Benthic Faunal Sampling Adjacent to the Sand Island Ocean Outfall, O’ahu, Hawai’i, September 2008

Benthic Faunal Sampling Adjacent to the Sand Island Ocean Outfall, O’ahu, Hawai’i, September 2008

Benthic Faunal Sampling Adjacent to the Sand Island Ocean Outfall, O’ahu, Hawai’i, September 2008

William G. Ambrose, Jr., Julie H. Bailey-Brock, William J. Cooke, Regina K. Kawamoto

March 2009, xiv + 255 pp.


In September 2008 benthic fauna near the Sand Island ocean outfall was sampled at 15 stations established in 1999. Five stations were located on each of three transects along isobaths of approximately 20 m (transect C), 50 m (transect D), and 100 m (transect E) of depth. Each transect included two stations near the diffuser at or inshore of the boundary of the zone of mixing (ZOM) and three stations beyond the diffuser at distances of approximately 2.4 to 2.7 km west, 4.4 to 4.9 km east, and 5.9 to 6.5 km east of the center of the diffuser. The 2008 survey followed the design initiated in 1999 and repeated in 2000, 2002, 2004, 2005, 2006, and 2007.

The 11 surveys at this site prior to 1999 were based on seven stations located on one transect (herein called transect B) at 58 to 77 m of depth, the approximate depth of the ocean outfall diffuser.

Measurements of total organic carbon, total Kjeldahl nitrogen, and oxidation-reduction potential showed no indication of significant organic buildup in sediments at any station. The biological Data indicate that discharge from the Sand Island ocean outfall is having few, if any, negative effects of on the macrobenthic community.

Most statistically significant differences in nonmollusk abundance and taxa richness among the 15 stations were associated with differences among the three transects and reflect the influence of depth-related factors. The abundance and number of taxa of crustaceans, polychaetes, and all nonmollusks were usually significantly greater at several transect D stations than at many stations on transects C and E. There was no significant difference in the number of individuals or taxa of mollusks among transects. When stations were pooled by proximity to the outfall, there were no significant differences between the near-diffuser station group and the beyond-diffuser station group in the abundance or taxa richness of crustaceans, polychaetes, and all nonmollusks. Composition, diversity, and evenness of these taxa were also more closely associated with water depth than proximity to the outfall. Cluster analysis of nonmollusk taxa composition and abundance resulted in station groups that were associated primarily with the three transects. Nonmetric multidimensional scaling of nonmollusk abundance showed an even clearer separation of stations by transects than that shown by cluster analysis.

There were no significant differences in mollusk abundance or taxa richness among transects or between near- and beyond-diffuser stations. Depth-related differences in mollusk taxa composition resulted in station clusters generally associated with transects. There was no between-transect grouping of stations that might reflect a common influence of the outfall on mollusks. Nonmetric multidimensional scaling of mollusk abundance also showed a clear separation of stations by transects.

Examination of long-term patterns confirmed this year’s pattern; the abundance and taxa richness of crustaceans and other nonmollusks were generally greater at transect D than at transects C and E in years when there were significant differences. There is no indication of the outfall effluent having a negative effect on the macrobenthos over the 19 years of study. In summary, the response patterns of benthic fauna near the Sand Island ocean outfall in 2008 showed little or no indication of a significant influence by the outfall effluent.