Reef fish functional trait responses to wastewater-driven environmental gradients

Wastewater pollution is a persistent but understudied driver of coral reef degradation. Here, we investigate how reef fish assemblages respond to environmental gradients shaped by chronic wastewater exposure, combining trait-based analyses with long-term environmental data. We surveyed reef fish at seven sites along the leeward coast of San Andrés Island (Colombian Caribbean), quantifying variation in species richness, abundance, and eight functional traits. Environmental conditions were characterized using 13 years of water quality data, complemented by in situ measurements of benthic cover and algal nitrogen stable isotopes (δ¹⁵N). We calculated functional indices such as Functional Richness (FRic) and assessed trait-environment relationships. Impacted sites exhibited lower species richness and reduced FRic, driven by the absence of species occupying peripheral regions of functional space, including very mobile predators, demersal spawners, and small-bodied herbivores. Shifts in trait composition were explained by the combined influence of water quality variables and benthic composition (e.g. hard coral cover) along the pollution gradient. By demonstrating that wastewater exposure selectively erodes functional trait combinations underpinning key ecosystem processes, such as nutrient cycling, top-down control, and coral-fish mutualisms, this study highlights the importance of integrating functional metrics into reef monitoring and wastewater management strategies.