Upwelling as the major source of nitrogen for shallow and deep reef-building corals across an oceanic atoll system

Oceanographic processes shape coral reefs worldwide by redistributing inorganic nutrients and particulate resources over depth. Deep-water upwelling occurs frequently in coral reef ecosystems, but its impact on coral nutrition remains unclear. This study investigated the influence of upwelling on the trophic ecology of three common reef-building corals (Galaxea fascicularis, Pachyseris speciosa and Pocillopora verrucosa) from different reef depths (shallow reef, 10 m, vs. deep reef, 30 m) and reef exposures (oceanic rim vs. Inner Sea) across > 250 km of the Maldives archipelago, Indian Ocean. Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of coral hosts, their symbionts and particulate organic matter (POM) were used to characterize coral trophic strategies. Across the Maldives, consistent mean δ15N values were recorded in hosts (5.5‰) and symbionts (5.2‰) of the three coral species from shallow and deep reefs of oceanic and Inner Sea reef exposures. Coral hosts, symbionts and POM from both depths had δ15N values that were consistent with the isotopic signature of a deep-water nitrate source transported to surface waters via upwelling. In contrast, a wide range of δ13C values (~10‰) revealed different trophic strategies and isotopic niches among the coral species. Different mean δ13C values of G. fascicularis indicated greater symbiont autotrophy in corals from shallow (-15.5‰) compared to deep reefs (-17.6‰). Conversely, the mean δ13C values of P. speciosa (-15.1‰) and P. verrucosa (-17.7‰) were not affected by reef depth. These corals maintained consistent trophic strategies over depth, with P. speciosa relying more on autotrophy compared to P. verrucosa. Despite different reef exposure to oceanic waters, coral host and POM δ15N and δ13C values did not differ between oceanic and Inner Sea reef exposures. Nutritional resources appear to be homogenous in the central Maldives due to atoll-wide water circulation. However, species-specific trophic strategies resulted in diverse patterns of δ13C values over depth. Because heterotrophic feeding has been linked to coral host survival through coral bleaching events, understanding the trophic ecology of corals within the reef assemblage can provide insight into species resilience under ocean warming conditions. As a member of the typically competitive Pocilloporidae family, the dependence of P. verrucosa on heterotrophy may help this coral be a future “winner” under sustained ocean warming. A plain language summary is available for this article.