WRRCTR No.124 Ecological Response to Relaxation of Sewage Stress off Sand Island, O'ahu, Hawai'i

Abstract

The largest domestic sewage outfall in Hawai'i discharged 3 m^3/s (62 mgd) of raw sewage in 10 m of water approximately 1 000 m off Sand Island, O'ahu, from 1955 to 1977. Results of an ecological field study of epibenthic communities in proximity to Sand Island conducted in 1975 and 1979 show clear patterns of community change associated with both sewage impact and relaxation of this stress; negative community effects attributable to sewage input decreased, and degree of recovery of community structure increased with distance from the point source of discharge. Sewage discharge had an impact up to 5 800 m west and 1 900 m east of the outfall. This elliptical area of influence is asymmetrical to the west due to the prevailing current pattern which carried the sewage-laden plume to the southwest. Following sewage abatement two distinct zones of impact are distinguished by the degree of physical degradation of the benthic reef structure. A high impact zone extending some 500 m east and 1000 west of the outfall is now characterized by a complete biochemical reduction of the reef structure to a pitted, flat carbonate pavement covered presently with sediment-bound algal turf and few benthic faunal colonizers. In the zone of intermediate impact the old reef framework is largely intact, though devoid of most living corals. Instead, a veneer of encrusting coralline algae covers most of the reef framework. Patterns of occurrence and diversity of reef fish show characteristics of response to sewage stress similar to attached invertebrates except that the area of influence is of much smaller extent.