Project Report PR-94-18
Benthic Sampling Adjacent to the Waianae Ocean Outfall, Oahu, Hawaii, June 1993
Anthony R. Russo, E. Alison Kay, Julie H. Bailey-Brock, and William J. Cooke
April 1994
ABSTRACT
In June 1993, bottom samples were collected by scuba divers near the Waianae Ocean Outfall at a depth of approximately 34 m. The Waianae outfall is located on the leeward side of the island of Oahu, Hawaii. The six sampling stations chosen were (1) Station Z, located in the zone of initial dilution (ZID) at the diffuser; (2) Station ZE, located on the southeast boundary of the ZID, about 30 m from the diffuser; (3) Station ZW, located about 60 m southwest of the diffuser; (4) Station W1, located about 2.5 km southeast of the diffuser; (5) Station W2, located about 1.0 km southeast of the diffuser; and (6) Station W9, located about 2.5 km northwest of the diffuser. The bottom at all stations was comprised of more than 90% fine to coarse sand. Station W1 had the coarsest sediments with a relatively high percentage of coral rubble. Oxidation-reduction potential and total volatile solid measurements indicated a nonreducing benthic environment at all stations. A total of 6,811 nonmollusk individuals representing 143 taxa and 6,709 mollusk individuals representing 103 species were recorded. The sediments around the outfall were rich in nematode, oligochaete, polychaete, crustacean, and molluscan fauna. Polychaetes represented 41% (80 species) and crustaceans 15% (51 taxa) of total nonmollusk abundance. Gastropods comprised more than 90% of total mollusk fauna. The highest abundance of nonmollusks was recorded at ZID station ZE and the lowest at reference stations W1 and W9. Nonmollusk taxa richness was greatest at ZID stations ZW and Z and least at Stations W1 and W9. Species richness of the mollusk component was lowest at Stations W2 and highest at Station ZE. Highest mollusk abundance was recorded at far-field station W1 and lowest at Station ZE. The species composition of all faunal components was generally similar at all stations. Stations Z and ZW were most similar in nonmollusk taxa composition, and Stations W9 and ZE, as a group, were most dissimilar to the remaining stations. There was a steady and consistent increase in nonmollusk taxa richness at all stations from 1989 to 1992, followed by a general decrease in 1993. The ZID and non-ZID stations showed parallel shifts in nonmollusk taxa richness from 1989 to 1993. Mean nonmollusk abundance shifted similarly for ZID and non-ZID station groups among the years of survey, except in 1993 when non-ZID stations decreased in abundance relative to ZID stations. The 1993 Waianae benthic Data did not suggest any differential reduction of species diversity at ZID stations relative to non-ZID stations. In fact, species richness was, in some cases, higher at ZID stations than at non-ZID stations. There was no large increase in abundance of a few species at ZID stations relative to non-ZID stations, as expected from the Pearson and Rosenberg (1978) model of pollution. Sediments were clean, total volatile solids were low (<4%), and ORP readings indicated a nonreducing sedimentary environment. All the results suggest that, temporally (1989 to 1993) and spatially, there was no detrimental effect by sewage effluent from the Waianae Ocean Outfall on the biologically indigenous benthic populations at the stations surveyed.