Monitoring nitrogen pollution using isotopic composition of macrophytes in oligotrophic tropical coastal areas: A short review

This review explores the application of nitrogen stable isotopes measurements as a tool for monitoring nitrogenous nutrients inputs in coastal zones, focusing on oligotrophic tropical ecosystems. Pulsed nutrient enrichment leads to the proliferation of macroalgae, compromising the health of coral reefs and seagrass beds. Through the analysis of 52 studies, we compile isotopic signatures measured in macroalgae and seagrass, while proposing reference thresholds to identify nitrogen pollution sources in tropical areas. By standardizing these isotopic compositions, our review aims to provide useful benchmarks for researchers and managers of coastal ecosystems. The main pollution sources identified include wastewater, groundwater, aquaculture, and fertilizers, each characterized by distinct δ 15 N ranges. Among these, wastewater and sewage show the highest isotopic values (>10 ‰), while inorganic fertilizers exhibit the lowest (down to −4.5 ‰). Macroalgae, particularly Dictyota and Rhodophyta species (e.g. Laurencia ), were most frequently used as bioindicators due to their wide distribution and responsiveness to nutrient variation. Thalassia testudinum emerged as the most commonly used seagrass species across studies. However, seagrasses overall were less frequently employed than macroalgae, and typically displayed less sensitively to nitrogen enrichment in their isotopic signatures. The review highlights practical aspects of isotopic monitoring, including macrophyte selection, caging techniques, sample preparation, and environmental variables affecting interpretation such as seasonality, depth, and hydrodynamic conditions. Caging, though underused in seagrass studies, proved essential in areas lacking native bioindicators. Finally, we provide recommendations for future studies to enhance consistency and standardization allowing temporal and intersites comparisons. This approach offers a cost-effective and sensitive tool for detecting nutrient inputs in vulnerable tropical ecosystems. • Synthesis of 52 studies defines indicative δ 15 N ranges for major N sources. • δ 15 N in macrophytes is a robust tracer of nitrogen pollution in tropical coasts. • This review provides guidelines to perform isotopic monitoring of nitrogen pollution. • Macroalgae provide higher sensitivity than seagrasses for detecting nitrogen pollution. • Wastewater exhibits the highest δ 15 N signatures, fertilizers the lowest.