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Mobile predators serve important ecological functions, including acting as nutrient vectors between different ecosystems. In coral reefs, pelagic nutrient subsidies are believed to play an increasingly important role under ongoing and projected environmental changes. Here, we combine visual sightings with passive acoustic monitoring to report habitat use and behaviour by cetaceans within atoll lagoons in the Maldives and Chagos archipelagoes. We demonstrate that spinner dolphins (Stenella longirostris) are the most widely distributed and numerically abundant cetacean inside these atolls (>90% of all individual cetaceans by numbers). Our visual and acoustic observations both provide evidence of a regular diurnal behaviour, where dolphins enter the lagoons during the morning, for day-time resting, and exit during the afternoon, for night-time foraging offshore. Using standard metabolic models and timing of lagoonal residencies, we estimate that a dolphin pod would deposit approximately 288 AE 17 kg year −1 of nitrogen of primarily mesopelagic origin inside the lagoons. The nitrogen deposited inside an atoll lagoon by a dolphin pod resident year-round will therefore likely enhance coral reef productivity and resilience and suggests that these dolphins play a role in making pelagic energy and nutrients available to coral reefs. The absence of any acoustic detections following the reversal of the monsoon winds suggests that the short-to-medium-term residency of the dolphins is sensitive to seasonal productivity dynamics.

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