Experimental nutrient enrichment during thermal stress reduces bleaching severity in an oligotrophic reef setting, Maldives

Coral bleaching events are intensifying with rising sea surface temperatures, yet the role of nutrient availability in influencing coral responses to thermal stress remains complex. Excessive nutrient enrichment often increases bleaching susceptibility, but moderate, balanced enrichment may provide physiological benefits. We tested this through a six-month in situ enrichment experiment on an oligotrophic reef in the Maldives during a mild bleaching event. Diffusers supplied either nitrogen-rich (High N; elevated N relative to P) or balanced nitrogen-phosphorus (High P; relative to typical Redfield N:P ratios) fertiliser to Pocillopora spp. colonies, and responses were assessed using bleaching surveys and stable isotope analyses (δ¹⁵N, δ¹³C, and C:N ratios) of host tissue and algal symbionts to monitor changes nitrogen assimilation strategies. Enriched colonies bleached significantly less than controls during peak heat stress. Isotopic signatures shifted toward fertiliser values, with δ¹⁵N reduced by up to 2 ‰ in symbionts and ∼ 1 ‰ in tissue, indicating assimilation of external nitrogen. Symbiont C:N ratios also declined under enrichment, consistent with reduced N limitation and altered metabolic balance within the symbiosis. Together, these results provide the first field evidence from the Indian Ocean that short-term nutrient enrichment in oligotrophic reef environments can reduce bleaching severity under mild heat stress by enhancing metabolic flexibility, i.e., the capacity of corals and their symbionts to adjust nutrient assimilation and internal resource use under stress. However, the long-term consequences of sustained or unbalanced nutrient exposure remain uncertain, highlighting the need for caution in management applications.