Spinner dolphin residency in tropical atoll lagoons: Diurnal presence, seasonal variability and implications for nutrient dynamics

Abstract

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.

Cetacean‐mediated vertical nitrogen transport in the oceanic realm

“…However, toothed cetaceans in the Gulf of Maine ( n = 110,458 individuals) only contribute 12% of the daily nitrogen export, while pinnipeds (absent in the Gulf of Mexico) and baleen whales (only represented by Rice's whale in the Gulf of Mexico) contribute to the rest (Roman and McCarthy 2010). The per capita cetacean nitrogen export for in the Gulf of Mexico is 1.1 × 10 4 ± 2.3 × 10 3 mmol N ind −1 d −1 (0.15 ± 0.03 kg N d −1 ) is 3 and 2.1 times greater than the per capita nitrogen export rate for toothed cetaceans in the Gulf of Maine (0.05 kg N d −1 ; Roman and McCarthy 2010) and spinner dolphins in the Maldives and Chagos archipelagos (0.07 kg N d −1 ; Letessier et al 2022), respectively. This is likely due to differences in odontocete community compositions and abundances between the three locations and the presence of deep‐diving whales (particularly sperm whales) in the Gulf of Mexico.…”

“…In the Gulf of Maine, cetaceans and pinnipeds add 2.3 Â 10 4 mt N yr À1 , enough to significantly increase the primary production in the region (Roman and McCarthy 2010). However, toothed cetaceans in the Gulf of Maine (n = 110,458 individuals) only Letessier et al 2022), respectively. This is likely due to differences in odontocete community compositions and abundances between the three locations and the presence of deep-diving whales (particularly sperm whales) in the Gulf of Mexico.…”

Contrasting ecological information content in whaling archives with modern cetacean surveys for conservation planning and identification of historical distribution changes

“…These locations, associated with the South West Indian Ridge and in the central Indian Ocean near and adjacent to the Chagos Archipelago, should be prioritized for further investigation and considered for promotion to IMMA–AOI. To this effect, we are expanding our spatial analysis through novel surveys (Letessier et al., 2022) and by considering other cetacean taxa.…”

Composition of cetacean communities worldwide shapes their contribution to ocean nutrient cycling

“…This can be particularly important for ecosystem functioning when the intrinsic nutrient characteristics of the donor and recipient ecosystems are significantly different 24 , e.g. diurnal movements of spinner dolphins ( Stenella longirostris ) between offshore waters, where they feed, and lagoons, where they rest, in the Maldives and the Chagos archipelagoes 53 .…”