Reef fish play central roles in nutrient cycling on coral reefs through feeding, digestion, and waste expulsion, yet few studies have traced the complete pathway of nutrient movement across these stages. Therefore, the quantity, form, and fate of fish-derived nutrients remain poorly quantified, particularly regarding their bioavailability to benthic organisms and contribution to reef productivity. Here, we used a labeled isotope experiment at Heron Island (Australia) to examine how coral-feeding fish mediate nutrient transfer to reef-building corals and calcareous macroalgae. In a three-phase design, Acropora aspera fragments were 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. Corals rapidly assimilated both isotopes during the pulse, and enrichment was detectable in fish stomachs post-feeding. However, no enrichment was observed in fish tissue, faeces, or recipient corals and macroalgae. This likely reflects dilution of isotopic signals by previously ingested reef material, low and variable feeding rates, and gut retention times exceeding experiment duration. Nonetheless, early-phase success highlights the potential of isotope tracing to capture nutrient transfers across multi-step trophic pathways. This study reveals methodological challenges and offers recommendations for future efforts to quantify fish-mediated nutrient fluxes.
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