Date of Award

Fall 12-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Science

Committee Chair

Scott P. Milroy

Committee Chair Department

Marine Science

Committee Member 2

Vernon Asper

Committee Member 2 Department

Marine Science

Committee Member 3

Charlotte Brunner

Committee Member 3 Department

Marine Science

Committee Member 4

Chet F. Rakocinski

Committee Member 4 Department

Coastal Sciences, Gulf Coast Research Laboratory

Committee Member 5

Kevin Briggs

Abstract

The Pearson and Rosenberg (P-R) conceptual model of macrobenthic succession was used to assess the impact of hypoxia (dissolved oxygen [DO] ≤ 2 mg/L) on the macrobenthic community on the continental shelf of northern Gulf of Mexico for the first time. The model uses a stress-response relationship between environmental parameters and the macrobenthic community to determine the ecological condition of the benthic habitat. The ecological significance of dissolved oxygen in a benthic habitat is well understood. In addition, the annual recurrence of bottom-water hypoxia on the Louisiana/Texas shelf during summer months is well documented.

The P-R model illustrates the decreasing impact of organic enrichment on the macrobenthic community distally from the source. To test the underlying principles of the P-R model using bottom-water concentration of dissolved oxygen as a proxy for distance from the source of organic enrichment, four sites were chosen based on the frequency of hypoxia occurrence in the hypoxic zone of the northern Gulf of Mexico. The chosen sites were arrayed from west to east on the Louisiana continental shelf, where site A was farthest from the Mississippi River and least impacted by seasonal hypoxia with75% annual exposure time. Site C was impacted by seasonal hypoxia between 25% and 50% of the time. Site B was impacted by seasonal hypoxia between 50% and 75% of the time. Sites B and C were situated near the vicinity of the Atchafalaya River between sites A and D.

To measure the spatio-temporal effect of hypoxia on the macrobenthic community, samples were collected during early spring (April 2009), late summer (September 2009), and mid-summer (August 2010). Results indicated that the macrobenthos collected during mid-summer were the most stressed community, with the exception of site C. The cumulative effect of annual hypoxia on the macrobenthos at each site was apparent from the species diversity, abundance, and biomass.

High values of species diversity, richness and evenness at site A and low values at site D indicated the existence of a westward-diminishing hypoxic gradient on the Louisiana/Texas shelf interrupted by the Atchafalaya River discharge. Assessment of successional stages of the benthic communities of the four sites with the multivariate analyses of nonmetric multi-dimensional scaling and principal component analysis identified the spatial gradient of hypoxia. Examination of the macrobenthos and available environmental data with canonical correspondence analysis indicated that sedimentary organic carbon, as well as bottom-water dissolved oxygen, had a strong impact on the benthic community structure. The study successfully demonstrated the applicability of the Pearson-Rosenberg model on the Louisiana/Texas continental shelf and the results were consistent with the predictions of the model. Site A was in the advanced phase of stage II succession; whereas sites B and C were in the intermediate phase of stage II succession, and site D was in the intermediate phase of stage I succession in the Pearson-Rosenberg successional continuum.

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