Local baselines reveal elevated groundwater nutrients, localised coastal enrichment, and broadly oligotrophic reef conditions in Laamu Atoll, Maldives

Introduction 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). Methods We assessed dissolved inorganic nutrients (total oxidised nitrogen, ammonium and phosphate), N:P ratios, and isotope-based tracers (δ 15 N, 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. Results 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. Exploratory δ 15 N analysis in seagrass and macroalgal tissue showed weak group-level differences but localised enrichment near fish-processing and relative depletion near agricultural zones, while C:N ratios differed most strongly by species, with a secondary, weaker signal by land-use type. Discussion Together, these cross-domain nutrient gradients demonstrate clear spatial differences among groundwater, coastal-surface, and reef environments, consistent with localised nutrient enrichment in some nearshore settings and strong hydrodynamic control across the wider atoll. This study provides the first integrated nutrient baseline for Laamu Atoll, complemented by exploratory isotope data from a subset of islands, and contributes to the limited body of nutrient monitoring data available for coral reef systems in SIDS, supporting future monitoring and management of nutrient pressures associated with wastewater and agriculture.