Climate change may be expanding low-oxygen zones in the oceans and making some marine ecosystems less habitable, according to a new study in Science published online on June 9.
"The growth of low-oxygen regions is cause for concern because of the detrimental effects on marine populations—entire ecosystems can die off when marine life cannot escape the low-oxygen water," says lead author Curtis Deutsch, assistant professor at the University of California, Los Angeles, in a press release.
"There are widespread areas of the ocean where marine life has had to flee or develop very peculiar adaptations to survive in low-oxygen conditions."
Low-oxygen zones are created by oxygen-consuming bacteria. The team of researchers used a computer model to show that these zones change their depths, and consequently their sizes, in response to climate fluctuations.
Normally, atmospheric oxygen dissolves in surface water and is carried to deeper ocean regions via currents. However, with increased environmental temperatures, oxygen becomes less soluble.
Moreover, warmer temperatures increase the buoyancy of ocean surface layers, thereby reducing the ability of the ocean currents to circulate the oxygen into deeper layers.
Low-oxygen zones can thus rise to shallower ocean depths. In these warmer temperatures, bacteria become more active, and the zone effectively enlarges.
"We have shown for the first time that these low-oxygen regions are intrinsically very sensitive to small changes in climate, says Deutsch. "That is what makes the growth and shrinkage of these low-oxygen regions so dramatic."Meanwhile, when oxygen is depleted, marine bacteria begin to consume nitrogen, a nutrient vital to algae at the base of the marine food chain.
"We found there is a mechanism that connects climate and its effect on oxygen to the removal of nitrogen from the ocean," says Deutsch. "Our climate acts to change the total amount of nutrients in the ocean over the timescale of decades."