Award: PLR-1316141

Intellectual Merit: Climate change and predator loss are reshaping global ecosystems, yet the combined impacts of these stressors are rarely considered. Thus, over the 3-year duration of this project, we studied how kelp forests of the Aleutian Archipelago (Alaska) are being altered by the combined influence of ocean warming, ocean acidification, and the loss of a keystone predator (the sea otter). We focused on impacts to Clathromorphum nereostratum, a long-lived (millennia in some cases) calcifying alga that forms the structural base of the ecosystem and contains an archive of past temperature, structural integrity, and grazing intensity in its calcified matrix. Using field observations, we revealed that with the recent collapse of sea otter populations and resulting proliferation of sea urchins in the ecosystem, long-livedClathromorphum reefs are now being rapidly eroded by urchin grazing. In much the same way that tree rings have been used to reconstruct longer term historical events in terrestrial ecosystems, we then used urchin grazing scars that are faithfully preserved in Clathromorphum?s calcified matrix (with annual resolution) toreconstruct past grazing intensity in the ecosystem. Our reconstructionsestablish that reef bioerosion over the past half-century was strongly linked to the wax and wane of sea otters and the trophic cascade that ensued. We then went on to show experimentally that ocean warming and acidification have recently accelerated the process of reef bioerosion, and will continue to do so in the near-future. Our findings thus indicate that without sea otter recovery, Clathromorphumreefs will become extinct sooner than predicted from climate change alone. Such findings highlight the importance of large predators in nature. They also indicate that scientists will benefit from incorporating predator-prey interactions into climate change studies, given that such interactions can strongly magnify or attenuate the physiological impacts of climate change in marine food webs. Broader Impacts:One postdoctoral researcher, three graduate students, and two undergraduate students received education experiences and technical training as part of this project. This group of trainees were comprised of ~65% females, with multiple individuals hailing from groups underrepresented in the sciences. To disseminate our findings, we have given numerous talks based on the results of our work, including public presentations (i.e., local science cafés and public forums), talks at scientific meetings (i.e., two Gordon Research Conferences and a Pew Charitable Trust meeting), departmental seminars at universities (University of Maine, University of California Santa Cruz, University of Michigan, Northeastern University, Harvard University) including non-traditional student universities (University of Massachusetts – Boston, Maine Maritime Academy), lectures to undergraduate classes (University of Maine, Colby College), and presentations to state and federal agencies (Maine Department of Marine Resources, U.S. State Department). To disseminate our findings more broadly, we collaborated with a documentary film maker (Compass Light Productions) to capture the essence of our work and make it available to the general public. We also helped publish an article through NSF?s Polar Program, and have worked closely with National Geographic Kids to disseminate our results in a cover story for the magazine (slated for release in August 2019). Several manuscripts (including one in review at Science) have been produced from this research. We anticipate their publication (and associated popular media articles) in 2019 and 2020. Last Modified: 01/29/2019 Submitted by: Robert S Steneck