Anthropogenic nitrogen inputs to Saipan’s coastal waters: Exploring sources, pathways, and ecosystem resilience

Abstract

Over the last century, humans have altered the nitrogen (N) cycle to an unprecedented degree, and one of the main detrimental effects has been increasing N inputs from anthropogenic sources to the ocean. This N can harm coastal coral reefs and seagrass ecosystems, which are adapted to oligotrophic conditions. In this study, we investigate the role of submarine groundwater discharge (SGD) in transporting anthropogenic N to the coastal ocean in Saipan (Commonwealth of the Northern Mariana Islands, USA). Saipan’s limited wastewater infrastructure and carbonate geology hasten the transport of sewage-derived N to the shore. A shore-parallel radon survey was used to identify hotspots of groundwater discharge along the island’s western coastline, and radon was used in combination with radium isotopes to estimate coastal ocean water residence times and SGD fluxes. Nutrient (nitrate, nitrite, ammonium, and phosphate) concentrations, other water quality parameters, and nitrogen isotope signatures in water, macroalgae and seagrass tissue were measured concurrently. Additionally, we explored the role of seagrasses in mitigating the negative effects of sewage pollution and ocean acidification by looking at correlations between seagrass presence, ocean pH, and concentrations of the fecal indicator Enterococcus. This study will help inform environmental management efforts in Saipan and similar areas worldwide.