May 4, 2017 full article here
The amount of dissolved oxygen contained in the water — an important measure of ocean health — has been declining for more than 20 years, reveals a new analysis of decades of data on oceans across the globe.
…Falling oxygen levels in water have the potential to impact the habitat of marine organisms worldwide and in recent years led to more frequent “hypoxic events” that killed or displaced populations of fish, crabs and many other organisms.
Researchers have for years anticipated that rising water temperatures would affect the amount of oxygen in the oceans, since warmer water is capable of holding less dissolved gas than colder water. But the data showed that ocean oxygen was falling more rapidly than the corresponding rise in water temperature.
“The trend of oxygen falling is about two to three times faster than what we predicted from the decrease of solubility associated with the ocean warming,” Ito said. “This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice.”….
….They found that air pollution drifting from East Asia out over the world’s largest ocean contributed to oxygen levels falling in tropical waters thousands of miles away. Once ocean currents carried the iron and nitrogen pollution to the tropics, photosynthesizing phytoplankton went into overdrive consuming the excess nutrients. But rather than increasing oxygen, the net result of the chain reaction was the depletion oxygen in subsurface water. That, too, is likely a contributing factor in waters across the globe, Ito said.
Takamitsu Ito, Shoshiro Minobe, Matthew C. Long, Curtis Deutsch. Upper Ocean O2 trends: 1958-2015. Geophysical Research Letters, 2017; DOI: 10.1002/2017GL073613