Macrobenthos Community Succession in the Northern Gulf of Mexico Hypoxic Regions: Testing the Pearson-Rosenberg Model

The Pearson and Rosenberg (P-R) conceptual model of macrobenthic successionwas used to assess the impact of hypoxia (dissolved oxygen concentration <= 2mg L-1) on the macrobenthic community on the continental shelf of the northern Gulf of Mexico. The stress-response relationship between bottom-water oxygen concentration and the macrobenthic community was used to determine the ecological condition of the benthic habitat. In lieu of testing the effect of decreasing influence of organic enrichment on the macrobenthic community as a function of distance from the source of organic pollution, we tested the effect of bottom-water dissolved oxygen concentration as a proxy for distance from the source. To include the seasonal response in the long-term cumulative effect of recurring hypoxia on the macrobenthic community, samples were collected during early spring (April 2009), late summer (September 2009), and midsummer (August 2010).The cumulative effect of annual hypoxia on the macrobenthos at each site was apparent from the species diversity, abundance, and biomass. High diversity values at site H7 and low diversity values at site A6 indicated the existence of a westward-diminishing hypoxic gradient on the Louisiana/Texas shelf, interrupted by the Atchafalaya River discharge. Unsurprisingly, the macrobenthos collected during midsummer were generally the most stressed community, with the exception of site E4. Assessment of successional stages of the benthic communities of the four sites with multivariate analyses of nonmetric multidimensional scaling, distance-based redundancy analysis, and principal component analysis identified the influence of bottom-water hypoxia. The study demonstrated the applicability of the P-R model in predicting successional change in the macrobenthic community on the Louisiana continental shelf. Site H7 was in the advanced phase of stage II, sites E4 and D5 were in the intermediate phase of stage II, and site A6 was in the intermediate phase of stage I in the P-R successional continuum.