Stable isotope and fatty acid biomarkers of seagrass, epiphytic, and algal organic matter to consumers in a pristine seagrass ecosystem

Abstract

The relative importance of the identity and abundance of primary producers in structuring trophic ecology, particularly in seagrass-dominated ecosystems, remains unclear. We assessed the contributions of seagrass, epiphytes, macroalgae, and other primary producers to the diets of resident animals in the nearly pristine seagrass-dominated environment of Shark Bay, Australia, by combining fatty acid composition with carbon, nitrogen, and sulfur stable isotopes of primary producers and consumers. Overall, mixed inputs of these primary producers fuel secondary production, with tropical detrital seagrass inputs supporting most fish species, likely through benthic intermediates. Epiphytic organic matter inputs were most closely associated with snails, whereas seagrass detritus, macroalgae, gelatinous zooplankton, and/or phytoplankton may all contribute to higher trophic levels including sea turtles and sharks. The fatty acid and isotope data suggest that diets of large-bodied consumers were highly variable – future food web studies need to incorporate large sample sizes to account for this variability.