London, Nov 15 : A new research has determined that the rising levels of carbon dioxide (CO2) will make oceans more hostile to life, with the volume of oxygen-depleted 'dead zones' in tropical oceans set to expand rapidly by as much as 50 percent before the end of the century.
According to a report in Nature News, Andreas Oschlies of the Leibniz Institute of Marine Sciences in Kiel, Germany, led the research.
At depths between several tens and hundreds of meters, 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.
Now, Oschlies and his team has 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 ocean.
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," said 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," he added.
Sporadic measurements in the tropical Atlantic and Pacific suggest oxygen-depleted zones have been slowly expanding over the past 50 years.
But none of the previously assumed physical causes, such as ocean warming and reduced circulation, completely accounts for the effect.
This prompted Oschlies and his colleagues to examine how rising CO2 levels would affect the ocean's biology.
"Nobody really has ever modelled the feedback of rising CO2 on oceanic oxygen concentrations in such a credible way," said Gian-Kaspar Plattner, a carbon-cycle modeller at the Swiss Federal Institute of Technology Zurich (ETH).
"A 50 percent volume increase of oxygen-poor zones is much more than I would have expected," he added.
According to Oschlies, for now, local fisheries may not feel any downturn because fish stocks can probably evade the dead zones by moving further up in the water column.
But, if oxygen and nutrient levels continue to drop, that could hit the region hard within a few decades, he added.