sharks are now listed as threatened species, according to IUCN.
Marilaure Grégoire is the lead author on Chapter 1 of this report: ”What is ocean deoxygenation?”
The main causes of this deoxygenation are eutrophication and warming of ocean waters. Eutrophication results from nutrient runoff from coastal areas and nitrogen deposition from fossil fuel use.
According to IUCN experts, the oceans could lose another 3-4% of their oxygen content by 2100 if greenhouse gas emissions increase at the same rate as at present.
Oxygen losses are concentrated in the first 1000 metres of the water column, where the abundance and diversity of fish species are highest.
If in the 1960s, there were 45 coastal sites in the world lacking oxygen, there are now about 700 of them, according to the data presented by the researchers. Over the past fifty years, the volumes of anoxic water, i. e. completely empty of oxygen, have quadrupled.
These changes lead to a disruption of the balance of underwater life and favour, for example, hypoxia-tolerant species such as germs, jellyfish and some squid, to the detriment of most fish species, which are sensitive to hypoxia. This weakens all marine ecosystems and coastal human populations, which are highly dependent on fisheries resources, particularly in Africa and Asia.
Marilaure Grégoire stresses the importance of the Marine Copernicus Environmental and Monitoring Service for delivering high-quality forecasts of low oxygen zones to support the decision making process. Long term projections, as those realized in the frame of IPCC, are also essential for projecting ocean deoxygenation at the end of this century and its consequences on the global biogeochemical cycles of carbon, nitrogen and phosphorus.
While working to improve data collection and models simulating changes in marine ecosystems, IUCN experts are advocating for a significant and rapid reduction in GHGs.
Global distribution of deoxygenation in the coastal and global ocean (from Breitburg et al., 2018). In the coastal area, more than 500 sites have
been inventoried with low oxygen conditions while in the open ocean the extent of low oxygen waters amounts to several millions km3.