TROPICAL STORM BARRY STIRS IT UP, BUT DEAD ZONE RETURNS

August 16, 2001; Press Release

Earlier reports this summer documented the dead zone off Louisiana and Texas to be the largest ever, stretching from the Mississippi River delta onto the upper Texas coast.  The record-breaking size was just over 8,000 sq. mi., approaching the size of the state of Massachusetts. 
Tropical Storm Barry approached the Louisiana coast on August 4-5, but then turned to the northeastern Gulf.  While nearing Louisiana the storm generated 12-foot waves off TerrebonneBay and eliminated the low oxygen in places along the Louisiana coast.  Waves from tropical storms, hurricanes, or cold fronts will break down the structure of the water column that supports the development of hypoxia, and allows mixing of oxygen from the surface waters to the bottom. Hypoxia has built back up following Tropical Storm Barry, at least off the southeastern Louisiana coast.  A few isolated stations from Barataria to TerrebonneBay were low in oxygen in the bottom waters the week of August 6 following the storm.  An extensive area built back up off TerrebonneBay by August 15, reported Dr. Nancy Rabalais of the Louisiana Universities Marine Consortium.  The same broad area covered in the July 20-26 cruise has not been resampled.
Her research team of scientists from Louisiana Universities Marine Consortium and LouisianaStateUniversity conducted oxygen surveys along a 25-mile line seaward off TerrebonneBay on Wednesday.  The oxygen levels were below 2 mg/l, or ppm, at 7 of the 9 stations sampled in water depths of 15 to 90 feet.  The low oxygen was confined to water depths of 75 feet or less. 
The usual distribution of fish was seen by research scuba divers who worked to replace 3 oxygen meters moored at a station in 67 ft of water near an offshore oil platform.  There were many fish in the upper half of the water column, the typical spadefish, sheepshead, and snapper, but none below about 30 feet down to the bottom at 67 feet. 
The oxygen data recovered from the meters indicated that the bottom waters were devoid of oxygen, close to zero or anoxic, since the shelfwide cruise that mapped the dead zone was completed on July 26, up until the waves from Tropical Storm Barry mixed enough oxygen into the lower water column to bring the oxygen levels to above 5 ppm, plenty of oxygen for the fish that reside at the offshore oil platform where the meters are deployed.  The oxygen remained plentiful for 6 days, but declined rapidly as seas calmed.  By August 12, the oxygen again fell below 2 ppm and remained low through Wednesday’s sampling.
Chantal, the Atlantic‘s third tropical storm of the season, came to life Thursday on a collision course with the Caribbean islands and as yet undetermined course.  Should this storm enter the Gulf of Mexico, another mixing event in the waters of the dead zone would be possible with another short-term cure for the low oxygen conditions.
The hypoxia forms during most springs when the Mississippi River delivers its highest load of freshwater.  The accompanying nutrients, particularly nitrogen and phosphorus, stimulate the growth of microscopic plants, the phytoplankton.  The decomposition of this organic matter after it sinks to the bottom uses up the oxygen to the point that it becomes lower than what is necessary to sustain the life of most marine animals.  The low oxygen will persist into late summer barring the passage of tropical storms or hurricanes.
The LUMCON/LSU research program is funded by the National Oceanic and Atmospheric Administration, Coastal Ocean Program.  For further information contact Nancy Rabalais, LUMCON, 985-851-2836, nrabalais@lumcon.edu.