<p>Submarine groundwater discharge (SGD) carries water, nutrients, and pollutants into the lagoons and coastal waters of oceanic islands. This study investigated SGD into Nu’uuli Pala Lagoon on Tutuila, the largest and most populated island of American Samoa. Using the natural tracers radium and radon, we characterized spatial and temporal variability of SGD into the Pala, quantified SGD-related fluxes of water and nutrients, compared these fluxes to those from rivers and streams, and investigated nitrogen sources to primary producers. SGD was localized in two areas: springs on the rocky western shore and muddy tidal creeks on the northern shore. These two SGD pathways had distinct radium and radon activities, radium isotope ratios, salinities, and patterns of seasonal variability. SGD was the dominant source of water and nutrients to the Pala in the dry season and a major source during the rainy season, when SGD was somewhat lower and river flow was substantially higher. Radium-derived Pala water ages suggested substantially slower circulation and longer Pala residence time in the dry season than in the rainy season. Nitrogen and oxygen isotope signatures of dissolved nitrate were consistent with sewage, manure, soil, or fertilizer sources, and spatial trends in the δ¹⁵N of macroalgae suggested a terrestrial nitrogen source supporting macroalgal growth. This study advances understanding of the hydrology, water quality, and ecosystem function of the Pala, with implications for conservation and environmental management.</p>