Project Report PR-2000-01
Benthic Faunal Sampling Adjacent to Barbers Point Ocean Outfall, Oahu, Hawaii, April 1999
Swartz, Richard C., Julie H. Bailey – Brock, William J. Cooke, and E. Alison Kay
Benthic infauna in the vicinity of the Barbers Point Ocean Outfall was sampled at seven stations on 13 and 14 April 1999 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 at all stations, although the proportion of medium and coarse sand was greater at Stations HB1, HB2, HB4, HZ, and HB7 than at Stations HB3, HB4, and HB6. Total organic carbon in the sediments at all stations was less than 0.02%. Values for oxidation-reduction potential and sediment oil and grease (O&G) showed no indication of significant organic buildup in sediments at any station except Station HB2. The high O&G values at Station HB2 were not associated with any biological alterations. A total of 9,679 nonmollusk individuals from 183 taxa were collected. Polychaetes represented 44.0%, nematodes 17.2%, crustaceans 14.2%, oligochaetes 9.9%, and sipunculans 9.5% of total nonmollusk abundance. Mean total nonmollusk abundance ranged from 152.6 individuals per sample (33,639/m2, at Station HB1) to 355.8 individuals per sample (78,431/m2, at Station HB6). Mean crustacean abundance ranged from 16.2 (3,571/m2, at Station HB3) to 65.6 (14,461/m2, at Station HB7). Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. Mean mollusk abundance ranged from 139.0/10 cm3 (at Station HB6) to 317.6/10 cm3 (at Station HZ). There were no significant differences among the seven stations in mean nonmollusk abundance, number of nonmollusk taxa, crustacean abundance, or number of crustacean taxa. There were significant differences in mollusk abundance and richness, but they do not indicate a spatial pattern related to the outfall. For example, reference station HB7 and ZID station HZ had more molluscan individuals and taxa than the other reference stations and the ZID-boundary stations. 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 that began in 1994, reversed itself in 1998 and 1999, when the density of crustaceans increased substantially over the 1997 level. Mean crustacean abundance averaged over the entire study period (1986 to 1999) was significantly lower at ZIDmarked-boundary station HB3 than at reference station HB6. However, for the 1999 collection the difference between these two stations was not significant. Both diversity and evenness values for both nonmollusks and mollusks were generally similar among all stations in 1999. 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 alteration of the benthic community composition related to station proximity to the diffuser.