Oregon's Mysterious Dead Zone Confounds Scientists

Unexplained low oxygen levels in Oregon's coastal waters are massacring the region's marine life every summer. 

Francis Chan of Oregon State University first observed the dead zone in July 2002. Compared to the normal 5 to 8 milliliters of oxygen per liter of water found along the coast, Chan and his team measured less than 1.43 milliliters per liter -- way too low for most animals to survive. 

Poorly oxygenated, or hypoxic, coastal waters are common in regions fed by nutrient-rich runoff, generally from agricultural fertilizers. Once plankton get a whiff of the chemicals, they go on a feeding frenzy that also sucks up much of the water's dissolved oxygen.

The stretch of water where the Mississippi River empties into the Gulf of Mexico is famous for having a dead zone that can be about the size of New Jersey.

But Oregon does not have enough farming to account for its recurring dead zone, leaving Chan and other scientists to search for something new. In a recent Nature News article, the researchers shared some recent breakthroughs. 

Before the team could determine the cause of hypoxia, they needed to figure out where the depleted water was coming from. After evaluating near-shore options, such as changing winds and phytoplankton communities, he turned his attention to Oregon's offshore permanent deep oxygen minimum zone (OMZ). 

This environment sits so deep under the ocean surface, between about 2,000 and 4,000 feet down, that it rarely mixes with the oxygen-rich shallow waters. The OMZ has an average of .5 milliliters of oxygen per liter of water. In recent decades, the zone has grown shallower. From the Nature News article:

 "The researchers pulled together 30 years of offshore recordings and, in an unpublished preliminary analysis, found that the concentrations of the gas have dropped by .5 milliliters per liter and are now 2 milliliters per liter -- dangerously close to hypoxic conditions. These are the very waters that well up onto the continental shelf in the spring and the summer. Chan estimates that this oxygen decline above the OMZ has pushed the chances of seeing hypoxia in near-shore waters from 10 percent to roughly 60 percent each year."

It appears that scientists have found their source. Oregon is not the only current case of OMZ expansion, either. This phenomenon is being observed all around the world, from Namibia to Peru. 

Uncovering this global trend, however, has not yet helped scientists answer the nagging "why": are OMZs growing due to natural climate variations or global warming? 

Chan suspects global warming as the cause, because numerous climate models predict that there will be between one and seven percent depletion in oxygen levels in the world's oceans over the next century. There is also evidence to suggest warmer oceans won't mix their waters as well. Deep water will become increasingly oxygen-poor, and when it does finally circulate up into coastal areas, it will wreak havoc with marine ecosystems.

"We need more evidence to conclusively fingerprint global warming -- and that requires a full-time presence in the ocean," Chan explained, something not entirely out of his reach thanks to a new wave of National Science Foundation funding. 

Image: Jami Dwyer, Flickr