Summer 2002 Hypoxia

Once again the estimated size of the Dead Zone off Louisiana and Texas has grown past previous years at 22,000 sq. km (= 8,500 sq. mi.), reports Dr. Nancy Rabalais who just completed this summer’s mapping. The area it stretches across the Gulf of Mexico sea floor is larger than the state of Massachusetts. This year’s zone also extends well into Texas waters. “Because of time constraints, we weren’t able to map the entire extent into Texas,” reported Nancy Rabalais, chief scientist. The zone stretches from the Mississippi River delta across the Louisiana coast, and onto the upper Texas coast near Galveston, from very near shore to as far as 25 miles off Grand Isle and 60 miles off Cameron, in water depths of 10 to 120 feet. The scientific word for the commonly named Dead Zone is “hypoxia,” or low oxygen. Dead Zone refers to the failure to capture fish, shrimp, and crabs in bottom-dragging trawls when the oxygen concentration falls below a critical level in water above the seabed. Higher in the water column, however, there is sufficient oxygen to support sizeable numbers of fish and swimming crabs, so that the term “dead zone” is not entirely accurate. Still, the area is large enough to garner a high level of public attention if only because of the lack of catchable bottom-dwelling fish and shrimp. Shrimp trawlers were a rare site for the scientists that were mapping the Dead Zone over the last 6 days. The seasonal formation and persistence of hypoxia are influenced by the discharges of the Mississippi and Atchafalaya Rivers. The fresh water from the rivers forms a fresher layer above the saltier Gulf waters, and the resulting two-layer system inhibits the oxygen in the surface waters from penetrating to depth. Nutrients, particularly nitrogen and phosphorus, stimulate the growth of microscopic plants, the phytoplankton. These single celled plants either end up in the food web, which off Louisiana supports valuable commercial fisheries, or end up as organic debris on the sea floor. The decomposition of this organic matter by bacteria uses up the oxygen to the point that it becomes depleted and lower than what is necessary to sustain the life of most marine animals. The hypoxia will persist through the remainder of the summer until the waters are mixed up by either tropical storms or a series of cold fronts in the fall. “The sequence of events that led to this year’s hypoxia was a bit abnormal,” reported Rabalais. Spring weather was windier than normal, and the two-layer system that supports hypoxia did not develop to its maximum strength until June, unlike other years when hypoxia occurs earlier. Also, the Mississippi River discharge, which stimulates the algal growth, peaked three times in 2002, February, April, and as recently as early June, pouring additional fresh water and nutrients into the Gulf at a critical time for the formation of summer hypoxia. The scientific party from the Louisiana Universities Marine Consortium, Louisiana State University, and Woods Hole Oceanographic Institution mapped the area along 90 stations from the Mississippi River to just west of High Island, Texas, over a 6-day period from July 21-26. The research is funded by the National Oceanic and Atmospheric Administration, Coastal Ocean Program. A White House-level assessment of the causes and consequences of hypoxia concluded that an almost 3-fold increase in nitrogen load to the Gulf since the 1950s has driven the long-term increase in hypoxia since the middle of the last century. The Task Force that oversaw the assessment completed during the Clinton Administration continues under the Bush administration in working to meet the goals of the Hypoxia Action Plan that calls for reducing the size of hypoxia to 5,000 sq. km. (approx. 2,000 sq. mi.) by 2015 through reducing nitrogen loading from the Mississippi River by 30%.

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