REEF PLANTS SHOW THAT GROUNDWATER DISCHARGE IS A MAJOR SOURCE OF ANTHROPOGENIC NITROGEN FOR COASTAL ECOSYSTEMS IN HAWAII

Submarine groundwater discharge (SGD) can transport land-based contaminants to shallow coastal ecosystems. The purpose of this study was to investigate the effects of SGD on reef organisms impacted by various levels and types of land-based anthropogenic nutrient loading. Algal bioassays, benthic community analyses, and conventional geochemical methods were used to compare water quality and measures of reef health on Maui and Oahu. Tissue nitrogen (N) parameters (δ¹⁵N and N %) of deployed and shore-collected reef algae were compared with nutrient and δ¹⁵N values of coastal groundwater and marine surface waters. Nearshore benthic cover was estimated using digital photographs and a point-intercept analysis. SGD flux was estimated using radon gas concentrations in surface water. In general, reefs adjacent to sugarcane farms and/or wastewater injection wells had the greatest abundance of macroalgae, the least community diversity, and the highest concentrations of N in algal tissues, coastal groundwater, and marine surface waters compared to relatively unimpacted locations. SGD flux and quality was variable among locations with clear onshore-offshore gradients at all locations. Tissue δ¹⁵N values were highest at sites adjacent to municipal wastewater facilities using sewage injection wells. It is clear SGD represents a significant nexus linking land use, water quality and reef health in Hawaii. Minimizing contaminant loads to coastal aquifers will reduce pollutant delivery to nearshore reefs in areas with SGD flux.