Changes in pollution indicators at the Sand Island sewage outfall

Comparison of benthic communities off the Sand Island Ocean Outfall was undertaken to recognize pollution indicator species for Hawaiian waters. The primary treatment sewage is discharged off the south shore of Oahu at 70 m depth. A historical data set spanning 15 years for seven sites at 70 m (4 ZID and 3 far field) and two studies at 20, 50, and 100 m were compared. Polychaetes collected with a 7.6 cm diameter sediment core were sorted and five indicator species tentatively identified. Neanthes arenaceodentata (F. Nereididae) and Ophryotrocha adherens (F. Dorvilleidae) have been dominant at ZID stations. Since 1993, N. arenaceodentata has virtually disappeared from the ZID while o. adherens became abundant and continues to flourish there. The well known indicators within the Capitella capitata complex (F. Capitellidae) are present at ZID and far field sites. Two sabellids, Euchone sp. B and Augeneriella dubia are inversely distributed, the smaller Euchone sp. B at the far field stations and larger A. dubia at the ZID. The former is most likely restricted to the greater proportion of fine sediment particles at two far field sites. Pionosyllis heterocirrata (F. Syllidae) is the most abundant and widespread polychaete off Oahu's south shore. These indicator species share life history and feeding traits that lead to rapid growth and onset of reproduction, and produce benthic juveniles that recruit to the parent population leading to high levels of abundance. The success of indicator species is due to rapid use of available food resources and resulting increase in abundance. O. adherens is the most abundant indicator species at the ZID, P heterocirrata is the most ubiquitous species at all sites and should always be present in these sediments. No pollution level effects on the benthic assemblages seem to be present, but fluctuations in indicator abundances may indicate a response to organic materials such as sewage outfalls. Future changes in benthic assemblages on Oahu's south shore need to consider all possible organic sources including sewage effluent, fish farm wastes, land run off, canal and harbor drainage and the effects of beach nourishment projects.