Project Report PR-98-02
Benthic Sampling Adjacent to the Waianae Ocean Outfall, Oahu, Hawaii, October 1996
Anthony R. Russo, E. Alison Kay, Julie H. Bailey – Brock, and William J. Cooke
August 1997
ABSTRACT
In October 1996, 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 6,949 nonmollusk individuals representing 145 taxa and 8,723 mollusk individuals representing 153 species were recorded. The sediments around the outfall were rich in nematode, oligochaete, polychaete, crustacean, and molluscan fauna. Polychaetes represented 36.8% and crustaceans 9.3% of total nonmollusk abundance. Polychaetes were represented by 80 species and crustaceans by 43. Gastropods comprised more than 90% of total molluscan fauna. The highest mean abundance of nonmollusks was recorded at ZID station ZE and the lowest at ZID station ZW. Mean nonmollusk taxa richness was greatest at station W2, followed by stations Z and ZW, and least at station ZE. Highest mean mollusk abundance was recorded at far-field station W9 and lowest at Station W2. Mean species richness for the mollusk component was highest at Station W9 and lowest at Station W2. The nonmollusk taxa composition was generally similar at all stations. Station ZE was most dissimilar in nonmollusk taxa composition, whereas Stations W1, Z, and W9 were most similar. For mollusks, Station ZE was most dissimilar to the other stations, whereas Stations W1, W2, and W9 were most similar. From 1995 to 1996, there was an increase in nonmollusk taxa richness at Stations W1, W9, and ZW and a decrease at the remaining stations. 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 and 1996. 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 and 1996 nonmollusk abundance values for the two groups converged. In 1996 total volatile solids, total organic carbon, and total nitrogen concentrations in the sediments near the outfall discharge were lower than in 1995. This is probably due to the shift from primary to secondary treatment of sewage at the Waianae Wastewater Treatment Plant in early 1996. 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 1996 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.