Transient structure in benthic communities : the effects of oxygen stress, burial and high rates of sedimentation

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1975

This work was funded by a Woods Hole OceanographicInstitution Predoctoral Fellowship, N.S.F. Grant GA-3655l,and N.S.F. Grant GA-33502.

The influence of natural short-term fluctuations inenvironmental parameters on three components of transientbenthic invertebrate community structure: abundance ofindividuals and species, biomass of individuals, andspecies diversity, were investigated in this study. Theeffect of low dissolved-oxygen on transient benthiccommunity structure was studied with samples fromGolfo Dulce, an intermittently anoxic basin off thewest coast of Costa Rica and the Posa de Cariaco, ananoxoic trench off the north coast of Venezuela.Periodic fluctuations in oxygen concentration wereaccompanied by a community numerically dominated by asingle polychaete species and low species diversity.As the frequency of fluctuations in oxygen concentrationdecreased, the number of species and individuals inthe community increased with a corresponding increasein species diversity.In contrast to fluctuating oxygen conditions whicheliminated many species from the community, fluctuatingamounts of suspended matter in the bottom water allowedone species to proliferate while maintaining the totalspecies list length. High rates of terrigenioussedimentation occurring naturally off the SpanishSahara coast produced conditions which apparentlyhampered the feeding mechanisms of a spionidpolychaete. Further offshore, where the diversityshould be expected to increase, the spionids were ableto flourish. The result was greater numerical abundanceand biomass offshore and a lower transient diversityvalue. Results of simulation of catastophic burial byin situ burial of small isolated portions of Buzzards Baysediment indicated that sedimentation rates recorded offSpanish Sahara would not eliminate species by burial.However, the small size of the organisms found offSpanish Sahara is probably a result of the constantexpenditure of energy for escape.In regions of fluctuating environmental conditions,diversity values are low, principally because of dominanceby a single species. Increasingly stable conditions, eventhough stressful, result in a more even distribution ofindividuals among the species present and a correspondinglyhigh transient value.