Ocean deoxygenation is a term that has been suggested to describe the expansion of oxygen minimum zones in the world's oceans as a consequence of anthropogenic emissions of carbon dioxide.<sup class="reference plainlinks nourlexpansion" id="ref_stramma">[1][2]
Oceanographers and others have discussed what phrase best describes the phenomenon to non-specialists. Among the options considered have been 'ocean suffocation' (which was used in a news report from May 2008[3]), 'ocean oxygen deprivation',[4] 'decline in ocean oxygen', 'marine deoxygenation', 'ocean oxygen depletion' and 'ocean hypoxia'.
Implications
Ocean deoxygenation poses implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitats.
Most of the excess heat from CO2 and other greenhouse gas emissions is absorbed by the oceans. Warmer oceans cause deoxygenation both because oxygen is less soluble in warmer water , and though temperature driven stratification of the ocean which inhibits the production of oxygen from photosynthesis.
The ocean surface stratifies as the atmosphere and ocean warms causing ice melt and glacial runoff. This results in a less salty and therefore a less dense layer that floats on top . Also the warmer waters themselves are less dense. This stratification inhibits the upwelling of nutrients (the ocean constantly recycles its nutrients) into the upper layer of the ocean . This is where the majority of oceanic photosynthesis (such as by phytoplankton) occurs . This decrease in nutrient supply is likely to decrease rates of photosynthesis in the surface ocean, which is responsible for approximately half of the oxygen produced globally . Increased stratification can also decrease the supply of oxygen to the interior of the ocean. Warmer waters also increase the metabolism of marine organisms, leading to increased respiration rates. In the surface ocean, increased respiration will likely lead to lower net oxygen production, and thus less oxygen transferred to the atmosphere. In the interior ocean, the combination of increased respiration and decreased oxygen supply from surface waters can draw oxygen down to hypoxic or anoxic levels. Not only are low levels of oxygen lethal to fish and other upper trophic level species, they can change the microbially mediated cycling of globally important elements such as nitrogen; nitrate replaces oxygen as the primary microbial electron acceptor at very low oxygen concentrations. All this, increased demand on herbivores, decreased nutrient supply, decreased dissolved oxygen, etc, result in catastrophic food web mismatches .
Ocean model simulations predict a decline of up to 7% in the global ocean O2 content over the next hundred years. The decline of oxygen is projected to continue for a thousand years or more.
See also
- Anoxic event
- Anoxic waters
- Dead zone (ecology)
- Ocean acidification
References
- 1.^ Expanding Oxygen-Minimum Zones in the Tropical Oceans by L. Stramma et al. doi:10.1126/science.1153847
- 2.^ [Biotic and Human Vulnerability to Projected Changes in Ocean Biogeochemistry over the 21st Century by Mora et al. Plos Biology, 11 e1001682, 2013 doi:10.1371/journal.pbio.1001682
- 3.^ Ocean Dead Zones Growing; May Be Linked to Warming National Geographic News, May 1, 2008
- 4.^ A problem without a name The Book of Barely Imagined Beings, Oct 13, 2008
External links
- Ocean Deoxygenation
- NASA: Ocean Deoxygenation: Past, Present, and Future (2012)
- Video: Ocean Deoxygenation, Our Ocean's Oxygen Supply & Demand Issue
