Mississippi River floods may bring biggest dead zone
The National Oceanic and Atmospheric Administration predicts the deadly low-oxygen waters to spread across an expanse of the Gulf of Mexico seafloor equivalent to the geographical size of New Hampshire. To date, the largest low-oxygen, or hypoxia, zone in the gulf came in 2002, sprawling across 8,400 square miles.
Such dead zones are a marine reaction to the nutrient-enriched runoff from fertilized farm fields, animal feed lots and even city sewage systems. These low-oxygen waters can kill fish, shrimp and other marine life that get trapped or fail to swim or scuttle away.
The culprit in many marine die-offs is usually decaying microorganisms. Although the oceans are awash in algae, riotous blooms occur in the spring when waters are supercharged by nutrients, particularly nitrogen and phosphorous. The algae die and sink to the bottom. Then bacteria take over, breaking down the plant matter and sucking the oxygen out of seawater. That leaves little or none for fish or other marine life.
Hundreds of dead zones have been identified all over the world. Robert Diaz, a professor at Virginia Institute of Marine Science, has reported to Congress and the White House that nearly half of U.S. bays, estuaries and other waterways surveyed have suffered from life-choking, low-oxygen waters.
Scientists expect dead zones to expand with changing landscapes and surging flood waters, such as this spring's record-setting deluge in the Mississippi basin. Although scientists are careful not to link any specific flood to climate change, they agree that extreme weather events are likely to become more common as the planet warms.
Whether this spring's polluted runoff will create a dead zone for the record books will be confirmed when scientists, organized by Nancy Rabalais, director of Louisiana Universities Marine Consortium, complete underwater surveys in late July and early August.
-- Kenneth R. Weiss
Chart on top: The annual measurements of the size of the Gulf of Mexico's hypoxia zone and the 2011 forecast. The dark gray represents the range of forecasts.
Credit: Nancy Rabalais/Louisiana Universities Marine Consortium, National Oceanic and Atmospheric Administration