Importance of Mangrove Carbon for Aquatic Food Webs in Wet-Dry Tropical Estuaries

Mangroves are traditionally considered to provide important nutrition to tropical estuarine consumers. However, there is still controversy about this, and the extent and importance of these inputs are largely unquantified. In particular, there is no information for food webs of small estuaries that dominate wet-dry tropical coasts, where freshwater inflow is intermittent, leading to highly seasonal inputs of nutrients from terrestrial systems. Since the relative importance of the different sources depends on the type and extent of different habitats and on hydrological and topographic conditions, results from other regions/type of systems cannot be extrapolated to these estuaries. Here, delta C-13 is used to determine the importance of mangrove-derived carbon for Penaeus merguiensis (detritivore; shrimp), Ambassis vachellii (planktivore; fish), and Leiognathus equulus (benthivore; fish) from six small wet-dry tropical estuaries that differ in mangrove (C-3) cover and in type of terrestrial vegetation adjacent to the estuary. Bayesian mixing models confirmed that mangrove material was important to consumers in all estuaries. There was a gradient in this importance that agreed with the extent of mangrove forests in the estuaries, as C-3 sources were the most important contributors to animals from the three estuaries with the greatest (>40%) mangrove cover. There was also evidence of incorporation of C-3 material for the three estuaries with lower (<30 %) mangrove cover. Since these latter estuaries had no adjacent terrestrial C-3 forests, the detected C-3 influence can only be of mangrove origin. This shows that mangroves are important contributors to these food webs, underlining the importance of mangroves in supporting estuarine nursery ground value and fisheries productivity.