CO2 and Oxygen-Depletion in Oceans

Nature-Published online 14 November 2008

News

Marine dead zones set to expand rapidly

Rising carbon dioxide levels will make oceans
more hostile to life.

Quirin Schiermeier

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.”

Dramatic result

Sporadic measurements in the tropical Atlantic
and Pacific suggest oxygen-depleted zones have
been slowly expanding over the past 50 years2.
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 the ocean’s biology would be affected
by rising CO2 levels. Their results are published
in the journal Global Biogeochemical Cycles.

“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.”

Meanwhile, researchers aboard two German research
vessels, the Meteor and the Maria S. Merian, are
investigating the matter further. In the waters
off western Africa and Peru, which are rich in
marine life, teams from the University of Kiel
and the Leibniz Institute of Marine Sciences,
also in Kiel, will study the physical and
biological processes that are thought to drive
oxygen loss in tropical oceans.

The coastal economies in these regions rely
heavily on fishing. For now, says Oschlies, 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.

Oceanic oxygen levels have varied considerably
throughout Earth’s past. During the end of the
Permian period, around 250 million years ago,
catastrophic oxygen losses triggered mass
extinction of terrestrial and marine life.

References:
1. Oschlies, A., Schulz, K., Riebesell,
U. & Schmittner, A. Glob. Biogeochem. Cycles
doi:10.1029/2007GB003147 (2008)
2. Stramma, L., Johnson, G., Sprintall, J. & Mohrholz, V. Science 320,

—————————————————————————————————————-

Bookmark the permalink.

Comments are closed