Using computer model predictions, scientists have determined that oxygen-depleted zones in tropical oceans are expanding, possibly because of climate change.
The research team consisted of an international team of physical oceanographers including a researcher from Scripps Institution of Oceanography at UC San Diego.
The team selected ocean regions for which they could obtain the greatest amount of data to document the decline in oxygen.
Some of the more recent data came from oxygen sensors which have been added to about 150 of the profiling floats used in Argo, a worldwide network of sensors that track basic ocean conditions such as temperature and salinity.
They discovered that oxygen-poor regions of tropical oceans are expanding as the oceans warm, limiting the areas in which predatory fishes and other marine organisms can live or enter in search of food.
The researchers found through analysis of a database of ocean oxygen measurements that levels in tropical oceans at a depth of 300 to 700 meters (985 to 2,300 feet) have declined during the past 50 years.
"We found the largest reduction in a depth of 300 to 700 meters (985 to 2,300 feet) in the tropical northeast Atlantic, whereas the changes in the eastern Indian Ocean were much less pronounced," said Lothar Stramma from the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel, Germany.
"Whether or not these observed changes in oxygen can be attributed to global warming alone is still unresolved. The reduction in oxygen may also be caused by natural processes on shorter time scales," he added.
According to Janet Sprintall, a physical oceanographer at Scripps Oceanography, the oxygen-poor areas have the potential to move into coastal areas via currents that flow from the mid-depth tropical oceans, where the oxygen changes were observed, and along the west coast of continents.
"The width of the low-oxygen zone is expanding deeper but also shoaling toward the ocean surface," she said.
The ecological impacts of this increase could have substantial biological and economical consequences.
Accoridng to Lisa Levin, a biological oceanographer at Scripps Oceanography who studies oxygen-minimum zones that intercept the seafloor, an expansion of oxygen-minimum zones in the oceans could lead to diminished biodiversity and to the expanded distributions of organisms that have adapted to live in hypoxic, or oxygen-poor waters.
"Thicker oxygen minimum zones could affect nutrient cycling, predator-prey relationships and plankton migrations," said Levin. "Where the expanding oxygen-minimum zones impinge on continental margins, we could see huge ecosystem changes," she added.