Benthic community response to hypoxia: Baseline data

Abstract

World wide more than 400 aquatic systems are hypoxic, affecting an area of more than 245,000 square kilometers. The hypoxic area in Gulf of Mexico is the second largest in the world next to the Baltic Sea basin. In the northern Gulf of Mexico hypoxia is a phenomenon which occurs when the seasonal thermohaline stratification in the water column develops. Under normoxic or hypoxic conditions benthic organisms affect the physicochemical properties of sediment by their usual biological activity such as feeding, locomotion, and burrowing. According to the Pearson-Rosenberg model, changes in the benthos due to hypoxia occur at the community level of organization, with the pioneering community having different biological behavior than the equilibrium community, which normally exists without hypoxic stress. These changes as a response to low oxygen include numerical density, species diversity, organism size, depth of bioturbation, and number of functional groups-all factors which ultimately can affect the physicochemical properties of sediment. Thus, to some extent there may be a feedback mechanism that conditions the sediment properties for the particular type of benthic community. The types of sediment properties that can affect the density and diversity of benthos include grain size distribution, bulk density, and concentration of organic matter. To study these changes, macrobenthos and sediment samples were collected from the northern Gulf of Mexico between the Atchafalaya and Mississippi Rivers. Four provinces were chosen based on the frequency of occurrence of hypoxic events for a comparison between pre-hypoxic conditions in early spring and hypoxic conditions in late summer. The macrobenthos data will be compared with the sediment properties of grain size, organic matter concentration, sedimentation rate, and depth of the redox potential discontinuity to help explain the variability in the biological data among provinces. The macrobenthos data will be statistically analyzed for species richness using Hurlbert rarefaction curves that enable the calculation of the richness for a set number of species, organism sizes, bioturbation depths, and functional groups. The macrobenthos data will be correlated with sediment properties using the principal component analysis. These baseline data will be compared with similar data collected in September for assessing the effects of hypoxia on benthic community structure in the northern Gulf of Mexico.