Polychaetes Associated with a Tropical Ocean Outfall: Synthesis of a Biomonitoring Program off O'ahu, Hawai'i

Abstract

A comparison of benthic polychaete communities off the Sand Island Wastewater Outfall was undertaken to recognize organic enrichment indicator species for Hawaiian waters. Primary-treatment sewage is discharged off the south shore of O'ahu at 70 m depth. A historical data set spanning 9 yr for seven sites at 70 m and two recent studies at 20, 50, and 100 m depths were analyzed. Geochemical data did not support the assumption that the outfall is an important source of organic enrichment in nutrient-poor sandy sediments within oligotrophic tropical waters. Five polychaete species, however, appeared particularly sensitive, positively or negatively, to environmental conditions near the outfall. Neanthes arenaceodentata (Nereididae) and Ophryotrocha adherens (Dorvilleidae) have been dominant at sites within the outfall's zone of initial dilution (ZID). Since 1993, N arenaceodentata has virtually disappeared, and 0. adherens concurrently became abundant and continued to flourish at ZID sites. Well known indicators within the Capitella capitata complex (Capitellidae) were present at ZID and control (far field) sites though their ZID abundance was greater. Two sabellids, Euchone sp. Band Augeneriella dubia were inversely distributed, the smaller Euchone sp. B at far field sites and larger A. dubia within ZID stations. The former was most likely restricted to a greater proportion of fine sediment particles at two far field sites. The most abundant and widespread polychaete off O'ahu's south shore was Pionosyllis heterocirrata (Syllidae), which does not seem to represent a sensitive indicator species. Ophryotrocha adherens was the most abundant indicator species within the ZID; P. heterocirrata was the most ubiquitous species at all sites and should always be expected in these sediments. Traditional measurements of numerical abundance, species richness, and diversity (H') have not shown a clear distinction between ZID and far field sites in annual analyses. An examination of composited data over an 11-yr period does support such a distinction. Multidimensional scaling (MDS) analyses clearly delineate different assemblages. We suggest that MDS analyses are sensitive to the community differences present near the outfall. The ZID community is clearly contained within the Environmental Protection Agency-approved ZID boundary. Because each ZID and far field site supports a diverse and coarsely similar polychaete fauna, no pollution level effects seem to be present.