U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameEffects of seasonal hypoxia on continental shelf benthos.
Document AuthorRabalais, N.N.
Smith, L.E.
Harper, Jr., D.E.
Justić, D.
Short DescriptionPp 211-240 in N. N. Rabalais and R. E. Turner (eds.), Coastal Hypoxia: Consequences for Living Resources and Ecosystems. Coastal and Estuarine Studies 58, American Geophysical Union, Washington, D.C.
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2001

Abstract: The benthic communities were characterized for two areas of the southeastern Lousiana continental shelf – one near the Mississippi River delta with silty sediments and intermittently affected by hypoxia on times scales of days to weeks and another farther from the Mississippi River delta in sandier sediments but affected by severe seasonal hypoxia lasting several months. The composition of the benthic communities reflected differences in sedimentary regime, seasonal input of organic material and seasonally severe hypoxia/anoxia. Decreases in specific richness, abundance and biomass of organisms were dramatic at the stations affected by severe hypoxia/anoxia, and lower than most literature values for similar habitats. Although there were summer/fall declines in the populations at the intermittently hypoxic site, these were not obviously related to changes in oxygen. Some macroinfauna, the polychaetes Ampharete and Magelona and the sipunculan Aspidosiphon, were capable of surviving extremely low dissolved oxygen concentrations and/or high hydrogen sulfide concentrations. Abundance of macroinfauna, primarly opportunistic polychaetes (similar to the spring), increased in the fall following the dissipation of hypoxia, but the numbers of individuals were only slightly greater than the summer depressed fauna and resulted in no or a negligible increase in biomass. Fewer taxonomic groups characterized the severely affected stations throughout the year. Long-lived, higher biomass and direct-developing species were never members of the severely affected community. Suitable feeding habitat (in terms of severely reduced populations of macroinfauna that may characterize substantial areas of the seabed) is thus removed from the foraging base of demersal organisms, including the commercially important penaeid shrimps.