Oxygen is an essential necessity of life on land. The same applies for almost all organisms in the ocean. However, the oxygen supply in the oceans is threatened by global warming in two ways: Warmer surface waters take up less oxygen than colder waters. In addition, warmer water stabilizes the stratification of the ocean. This weakens the circulation connecting the surface with the deep ocean and less oxygen is transported into the deep sea. Therefore, many models predict a decrease in global oceanic oxygen inventory of the oceans due to global warming. The first global evaluation of millions of oxygen measurements seems to confirm this trend and points to first impacts of global change.
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| A CTD-rosette is being lowered into the seawater from the German RV METEOR. For the new study the authors have analysed hundreds of thousands of historical and current oxygen measurements [Credit: Martin Visbeck, GEOMAR] |
"Since large fishes in particular avoid or do not survive in areas with low oxygen content, these changes can have far-reaching biological consequences," says Dr. Schmidtko, the lead-author of the study.
The researchers used all historic oxygen data available around the world for their work, supplemented it with current measurements and refined the interpolation procedures to more accurately reconstruct the development of the oxygen budget over the past 50 years. In some areas previous research had already shown a decrease in oxygen.
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| Changes of dissolved oxygen in the global ocean in percent [Credit: GEOMAR] |
The study also shows that, with the exception of a few regions, the oxygen content decreased throughout the entire ocean during the period investigated. The greatest loss was found in the North Pacific.
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| Mean Oxygen Concentration of the Water Column [Credit: GEOMAR] |
"However, with measurements alone, we cannot explain all the causes," adds Professor Martin Visbeck, "natural processes occurring on time scales of a few decades may also have contributed to the observed decrease."
However, the results of the research are consistent with most model calculations that predict a further decrease in oxygen in the oceans due to higher atmospheric carbon dioxide concentrations and consequently higher global temperatures.
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| Large fish like this Marlin are dependent on sufficient oxygen supply. Previous studies have already shown that reduction in oxygen content is limiting their habitat [Credit: Bill Boyce, GEOMAR] |
"From the beginning of March onwards, four expeditions aboard the German research vessel METEOR will investigate the tropical oxygen minimum zone in the eastern Pacific off Peru. We hope to obtain further data on regional development which will also help us to better understand the global trends," emphasizes Dr. Stramma, the expedition coordinator for the SFB.
Source: Helmholtz Centre for Ocean Research Kiel (GEOMAR) [February 15, 2017]