Consumer-driven nutrient fluxes are increasingly recognised as important pathways linking pelagic production to benthic ecosystem processes on coral reefs. Among these, the close association between planktivorous damselfishes and branching corals creates a unique biogeochemical pathway linking the recycling of pelagic nutrients to coral productivity. This pathway may become increasingly important as ocean warming alters nutrient dynamics on coral reefs. However, the supply of nutrients from damselfishes remains poorly documented, particularly under natural reef conditions and during periods of ocean warming. Furthermore, the pathway linking fish-derived nutrients to reef-building corals has yet to be directly demonstrated. To address these knowledge gaps, we quantified in situ nutrient supply by damselfishes during the 2025–26 Ningaloo marine heatwave and used stable isotope tracing to directly demonstrate nutrient transfer to corals.
We measured ammonium and phosphate excretion across natural size classes of Dascyllus reticulatus (n = 70) in the Ningaloo Marine Park. Despite their relatively small body size, damselfishes supplied substantial quantities of bioavailable nitrogen. Ammonium excretion scaled predictably with body mass (mass0.75), consistent with metabolic theory, providing a quantitative framework for predicting nutrient supply from fish size structure. Damselfish aggregations generated ecologically significant nitrogen hotspots that extended up to one metre from coral habitats while improving surrounding water nutrient stoichiometry. To directly demonstrate nutrient transfer from fishes to corals, we used stable isotope tracing to track nitrogen from damselfishes into corals. Corals rapidly assimilated labelled ammonium into both host tissue and symbiotic algae.
These findings represent the first direct demonstration and quantification of the biogeochemical pathway linking fish-mediated nutrient recycling to reef-building corals. Combined with benchmark estimates of nutrient excretion under marine heatwave conditions, this work provides a quantitative framework for evaluating the contribution of fish-mediated nutrient fluxes to coral reef ecosystem functioning under future ocean warming.