Project Report PR-99-06
Benthic Sampling Adjacent to the Waianae Ocean Outfall, Oahu, Hawaii, May 1998
Anthony R. Russo, E. Alison Kay, Julie H. Bailey – Brock, and William J. Cooke
November 1998
ABSTRACT
In May 1998, 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 5,229 nonmollusk individuals representing 158 taxa and 9,512 mollusk individuals representing 181 taxa were recorded. The sediments around the outfall were rich in nematode, oligochaete, polychaete, crustacean, and molluscan fauna. Polychaetes represented 35.2% and crustaceans 15.1% of total nonmollusk abundance. Polychaetes were represented by 90 taxa and crustaceans by 45 taxa. Gastropods comprised more than 90% of the molluscan fauna. The highest mean abundance of nonmollusks was recorded at reference station W9 and the lowest at reference station W1. Mean nonmollusk taxa richness was greatest at Station W2 and least at Station W1. Highest mean mollusk abundance was recorded at reference station W1 and lowest at Station W2. Mean taxa richness for the mollusk component was highest at Stations W9 and W1 (same amount for both) and lowest at Station W2. The nonmollusk taxa composition was similar at Stations ZE and Z. These stations, taken as a group, were most dissimilar to the remaining group of stations comprised of Stations W9, W1, W2, ZW, and Z. For mollusks, Stations Z and W1 were most similar, whereas Station ZE was most dissimilar to the other stations in taxa composition. In 1998 there was a decrease in mean nonmollusk taxa at all stations except ZW and W2. 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 close in 1995, 1996, 1997, and 1998. 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, 1997, and 1998 nonmollusk abundance values for the two groups remained close. Annual averages for total suspended solids, total phosphorous, and total nitrogen concentrations in the outfall effluent were essentially the same in 1997 and 1998. At all stations the sediment was oxygen-rich, as indicated by positive oxidation-reduction potential readings. Mean taxa richness between ZID (27 taxa) and non-ZID (31 taxa) station groups was not significantly different. Cluster analysis indicated there was no pattern or grouping of stations, according to taxa composition, which showed an interaction with the sewage discharge. Near the outfall discharge, no large increase in abundance was found among some taxa relative to others; the equitability of relative abundances was high at all stations. There was no large decrease in taxa diversity at stations near the diffuser, as predicted by the Pearson – Rosenberg pollution model. Thus all of the results of the 1998 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.