Project Report PR-94-11
Benthic Sampling in the Vicinity of the Mokapu Ocean Outfall, Oahu, Hawaii, March 1992
Anthony R. Russo, E. Alison Kay, Julie H. Bailey – Brock, and William J. Cooke
In March 1992, the 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 sampling stations were chosen along the 34-m diffuser isobath: 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 >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. All sediments were clean, devoid of particulates, and considered non-reducing. A total of 4,972 nonmollusks (197 taxa) and 5,755 mollusks (105 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 between 1986 and 1992 studies at the same stations showed a large decline in nonmollusk abundances, especially for crustaceans. Mollusks generally increased between years. Nonmollusk species richness declined between years but is only significant at Station A, the northernmost station, which is most affected by wave scour. Differences in sampling seasons between 1986 (September) and 1992 (March) studies may be the reason for differences seen in benthic community distribution and abundance, reflecting disturbances in the sedimentary environment because of severe winter storms from November 1991 to February 1992. 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 Data from season to season and year to year. Whatever the effect, if any, of the Mokapu outfall on the benthic community in the area, fluctuations in community abundance and lesser variations in community diversity were seen at all stations, including those located 1 km or farther from the ZID. There was no pattern or grouping of stations for nonmollusk or mollusk species composition to indicate an outfall effect on the benthos. Benthic communities near the Mokapu outfall may be structured more by seasonal sedimentary disturbances than by the outfall discharge.