Abstract:
Coral reef productivity depends on tightly coupled nutrient transfers between organisms. Fish make up a major pool of biomass in reef ecosystems, and thus play key roles in regulating nutrient storage, movement and transformation. However, the bioavailability of fish-derived waste as a nutrient source to the benthos remains poorly quantified. In this ongoing study, we use a laboratory-based stable isotope experiment on Heron Island (Australia) to investigate how coral-feeding fish mediate the redistribution of nutrients to reef-building corals and calcareous macroalgae. In a three-phase design, Acropora aspera fragments are first enriched with 13C and 15N, then fed to the butterflyfish Chaetodon melannotus, and the resulting waste products introduced to unenriched A. aspera fragments and Halimeda thalli to evaluate nutrient uptake. Isotope enrichment and elemental ratios are measured across different phases, and isotopic signatures are expected to reflect nutrient transfer from A. aspera to C. melannotus (via ingestion) and from C. melannotus to A. aspera and Halimeda algae (via excretion and egestion). Based on previous studies we anticipate isotope enrichment to vary by element (N > C), reflecting nitrogen’s limited availability on reefs, and among taxa and compartments (symbiont > coral host > macroalgae), due to differences in photosynthetic capacity and nutrient uptake rates. We also anticipate a decline in isotope enrichment across experimental phases, driven by incomplete transfer efficiency, partial assimilation at each stage, and losses through respiration. These patterns will be used to assess the extent to which corallivore ingestion, excretion and egestion support the metabolic requirements of reef-building corals versus competitive macroalgae, thus contributing to reef productivity and accretion. This study is expected to improve future models of reef nutrient dynamics and inform our understanding of ecosystem functioning under changing environmental conditions.