Creature feature: Dead zones vs. killer hurricanes

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The seasonal low-oxygen ‘dead zone’ off the coast of Louisiana and Texas didn’t live up to the forecast as the largest in history. Instead, it was merely the second largest.

The reason appears to be that Hurricane Dolly swept through the area, stirring up and re-oxygenating the waters just before scientists could complete their annual survey.

‘If it were not for Hurricane Dolly, the size of the dead zone would have been substantially larger,’ said Nancy Rabalais, director of the Louisiana Universities Marine Consortium. Nonetheless, she added, ‘an amazingly large area of [low-oxygen] hypoxia persisted.’

R. Eugene Turner of Louisiana State University had predicted that the dead zone would be the largest ever recorded because of the 37% increase this year in fertilizer runoff -- a form of nitrogen, mostly -- that spilled out of the Mississippi and Atchafalaya rivers into the Gulf of Mexico.

This nutrient pollution causes enormous blooms of algae, which then die and sink to the bottom. Bacteria then moves in as part of the decomposition of the plant material, stripping oxygen from seawater hovering near the ocean bottom. When dissolved oxygen drops to a certain level, fish, shellfish and most other forms of marine life have to flee or they suffocate and die.

Scientists such as Turner and Rabalais have been predicting an ever-expanding dead zone, in part because grain farmers are growing extra corn for ethanol. Nearly all of the nation’s heartland drains into the Gulf. ‘The intensive farming of more land, including crops used for biofuels, has definitely contributed to this high nitrogen loading rate,’ Turner said.

Instead of the predicted 8,800 square miles, the affected waters stretched over 7,988 square miles. That’s an area larger that the land mass of Connecticut and getting closer to the size of New Jersey.

Marine scientists and government officials have been trying to work with farmers to change their practices, such as using less fertilizer and applying it in the spring instead of the fall to reduce nutrient-laden discharges into streams and rivers. So far, they have not been successful.

-- Kenneth R. Weiss

Graphics source: Louisiana Universities Marine Consortium

The largest recorded dead zone occurred in 2002, It measured 8,481 square miles.