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 (δ(13)C) and nitrogen (δ(15)N) isotopic compositions of coral host tissues and photosymbionts 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 tissues and photosymbionts. Samples from the site with higher level of suspended particulate matter were (13)C-depleted and (15)N-enriched relative to corals and photosymbionts from less turbid sites. However, differences in both δ(13)C and δ(15)N 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 δ(13)C and enriched δ(15)N 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 δ(13)C and δ(15)N 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.