Spatial, seasonal, and ontogenetic variations in the significance of detrital pathways and terrestrial carbon for a benthic shark, Chiloscyllium plagiosum (Hemiscylliidae), in a tropical estuary

Abstract

Using a combination of stable isotope analyses (SIA), fatty acid profiling (FAP), and FA‐specific SIA, we investigated the relative importance of terrestrial‐ and marine‐derived carbon, and spatial (inshore vs. offshore), seasonal (wet vs. dry season), and ontogenetic variations, in the relative contribution of detrital pathways to the white‐spotted bamboo shark, Chiloscyllium plagiosum in the Pearl River estuary. SIA and FAP suggested the sharks acquired carbon from both marine‐ and terrestrial‐derived detritus via consumption of polychaetes and small crustaceans, with carbon from terrestrial sources being more important to juveniles (25–50 cm long) at inshore locations. Juveniles generally had significantly higher levels of diatom FA, bacterial FA, and long‐chain‐carbon–saturated FA biomarkers than adults (> 65 cm), especially during the wet season, suggesting that they relied on primary consumers which assimilated a mixture of autotrophic and detrital carbon sources. Levels of zooplankton and animal‐derived FA biomarkers generally increased with shark size, indicating an ontogenetic dietary shift. Utilizing the results of FA‐specific SIA (i.e., δ13C values of individual bacterial FA of detritus) from four sources (i.e., terrestrial plant litter, macroalgae, phytoplankton, and sediments), Bayesian mixing models revealed juveniles in the inner estuary assimilated significantly higher amounts of terrestrial carbon (37–45%) than adults (22–37%) or sharks in the outer estuary (1–36%). The combined use of multiple dietary tracers effectively revealed the interaction of various detrital pathways to marine predators in the Pearl River estuary, and highlighted the importance of terrestrial–marine linkages for interpretation of energy flow in coastal ecosystems.