PROJECT REPORT PR-2010-01
Benthic Faunal Sampling Adjacent to the Barbers Point Ocean Outfall, O’ahu, Hawai’i, March 2009
William G. Ambrose, Jr., Julie H. Bailey-Brock, William J. Cooke, Cynthia L. Hunter, and Regina K. Kawamoto
July 2009, xiii + 204 pp.
Benthic fauna in the vicinity of the Barbers Point ocean outfall was sampled at seven stations on 13-16 March 2009 with a modified van Veen grab sampler. The stations are 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 (>92%) sand (coarse, medium, and fine) at all stations. The coarse-sediment fraction was moderately higher and the fine-sand fraction moderately lower at stations HB1, H2, HB4, and HB7 than at the other stations while the proportion of fine sand was higher at HB3, HZ, and HB6 than at other stations. The silt-clay fraction averaged 6% over all stations with little difference among stations (range 3.4-10.2%). Total organic carbon (TOC) in the sediments was ≤0.84% at all stations (range 0.2-0.84%). There was a significant difference among stations in mean oil and grease (O&G) measurements; the mean concentration of oil and grease was significantly higher at reference station HB7 than all other stations and significantly lower at reference station HB1 than at ZID stations HB4 and HZ. Values for oxidation-reduction potential (ORP) showed no evidence of reducing conditions at the sediment surface at any station. The ORP at station HB4 was, however, significantly greater than at stations HB1, HB2, and HB7 and station HB1 had a significantly lower ORP value than station HZ. Measurements of physical parameters continue to show no evidence of a buildup of organic matter in the vicinity of the Barbers Point ocean outfall.
A total of 7,332 nonmollusk individuals from 208 taxa were collected. Polychaetes represented 40.2%, nematodes 16.3%, crustaceans 14.1%, oligochaetes 9.9%, and sipunculans 6.4% of total nonmollusk abundance.
Mean total nonmollusk abundance ranged from 152.4 individuals per sample (33,595/m2 at HB7) to 298.2 individuals per sample (65,723/m2 at HZ). There were no significant differences in the abundance or taxa richness of nonmollusks among stations.
Mean crustacean abundance ranged from 14.2 (3,130/m2 at HB4) to 51. (11,328/m2 at HB2). Both crustacean abundance and taxa richness increased in 2009 from 2008 but not significantly. There was no significant difference in mean crustacean abundance among the seven stations in 2009.
The only significant difference in mean number of crustacean taxa among stations was between one ZID station (HB4) where significantly fewer crustacean taxa were found than at the other ZID stations (HB2, HB3, and HZ). There is a historic pattern of reductions in crustacean abundance and taxa richness at the four ZID stations relative to each of the reference stations. The differences, however, are usually not statistically significant and the pattern has not been observed in every sampling year. This year the two highest abundances of Crustaceans were at ZID stations HB2 and HZ. The collection of a variety of pollution-sensitive amphipod taxa at the ZID stations in 2009 and earlier years indicates that the diminished crustacean fauna at these stations may be caused by a noncontaminant factor and the high abundances this year at all the ZID stations except HB4 and the lack of significant differences in the number of crustacean taxa among stations does not indicate any negative effect of the diffuser on the crustacean community.
Mollusks were analyzed separately because they represent time-averaged collections of live and dead shells. A total of 7,992 mollusks representing 161 taxa were collected. Mean mollusk abundance ranged from 154.4 individuals/10 cm3 (at HB3) to 321.0 individuals/10 cm3 (at HB7). There were no significant differences in abundance or taxa richness of mollusks among stations.
Diversity and evenness values were similar among all stations in 2009. Although differences among stations in diversity and evenness were small, the lowest values occurred at ZID station HB4 and the highest at ZID station HB2 for nonmollusks. For mollusks, the highest evenness and richness values were recorded at reference station HB6 and ZID station HB4 and the lowest values at reference stations HB1 and HB7.
Separate cluster and non-metric multidimensional scaling analyses of mollusk and nonmollusk Data revealed no impact of the outfall on species composition. For nonmollusks, ZID stations HB3 and HZ were most similar in community composition. The composition of mollusks was similar at reference station HB6 and ZID stations HB3, HB4, and HZ.
Other than a diminished richness of crustacean taxa and slightly lower Shannon-Wiener diversity and evenness indices for nonmollusks at ZID-boundary station HB4 than other stations, there is no indication of any marked alteration of the benthic community composition related to station proximity to the diffuser. The analyses of the all faunal groups clearly demonstrate the presence of a diverse and abundant macrobenthos within and near the ZID of the Barbers Point ocean outfall.