Linking coral stress to groundwater phosphorus loading: a model-based novel framework for water resource management and coral reef protection

Excess nutrient enrichment threatens coral reef ecosystems, yet establishing management targets remains challenging especially in data-scarce and capacity-limited small island developing states. Addressing this, we developed a novel first-order modelling framework linking groundwater-derived nutrient loading directly to exchangeable phosphate in seawater (EPS). We applied a tidally forced, single-layer saltwater intrusion model to a representative reef island. Validated against hydraulic heads and electrical resistivity surveys, the model identified discharge hotspots coinciding with elevated EPS. Integrating discharge fluxes with phosphorus concentrations revealed a power-law relationship between nutrient load and EPS. This allowed for the translation of an ecological constraint – the juvenile coral survival threshold (EPS = 0.5 µg g−1) – into a quantitative phosphorus load target. This study demonstrates that submarine groundwater discharge dynamics can be translated into operational water resources management criteria in a scalable but site-specific manner, providing a critical tool for defining ecologically defensible nutrient limits tailored to local reef resilience.