Ontogeny drives allochthonous trophic support of snappers: Seascape connectivity along the mangrove-seagrass-coral reef continuum of a tropical marine protected area

Ecosystems are connected through the movement of organisms, nutrients, energy, and matter. The connection and dependence between aquatic habitats enhances our capacity to understand the functioning of the seascape. In this paper, we investigated the trophic ecology of two snapper species, Lutjanus analis and Lutjanus alexandrei, along a tropical coastal seascape (inner estuary, outer estuary, marine beach, and coastal reefs). To quantify and compare trophic (isotopic) niches, primary trophic support, and trophic connectivity along the ontogeny, delta(13C) and delta N-15 were used to construct isotopic metrics and mixing models for assessing consumers' isotopic niches and the relative contributions of primary sources supporting them. The isotopic niche was wider for fishes from coastal reefs, but narrower for those from inner estuary, suggesting diversification on trophic support when these species move out from the estuary to the sea. Mixing models showed that smaller individuals generally showed their previous habitat support and as the fish start to feed locally and grow, they change tissues and reflect local sources. Inside the estuary, larger fish started to show marine support while smaller fish from coastal reefs still reflected sources from previous habitats. Specifically, the models revealed that smaller individuals of L. alexandrei retained their mangrove and estuarine particulate organic matter previous support, while the L. analis model was inconclusive but converged with stable isotope niche metrics, which indicated high variability that could be related to periphyton sources from seagrass beds. The allochthonous support seems to occur either by movement or recent migrations of organisms instead of organic matter flow promoted by abiotic vectors. These results suggest the importance of maintaining habitat connections for these valuable commercial species. Future studies should focus on other sites with seagrass beds to fully understand the habitat and resource use, as well as the movements among them, especially for L. analis.