A trophic study of a marine ecosystem off southeastern Australia using stable isotopes of carbon and nitrogen

The stable isotopes of carbon and nitrogen were used to investigate trophic relationships of fish and invertebrates on the continental shelf of southeastern Australia. We examined 87 fish species, marine mammals, penguins, invertebrates, algal samples, suspended particulate organic matter (POM), and sediments. The main source of primary production is oceanic phytoplankton, although there is evidence of a seagrass contribution to the benthos. Marine mammals, penguins, some benthic invertebrates, and some benthic fish filled the highest trophic positions. Sources of variability in isotope results included temporal (POM, some fish) and spatial (POM, sediments) elements, bottom depth (some fish), and size (some fish). Fish had muscle δ 15 N values of 9.6‰ to 14.7‰ and δ 13 C values of –20.6‰ to –14.6‰. Cluster analysis on fish stable isotope results produced fish groups that could be interpreted with trophic and habitat information. Trophic relationships in fish, suggested by stable isotope results, were supported by stomach contents analysis. Stable isotope results may indicate more representative trophic relationships, as stomach contents analyses tend to group prey by taxon rather than by trophodynamic position. In fish and invertebrates, stable isotope results related more to functional patterns of feeding than to taxonomic relationships.