2 very different forecasts for Gulf dead zone

Published: Jun. 21, 2012 at 8:41 PM CDT|Updated: Jul. 5, 2012 at 4:41 PM CDT
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File/AP Photo
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NEW ORLEANS (AP) - Scientists from Louisiana and Michigan have wildly different predictions for the size of this year's "dead zone" of low-oxygen water in the Gulf of Mexico.

Forecasts released Thursday say it will either be the smallest in nearly a quarter century at just under 1,200 square miles, or five times that big and a bit above average.

The forecasts are based on different hypotheses about how the system works.

The larger estimate comes from the Louisiana Universities Marine Consortium (LUMCON) in Cocodrie and Louisiana State University. They estimate that the area where oxygen levels are too low to sustain life will cover about 6,210 square miles off Louisiana and Texas. That's a bit larger than the state of Connecticut - above the 27-year average but nowhere near a record.

Donald Scavia and Mary Anne Evans of the University of Michigan made the smaller prediction, which was based mainly on the amount of nutrients carried down the Mississippi River. That "nutrient load" is the smallest since 1988, so the hypoxic zone that LUMCON's Nancy Rabalais has measured every July since 1985 also will probably be the smallest in 24 years, they say.

Rabalais' forecast is based on a model that also considers environmental changes that can carry over from year to year, even over decades, and assumes that the ecosystem is becoming more sensitive to nitrogen.

The dead zone is created because fertilizer and other nutrients swept into the Gulf by the Mississippi River feed huge numbers of microscopic organisms. They die and fall to the bottom, where their decomposition uses up oxygen. Without wind-driven waves to mix oxygen back into the water, the hypoxic area grows.

Last year's hypoxic zone was about 6,765 square miles. The record is 8,400.

Rabalais said she and her graduate students saw the effect of waves last weekend off the Atchafalaya River and Terrebonne Parish, where LUMCON makes monthly measurements.

Dropping their oxygen meter 65 feet to the bottom, they found readings close to zero parts per million. At under 2 parts per million, oxygen levels are considered too low for ocean life.

However, Saturday brought rough weather with 5- to 6-foot seas. By late Sunday and Monday, she said, readings ranged from well above 2 ppm to nearly 7 ppm. "That lasted about a day or two," Rabalais said.

That's why a tropical storm shortly before the annual July cruise to measure the dead zone could leave a much smaller hypoxic area than predicted, she said.

If the weather stays calm, this year's annual cruise to measure the dead zone could indicate which model is more accurate, Rabalais said.

"This year will be a good test. It'll be very interesting," she said.


Online: -Mexico-Hypoxic-Forecast.pdf

(Copyright 2012 by The Associated Press. All Rights Reserved.)