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 Science 1. 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.
It has been shown that, as CO2 levels in the atmosphere increase, oxygen levels are decreasing. Expecting the oceans to act as a carbon sink may have the same outcome. This article doesn't mention the effect of increasing CO2 levels in the ocean in relation to oxygen depletion but I think that it would make an interesting if not necessary addition to this research based on current conditions.
Although fisheries are uncommon under 300 m depth, the effect on fish populations can occur up to 2500 m. In fact, commercial fish depletion at shallower depths can be correlated with fish reduction from much lower ones. It should be payed attention to the vertical ecosystems range on any issue that concerns marine policies.
It is amazing to me that there are still people who do not believe in Global Warming! (i.e. Also amazing that some people do not believe the holocaust, moon landing, the world is round, etc... ad nauseam) How but some images and maps showing the areas. It would be great if some scientific body, agency, person could take images with the 'right' kinds of satellites to show these regions over time; and from one year to the next. (If the images already exist, why no links to them with the article? There has to be something!) We might not have images for as far back as we might like to go, but we should be cataloging these changes over time for future generations. Even correlating the images with Nasa images of the planet; Sun Spot activity; Magnetic Field variation; CO2 levels and of course Ice Core Samples, etc.... Than perhaps the many hypothesis that come from this and other similar news worthy events might be more significant. Perhaps even significant enough to make the correct changes and improve everyone's future. Just a thought!
maybe the water isnt heating up because of global warming at all. {Remember when} The tsunami hit? At that time some people beleived that Some sort of nuclear explosion or something real big caused that tsunami and it gave off radiation and killed alot of animalsm. It also made certain animals such as whales and dolphins come up from the depths and travel down rivers to get away from the unlivable water. I dont know this for a fact But I think I remember coast to coast am having an hour or so discussion about this. And I am not sure who the interview was with. That didnt sound like a global warming to me.