Project Report PR-98-13
Benthic Faunal Sampling Adjacent to Barbers Point Ocean Outfall, Oahu, Hawaii, January 1998
Richard C. Swartz, Julie H. Bailey – Brock, William J. Cooke, and E. Alison Kay
June 1998
ABSTRACT
Benthic infauna in the vicinity of the Barbers Point Ocean Outfall was sampled at seven stations on 10 and 12 January 1998 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 than other stations, and Station HB2 had relatively finer sediments. Total organic carbon in the sediments at all stations was near or 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 8,373 nonmollusk individuals from 140 taxa were collected. Polychaetes represented 42.1%, nematodes 24.6%, oligochaetes 11.5%, sipunculans 9.8%, and crustaceans 8.3% of total nonmollusk abundance. Mean total nonmollusk abundance ranged from 140.4 individuals per sample (30,949/m2) at Station HB2 to 392.2 individuals per sample (86,455/m2) at Station HB6. Mean crustacean abundances ranged from 4.0 (882/m2) at Station HB3 to 32.2 (7,098/m2) at Station HB6. Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. Mean mollusk densities ranged from 229.0 at Station HB7 to 414.0 at Station HB1. From comparisons of nonmollusk abundance among stations, reference station HB6 had significantly greater mean abundances than reference stations HB1 and HB7 and ZID-boundary stations HB2 and HB3. 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 increased abundance for mollusks. A temporal trend of decreased abundance for crustaceans began in 1994, but reversed itself in 1998 when the density of crustaceans increased substantially over the 1997 level. In 1998 the mean number of nonmollusk taxa at Station HB3 was significantly less than at Stations HZ and HB6, and the mean number of nonmollusk taxa at Station HB2 was significantly less than at Station HZ. The mean number of crustacean taxa was significantly less at Station HB3 than at Stations HB1, HZ, HB6, and HB7. Station HB2 had significantly fewer mollusk taxa than all other stations, and Station HZ had significantly fewer mollusk taxa than Station HB1. Mean crustacean abundance and taxa richness averaged over the entire study period and in 1998 were lower at two stations near the diffuser relative to reference stations. Although most station differences in crustaceans were not significant, the pattern may indicate a trend related to proximity to the diffuser. However, this trend is not consistent for all crustacean parameters, e.g., the total number of crustacean taxa collected at ZID station HZ was greater than at two of the reference stations in 1998. Both diversity and evenness values were generally similar among all stations in 1998 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.