Warm waters: climate change models predict an expansion of oxygen-poor water.

Low-oxygen 'underwater deserts' in the tropical oceans have expanded over the past 50 years, according to new measurements. The most likely cause of the change is global warming, and climate models predict that the trend will continue, potentially threatening marine ecosystems.

The discovery concerns a layer of the ocean called the 'oxygen-minimum zone', where concentrations of dissolved oxygen are particularly low. The new study shows that this zone has been expanding both upwards and downwards into the adjacent layers in tropical waters.

Climate models predict that warming of the sea's surface as a result of human activity will hamper the mixing of oceanic waters, preventing dissolved oxygen from mixing evenly through the water column. The new results suggest that this process has already begun.

Researchers led by Lothar Stramma of the University of Kiel, Germany, measured the oxygenation of the oceans at depths of between 300 and 700 metres during a series of observation cruises in tropical regions of the world's three main oceans. They added their new data to previous oxygen measurements to build up a picture of the trend over the past 50 years.

Overall levels of oxygen have dropped in these zones, Stramma and his colleagues report in Science1. Regions of the eastern tropical Pacific Ocean and the northern reaches of the Indian Ocean are now classed as 'suboxic', meaning that the amount of oxygen has dropped sufficiently to harm the functioning of ecosystems.

Starving waters

In suboxic waters, nitrogen cannot react with oxygen to form biologically available nitrate. This means that organisms at the base of food chains, such as plankton, do not get enough nutrients to survive, Stramma explains.

The ultimate effect on commercially important ecosystems such as fisheries are difficult to predict, Stramma adds. "There are many complicated mechanisms involved that we need to understand better to predict changes for the future," he says. "I see our results as a starting point to be able some day to tell what changes in biogeochemistry, biology and fisheries we have to expect."

Any effect on fisheries is likely to be indirect, because these low-oxygen zones are far from the coastal waters that host most commercial fishing, suggests Andrew Solow, director of the Marine Policy Center at Woods Hole Oceanographic Institution in Massachusetts. "I don't know many fisheries that take place between 300 and 700 metres in the tropical ocean," he says.

These 'underwater deserts' should not be confused with the 'dead zones' created in coastal waters, most famously in the Gulf of Mexico, by runoff of nitrogen-rich fertilizer, Solow adds. Coastal waters lose their oxygen as a result of booms in phytoplankton growth; when these organisms die, they provide food for microbes that suck up all of the oxygen.

Going down

"It's a worrying trend," comments Laurence Mee, director of the Marine Institute at the University of Plymouth, UK. "This is one more piece in the argument that we need to do something about climate change."

Mee agrees that it's quite difficult to say whether, or how much, the decline in oxygen levels will affect ecosystems, including economically important ones. If the low-oxygen zones extend closer to the surface, they may reach the shallow, sunlit waters where many valuable fish species live. "When you start to mess around with the food chain, it has all kinds of knock-on effects that we don't know about yet," Mee says.

Team member Gregory Johnson of the National Oceanic and Atmospheric Administration in Seattle says that he and his colleagues now plan to take more measurements, to see whether the low-oxygen zones are spreading across the oceans, or spreading upwards and downwards within the water column.