Food web structure in a near-pristine mangrove area of the Australian Wet Tropics

Abstract

Carbon and nitrogen isotopic composition was used to identify the main sources of carbon and describe the main trophic pathways in Deluge Inlet, a near-pristine mangrove estuary in tropical north Queensland, Australia. Producers' δ 13C varied from −28.9‰ for mangroves to −18.6‰ for seagrass. Animals were also well separated in δ 13C (−25.4‰ to −16.3‰ for invertebrates and −25.2‰ to −17.2‰ for fish), suggesting considerable differences in ultimate sources of carbon, from a substantial reliance on mangrove carbon to an almost exclusive reliance on seagrass. In general, invertebrates had lower δ 15N than fish, indicating lower trophic levels. Among fish, δ 15N values reflected well the assumed trophic levels, as species from lower trophic levels had lower δ 15N than species from higher trophic levels. Trophic levels and trophic length were estimated based on δ 15N of invertebrate primary consumers (6.1‰), with results suggesting a food web with four trophic levels. There was also evidence of a high level of diet overlap between fish species, as indicated by similarities in δ 13C for fish species of higher trophic levels. Stable isotope data was also useful to construct a general model for this food web, where five main trophic pathways were identified: one based on both mangrove and microphytobenthos, one on plankton, two on both microphytobenthos and seagrass, and one based mainly on seagrass. This model again suggested the presence of four trophic levels, in agreement with the value calculated based on the difference in δ 15N between invertebrate primary consumers and top piscivores.