Researchers: Dead zone formed near MRGO rock barrier

Researchers: Dead zone formed near MRGO rock barrier

A 16-mile-long "dead zone" of low-oxygen water formed this summer downstream of the Mississippi River Gulf Outlet’s new rock barrier, but researchers don’t know why the zone formed, how long it will stay or if it will become a seasonal occurrence. 
 
They also aren’t sure whether it is something new or just never noticed before, said Chris Swarzenski of the U.S. Geological Survey’s Louisiana Water Science Center. 
 
Testing for salinity and dissolved oxygen began about a year ago, before work began on the barrier, with money from the Louisiana Coastal Area Science Board. 
 
Follow-up monitoring – both spot water tests at sections of the river and continuous salinity monitoring both above and below the barrier – has shown some changes in the channel, Swarzenski said. 
 
The Mississippi River Gulf Outlet navigation channel, MRGO for short, was built in the 1950s and 1960s as a shortcut between the Gulf of Mexico and the Port of New Orleans. Damage from Hurricane Katrina in 2005 prompted Congress to order it closed in 2008. 
 
The rock barrier to close it off was started in January, mostly complete by April and finished in July. 
 
Water got less salty above the barrier, and saltier below it. But dissolved oxygen levels changed, too. 
 
"Before June, we usually found plenty of oxygen," Swarzenski said. 
 
In June, he found the low-oxygen area stretching 16 miles south of the barrier, from 5 feet below the surface to the bottom of the channel bed, anywhere from 35 to 43 feet deep. 
 
"What caught us by surprise was it was downstream," Swarzenski said. A dead zone upstream seemed more likely because the barrier would have cut off some circulation. 
 
So far, no one seems to have a sound explanation of why that dead zone appeared where it did, he said. 
 
Greg Miller, U.S. Army Corps of Engineers Senior Project Manager, said monitoring will help the corps find out where and how the area of low oxygen is forming and if it’s temporary. 
 
If it appears permanent, staff will have to look at possible solutions, he said. 
 
However, he said, it’s a little early to reach a conclusion on what the low-oxygen area means or how long it will stay around and if the salinity reduction provided by the barrier has had benefits across the basin. 
 
"We haven’t had any evidence to suggest that it’s had an impact on fisheries," Miller said. "We’ve gotten numerous reports of people congregating at the structure to fish." 
 
Swarzenski said another round of monitoring is expected to be done in a couple of weeks and it will be interesting to see if the area of low oxygen has grown or shrunk. 
 
He said the corps is paying for more monitoring to see whether the dead zone area is permanent or temporary. 
 
In addition, there’s some indication that something similar might be starting to form north of the barrier as well, he said. 
 
"So there is something going on upstream," he said. 
 
He also cautioned that there’s very little dissolved oxygen baseline information for the channel, except for the fact that commercial fishermen have used the channel for shrimping and other fishing, so a dead zone wouldn’t be likely. 
 
John Lopez, Lake Pontchartrain Basin Foundation Coastal Sustainability Program director, agreed. He said there could have been localized areas of low-oxygen water in the channel before. 
 
"What the conditions were there before this dam are just speculation," he said in regard to the barrier construction and water quality. "That doesn’t necessarily mean this is a permanent condition."