Ocean deoxygenation

Ocean deoxygenation is the expansion of oxygen minimum zones in the world's oceans as a consequence of anthropogenic emissions of carbon dioxide. The change has been fairly rapid and poses a threat to fish and other types of marine life, as well as to people who depend on marine life for nutrition or livelihood.[1]

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), "ocean oxygen deprivation", "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.[2][3]

Most of the excess heat from CO2 and other greenhouse gas emissions is absorbed by the oceans.[4] Warmer oceans cause deoxygenation both because oxygen is less soluble in warmer water,[5] and through temperature driven stratification of the ocean which inhibits the production of oxygen from photosynthesis.[6]

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.[7] 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.[8] This is where the majority of oceanic photosynthesis (such as by phytoplankton) occurs.[9] 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.[10] Increased stratification can also decrease the supply of oxygen to the interior of the ocean. Warmer waters also increase the metabolism of marine organisms,[11] 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.[12][13]

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.[14]

See also

References

  1. ↑ Oceans suffocating as huge dead zones quadruple since 1950, scientists warn The Guardian, 2018
  2. ↑ Putic, George. "Scientists: Oceans Rapidly Losing Oxygen".
  3. ↑ "Global Warming Is Starving West Coast Waters of Oxygen".
  4. ↑ Levitus, Sydney, et al. "Warming of the world ocean." Science 287.5461 (2000): 2225–2229
  5. ↑ https://www.ysi.com/File%20Library/Documents/Technical%20Notes/DO-Oxygen-Solubility-Table.pdf
  6. ↑ "Climate-driven trends in contemporary ocean productivity." Nature 444.7120 (2006): 752–755
  7. ↑ Sigman, Daniel M., Samuel L. Jaccard, and Gerald H. Haug. "Polar ocean stratification in a cold climate." Nature 428.6978 (2004): 59–63
  8. ↑ Arrigo, Kevin R., et al. "Phytoplankton community structure and the drawdown of nutrients and CO 2 in the Southern Ocean." Science 283.5400 (1999): 365–367. Behrenfeld, Michael J., et al. "Climate-driven trends in contemporary ocean productivity." Nature 444.7120 (2006): 752–755
  9. ↑ Cermeño, Pedro, et al. "The role of nutricline depth in regulating the ocean carbon cycle." Proceedings of the National Academy of Sciences 105.51 (2008): 20344-20349
  10. ↑ Cermeño, Pedro, et al. "The role of nutricline depth in regulating the ocean carbon cycle." Proceedings of the National Academy of Sciences 105.51 (2008): 20344-20349
  11. ↑ Gillooly, James F., et al. "Effects of size and temperature on metabolic rate." science 293.5538 (2001): 2248–2251
  12. ↑ Nagelkerken Global alteration of ocean ecosystem functioning due to increasing human CO2 emissions, PNAS vol. 112 no. 43, 2015
  13. ↑ Goldenberg, Silvan U., et al. "Boosted food web productivity through ocean acidification collapses under warming." Global Change Biology (2017)
  14. ↑ Ralph F. Keeling, Arne Kortzinger, Nicolas Gruber (2010). "Ocean Deoxygenation in a Warming World" (PDF). Annual Review of Marine Science. 2: 199–229. Bibcode:2010ARMS....2..199K. doi:10.1146/annurev.marine.010908.163855. PMID 21141663. Archived from the original (PDF) on 2016-03-01.
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