HR: 11:35h
AN: OS52A-06    [Abstracts]
TI: Louisiana Shelf Hypoxic Zone Mixing Rates determined from Radium and Radon
AU: * Smoak, J M
EM: smoak@stpt.usf.edu
AF: Environmental Science, University of South Florida, St. Petersburg, FL 33701, United States
AU: Cable, J E
EM: jcable@lsu.edu
AF: Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Rabalais, N N
EM: nrabalais@lumcon.edu
AF: Louisiana Universities Marine Consortium, 8124 Hwy. 56, Chauvin, LA 70344, United States
AU: Svoboda, C C
EM: chclayto@mail.usf.edu
AF: Environmental Science, University of South Florida, St. Petersburg, FL 33701, United States
AB: Hypoxic conditions are observed in bottom waters of the inner continental shelf off the coast of Louisiana during the summer months when spring floods from the Mississippi River and fewer frontal storms create a stratified coastal system. Hypoxia is established after spring phytoplankton blooms when organic matter sources to the sediments are high. Decomposition of this organic matter supplied from the overlying water column and from underlying sediments consumes more oxygen than is produced or supplied via water mass mixing. Models have shown oxygen concentrations to be very sensitive to the mixing rate. We use naturally occurring isotopes of radium, ^{224,223,228,226}Ra (t_1/2 = 3.6 d, 11 d, 5.7 y, and 1620 y), and radon, ²²²Rn (t_1/2 = 3.8 d), to calculate the mixing rates across the boundary of oxygenated surface water and the bottom hypoxic zone. We determine an eddy diffusion rate of 3.8 x 10^-4 to 3.5 x 10^-5 m²/s. This rate is greater than what was previously used in models of the open ocean, but may be of a similar magnitude to near- bottom vertical eddy diffusivity. A higher mixing rate requires great consumption of oxygen to maintain the hypoxic conditions. This suggests the hypoxic conditions may be more sensitive to organic matter input (i.e. nutrient supply) than previously thought.
DE: 4211 Benthic boundary layers
DE: 4217 Coastal processes
DE: 4808 Chemical tracers
DE: 4834 Hypoxic environments (0404, 4802)
DE: 4860 Radioactivity and radioisotopes
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting