Terrestrial Nutrient and Sediment Fluxes to the Coastal Waters of West Maui, Hawai'i

Abstract

Water-quality degradation is often linked to land use practices in adjacent and upstream areas. Such linkages are here explored for the Lahaina District of Maui, Hawai'i, where severe algae blooms in 1989 and 1991 prompted public concern and a subsequent search for the factors contributing to algal growth. Because we expected that elevated nutrient levels might play a role in the blooms, this study examined the nutrient and sediment budgets from terrestrial sources entering the coastal waters. Although our work did not show any definitive causal relationship between algal growth and terrestrial nutrient and sediment loading, it clearly established that the principal agricultural activities in the area of sugarcane and pineapple cultivation contribute elevated loads of nutrients and sediments to the coastal waters. Likewise, disposal of treated domestic sewage effluent into subsurface injection wells contributes substantial nutrient loads to the coastal waters. Conversely, golf courses appear to have negligible impacts on the nutrient and sediment loading of coastal waters in the area. Finally, although groundwater discharges substantially greater annual nutrient loads than streamflow, the groundwater discharge is fairly evenly distributed in time and is dispersed over nearly 25 km of shoreline. Streamflow, however, often discharges intensely for short periods of time at a few discrete locations, and thus may have substantial impact locally on coastal water quality.