Terrestrial Nutrients and Dissolved Organic Matter Input to the Coral Reef Ecosystem via Submarine Springs

Submarine groundwater discharge (SGD) transports terrestrial nutrients and dissolved organic matter (DOM) to the ocean. An elevated concentration of nutrients and DOM can act as stressors enhancing coral disease and mortality, but only a few studies address the impacts of groundwater-borne nutrients and DOM on coral reef ecosystems. This study quantifies and characterizes nutrients, nitrate (NO3-) stable isotopes, and DOM molecular composition of coastal groundwater discharging to the reef ecosystem via submarine springs in Lombok, Indonesia. NO3- isotopic values point to both natural (soil) and anthropogenic (wastewater and fertilizer) origins of nutrients in the coastal aquifer. Submarine springs are fed by different groundwater sources and deliver land-based NO3-, dissolved silica, phosphate, and labile DOM to the reef water column. Terrestrial nutrients and DOM undergo rapid turnover in the reef water column due to biogeochemical processes and biological uptake. Meanwhile, reef and offshore water likely act as sources of more stable, reworked DOM formulas and its mineralization product, ammonium. We observed that submarine springs consistently deliver similar nutrient loadings, creating a long-term environmental threat to coral reef sustainability. This study emphasizes the importance of understanding coastal biogeochemistry and hydrological processes in sensitive tropical ecosystems, particularly those adjacent to modified land-use watersheds.