Project Report PR-98-11
Benthic Sampling Adjacent to the Waianae Ocean Outfall, Oahu, Hawaii, July 1997
Anthony R. Russo, E. Alison Kay, Julie H. Bailey-Brock, and William J. Cooke
In July 1997, bottom samples were collected by scuba divers near the Waianae Ocean Outfall at a depth of approximately 33 m. The Waianae outfall is located on the leeward side of the island of Oahu, Hawaii. The six stations sampled 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 W2 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 6,883 nonmollusk individuals representing 182 taxa and 12,964 mollusk individuals representing 206 species were recorded. The sediments around the outfall were rich in nematode, oligochaete, polychaete, crustacean, and molluscan fauna. Polychaetes represented 34.4% and crustaceans 20.9% of total nonmollusk abundance. Polychaetes were represented by 101 species and crustaceans by 61. Gastropods comprised more than 90% of total molluscan fauna. The highest mean abundance of nonmollusks was recorded at ZID station Z and the lowest at ZID station ZW. Mean nonmollusk taxa richness was greatest at Station W2 and least at Station ZW. Highest mean mollusk abundance was recorded at reference station W1 and lowest at Station W2. Mean species richness for the mollusk component was highest at Station W9 and lowest at Station ZW. The nonmollusk taxa composition was similar at all Stations ZE and ZW. These stations, taken as a group, were most dissimilar to the group of stations comprised of Stations W9, W1, W2, and Z. For mollusks, Stations Z and ZW were most similar. Stations ZE and W2 were also quite similar in taxa composition as were Stations W9 and W1. These three pairs of stations were all dissimilar to each other in mollusk taxa diversity. In 1997 there was an increase in nonmollusk taxa (excluding crustaceans) at Stations W1, W2, Z, and W9. From 1989 to 1993 the ZID and non-ZID station groups showed parallel shifts in nonmollusk taxa richness, whereas in 1994 non-ZID station group values increased and ZID station group values decreased, such that the values for the two groups converged. The values for both groups remained essentially the same in 1995, 1996, and 1997. Mean nonmollusk abundance shifted similarly for ZID and non-ZID station groups among the years of survey, except in 1993 when there was a decrease for the non-ZID station group relative to the ZID station group. In 1995, 1996, and 1997 nonmollusk abundance values for the two groups converged. Annual averages for total suspended solids, total phosphorous, and total nitrogen concentrations in the outfall effluent were slightly higher in 1997 than in 1996, whereas the annual average for biochemical oxygen demand was lower. At all stations the sediment was oxygen-rich, as indicated by positive oxidation-reduction potential readings. Mean species richness between ZID and non-ZID station groups was not statistically different. Cluster analysis indicated there was no pattern or grouping of stations, according to species composition, which showed an interaction with the sewage discharge. Near the outfall discharge, no large increase in abundance was found among some species relative to others; the equitability of relative abundances was high at all stations. There was no large decrease in species diversity at stations near the diffuser, as predicted by the Pearson – Rosenberg pollution model. Thus all of the results of the 1997 Waianae benthic study, along with results from previous years, suggest that no deleterious effects occur on the biologically indigenous populations near the outfall from sewage discharge.