VALIDATING COASTAL NUTRIENT FLUX MODELS USING STABLE ISOTOPES

Understanding the magnitude and distribution of groundwater nutrient fluxes to the coastal ecosystem is critical to developing measures to mitigate the adverse impacts. Nitrogen is frequently the limiting nutrient. By identifying the sources of the nitrogen, measures can be implemented to reduce the coastal nutrient load. However, installing a groundwater monitoring network extensive enough to link nitrogen sources to coastal discharge points is not economically feasible on a regional scale. Fortunately the two primary sources of nitrogen have contrasting isotopic compositions. Wastewater and animal wastes are isotopically heavier than most fertilizer and natural sources of nitrogen. Nitrogen in marine algae are in approximate equilibrium with the nitrogen composition of the surrounding waters. We merge existing coastal algal nitrogen isotopic compositions with available groundwater nitrogen data sets to investigate the linkage between the terrestrial nutrient sources and the coastal ecosystems in northwest Maui, Hawaii, USA. These data are imported into a groundwater flow and transport model that simulates the primary nitrogen sources as different species. The model results are validated using the simulated and measured isotopic composition of the coastal nitrogen flux.