Based on an article by Laura Poppick for Scientific America
A warming ocean loses oxygen for two reasons: First, the warmer a liquid becomes, the less gas it can hold. That is why carbonated beverages go flat faster when left in the sun, Oschlies says.
Second, as polar sea ice melts, it forms a layer of buoyant water at the sea surface above colder, more saline waters. This process creates a sort of lid that can keep currents from mixing surface water down to deeper depths. And because all oxygen enters this habitat at the surface—either directly from the atmosphere or from surface-dwelling phytoplankton producing it during photosynthesis—less mixing means less of it at depth.
Some coastal regions around the equator naturally are low-oxygen hotspots because they contain nutrient-rich waters where bacterial blooms consume oxygen as they break down dead marine life. But shifts in ecosystems elsewhere—including in the open ocean and around the poles—especially surprises and concerns Oschlies and others because these regions were not considered as vulnerable. Climate models projecting future change have also routinely underestimated the oxygen losses already observed around the world’s oceans, he and his colleagues reported in Nature last year—another reason why this trend calls for more attention, he says.
As oxygen-rich regions become scarcer, current fish habitats will also shrink and force economically important species—such as tuna, which globally generate an estimated $42 billion annually—into new ranges. In the northeastern tropical Atlantic researchers have found habitat for tuna as well as billfish fisheries shrank by 15 percent from 1960 to 2010 (pdf) due to oxygen loss.