Upwelling modulates seaweed biomolecule contents, altering seaweed-consumer interactions in a southwestern pacific coastal system.

Eastern Boundary Upwelling Systems (EBUS) are among the most productive marine ecosystems, largely because of the upwelling of cold, nutrient-rich seawater into coastal areas. However, the influence of upwelling on biomolecule concentrations in seaweed tissues, and how these changes affect herbivore feeding behavior, remains poorly understood. We addressed these questions by hypothesizing that physicochemical conditions associated with upwelling increase the concentration of essential biomolecules in seaweed tissues, indirectly modulating herbivore behavior. We first characterized multiple upwelling (UW) and downwelling (DW) sites and used UW-DW field transplants to examine variation in biomolecule concentrations and growth of the widespread southwestern Pacific seaweed Lessonia spicata. We then assessed how seaweed origin influenced the feeding behavior of two common sea urchins, Loxechinus albus and Tetrapygus niger. Seaweeds from UW sites contained higher protein and carbohydrate concentrations than those from DW sites, consistent with higher nutritional quality under natural UW conditions. However, after prolonged exposure to a common DW environment, biomolecule concentrations of UW-origin seaweeds converged toward those of DW-origin individuals. Blade length increased throughout the transplant experiment, but growth rate over the complete experimental period did not differ between origins. Seaweeds from UW sites were consumed at higher rates than those from DW sites, while the feeding responses of both sea urchin species depended on seaweed origin, consumer origin, and species identity. Together, these results indicate that upwelling-associated environmental conditions influence seaweed nutritional traits and consumer-seaweed interactions, while the convergence observed after transplantation highlights the plasticity of these traits under changing local conditions. These findings have implications for predicting ecological responses to future changes in coastal upwelling regimes.