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

Abstract

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.

Novel tri‐isotope ellipsoid approach reveals dietary variation in sympatric predators

“…Although δ 15 N values of a corallivore, Chaetodon meyeri , and a nocturnal planktivore, Myripristis violacea , were significantly higher in the outer atoll, the differences in mean values were small (~1‰) and isotopic values of all other prey species were similar between areas (Skinner et al, ). Furthermore, δ 13 C and δ 15 N values of coral host and particulate organic matter (POM) are consistent around the Maldives and do not vary between inner and outer atoll (Radice et al, ). This suggests that the differences in predator CR and NR ranges are a direct result of feeding on different combinations of prey, rather than differences in baseline isotope values.…”

“…With the exception of C. argus , all predators had larger isotopic niches in the outer atoll. Given the similarity in prey and primary producer isotope values between atoll areas (Radice et al, ; Skinner et al, ), it seems likely that this spatial variation in resource use is linked to variations in resource availability (Araújo et al, ). The oceanic rim reefs of the outer atoll had higher live branching coral and habitat structural complexity following the 2016 bleaching event compared with inner atoll reefs (first author, unpublished data).…”

Toward a standardised protocol for the stable isotope analysis of scleractinian corals

“…These studies have increased our understanding of the effects of environmental factors (such as depth, light, and turbidity) on the trophic status of corals, the feeding relationships of symbiotic organisms, and the energy sources used. Investigations of potential energy sources (e.g., resource availability in the reef water column and across environmental gradients) are critical when attempting to characterise the trophic status of corals …”

“…[1][2][3] In tropical coral reef habitats, many studies have examined the stable isotope compositions of scleractinian coral skeletal material, [4][5][6][7][8][9] while fewer have investigated in situ natural variations in stable isotopes in coral host tissues and their associated Symbiodiniaceae. [10][11][12][13][14][15] These studies have increased our understanding of the effects of environmental factors (such as depth, light, and turbidity) on the trophic status of corals, the feeding relationships of symbiotic organisms, and the energy sources used.…”

“…Investigations of potential energy sources (e.g., resource availability in the reef water column and across environmental gradients) are critical when attempting to characterise the trophic status of corals. 15,16 Carbon and nitrogen stable isotopes are the most commonly used food web tracers in marine ecology, especially in coral studies. [17][18][19] Carbon stable isotope ratios ( 13 C/ 12 C, expressed as δ 13 C values in per mil notation) are used to track the origin of carbon in the diet 20 and elucidate whether coral animal tissue derives more carbon from allochthonous sources (e.g., zooplankton) than from Symbiodiniaceae photosynthesis (i.e., photoassimilate translocation).…”