Storm-stirred waters help shrink Gulf’s low-oxygen dead zone

August 02, 2005; The Times-Picayune

But it’s still too big, marine scientists say

By Mark Schleifstein
Staff writer
The dead zone, large patches of water along Louisiana‘s coast containing very low levels of oxygen, was smaller than last year’s but, at 4,564 square miles, still more than double the goal set in a multi-year abatement plan.
Marine scientist Nancy Rabalais attributed the zone’s 1,200-square-mile shrinkage from 2004 to a temporary combination of weather conditions: reduced water flow from the Mississippi and Atchafalaya rivers in May and June and a series of hurricanes and tropical storms crossing the Gulf of Mexico that stirred some oxygen into the deeper water
Hypoxia, as the low-oxygen condition is called, is formed in ocean depths as shallow as 20 feet and as deep as 85 feet when nutrient-rich water from the rivers flows over denser, salt water in the Gulf.
Nitrogen and phosphorus in the fresh water, largely from Midwest farmland, spur blooms of algae that die, sink to the bottom and decay, using up oxygen. Higher-oxygen surface water is unable to mix with the deeper salt water until hurricanes or frontal systems stir them together.
Scientists believe fish and shrimp avoid the hypoxia areas, if they can. But bottom-living organisms that are a food source for the fish and shrimp are unable to escape and so they die.
A study published last year by fisheries scientists with National Oceanic and Atmospheric Administration, or NOAA, showed that fewer shrimp were landed in areas affected by low oxygen in Louisiana and Texas.
This year’s hypoxia area also is almost 2 ½ times as large as the goal included in a national plan that would reduce the five-year average hypoxia area to 1,930 square miles by 2015. That plan, sent to Congress in 2001 by a task force representing states along the river and several federal agencies, relies on voluntary actions to reduce nutrient pollution.
But even those voluntary actions are barely under way, and it shows, said Rabalais, who is director of the Louisiana Universities Marine Consortium.
"We can keep measuring it year after year," she said of her annual five-day research cruises in the Gulf, "but if we still allow nitrogen to flow off the land and the concentration is not going down, it’s not going to make a difference."
Delays in implementing the plan to reduce the use of nitrogen fertilizers in the Midwest are partly the result of efforts to lay a larger part of the blame for the dead zone on phosphorus-based fertilizers, which are not used as much in many parts of the Midwest, and on other sources of phosphorus, such as sewage treatment plants.
An Environmental Protection Agency study into that concern has indicated that phosphorus does have a small role, but that nitrogen is still the largest factor in causing the low-oxygen area. The study is expected to be included in a revision of the 2001 reduction plan next year.
Federal scientists this year also were unsuccessful in accurately predicting the size of the low-oxygen zone in advance of Rabalais’ cruise.
An NOAA model using information on the nutrients in the Mississippi and Atchafalya rivers predicted the hypoxia area would be less than 1,400 square miles. Rabalais said it failed to take into account an early high-river season this year, which resulted in nutrients getting dumped into the Gulf long before May.
A second model developed by LouisianaStateUniversity environmental studies Prof. Eugene Turner led to an oversized prediction of 6,200 square miles. Turner took into account both the early spring water and nutrients that might have been left over in Gulf sediments from last year.
Rabalais said Turner’s model was off because of the mixing caused by the tropical storms, but that if she had made her cruise a week later, it might have been accurate.
She said two surveys of the southwestern Louisiana shelf made in early July, before Hurricane Dennis, by TexasA&MUniversity and NOAA-Fisheries found hypoxia between the AtchafalayaRiver and the Calcasieu estuary near the state line.
Rabalais’ July 24-29 survey aboard the research vessel Pelican was conducted by scientists from LUMCON, LSU, Harvard School of Public Health and NichollsStateUniversity. It was paid for by NOAA’s Center for Sponsored Coastal Ocean Research.
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Mark Schleifstein can be reached at or (504) 826-3327.