The phenomenon is caused when salt water loses large amounts of oxygen, a condition known as hypoxia that is typically associated with an area off the Louisiana coast at the mouth of the Mississippi. The fresh water and salt water don't mix well -- like oil and water -- and keeps oxygen from filtering through to the sea bottom, causing problems for fish, shrimp, crabs and clams.
DiMarco, joined by researchers from Texas A&M, Texas A&M at Galveston and the University of Georgia, just returned from an examination of 74 sites between Terrebonne and Cameron, La. He said the most severe hypoxia levels were recorded in the mid-range depths, between 20 and 30 feet, as well as near the bottom of the sea floor at about 60 feet.
Some of the worst hypoxic levels occurred in the western gulf toward the border between Texas and Louisiana.
"We saw quite a few areas that had little or no oxygen at all at that site," he said. "This dead zone area is the strongest we've seen since 2004, and it's very likely the worst may be still to come.
"Since most of the water from the Midwest is still making its way down to the gulf, we believe that wide area of hypoxia will persist through August and likely until September, when it normally ends."
The area likely will be the largest ever recorded and last longer, with marine life affected for hundreds of miles off the Louisiana-Texas coast, said DiMarco, who last year discovered a similar dead zone off the Texas coast where the then rain-swollen Brazos River emptied into the gulf.
The zone off Louisiana reached a record 7,900 square miles in 2002. A recent estimate from the National Oceanic and Atmospheric Administration and Louisiana State University shows the zone, which has been monitored for about 25 years, could exceed 8,800 square miles this year, an area roughly the size of New Jersey .
DiMarco said a tropical storm or hurricane likely would have no impact on this year's zone, believed to be caused by nutrient pollution from fertilizers that empty into rivers and eventually reach the gulf.