Upwelling enhances planktivory in a benthic-feeding omnivorous damselfish on subtropical reefs

Reef ecosystems are fueled by both benthic and pelagic pathways, yet how the contribution of these sources varies across spatial and temporal scales remains a central question in reef ecology. Most research has focused on tropical coral reefs, whereas subtropical systems, defined by stronger oceanographic variability, have received far less attention. Coastal upwelling amplifies this variability and delivers nutrient-rich pelagic subsidies that may reshape food web dynamics, although their effects on reef trophic interactions remain largely unknown. Here, we investigated the influence of upwelling on multiple dimensions of the trophic niche by increasing plankton subsidies to the benthic-feeding omnivorous damselfish Stegastes pictus , integrating foraging behavior, diet composition, and stable isotope analysis in subtropical reefs of the Southwestern Atlantic, under contrasting non-upwelling and upwelling conditions. Although benthic feeding prevailed in both conditions, plankton feeding increased markedly during upwelling, with bite rates nearly tripling and dietary contribution rising from 0.1 to almost 20%. Stable isotope data and Bayesian mixing models indicated a consistent contribution of both benthic and pelagic sources across periods, reflecting longer-term pelagic integration. These findings reveal that upwelling creates transient windows of trophic opportunity, promoting pelagic food resource use and trophic niche expansion in benthic-feeding omnivorous fishes such as S. pictus . Environmental variability in subtropical systems thus provides high-quality pelagic resources that support generalist species with flexible diets, enhancing trophic connectivity, resilience, and the overall stability of subtropical reef food webs.