NOAA, LOUISIANA SCIENTISTS ISSUE FIRST-EVER “DEAD ZONE” FORECAST
July 25, 2003 ó A team of scientists from NOAA, the Louisiana Universities Marine Consortium (LUMCON), and Louisiana State University is forecasting that the size of the “Dead Zone” off the coast of Louisiana and Texas this summer should be between 4,770 and 6,900 square miles, an area approximately the size of the state of Connecticut. (Click NOAA image for larger view of “dead zone” in the Gulf of Mexico in July 2002. Please credit “NOAA.”)
The “Dead Zone” is the name for the seasonal change in areas of the Gulf of Mexico where algal growth, stimulated by input of nutrients such as nitrogen and phosphates from the Mississippi and Atchafalaya rivers, settles and decays in the bottom waters, leading to decreased oxygen levels. Because the decaying algae consumes oxygen faster than it can be replenished from the surface, the result is oxygen levels too low (hypoxia) to support most life in a massive area two times larger than the entire surface of the Chesapeake Bay.
The prediction for this summer is an area slightly larger than the average annual hypoxia affected waters since 1990 of approximately 4,900 square miles. The forecast is based on nutrient loads from the Mississippi and Atchafalaya Rivers in May and June provided by the U.S. Geological Survey.
This is the first advance forecast of the annual hypoxic event in the Gulf of Mexico and is an example of an innovative environmental service—officially referred to as “ecological forecasting”—that NOAA scientists believe will become an important tool in coming years for both decision makers and the public. (Click NOAA image for larger view of “dead zone” in the Gulf of Mexico from June 11 through July 17, 2003. Please credit “NOAA.”)
Oceanographers have been exploring new ways to provide more accurate and timely forecasts of the “Dead Zone,” and their research has led to the development of a forecasting model that was published in the May 2003 issue of the peer-reviewed journal Limnology and Oceanography. The model is the first to predict directly the size of the hypoxic zone as a function of changes in nutrient loadings.
“By using this river dissolved oxygen model based on Mississippi River nutrient loadings in the northern Gulf of Mexico, our research team was also able to look back more than 30 years and determine that these now virtually perennial events were uncommon before the mid-1970s,” said lead author Donald Scavia, the chief scientist of the NOAA Ocean Service.
The northern Gulf of Mexico’s bottom-water summer hypoxic zone in recent years has extended roughly 375 miles westward from the mouth of the Mississippi River, in Louisiana, across the Texas border.
Between 1985 and 1992, the northern Gulf of Mexico hypoxic zone averaged 3,200 square miles (8,300 km2). But between 1993 and 2001 that hypoxic region nearly doubled—to an annual average of roughly 6,200 square miles (16,000 km2), according to research done by LSU scientist Nancy Rabalais, one of the report’s co-authors.
cruises to track development of hypoxia have been conducted monthly
since January. Rabalais, chief scientist for hypoxia research at LUMCON,
explains that the “algal
blooms that fuel the eventual summer hypoxia events were abundant
this spring when the Mississippi River discharge peaked. Another crest
of the river, in late May, provided more nutrients for another boost
of algal growth. Hypoxia began to form in April and May and was widespread
in early June.” Rabalais notes, however, that two recent tropical
storms have caused a shrinking of the hypoxic area where the storm passed
across the southeastern Louisiana shelf but that hypoxia was once again
rebuilding. This year’s research cruise to map the extent of the
hypoxic, or low oxygen, zone is currently underway.
This work evaluated the influence of nitrogen load and variations in ocean currents looking both backward and forward at the range of the Gulf’s hypoxic area relative to changing nitrogen loadings.
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