Project Report PR-97-08
Benthic Faunal Sampling Adjacent to Barbers Point Ocean Outfall, Oahu, Hawaii, January – February 1997
Walter G. Nelson, Julie H. Bailey – Brock, William J. Cooke, and E. Alison Kay
June 1997
ABSTRACT
Benthic infauna in the vicinity of the Barbers Point Ocean Outfall was sampled at seven stations on 30 and 31 January and 4 February 1997 with a modified van Veen grab sampler. The stations were located along the diffuser isobath (61 m) as follows: Station HZ within the zone of initial dilution (ZID); Stations HB2, HB3, and HB4 on the ZID boundary; Station HB6 at 0.5 km from the ZID; and Stations HB1 and HB7 at 3.5 km from the ZID. Sediments were predominantly (>90%) fine to coarse sands. Stations HB1 and HB7 had relatively coarser sediments, and Station HB2 had a higher percentage of silt and clay than the other stations. Total organic carbon in the sediments at all stations was below the detection limit of the analytical method used. Values for oxidation-reduction potential and sediment oil and grease showed no indication of significant organic buildup in sediments at any station. A total of 6,865 nonmollusk individuals from 138 taxa were collected. Polychaetes represented 41.0%, nematodes 28.2%, oligochaetes 13.4%, sipunculans 9.0%, and crustaceans 4.2% of total nonmollusk abundance. Mean total nonmollusk abundance ranged from 122.8 individuals per sample (22,554/m2) at Station HB2 to 259.4 individuals per sample (47,643/m2) at Station HB4. Mean crustacean abundances ranged from 4.2 (926/m2) at Station HB2 to 12.4 (2,733/m2) at Station HB4. Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. Mean mollusk densities ranged from 152.0 at Station HB2 to 308.0 at Station HB1. From comparisons of nonmollusk abundance among stations, ZID-boundary station HB4 had significantly greater mean abundances than ZID-boundary station HB2. There has been a significant trend of increased abundance for nonmollusks within the entire study area since 1990. Since 1994, there has been a trend of decreased abundance for crustaceans and a trend of increased abundance for mollusks. Significantly elevated abundances of nonmollusks over the entire study period have occurred at two stations near the diffuser relative not only to two of the reference stations but also to a third near-diffuser station. There were no significant differences among stations in number of nonmollusk or crustacean species. Although there were significant differences among stations in number of species for the mollusks, no clear pattern of differences related to proximity to the diffuser was seen among stations. Crustacean taxa richness averaged over the entire study period was lower at stations near the diffuser relative to reference stations. Although not all station differences were significant, the pattern may indicate a trend related to the diffuser. Both diversity and evenness values were generally similar among all stations in 1997 for both nonmollusks and mollusks. Cluster analysis of nonmollusk Data confirmed that all stations were relatively similar to one another in terms of species composition and relative abundance. There is no indication of any marked alteration of the benthic community composition related to station proximity to the diffuser.