Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Open-water continuous monitoring of DO concentrations at a single station (C6) in the Gulf of Mexico from 1989 to 2008 afforded an excellent opportunity to characterize short-term oxygen variability and to estimate the relative importance of key physical and biological factors controlling the development, persistence, and dissipation of hypoxia. I investigated temporal trends in three aspects of short-term DO variability: respiration rates (i.e., how quickly bottom waters become hypoxic), persistence of hypoxia, and the dissipation of hypoxia (i.e., re-aeration events). I identified the range of respiration rates present at the study site, and showed how these rates vary throughout the year and from year to year. Although a strong relationship between the persistence of hypoxia at station C6 and the areal extent of hypoxia in the NGOM was not present, both were statistically related to the monthly Mississippi River nitrogen flux. Using time-series analysis, I found no consistent periodicities in DO across the three depths (near-surface, mid-water-column, and near-bottom) or related to water levels (tides). I did find a diel signal in the DO that could be related to the diel pattern in available light for photosynthesis reaching the near-bottom. Percent of days exhibiting a diel pattern was 20 at the surface, 12 at the mid-water column, and 7 for the near-bottom measurements. Using regression trees and matching of the timing of events, I found that the density gradient was a good predictor of severe hypoxia (DO -1) and that 65% of the re-aeration events could be associated with wind stress events, cold fronts or tropical cyclones. These results suggest that continuous DO monitoring at a single location provides valuable information on short-term variability that will help in assessing the exposure and the resulting biological responses to hypoxia, in interpreting the possible variability around the annual hypoxia maps generated from the single, shelf-wide cruises, and as a basis for improving predictive models of hypoxia.
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Babin, Brenda Leroux, "Factors Affecting Short-term Oxygen Variability in the Northern Gulf of Mexico Hypoxic Zone" (2012). LSU Doctoral Dissertations. 466.