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

Abstract

The influence of natural short-term fluctuations in environmental parameters on three components of transient benthic invertebrate community structure: abundance of individuals and species, biomass of individuals, and species diversity, were investigated in this study. The effect of low dissolved-oxygen on transient benthic community structure was studied with samples from Golfo Dulce, an intermittently anoxic basin off the west coast of Costa Rica and the Posa de Cariaco, an anoxoic trench off the north coast of Venezuela. Periodic fluctuations in oxygen concentration were accompanied by a community numerically dominated by a single polychaete species and low species diversity. As the frequency of fluctuations in oxygen concentration decreased, the number of species and individuals in the community increased with a corresponding increase in species diversity. In contrast to fluctuating oxygen conditions which eliminated many species from the community, fluctuating amounts of suspended matter in the bottom water allowed one species to proliferate while maintaining the total species list length. High rates of terrigenious sedimentation occurring naturally off the Spanish Sahara coast produced conditions which apparently hampered the feeding mechanisms of a spionid polychaete. Further offshore, where the diversity should be expected to increase, the spionids were able to flourish. The result was greater numerical abundance and biomass offshore and a lower transient diversity value. Results of simulation of catastophic burial by in situ burial of small isolated portions of Buzzards Bay sediment indicated that sedimentation rates recorded off Spanish Sahara would not eliminate species by burial. However, the small size of the organisms found off Spanish Sahara is probably a result of the constant expenditure of energy for escape. In regions of fluctuating environmental conditions, diversity values are low, principally because of dominance by a single species. Increasingly stable conditions, even though stressful, result in a more even distribution of individuals among the species present and a correspondingly high transient value.