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 ( Engraul...