Project Report PR-99-10
Benthic Faunal Sampling Adjacent to Sand Island Ocean Outfall, Oahu, Hawaii, August 1998
Richard C. Swartz, Julie H. Bailey – Brock, William J. Cooke, E. Alison Kay
March 1999
ABSTRACT
Benthic fauna in the vicinity of the Sand Island Ocean Outfall was sampled at seven stations along the diffuser isobath in August 1998. Stations were located both within and on the boundary of the zone of initial dilution (ZID) and at distances of 1.2 to 2 km from the ZID boundary. Values for total organic carbon, total Kjeldahl nitrogen, and oxidation-reduction potential showed no indication of significant organic buildup in sediments at any station. Correlations between sediment oil and grease from Sand Island samples and both crustacean abundance and taxa richness showed no evidence of a negative relationship. There were no significant differences in mean nonmollusk abundance among the stations. The only significant difference in number of nonmollusk taxa was a reduction in richness at reference station B1. There were no significant differences among stations in mean crustacean abundance. ZID-boundary stations B3 and B4, within-ZID station Z, and reference station B1 had fewer crustacean taxa and noncopepod crustacean individuals than reference stations B2 and B6. There were no significant differences in the number of crustacean taxa and noncopepod crustaceans at reference station B1 and any of the four ZID-boundary and within-ZID stations. These comparisons indicate that the crustacean assemblage at the ZID and near-ZID stations was within the range of natural variability as defined by conditions at all three reference stations. There were significant differences among stations in mean mollusk abundance and number of mollusk taxa, but the differences showed no evidence of a negative effect of the diffuser effluent on mollusks. For example, mean mollusk abundance was significantly greater at reference station B2 and at ZID-boundary station B4 than at all other stations. Also, there were significantly more mollusk taxa at ZID station Z and ZID-boundary station B5 than at reference stations B1 and B6. Cluster analysis of nonmollusk species composition and abundance showed high similarity (>63%) among all stations. Reference and ZID stations were not segregated in the cluster pattern. There was no pattern of lower diversity or evenness for either nonmollusk or mollusk groups at ZID stations relative to reference stations. In fact, the highest diversity and evenness values for both mollusks and nonmollusks were observed at ZID-boundary station B5. The long-term pattern of lower species richness at Station B3, as compared to the other stations, was observed only for the crustacean component and not for other faunal components. The abundance and richness of amphipods and all crustaceans increased at all ZID and ZID-boundary stations in 1998. In summary, the response patterns of benthic fauna near the Sand Island Ocean Outfall showed little or no indication of a strong influence by the diffuser effluent.