Data Sheet 1_Local baselines reveal elevated groundwater nutrients, localised coastal enrichment, and broadly oligotrophic reef conditions in Laamu Atoll, Maldives.docx

Introduction<p>Nutrient enrichment in coral reef systems is increasingly recognised as a major driver of ecological change, yet integrated assessments across groundwater, coastal, and reef environments remain limited in small island developing states (SIDS).</p>Methods<p>We assessed dissolved inorganic nutrients (total oxidised nitrogen, ammonium and phosphate), N:P ratios, and isotope-based tracers (δ¹⁵N, C:N) across groundwater, coastal-surface, and reef domains of Laamu Atoll, Maldives, between 2022 and 2024 to characterise spatial nutrient patterns and establish a baseline for environmental monitoring.</p>Results<p>Groundwater contained substantially higher nutrient concentrations – typically one to two orders of magnitude – than coastal-surface and reef waters, with several wells exceeding national drinking-water standards for ammonium.

Coastal-surface waters showed localised nitrogen enrichment at several inhabited and poorly flushed shorelines, while reef waters remained characteristically oligotrophic with phosphate frequently below detection limits across most sites. N:P patterns suggested predominant, though not uniform, phosphorus limitation in groundwater, and more variable nutrient limitation across coastal and reef environments that differed by island and use-type context.