Project Report PR-99-02
Benthic Sampling in the Vicinity of the Mokapu Ocean Outfall, Oahu, Hawaii, March 1998
Anthony R. Russo, E. Alison Kay, Julie H. Bailey – Brock, William J. Cooke
In March 1998, City and County of Honolulu scuba divers collected bottom sediment samples for biological and geochemical analyses in the vicinity of the Mokapu Ocean Outfall on the windward coast of Oahu, Hawaii. Six stations along the 34-m diffuser isobath were sampled: Station B1 at 33 m north and Station B2 at 33 m south of the diffuser on the boundary of the zone of initial dilution (ZID), Station Z in the ZID, Station A at 1 km north and Station C at 1 km south of the diffuser, and Station D at 3.2 km south of the diffuser. The net current direction is to the north in this area. All stations had sediment fractions with greater than 90% sand. Sediment grain size was essentially the same at all stations. Oxidation-reduction potentials were positive and did not show major fluctuations among stations. Total organic carbon in the sediment was less than 1% (dry weight) at all stations. All sediments were clean, devoid of particulates, and considered non-reducing. A total of 7,576 nonmollusks (216 taxa) and 7,320 mollusks (206 taxa) were collected. The sediments were rich and diverse with polychaetes, crustaceans, nematodes, oligochaetes, and gastropod mollusks. The pattern of abundance among stations for nonmollusks, mollusks, and crustaceans was parallel except at Station D. Shifts in taxa richness for all three components of the benthos were similar among stations. Comparisons among 1986, 1992, and 1998 studies at the same stations showed a large decrease in nonmollusk abundances (especially for the crustacean component) from 1986 to 1992, followed by an increase in 1998 at Stations A, B1, B2, and D. Nonmollusk taxa richness decreased between 1986 and 1992, but the decrease was only significant at Station A, the northernmost station that is most affected by wave scour. Nonmollusk taxa richness increased from 1992 to 1998. Differences in sampling seasons between 1986 (post-summer) and 1992 (post-winter) may be the reason for differences seen in benthic community distribution and abundance for those years, reflecting disturbances in the sedimentary environment because of severe winter storms from November 1991 to February 1992. Mollusks generally increased in abundance and diversity among all years of study. Crustacean abundances were lower in 1992 than in 1986 but higher in 1998 than in 1992 at Stations A, Z, B2, and C. Increases in crustacean diversity in 1998 over those in 1992 were recorded at stations A, B2, C, and D. Faunal species diversity appears to be a better measure of changes in the benthic community than abundance patterns since benthic fauna reproduce rapidly, resulting in wide swings in abundance from season to season and from year to year. Whatever the effect, if any, of the Mokapu outfall discharge on the benthic community in the area, fluctuations in community abundance and lesser variations in the community diversity were seen at all stations, including those 1 km or farther from the ZID. There was no pattern or grouping of stations for nonmollusk or mollusk taxa composition which indicated an outfall effect on the benthos. ZID stations, taken as a group, did not differ significantly from non-ZID stations in mean abundance or mean taxa richness. Benthic communities near the Mokapu outfall may be structured more by seasonal sedimentary disturbances than by the outfall discharge.