Expanding Oxygen-Minimum Zones in the Tropical Oceans
Lothar Stramma,1*
Gregory C. Johnson,2
Janet Sprintall,3
Volker Mohrholz4
Oxygen-poor waters occupy large volumes of the intermediate-depth eastern tropical oceans. Oxygen-poor conditions have far-reaching impacts on ecosystems because important mobile macroorganisms avoid or cannot survive in hypoxic zones. Climate models predict declines in oceanic dissolved oxygen produced by global warming. We constructed 50-year time series of dissolved-oxygen concentration for select tropical oceanic regions by augmenting a historical database with recent measurements. These time series reveal vertical expansion of the intermediate-depth low-oxygen zones in the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.
1 Institut für Meereswissenschaften an der Universität Kiel (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany.
2 National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115, USA.
3 Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA.
4 Baltic Sea Research Institute Warnemünde, Post Office Box 301161, 18112 Rostock, Germany.
* To whom correspondence should be addressed. E-mail: lstramma{at}ifm-geomar.de
