The role of mangroves in early-life trophic interactions supporting fishery production in the Galapagos

Mangrove nurseries are crucial for fishery production, supplying energy and nutrients via leaf organic matter in food webs. Recent stable isotope studies emphasize the nutritional value of mangrove detritus, especially for leaf-eating crabs, while algae remain important for other consumers in the ecosystem. In the Galapagos Islands, protected shorelines with mature mangrove forests may significantly contribute to secondary production. In this study, we measured carbon (δ¹³C) and nitrogen (δ¹ 5 N) stable isotope ratios from primary producers (e.g., mangroves, epiphytes, benthic algae), coastal sediments, and consumers (including fish larvae, epibenthic, and zooplanktonic crustaceans). We used Bayesian mixing models to assess the relative importance of food sources across six protected shorelines on Santa Cruz and Isabela Islands, representing different biogeographic origins. We also examined environmental drivers and habitat conditions influencing spatial variation in resource partitioning among consumers. Our results reveal distinct dietary patterns: ‘δ¹³C-depleted’ consumers (e.g., crab larvae, post-flexion fish larvae) relied heavily on mangroves and epiphytes, with terrestrial inputs exceeding 25%; whereas ‘δ¹³C-enriched’ consumers (e.g., polychaetes, caridean shrimps) primarily fed on microphytobenthos, macroalgae, and ¹³C-rich organic matter, with terrestrial inputs < 25%. Opportunistic feeding is suggested for fishery-related taxa, such as spiny lobster larvae. Turbidity and nitrate concentration emerged as key drivers of resource partitioning, with spatial patterns more distinct within islands than between them. These results highlight the dominant role of local hydrophysical conditions and water quality in shaping mangrove nursery functions, surpassing the influence of broader regional factors. Our study also underscores the importance of internal nutrient recycling—particularly mediated by larval crabs—in sustaining early life stages of fish and other mid-trophic species. These findings call for a deeper understanding of how habitat conditions and environmental variability influence food availability at both local and regional scales.