Gulf dead zone larger than expected

Gulf dead zone larger than expected
Mapping of Dead Zone Completed
Louisiana Universities Marine Consortium Release
August 2, 2005

The coast wide extent of the Louisiana “dead zone” mapped this week is 11,840 square kilometers (or 4,564 square miles), slightly smaller than the size of Connecticut, reported Dr. Nancy Rabalais, Chief Scientist for Northern Gulf of Mexico Hypoxia Studies. The low oxygen waters extended from near the Mississippi River to the Louisiana/Texas border. The long-term average since mapping began in 1985 is 12,700 km2 (or 4,800 square miles).

The scientific word for the commonly named Dead Zone is `hypoxia,’ or low oxygen, which results in the failure to capture fish, shrimp, and crabs in bottom-dragging trawls when the oxygen falls below the critical level of 2 ppm. Higher in the water column and on both inshore and offshore sides of the hypoxic area, there is sufficient oxygen to support sizeable numbers of fish. The net result, however, is a sizeable stretch of the Louisiana coast unsuitable for supporting fish and shrimp.

The seasonal formation and persistence of hypoxia are influenced by the discharges and nutrient loads of the Mississippi and Atchafalaya rivers. The fresher water forms a layer above the saltier Gulf waters. Nitrogen and phosphorus in the river water stimulate the growth of microscopic plants, the phytoplankton. These algae are either transferred into the food web or end up as organic debris on the sea floor. Their decomposition by bacteria depletes oxygen in the lower waters until they no longer sustain the life of most marine animals.

Wikipedia: Dead zone Dead zones are hypoxic (low-oxygen) areas in the world’s oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. The term could as well apply to the identical phenomenon in large lakes. In March 2004, when the recently-established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003) it reported 146 dead zones in the world oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometer, but the largest dead zone covered 70,000 square kilometers. Aquatic and marine dead zones can be caused by the process of eutrophication, triggered by an excess of plant nutrients (nitrogen and phosphorus) from fertilizers, sewage, combustion emissions from vehicles, power generators, and factories. In a cascade of effects, the nutrients trigger a bloom of phytoplankton at the bottom of the marine food chain, allowing zooplankton to proliferate. As phytoplankton and zooplankton die and sink below the photic zone where photosynthesis can occur, a bloom of natural bacterial degradation exhausts the water’s dissolved oxygen. Dead zones can also be produced by the natural event of river flooding. Large amounts of fresh water empty into the ocean forming a thick layer of fresh water atop the denser salt water, effectively forming a barrier between the ocean water and oxygen in the atmosphere. (Osterman, 2004) Remains of organisms found within sediment layers near the mouth of the Mississippi River indicate four hypoxic events before the advent of artificial fertilizer. In these sediment layers, anoxia-tolerant species are the most prevalant remains found. The periods indicated by the sediment record correspond to historic records of high river flow recorded by instruments at Vicksburg, Mississippi. Reversal of dead zones Dead zones are not irreversible. The Black Sea dead zone, previously the largest dead zone in the world, largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Soviet Union and the demise of centrally planned economies in Eastern and Central Europe. Fishing has again become a major economic activity in the region. While the Black Sea “cleanup” was largely unintentional and involved a drop in hard-to-control fertilizer usage, the U.N. has advocated other cleanups by reducing large industrial emissions. From 1985-2000, the North Sea dead zone had nitrogen reduced by 37% when policy efforts by countries on the Rhine River reduced sewage and industrial emissions of nitrogen into the water. Excerpted from Wikipedia

Water depths affected were as shallow as 20 feet and as deep as 85 feet. The hypoxic zone was smaller between the Mississippi River and the Atchafalaya on the central Louisiana coast than between the Atchafalaya River delta and Calcasieu estuary to the west.

Steve DiMarco and colleagues at Texas A&M University and also the National Marine Fisheries Service groundfish survey both conducted cruises in the area Louisiana hypoxia in early July before the landfall of Hurricane Dennis on the Florida coast. Their sampling grids were limited to the southwestern Louisiana shelf, but they also found hypoxia between the Atchafalaya River and the Calcasieu estuary.

The size just mapped was smaller than predicted using a model developed by Dr. Eugene Turner of Louisiana State University, an investigator of the research team, that relates the size with the May nitrate load along with a term that adds the influence of the previous year’s nitrate load. Turner predicted a size of 6,200 square miles, which was larger than the measured size of 4,800 square miles. The smaller than predicted size was expected because of a tropical storm and hurricane that affected the area between the Mississippi and the Atchafalaya rivers earlier in July. While the two-layered system that supports the formation of hypoxia was redeveloping at the time of the mapping cruise, the oxygen level beneath that layer had not fallen below 2 ppm again. “I would predict that a somewhat larger area of hypoxia would have been mapped if the cruise had been conducted one week later than planned and therefore closer to the size modeled by Turner,” said Rabalais. Confirmation of this prediction may come from the oxygen measurements taken by the NMFS groundfish survey that finished their work on the southeast Louisiana coast on July 27 – August 31.

Scientists from the National Atmospheric and Oceanic Administration predicted this summer’s dead zone to be less than 1,400 square miles based on nutrient loads from the Mississippi and Atchafalaya rivers in May and June. While high in late winter and early spring, the nutrient loads were lower than average this year, probably due to below average precipitation across much of the Mississippi River Basin. The multiple models will help evaluate the influence of the nitrogen load and variations in the physical structure of the offshore coastal waters to improve assessments of hypoxia.

Additional research conducted on the cruise was focused on the effects of varying levels of hypoxia in controlling mercury methylation in bottom sediments. These studies were conducted by Dr. David Senn, Harvard School of Public Health, and funded by NOAA’s initiative on Oceans and Human Health.

The scientific party that mapped this year’s zone was from LUMCON, LSU, Harvard School of Public Health, and Nicholls State University, and was funded by NOAA’s Center for Sponsored Coastal Ocean Research, part of the Ocean Service’s National Centers for Coastal Ocean Science. The mapping was conducted from July 24-29 from aboard the research vessel, Pelican.

For further information contact Dr. Nancy Rabalais, LUMCON, 985-851-2801, nrabalais -AT lumcon.edu.

The original release appeared here on July 29, 2005.