Tropical Dead Zones Set to Expand by 50 Percent Under Climate Change


Image from Science

Dead zones are certainly no stranger to these pages. As Matthew quipped in a recent post, stories about the Gulf of Mexico's (in)famous dead zone have a way of turning up on TreeHugger, as if on cue, every summer. And while the general narrative has stayed the same -- large nutrient inputs derived from fertilizer and pesticide run-off turn once vibrant ecosystems into barren, lifeless deserts -- some new science suggests climate change will play a role in exacerbating an already dire situation, expanding the volume of dead zones in tropical oceans by up to 50 percent over the coming century.
Image from NASA

As Quirin Schiermeier reports in Nature, a new study published in the journal Global Biogeochemical Cycles has found that higher carbon dioxide levels will accelerate the expansion of "oxygen minimum zones" (OMZ) by fostering the rapid growth of bacterial populations in these waters. Though scientists have long suspected that these oxygen-depleted zones, found at depths between roughly 10 and 1,000 meters, were susceptible to higher CO2 levels, they had not been able to pinpoint the root of the problem.

According to Andreas Oschlies, the study's lead author and an oceanographer at the Leibniz Institute of Marine Sciences, the problem resides with decomposing bacteria. As more carbon dioxide dissolves into the water column, phytoplankton are able to increase their rate of photosynthesis, resulting in the production of more dissolved organic matter and the release of various nutrients.

The organic matter that eventually sinks out of the surface waters is decomposed by hordes of hungry decomposers, which consume large amounts of oxygen during the process (see: eutrophication). This deprives other organisms in the surrounding waters of the vital gas, which means they must either leave or, if they can't, die.

The depletion of already low dissolved oxygen levels causes the volume of the OMZs to grow, disrupting primary producers in this and surrounding ecosystems. While local fisheries may be able to avoid the problem for now -- they can always escape by moving up the water column -- they could begin to suffer if oxygen and nutrient levels continue to plummet. Entire regions of the ocean may eventually be devoid of life; another concern is that the activity of denitrifying bacteria, which are often found in OMZs, could result in the release of more nitrous oxide (N2O), an extremely potent greenhouse gas. (This already seems to be happening in the Arabian Sea, which is home to one of the world's largest OMZs.)

Indeed, as another study published in Geophysical Research Letters a few months ago demonstrated, ocean "deserts," areas of very low productivity, have also been expanding in the region. Unlike Oschlies' study, the authors found that the warming of sea surface waters was largely to blame for the growth of these deserts.

Via: Nature News: Marine dead zones set to expand rapidly
More about ocean dead zones
Ocean "Dead Zones" Increasing: 400 Oxygen-Deprived Areas Now Exist
The Formation of Dead Zones off Oregon and Washington is Tied to Climate Change
Corn Ethanol Worsens Gulf of Mexico 'Dead Zone'

Tags: Oceans