Table 1 also shows the amount of organic carbon that was intercepted in sedi- 

 ment trap deployments as a function of distance from the outfall. While oxygen 

 uptake (carbon utilization) increased by a factor of 13 between control and the 

 5 m outfall station, organic sedimentation increased by only a factor of 3. 

 Organic content of bottom sediment remained relatively constant throughout the 

 stations. 



All fluxes of nitrogeneous dissolved substances, NH^" 1 ", NO3" plus N02~, were posi- 

 tive, indicating release to the water column from the sediments. Mean NH4 + fluxes 

 were highest near the outfall, and decreased with distance from the source of 

 nutrient particulates. Ammonium flux at the station nearest the diffuser (2.2 mM 

 m^day-l) was very close to the ammonium flux measured in Kaneohe Bay (1.86 mM m2 

 day - '). Nitrate plus nitrite did not show the same distinct progressive decrease 

 in magnitude with distance from the outfall. 



Phosphate fluxes showed even higher variability since even the direction of flux 

 changed from station to station. Such high variability appears to indicate that net 

 phosphorous exchange is not significantly influenced by the effluent discharge. 



Data from the City and County of Honolulu show that the ratio of particulate 

 total N to P in the sewage effluent is about 5.5, while planktonic organic material 

 has a N:P ratio of 16:1 (Ryther and Dunstan, 1971). The ratio of total N to P 

 fluxed from the sediment at the Sand Island stations was 8.5. The low flux ratios 

 of N:P may be due to metabolism of exogeneous sewage rather than natural sedi- 

 menting organics. However, Nixon (1981) also has measured N:P ratios of 7:1 in 

 benthic fluxes in Naragansett Bay, Rhode Island. He theorizes that the low 

 nitrogen fluxes are due to sequential nitrification of ammonium produced in 

 aerobic metabolism, diffusion of nitrate and nitrite into the lower anaerobic 

 sediment column, and subsequent denitrification to di-nitrogen. The end product is 

 lost from the dissolved fixed nitrogen pool, and may be the cause for nitrogen 

 limitation in some shallow marine systems. 



Approximately 1()6 moles of particulate organic carbon are discharged each day 

 from the outfall. Extrapolation of sediment trap fluxes to the total area of im- 

 pact around the outfall structure indicate that only about 10^ moles of this par- 

 ticulate carbon reaches the benthic surface. If these fluxes are of the correct 

 order of magnitude, over 99.9% of the discharged material is dispersed in the 

 water column under the normal regime of tide and current flow. While the propor- 

 tions of material cycled within the benthic boundary layer are infinitesimally 

 small compared with total effluent volume, it is significant that there is any 

 increased- signal at all indicating that benthic nutrient metabolic process are 

 very sensitive to nutrient inputs. 



DISCUSSION 



Both community structure and community function analyses are useful approaches 

 to determining ecosystem response to nutrient subsidy. Structural analysis of 

 benthic macrofaunal community assemblages, including measures of species composition, 

 abundance, diversity, trophic position and biomass, are aimed at determining the 

 ultimate effect of a large scale nutrient subsidy on macrobiota. With this 

 approach, however, there are no provisions to link structural characteristics of 

 the community with changes in nutrient parameters via examination of metabolic 

 pathways that are changed by the perturbation. Unless the trophic response of the 

 macrofaunal community is extremely visible, as was the case at the shallow Sand 

 Island outfall, there is little or no understanding of the causative relationships 

 between biotic structure and the altered environment. 



A community metabolism approach can, however, be used successfully to assess 

 the effect of sewage stress or other nutrient subsidies on the marine ecosystem. 

 Quantitative determination of rates of processes that cycle materials provide a 



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