Complex food webs in highly diversified coral reefs: Insights from δ13C and δ15N stable isotopes

We studied the trophic network architecture of the coral reef ecosystem of the New Caledonian lagoon. To encompass the main tropic levels, we assayed carbon and nitrogen stable isotopes in various organic matter sources, intermediate consumers (invertebrates and fish), and 19 species of predatory fish (total of 1229 samples). At each level, wide range of variations for δ13C and δ15N suggested multiple sources for the OM, and complex trophic relationships among the different organisms. Despite this complexity, four trophic structures were identified. 1) The predominant reef benthic food web (R-BFW) based on the OM produced by algal turf supplies most of intermediate consumers and all anguilliform fish studied. 2) The sedimentary benthic food web (S-BFW), and 3) the lagoon pelagic food web (L-PFW), respectively based on sedimentary OM (SOM) and particulate OM (POM) involve a wide range of organisms and represent complementary food webs for most anguilliform fish. 4) Finally, the detrital benthic food web (D-BFB) where the OM issued from seagrass is indirectly utilised by reef consumers plays a secondary role. Anguilliform fish are widespread sedentary predators that live in various habitats; the species sampled were respectively associated with the three main trophic structures identified, depending on their diet and habitat. This study reveals several major structures that are inter-connected to form an overall complex trophic network in the coral reefs.