"Dead zones" containing too little oxygen for fish to breathe are growing as global temperatures increase.
Warmer water dissolves less oxygen, so as temperatures rise, oxygen vanishes from oceans. Marine biologists are warning that if dead zones continue expanding, oceanic "deserts" could massively deplete marine life and fish stocks.
Previous studies have shown that surface layers of the ocean can be depleted of oxygen by pollution draining out from rivers, as in the Gulf of Mexico. However the new study finds depletion at intermediate ocean depths, between 300 and 700 metres. There has also been evidence of oxygen depletion closer to the sea bed in some regions, such as the Arabian Sea, but no one has looked before in detail at intermediate depths.
"From our observations we can only tell what happened in the past 50 years, but we need to find out what will happen in the future," says Lothar Stramma of the University of Kiel, Germany.
Stramma's team used data from historical records of oceanic oxygen concentrations, collected mainly from research vessels. They combined it with recent data from buoys newly equipped to measure oxygen concentrations, as well as temperature and salinity. "We added our own data from recent cruises and floats where available to continue the older data set to the present," says Stramma.
The combined data set shows that, over the past 50 years, large volumes of ocean previously rich in oxygen have become "oxygen minimum zones" (OMZs) containing less than 120 micromoles of oxygen per kilogram of water. These are the concentrations at which fish, squid, crustaceans and other marine creatures begin to suffocate and die.
On average, the team calculate that oxygen dropped by between 0.90 and 0.34 micromoles per kilogram of ocean per year.
Worst affected of six areas sampled was a tropical region of the Atlantic Ocean to the west of Africa. Between 1960 and 2006, the layer with less than 90 micromoles of oxygen per kilogram of ocean grew dramatically - its vertical thickness increased by 85%, from 370 to 690 metres. The other region of particular concern was in the equatorial Pacific Ocean.
Andy Gooday of the UK's National Oceanography Centre in Southampton says that oxygen-deprived zones caused through pollution from human activity - such as those in the Gulf of Mexico or off the coast of Louisiana - are serious, but they're dwarfed in size by the OMZs in the current study. "They're far larger, and so could have a bigger impact if they expanded," he says.
But the ultimate impact and cause of the growing deserts is difficult to gauge, say the researchers.
Oxygen is delivered to intermediate levels by surface waters which carry more oxygen, and sink through being colder and denser. Since warm water carries less oxygen and sinks less through being lighter, climate change, which has been blamed for increases in sea surface temperatures, could possibly account for the growing OMZs, says Greg Johnson, a co-author at the US National Oceanic and Atmospheric Administration's Marine Environmental Laboratory in Seattle.
Gooday stresses that OMZs are a natural phenomenon, and have fluctuated in volume throughout history. They are influenced by natural climatic phenomena such as the cyclical El Niño weather patterns that whip up ocean currents every decade or so. Stramma's team acknowledge this, and point out that oxygen-depleted oceans driven by high levels of atmospheric carbon dioxide were to blame for huge Permian extinctions of marine creatures 250 million years ago.
The big questions are whether global warming is making the deserts larger today, and how this affects marine life.
Gooday, who has studied how low-oxygen conditions affect marine life at the sea bed, says that mobile creatures can cope best, because they can move elsewhere.
Oxygen-poor water tends to become rich in the organic matter of dead organisms, as there's less oxygen available to aid decay. This provides food for fish, although they take risks accessing it because of the lack of oxygen to breathe.
"They can swim in and feed and swim out again, figuratively 'holding their breath' till they get out," says Gooday. This explains why marine creatures tend to live at the upper and lower limits of the OMZs, where they can "dip in and dip out", he says.
Journal reference: Science, DOI: 10.1126/science.1153847
(c) New Scientist 1.05.08