Ocean deoxygenation is one of the most pernicious, yet under-reported side-effects of human-induced climate change. Scientists warn that the loss of oxygen from the world’s ocean is increasingly threatening fish species and disrupting ecosystems. A ground-breaking new report by the International Union for Conservation of Nature (IUCN) with leading scientists explores the causes and consequences of ocean deoxygenation and how we, as a planet, must react.
"Global warming has many negative consequences for the environment, one being the loss of oxygen in the worlds estuaries and oceans,” said University of Maryland Center for Environmental Science Professor Mike Roman. “This timely, comprehensive report documents the impacts to marine life that have occurred because the ocean is losing its breath."
Experts from the University of Maryland Center for Environmental Science’s Horn Point Laboratory, including Director Mike Roman, Professor Kenny Rose and Associate Professor Jamie Pierson, authored chapters in the report on ocean deoxygenation and its significance for estuarine and coastal plankton, the basis of marine ecosystem’s food web, and on fisheries.
Ocean regions with low oxygen concentrations are expanding, with around 700 sites worldwide now affected by low oxygen conditions—up from only 45 in the 1960s. In the same period, the volume of anoxic waters (areas completely depleted of oxygen) in the global ocean has quadrupled, according to the report. The ocean is expected to lose 3-4% of its oxygen inventory globally by the year 2100 under a business-as-usual scenario.
The major drivers of ocean oxygen loss are climate change and nutrient pollution. As the ocean warms, its waters hold less oxygen and become more buoyant, resulting in reduced mixing of the oxygen-rich water near the surface with the ocean depths, which contain less oxygen. Nutrient pollution causes oxygen loss in coastal waters as fertilizer, sewage, animal and aquaculture waste cause excessive growth of algae, which uses up oxygen as they decompose.
Deoxygenation is starting to alter the balance of marine life, favoring low-oxygen tolerant species (e.g. microbes, jellyfish and some squid) at the expense of low-oxygen sensitive ones, including most fish). Species groups such as tuna, marlin and sharks are particularly sensitive to low oxygen because of their large size and energy demands. These species are starting to be driven into increasingly shallow surface layers of oxygen-rich water, making them more vulnerable to overfishing. Very low ocean oxygen can also affect basic processes like the cycling of elements crucial for life on Earth, such as nitrogen and phosphorous, the report warns.