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Geological Society, London, Special Publications; 1991; v. 58; p. 49-64;
DOI: 10.1144/GSL.SP.1991.058.01.04
© 1991 Geological Society of London

Modern Shelf Anoxia

Recovery responses of two benthic assemblages following an acute hypoxic event on the Texas continental shelf, northwestern Gulf of Mexico

Donald E. Harper, Jr, Larry D. McKinney1, James M. Nance2 & Robert R. Salzer3

Marine Laboratory, Texas A&M University at Galveston, Galveston, Texas 77551, USA

A hypoxic/anoxic event occurred in the northwestern Gulf of Mexico between May and July 1979. This event, associated with warm water temperatures, water column stratification and the decay of a phytoplankton bloom, led to the production of hydrogen sulphide which moved above the sediment—water interface and into the lower 2 metres of the water column. Regular monthly sampling at two sites (15 and 21 m depths) off Freeport, Texas, showed that abundances of benthic organisms declined precipitously during the event. Divers inspecting the study areas during the event observed ‘cottony’ mats, presumed to be sulphur bacteria colonies, covering large areas of bottom, and numerous bodies of dead benthic invertebrates.

Recovery of the two communities was markedly different. The assemblage in deeper water apparently stabilized within a year; the species of polychaetous annelids that were dominant before the event quickly returned to dominance and there was very little evidence of succession of different species during the recovery process. The shallower water assemblage, however, underwent a very different recovery process. Following the hypoxic event, polychaete dominance was greatly reduced, and there occurred successional dominance which involved several species in different taxa. Each of these species underwent a ‘bloom’ and constituted the numerical dominant for 1 to 3 months, and then declined and was replaced by another species’ ‘bloom’. This sequential dominance pattern persisted until about May 1981, two years after the hypoxic event, at which time polychates again became, and remained, numerically dominant.


1 Present address: Texas Parks and Wildlife Dept., 4200 Smith School Road, Austin, TX 77843, USA.

2 Present address: National Marine Fisheries Service, 4700 Avenue U, Galveston, TX 77551, USA.

3 Present address: Rt. 4, Box 275, Alvin, TX 77511, USA.




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