Land-based Nutrient Flux to a Fringing Reef: Insights from Ofu Island, American Samoa

Nancy Grumet Prouty^1*Ferdinand K. J. Oberle¹Olivia M Cheriton¹Lauren T. Toth²Eric K. Brown³Curt D Storlazzi¹

• ¹Pacific Coastal and Marine Science Center, U.S. Geological Survey, Santa Cruz, California, United States
• ²St. Petersburg Coastal and Marine Science Center, U.S. Department of the Interior, United States Geological Survey, St. Petersburg, Florida, United States
• ³U.S. National Park Service, National Park of American Samoa, Pago Pago, American Samoa

Submarine groundwater discharge (SGD) can be a critical driver of nutrient transport to coral reef ecosystems, shaping water quality, primary productivity, and overall reef health. This study quantifies SGD fluxes and associated nutrient dynamics in two reef flat pools within the National Park of American Samoa on Ofu Island: Papaloloa Point and Fatuana Point. A multi-method approach integrating multichannel electrical resistivity tomography (ERT), unoccupied aerial system-based thermal infrared (UAS-TIR) surveys, radon-based SGD measurements, and discrete water sampling was used to assess SGD rates and nutrient contributions to nearshore pools. UAS-TIR imagery revealed cooler sea-surface temperatures relative to ambient seawater emanating from coastal point sources in both pools, indicative of SGD, with higher fluxes observed at Papaloloa Point. Time-series radon measurements revealed a strong inverse correlation between calculated SGD rates and tidal stage, further indicative of SGD. The SGD response was immediate (i.e., in phase with the tides) at Papaloloa Point, likely due to its highly permeable calcareous sand and gravel substrate. In contrast, a 2-3-hr lag in SGD response at Fatuana Point indicates discharge from a more inland aquifer with lower transmissivity. Nutrient concentrations correlated directly with temperature and salinity, confirming SGD as the dominant nutrient transport mechanism, whereas isotopic analyses demonstrated inputs from both groundwater and potential anthropogenic sources. These findings underscore the complexity of SGD-mediated nutrient dynamics in coral-reef environments and emphasize the need for an integrated geochemical and geophysical approach to support effective coral-reef conservation and management strategies.