Monsoons and habitat influence trophic pathways and the importance of terrestrial-marine linkages for estuary sharks

Tropical estuaries often receive enhanced fluxes of terrestrial derived organic matter and phytoplankton during the wet season, and such monsoonal events may significantly influence the trophic dynamics of these systems. This study examined spatio-temporal terrestrial-marine linkages in a tropical estuary, the Pearl River Estuary (PRE), southern China, by investigating trophic pathways leading to estuary sharks. We investigated spatial (inshore vs. offshore) and seasonal (wet vs. dry season) variation in the relative importance of terrestrial-and marine-derived carbon, so as to assess the contribution of detrital pathways to the pelagic spadenose shark, Scoliodon laticaudus; ontogentic changes in shark diets were also documented. Stable isotope analyses (SIA) and fatty acid (FA) profiling indicated that spadenose sharks assimilated both marine and terrestrial carbon via consumption of zooplantivorous fish and shrimps. Detrital carbon sources were more important to juvenile and pre-mature sharks at inshore locations, especially during the wet season when river discharge increased and terrestrial detritus was more abundant. Ontogenetic dietary shifts were evident: juvenile and pre-mature sharks had significantly higher levels of bacterial (detrital) FA than adults which contained more animal-derived FA. Inshore sharks, with more depleted delta C-13 signatures, relied more on terrestrial carbon than sharks offshore. Comparison of spadenose shark FA profiles with those of the sympatric, white-spotted bamboo shark (Chiloscyllium plagiosum)-a benthic predator that acquires detrital carbon via consumption of polychaetes and crustaceans-revealed that they made greater use of detrital carbon sources. However, spadenose sharks in the inner estuary assimilated higher proportions of terrestrial detritus (44-56%) than bamboo sharks (31-45%). The importance of terrestrial detritus for both shark species demonstrated the important contribution of terrestrial detritus to both pelagic and benthic food webs in the PRE. Terrestrial-marine linkages are therefore of great significance, particularly during the wet season, in this estuarine system, which serves as feeding and nursery grounds for both shark species, and trophic subsidies from land are likely to be important for marine predators in other tropical estuaries.