Spatial and temporal variations in stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition of symbiotic scleractinian corals 

Tropical scleractinian corals are considered autotrophic as they rely mainly on photosynthesis-derived nutrients transferred from their photosymbionts. Corals are also able to capture and ingest suspended particulate organic matter, so heterotrophy can be an important supplementary trophic pathway to optimize coral fitness. The aim of this in situ study was to elucidate the trophic status of 10 coral species under contrasted environmental conditions in a French Polynesian lagoon. Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions of coral host tissues and photosymbiont were determined at 3 different fringing reefs during wet and dry seasons. Our results highlighted spatial variability in stable isotopic compositions of both coral host tissue and photosymbiont. Samples from the site with higher level of suspended particulate matter were ¹³C-depleted and ¹⁵N-enriched relative to corals and photosymbionts from less turbid sites. However, differences in both δ¹³C and δ¹⁵N between coral host tissues and their photosymbionts (Δ^{host-photosymbionts 13}C and Δ^{host-photosymbionts 15}N) were small (0.27 ± 0.76‰ and 1.40 ± 0.90‰, respectively) and similar at all sites, thus indicating no general increases in the heterotrophic pathway. Depleted δ¹³C and enriched δ¹⁵N values of coral host tissues measured at the most turbid site were explained by changes in isotopic composition of the inorganic nutrients taken up by photosymbionts and also by changes in rate of isotopic fractionation with environmental conditions. Our results also highlighted a lack of significant temporal variations in δ¹³C and δ¹⁵N values of coral host and photosymbiont tissues and in Δ^{host-photosymbionts 13}C and Δ^{host-photosymbionts 15}N values. This temporal stability indicated that corals remained principally autotrophic even during the wet season when photosymbiont densities were lower and the concentrations of phytoplankton were higher. Increased coral heterotrophy with higher food availability thus appears to be species-specific.