Unraveling carbon flow pathways in coral reef seascapes with compound-specific stable isotope analysis
Dr. Simon R. Thorrold (WHOI), Dr. Michael L. Berumen (KAUST and WHOI)
Quantifying carbon flow on coral reefs is fundamental to the development of models capable of predicting the response of reef ecosystems to threats, including overfishing and global climate change. For instance, it is often assumed that water column-based phytoplankton production is the dominant carbon source fuelling large predatory fish, such as snapper and grouper, on coral reefs. However, recycled carbon passing through microbial/detrital pathways may also supply significant amounts of carbon to higher trophic levels. We are using amino acid δ13C values to examine diet and food web structure supporting commercially and ecologically important fishes on coral reefs along a cross-shelf gradient in the northern Farasan Banks, Red Sea. Our results highlight the remarkable complexity of food webs on coral reefs. Coralivorous butterflyfish, planktivorous damselfish and herbivorous damselfish plotted close to coral tissue, zooplankton and macroalgal profiles, respectively. One of the more fascinating observations in this system was the apparent prey resource partitioning of two congeneric snapper species (Lutjanus kasmira and L. ehrenbergii) that commonly occur together in mixed species schools. While L. kasmira apparently feeds on C derived from phytoplankton, L. ehrenbergii relies almost entirely on C that has been recycled through a macroalgal/detritus pathway. Non-essential amino acid δ13C values showed quite different patterns from the essential amino acids, with evidence of biosynthesis and isotopic routing. The herbivorous damselfish, Stegastes nigricans, had an essential amino acid profile very similar to that of its farmed algae, Womersleyella setacea; however, there was a significant fractionation between diet and consumer for several non-essential amino acids associated with the glycolysis cycle. The high degree of biosynthesis of non-essential amino acids likely reflects that S. nigricans was feeding on a low protein, low quality diet. Conversely, the planktivorous damselfish, Amblyglyphidodon indicus, and the carnivorous snappers, L.kasmira and L. ehrenbergii, showed tight correlations between diet and consumer for essential and non-essential amino acids, suggesting a higher quality diet and enhanced isotopic routing of dietary proteins. Compound-specific stable isotope analyses represent a powerful new tool for tracing the origin of nutrients fuelling reef ecosystems.