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American Association for the Advancement of Science

A lack of oxygen in the oceans

Global distribution of the 400+ marine systems with dead zones caused by increased eutrophication. Their distribution matches the current human "footprint" in the northern hemisphere. In the southern hemisphere, dead zones have only been reported recently. [Image courtesy of Science/AAAS]

Imagine there was a room in your home that you could not breathe in. Or try to picture a neighborhood in your town that does not have enough oxygen. Whenever you went into that room or neighborhood, you simply could not take a breath.

That's kind of like the situation that some fish are in because there are now many places underwater that do not have enough oxygen to support life. In these "oxygen minimum zones," or OMZs, marine organisms start behaving strangely. Some of them abandon their burrows and move somewhere else, and some of them suffocate and die. For that reason, these OMZs are also called "dead zones." But the really bad news is that these dead zones are multiplying.

Researchers Robert Diaz and Rutger Rosenberg have reviewed the latest studies on this subject and found that since the 1960's, the number of dead zones in the world's oceans has approximately doubled every 10 years. Back in 1995, scientists knew about 46 dead zones in the world's oceans, but today they have noted more than 400. Diaz and Rosenberg predict that this decrease of dissolved oxygen in the oceans is going to be one of the most serious environmental problems of the 21st century for humans to deal with. They also say that, for the most part, we are to blame.

The main cause of the OMZs is something called increased eutrophication. It means an increase of nutrients in the water -- usually nitrogen or phosphorus -- that triggers a frenzy of action from microscopic organisms. These microscopic organisms thrive on the added nutrients in the water, but in the process, they also use up all the oxygen in the area. This increased eutrophication often leads to poor water quality, and sometimes marine dead zones.

Most of the increased eutrophication is due to organic waste and fertilizer runoff from humans into rivers and streams. These human-made chemicals find their ways into the ocean, and deplete the oxygen as described. So the two researchers, Diaz and Rosenberg, argue that humans must find a way to keep fertilizers and other organic waste out of the sea. Otherwise, these marine dead zones will continue to spread across the globe and destroy more ecosystems.

Read more about the expansion of dead zones in the 15 August issue of the journal Science.