Award: OCE-1737071

A well-known pattern in coastal marine systems is a positive association between the organic material, or biomass, of primary producers and the occurrence or intensity of upwelling. This pattern is assumed to be caused by the increase in nutrient concentration associated with upwelling, enabling higher primary production and thus greater standing seaweed biomass. However, upwelling also causes large, rapid declines in seawater temperature. Because the metabolism of fish and invertebrate herbivores is temperature-dependent, cooler upwelled water could reduce the metabolism of this consumers and their feeding intensity. This could in turn lead to increased standing seaweed biomass. Therefore, upwelling could influence both bottom-up and top-down control of populations and communities of primary producers. The purpose of this study is to test the hypothesis that feeding intensity and seaweed biomass is, in part, regulated by temperature via the temperature-dependence of metabolism. More broadly, we investigated the role of temperature in regulating the composition and functioning of Galápagos’ unique marine ecosystem. We took advantage of the strong spatiotemporal variance in water temperature in the Galápagos Islands to compare primary production and top-down control (both herbivory and carnivory) across a natural temperature gradient. We combined field monitoring, statistical modeling, predation assays, populations-specific metabolic measurements, and in situ consumer exclusion and nutrient addition treatments to measure the effects of temperature on patterns and processes in shallow subtidal communities. We also tested the hypothesis that herbivores populations at warmer sites and/or during warmer seasons are less thermally sensitive, potentially due to acclimatization or adaptation mechanisms. Finally, we performed a series of laboratory-based experiments to measure the effect of near-future temperatures on herbivores, seaweed, and herbivory. Primary findings of the overall investigation include: 1) Sea urchins, key herbivores of the Galápagos marine system, are adapted to the local thermal conditions of the environment in which they live. Urchin populations from warmer sites and/or during warmer seasons are less thermally sensitive (i.e., potentially due to acclimatization or adaptation). 2) Consumption by herbivores (especially by pencil sea urchins) has a strong top-down effect on the seaweed community (regardless of other environmental condition). This is somewhat surprising because in a highly productive "upwelling system", like the Galápagos, many scientists believe that oceanographic processes are the most important drivers. 3) We found that temperature also influences carnivory by common benthic predators including whelks and sea stars of habitat-forming prey (barnacles). 4) Higher temperatures and anthropogenic warming could alter species composition by reducing the abundance and productivity of benthic macroalgae. Project outcomes and broader impacts include the publication of 25 articles in peer-reviewed scientific journals, the taxonomic description of the seaweeds of the Galapagos, the training of dozens of students, and materials explaining the work to the general public including this video originally published in the Washington Post: https://youtu.be/BVBcKgbEtnk Last Modified: 01/12/2023 Submitted by: John F Bruno