WRRCTR No.158 Micromolluscan Assemblages in Māmala Bay, O'ahu, 1974-1982

Abstract

The benthic communities in Mamala Bay, Oahu have been monitored since 1974 to assess the extent to which the introduction of sewage effluent at about a 70-m depth has affected the system. The index organisms utilized are micromollusks, mollusks less than about 7 mm in greatest dimension. Replicate sediment samples were obtained by modified Petersen dredge from 11 stations at 15- to 120-m depths between 1974 and 1981; samples from the diffuser and the ZID boundary were obtained by submersible-operated grab in 1982. Aliquots of sediment were sorted for micromollusks and analyzed by Q-mode cluster analysis, relative species composition, relative abundance, species diversity, and dominance. Three micromolluscan assemblages were distinguished. A Tricolia-rissoid dominated assemblage is an algal and rubble-associated assemblage of micro herbivores derivative of the reef. An infaunal assemblage with about 30% of the mollusks' carnivorous, parasitic and/or deposit feeders is associated with the occurrence of beds of the bivalve Pinna. The habits of the mollusks of the third assemblage, the dialid assemblage, are not known. Mean micromolluscan abundance tends to increase with depth; number of species and species diversity decreases with depth. Abundance is not significantly different between pre- and post-discharge stations. The Tricolia assemblage was the most frequently occurring assemblage under pre-discharge conditions; the infaunal assemblage the most frequently occurring assemblage under post-discharge conditions. In the submersible-obtained samples, only the dialid assemblage was represented at the ports of the diffuser and only the Tricolia assemblage at the ZID boundary. It is suggested that the micromolluscan assemblages in Mamala' Bay are distributed in a continuum throughout the bay, their boundaries associated with different types of sediment. No major changes in the benthos associated with the introduction of sewage effluent were detected in this study, but variations in pattern of assemblage occurrence may be the result of changes in sediments associated with diffuser activity.