| Hypoxia Offshore the Atchafalaya and Mississippi Rivers |
NO PRESENTATION AVAILABLE
Nancy N. Rabalais and R. Eugene Turner nrabalais@lumcon.edu
Louisiana Universities Marine Consortium, 8124 Hwy. 56, Chauvin, LA 70344
Coastal Ecology Institute and Department of Oceanography and Coastal Science, Louisiana State University, Baton Rouge, LA 70803
Abstract
The Mississippi and Atchafalaya rivers are the primary sources of fresh water, nitrogen and phosphorus to the northern Gulf of Mexico. The MR empties onto a relatively deep shelf and the plume separates rapidly from the bottom and expands laterally as its waters flow from east to west. The AR enters into Atchafalaya Bay, a broad shallow embayment, then to a shallow, broad continental shelf. The influence of the Mississippi and Atchafalaya outflows is evident in downplume surface waters in salinity, suspended sediments, nutrients, chlorophyll biomass and bottom water hypoxia. Seasonal hypoxia in this region is the result of the strong and persistent stratification coupled with the high organic production that is fueled by river‐derived nutrients. Data transects off Terrebonne Bay (TB) and Atchafalaya Bay (AB) depict long‐term conditions, with regard to salinity, nutrients, chlorophyll biomass, and hypoxia. Lower surface salinity is more pronounced on TB than on AB, but each shows a spring‐summer lowering of salinity across the shelf. The surface dissolved inorganic nitrogen is composed primarily of nitrate‐N for both transects with high nearshore concentrations on AB and periodic peaks in ammonium‐N. Higher concentrations of chlorophyll a in surface waters on TB occur after the winter‐early spring peak in nitrate‐N. The general pattern for AB is for higher chlorophyll a in surface waters nearshore to mid‐transect increasing through spring to maximal concentrations in summer. The development of hypoxia on TB occurs earlier in spring compared to AB in summer and extends into September as opposed to August for AB.
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