Seagrass is an early responder to nitrogen enrichment in oligotrophic oceanic coral atoll environments.

Seagrass meadows in oceanic coral atoll settings play key roles in sediment stabilisation and biodiversity enhancement. However, seagrass is susceptible to anthropogenic disturbance including nutrient input. Here, we experimentally enriched Thalassia hemprichii meadows with N-based fertiliser in two similar reef-top settings with different recent histories of disturbance, in Laamu atoll, Maldives. Enrichment resulted in enhanced overall leaf production, but in different ways at the two sites. At the 'more impacted', and slightly deeper site, the dominant effect of high-nutrient treatment was growth of new leaves, but this also coincided with cyanobacteria and filamentous green-algal blooms. In contrast, the 'less impacted' site, leaf extension rates (tagged and new) significantly increased. Isotopic analysis confirmed fertiliser uptake into biomass. Our findings highlight that the response of seagrass to nutrient enrichment is context dependent. This study, the first of its kind in the Indian Ocean, highlights the need and opportunity for nutrient and seagrass monitoring and associated policy development.