Carbon dynamics of Deep Bay, eastern Pearl River estuary, China. II: Trophic relationship based on carbon- and nitrogen-stable isotopes

A study of the sources of nutrients supporting the consumer community in Deep Bay, a shallow mangrove-fringed embayment in the eastern Pearl River estuary, was conducted using the stable isotopes of carbon and nitrogen. The delta C-13 and delta N-15 signatures of the potential natural (3 species of mangroves, macroalgae, benthic microalgae, seston in mangrove creeks and in Deep Bay) and anthropogenic (POM from a local river) nutrient sources were measured, as were those of 22 species of consumers from 20 stations in 4 zones (mangrove forests, inner and outer bay, the estuary) at different distances from the mangroves. While the stable-isotope signatures of the nutrient sources (except the macroalgae Enteromorpha spp.) are sufficiently different to allow their use in tracing trophic relationships, the consumers studied had generally uniform delta C-13 (approximate to -20 +/- 1 parts per thousand) and delta N-15 (approximate to +12 +/- 1 parts per thousand) signatures. There is, however, a significant increasing trend in the delta C-13 values of consumers at increasing distance from the head of the bay, with the inner-bay consumers having a significantly more negative mean value than their counterparts in the pearl River estuary. By contrast, delta N-15 signatures were generally not significantly different among the 3 sampling zones, probably reflecting the similar trophic positions of the species studied. Species that are known to feed at other trophic levels did, however, demonstrate considerable deviations from the overall mean for all consumers. Data from this dual isotope study suggest that the consumer community in Deep Bay and the Pearl River estuary was probably supported mainly by anthropogenic POM input from the local catchment and benthic algae on the mudflat, with a dominance of the latter in the food chains starting from short distances offshore. The results of this study generally agree with the estimations made in a previous mass-balance calculation, which indicated that the contribution from mangrove carbon played a minor trophic role in the system, although the importance of benthic algae as sources of carbon have been under-estimated in the mass-balance calculations. The extensive mudflats in Deep Bay therefore form the basis of not only the feeding habitat for shorebirds utilising the bay as a refueling ground, but also the actual energy source for the system.