Marine dead zones set to expand rapidly

NatureNews: Rising levels of carbon dioxide could increase the volume of oxygen-depleted 'dead zones' in tropical oceans by as much as 50% before the end of the century — with dire consequences for the health of ecosystems in some of the world's most productive fishing grounds.

At depths between several tens and hundreds of metres, large parts of the tropical oceans are poorly supplied with dissolved oxygen, and are therefore hostile to most marine life. Scientists suspect that these zones are sensitive to climate change, but previous studies have arrived at conflicting conclusions regarding exactly how and why a more CO2-rich world affects oceanic oxygen content.

A team led by Andreas Oschlies of the Leibniz Institute of Marine Sciences in Kiel, Germany, has now used a global model of climate, ocean circulation and biogeochemical cycling to extrapolate existing experimental results of the effects of altered carbon and nutrient chemistry on dissolved oxygen to the global ocean1. They found that a CO2-rich world will only have a small impact on waters at middle and high latitudes. But in all tropical oceans the volume of 'oxygen-minimum' zones will substantially increase as ocean bacteria feed on the algae that will flourish as a result of the elevated CO2 levels.

"Carbon dioxide fertilizes biological production," says Oschlies. "It's really like junk food for plants. When the carbon-fattened excess biomass sinks it gets decomposed by bacteria which first consume the oxygen, and then the nutrients."

…"Nobody really has ever modelled the feedback of rising CO2 on oceanic oxygen concentrations in such a credible way," says Gian-Kaspar Plattner, a carbon-cycle modeller at the Swiss Federal Institute of Technology Zürich (ETH). "A 50% volume increase of oxygen-poor zones is much more than I would have expected. But further studies, with different climate parameters, are needed to add robustness to these results and reduce uncertainty."

Arnold Böcklin, The Isle of the Dead, 1901