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Historical Trends in Nitrogen to Phosphorus Ratios in the Gulf of Mexico as Related to the "Dead Zone"
Produce and Refine Short Term Ecological Forecasts for the Extent of the Gulf of Mexico Hypoxic Zone
Abstract
Coastal hypoxia is a widespread environmental problem in the United States. Forty out of one hundred thirty eight estuaries in the U.S. exhibit moderate to severe hypoxia. In addition to faunal mortalities, persistent low oxygen conditions can also reduce critical habitat areas. Much publicity, research and policy attention has been given to the extensive hypoxic zone in the Gulf of Mexico commonly referred to as the “Dead Zone.” Bottom water oxygen concentrations of less than 2 mg/L form in large areas off the Louisiana coast annually during the spring and summer. The size of this hypoxic area averaged 8,300 km2 from 1985 to 1992 and increased to an average of 16,000 km2 from 1993-2001, with a maximum size of over 20,000 km2 in 2003.
Using a biogeochemical model, NOAA scientists annually predict the size of this area of hypoxia. The model is based on Scavia et al. (2003) and was constructed using 18 years of field data. The model uses a suite of input parameters including: organic matter load (which is derived from USGS riverine nitrogen loading values from the Mississippi and Atchafalaya Rivers), a first order oxygen reaeration constant, a first order organic matter decomposition and downstream advection of sub pycnoclinal waters. Loading data from the USGS stations at St. Francisville and Melville are used for the loadings for the Mississippi and Atchafalaya Rivers, respectively. May and June riverine loading data (total nitrogen) is used to determine the biochemical oxygen demand load because nitrogen is the limiting nutrient for algal production. There is considerable uncertainty in the advection term and this uncertainty is quantified via Monte Carlo analysis.
Objectives
The objective of this study to be able to quantitatively forecast the size of the Gulf of Mexico hypoxic zone during its peak size in July.
Time Frame
Ongoing.
For More Information
Dave Whitall@noaa.gov
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