Fish represent a major biomass pool on coral reefs and play key roles in nutrient storage, movement and transformation. However, the bioavailability of fish-derived waste as a nutrient source to the benthos remains poorly quantified. In this study, we use a stable isotope experiment on Heron Island (Australia) to investigate how coral-feeding fish mediate nutrient transfers to corals and calcareous macroalgae. In a three-phase design, Acropora aspera fragments are enriched with 13C and 15N, fed to the butterflyfish Chaetodon melannotus, and the resulting waste products introduced to unenriched A. aspera fragments and Halimeda thalli. Isotopic and elemental analyses assess nutrient uptake and redistribution across 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 (via excretion and egestion). Furthermore, we predict isotope enrichment to vary by element, among taxa and compartments, and across experimental phases. These patterns will help assess how corallivore ingestion, excretion and egestion support the metabolic requirements of reef-building corals versus competitive macroalgae, thus contributing to reef productivity. The study aims to improve models of reef nutrient dynamics and inform our understanding of ecosystem functioning under changing environmental conditions.
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