Hurricane keeps dead zone small

The huge "dead zone" of oxygen-poor water in the Gulf of Mexico failed to reach record size this year. Scientists had predicted that this hypoxic zone would swell to 8,800 square miles (Reuters, 15 July) due to floodwaters that poured tonnes of fertilizer into the Mississippi River, which empties the Midwest’s agricultural runoff into the Gulf. But it ended up rating a mere 7,988 square miles (still nearly the size of Israel) and thus ranks as second-biggest since scientists started tracking it in the 1980s. 2002 keeps its place as the worst year, with an 8,500-square-mile dead zone .

Increased corn farming, for ethanol, meant farmers used lots of fertilizer this season. When the rains hit, they rinsed the fertilizer into the river. This spring 83,000 tons of phosphorus rode the Mississippi to the Gulf, 85% higher than average levels. Those nutrients, as they do every year, sparked an algal bloom. When the algae die and sink to the bottom, bacteria feast on their remains. With so many bacteria slurping so many dead algae, the bacteria suck all the dissolved oxygen out of the water faster than it can diffuse back in. Fish and crustaceans rush toward airier waters, including the coastline, in an underwater stampede some Louisiana seafood lovers call a “jubilee.”

But this year, Hurricane Dolly stirred the dead zone like a big pot of soup, aerating water that would otherwise have been oxygenless. Thus by the time scientists finished measuring it, the zone was smaller than predicted.

It should shrink further in the fall, with cooler weather, fewer algae and more storms mixing the waters.

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