EFFECTS OF NUTRIENT SUBSIDIES FROM GROUNDWATER TO NEARSHORE MARINE ECOSYSTEMS OFF THE ISLAND OF HAWAII

Nonconservative fluxes of dissolved nutrients (Si, N and P) in groundwater and nearshore ocean water off two golf courses (Keauhou and Waikoloa) on the west coast of Hawaii were estimated using a one-dimensional mixing model. Groundwater flowing under the Keauhou course discharges into a semi-enclosed embayment with restricted circulation. Leaching of 10% of the N from golf course fertilizer accounted for an increase in groundwater N flux to the bay of 116%; leaching of 1% of the P from fertilizer accounted for an increase in groundwater P flux of 22%. Uptake of Si, N and P was not measurable within the bay; a buoyant surface layer (1 m thick) did not come into contact with benthic communities, and nutrient uptake by plankton was limited to no greater than 2% of the N and P delivered by groundwater flux. At Waikoloa, leaching of N and P from golf course fertilizers accounted for an increase in groundwater nutrient flux to brackish ponds near the coastline of 229% and 400%, respectively. Waikoloa is located on an exposed coastline with substantial turbulent mixing throughout the water column. Nutrient subsidies from golf course fertilizers were not distinguishable from natural groundwater input, and uptake of N in the nearshore zone equaled 80% of the groundwater flux at the shoreline. On a mass delivery basis, natural and anthropogenic nutrient fluxes to the nearshore ocean appear sufficient to cause alteration to community function and structure. Such effects, however, are not solely responses to nutrient loading. Consideration of physical processes, along with use of conservative tracers can provide a practical method for quantifying these effects of man's activities on the ocean. © 1992 Academic Press Limited.