Nutrient and light availability control food webs and fish larvae in the Peruvian upwelling system

Abstract Upwelling ecosystems sustain some of the world's most productive fisheries by supplying deep‐water nutrients to the sunlit ocean surface. Yet, how spatiotemporal variability in nutrient‐upwelling and light availability regulates trophic transfer to fish remains poorly understood. We investigated these dynamics in a 36‐d mesocosm experiment off Peru, using plankton food webs with fish larvae—including Peruvian anchovy ( Engraulis ringens )—as top consumers. Nutrient‐upwelling enhanced fish larval growth and survival by stimulating primary production. However, despite shortening food webs and promoting diatom‐based pathways, it did not improve food web efficiency from phytoplankton via zooplankton to fish. Surprisingly, fish also benefited from reduced light availability, even though it lowered primary production and lengthened food webs. This counterintuitive result was driven by higher food web efficiency under light limitation, likely mediated by a more nutritious food web base that boosted zooplankton prey. These effects were substantial: upwelling increased fish biomass by up to six‐fold, while light limitation tripled food web efficiency. Our study reveals how upwelling enhances the quantity and light limitation the quality of bottom‐up forcing—shaping fish recruitment success. Notably, food web efficiency was not dictated by food web length but by the match between resource availability and consumer needs. This allowed more complex food webs to outperform simpler ones. As climate change continues to alter physicochemical drivers in upwelling ecosystems, understanding these shifts in trophic transfer is key to adaptive fisheries management.