CONTRASTS IN BENTHIC ECOSYSTEM RESPONSE TO NUTRIENT SUBSIDY: 

 COMMUNITY STRUCTURE AND FUNCTION AT SAND ISLAND, HAWAII 



S. J. Dollar 

 Hawaii Institute of Marine Biology 

 Department of Oceanography 

 University of Hawaii 



INTRODUCTION 



Controlled technological events may provide a well-defined framework for experi- 

 ments that demonstrate community metabolic response to large-scale nutrient sub- 

 sidies (Smith, et al_. , 1981). Sand Island, Oahu is an Hawaiian site where a major 

 change in sewage discharge technology has presented an opportunity to compare 

 response patterns of two yery dissimilar benthic communities to essentially the 

 same effluent. From 1955 to 1977 the Sand Island sewage outfall discharged 

 approximately 2.3 x 10^ m^ raw effluent per day from a point source discharge at a 

 depth of 10 meters. Prior to discharge the receiving environment was a coral reef 

 typical of those off leeward areas of Hawaii. Two research programs were conducted 

 at the shallow discharge site, one in 1975 while sewage was still being discharged 

 (Grigg, 1975) and one in 1979, 1.5 years after sewage diversion (Dollar, 1979). 

 In 1977, in response to federal mandate, the Sand Island discharge was shifted to 

 a newly constructed multi-port deepwater outfall. The receiving environment, at 

 a depth of approximately 70 m is a homogeneous calcium carbonate sandy substratum. 

 For the first 5 years of operation effluent was discharged raw, while for the last 

 two years effluent has undergone primary treatment. In 1981, an ongoing research 

 program designed to examine the response of the benthic ecosystem to the Sand 

 Island outfall was begun. The questions posed in this study were based on the 

 functional metabolic approach developed by Smith, et a]_. (1981): can a total system 

 response be ascertained from a description of nutrTent addition, community meta- 

 bolism and resultant nutrient fluxes? The purpose of this paper is to present a 

 summary of results and a short discussion of the contrasting effects to the benthic 

 ecosystems resulting from the nutrient subsidies from the Sand Island sewage out- 

 falls. Of particular interest in this regard are the contrasting approaches to 

 defining ecosystem response by community structure and community function analyses. 



RESULTS 



Shallow Outfall 



The 1975 and 1979 benthic surveys at the shallow Sand Island Outfall consisted 

 of series of line transect and photographic quadrats used to estimate quantita- 

 tively the effects of sewage on macrobenthos at 28 stations ranging from 3000 m 

 east to 11,000 m west of the outfall. A clear pattern of community alteration 

 associated with the sewage impact was distinguished by two distinct zones; an 

 acute impact zone included approximately 4 km2 of bottom and was characterized by 

 a total lack of living reef coral. The epibenthos was dominated by large mounds of 

 deposit feeding worms ( Chaetopterus sp.) apparently adapted to the high organic 

 particulate loading from the outfall. The zone of intermediate impact covered 

 approximately 20 km2 in an asymmetric pattern reflecting prevailing current 

 patterns which transported the sewage laden plume to the southwest. Coral 

 mortality was high, but not total in this area. Macrobenthic species diversity 

 was highest in the zone of intermediate impact due to the co-occurrence of species 

 found in normal, unstressed communities and those species directly associated with 

 the particulate loading. 



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