WRRCTR No.101 Inventory of Benthic Organisms and Plankton at Mōkapu, Oahu

Abstract

An inventory of benthic and planktonic marine communities at Mōkapu Point, Oahu, compiled in July to August 1975, prior to the completion of an outfall and discharge of secondary sewage effluent, has been prepared for the City and County of Honolulu. Data were obtained from five transects between Mokapu Point on the northeastern tip of Oahu to Alāla Point, approximately 6 km (4 miles) south, at depths of from 6 to 30 m (20 to 100 ft). The area is exposed to predominant northeast tradewinds. Coral cover ranges from 20% at the 6 m depth stations to 85% at greater depths, with the most abundant corals at depths of from 12 to 18 m (40 to 60 ft). Four species of corals dominate the survey area, Pocillipora meandrina, Porites lobata, P. compressa, and Montipora sp. Most of the stations surveyed had little or no algal cover. Standing crop of fishes and mollusks decreases from stations A through C (north to south) and increases at stations D and E (the southernmost of the transects). The dominant fish are acanthurids and chaetodontids. The dominant mollusks are gastropods, primarily the archaeogastropods Tricolia variabilis and Leptothyra rubricincta, rissoids, cerithids, and dialids. Total numbers of fish are not dependent on coral cover, but standing crop of micromollusks decreases with increasing coral cover and depth. Standing crop of net phytoplankton cells (<100 μ) measure 2000 to 3000 cells per cm^3 but does not vary significantly with area. Macrozooplankton abundance decreases from north to south. Transect A is distinguished by an especially high energy environment, reflected by the low standing crop and mixed species composition of micromollusks. Transect B, the proposed outfall site, is hard and flat, and mostly devoid of coral cover; very few fish were seen at this station.