Presenting... 2009's very own "dead zone" in the Gulf of Mexico:

The National Oceanic and Atmospheric Administration just unveiled this year's estimate of the vast region in the Gulf where no aquatic life can survive, clocking in at some 6,000 square miles. The dead zone, which blossoms in the summer, is caused primarily by fertilizer runoff from farms, which splashes into the Mississippi River, bobs down to the Gulf, and then triggers algae blooms that choke off all oxygen in the area, wreaking havoc on, among other things, the region's $3-billion fishing industry.

The good news is that the 2009 dead zone is a fair size smaller than the record 8,481-square-mile version in 2002. This year's incarnation was originally predicted to break records, but, according to Nancy Rabalais of the Louisiana Universities Marine Consortium, freak weather patterns may have helped re-oxygenate the waters. (Curiously, other dead zones, such as the one that suddenly showed up off the Oregon coast six years ago, seem to be triggered not by fertilizer runoff but by shifting northerly winds, which may be linked to rising ocean temperatures.)

The not-so-good news is that the weird weather that kept the Gulf of Mexico's dead zone (relatively) contained aren't expected to repeat every year, and the underlying runoff problem hasn't gone away. It may even get worse: As Fred Below, a professor of crop physiology at University of Illinois Urbana-Champaign explains, federal incentives for corn ethanol will likely fuel the growth of the Gulf's dead zones, since it will entice more Midwestern farmers to switch over to growing corn, and corn requires more nitrogen fertilizer than other crops. Three guesses where that fertilizer ends up.

--Bradford Plumer