Project Report PR-98-08
Benthic Faunal Sampling Adjacent to Sand Island Ocean Outfall, Oahu, Hawaii, August 1997
Richard C. Swartz, Julie H. Bailey – Brock, William J. Cooke, and E. Alison Kay
March 1998
ABSTRACT
Benthic fauna in the vicinity of the Sand Island Ocean Outfall was sampled at seven stations along the diffuser isobath in August 1997. 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 species richness showed no evidence of a negative relationship. There were no significant differences in mean nonmollusk abundance or number of nonmollusk taxa among the stations. There were no significant differences among stations in mean crustacean abundance, although the mean number of noncopepod crustaceans was significantly less at within-ZID station Z and ZID-boundary station B3 than at reference station B6. There were significantly more crustacean taxa at all three reference stations (B1, B2, and B6) than at Station B3 and significantly more crustacean taxa at Station B6 than at Station Z. No other pairwise differences in stations for the nonmollusk or crustacean component of the benthic fauna were significant. Therefore, there was no general relationship between nearness to the diffuser and the abundance and richness of nonmollusks and crustaceans. 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 than at ZID station Z or reference stations B1 and B5. Also, there were significantly more mollusk taxa at ZID station Z than at all three reference stations (B1, B2, and B6). Cluster analysis of nonmollusk species composition and abundance showed high similarity (> 65%) among all stations. Two reference stations (B1 and B2) were linked together in a weakly defined subcluster, but the third reference station (B6) was clustered with the ZID and ZID-boundary stations. There was no pattern of lower diversity or evenness for either nonmollusk or mollusk groups at ZID or near-ZID stations relative to the other 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 response patterns of benthic fauna near the Sand Island Ocean Outfall showed little indication of a strong influence by the diffuser effluent.