Tracing carbon transfer and assimilation by invertebrates and fish across a tropical mangrove ecosystem using stable isotopes

Mangroves are highly productive ecosystems that exhibit a diverse range of habitats, including tidal creeks and flats, forest gaps and interior forest with varying understory light intensity, tidal dynamics, geomorphological settings, and overall biological production. Within mangrove ecosystems, invertebrates and fish feed on heterogeneous food sources, the occurrence of which is unevenly distributed across the system. This provides a basis for testing models of carbon transfer across mangrove ecosystems. We hypothesized that the carbon transfer and assimilation by fish and invertebrates will vary across the different mangrove habitats and that such variations can be predicted by their stable isotope compositions. We analysed delta C-13 and delta N-15 signatures of consumers and their potential organic carbon sources across a tropical mangrove ecosystem in Vietnam. The delta C-13 values of crabs and snails significantly decreased from the tidal flat to interior forest, indicating that variations in carbon transfer and assimilation occurred at small scales <30 m. Reduced variation in delta C-13 of suspension-feeding bivalves suggested that tidal water was a vector for large-scale transport of carbon across the mangrove ecosystem. An analysis of co-variance using habitat as a fixed factor and feeding habit and movement capacity of consumers as co-variates indicated that habitat and feeding types were major features that affected the delta C-13 values of invertebrates and fish. The findings demonstrate that carbon transfer and assimilation across mangrove ecosystems occur as a diverse combination of small (<30 m) and large (>30 m) scale processes.