Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Varying reproductive success under ocean warming and acidification across giant kelp (Macrocystis pyrifera) populations Jordan A. Hollarsmith a,b, ⁎ , Alejandro H. Buschmann c , Carolina Camus c , Edwin D. Grosholz a,b a University of California Bodega Marine Laboratory, 2099 Westshore Rd., Bodega Bay, CA 94923, USA b Department of Environmental Science and Policy, University of California, Davis, CA 95616, USA c Centro i~mar & CeBiB, Universidad de Los Lagos, Camino Chinquihue Km 6, Puerto Montt, Chile ARTICLE INFO Keywords: Giant kelp Global warming Ocean acidification Local adaptation Reproduction Gametophyte ABSTRACT Understanding how climate change may influence ecosystems depends substantially on its effects on foundation species, such as the ecologically important giant kelp (Macrocystis pyrifera). Despite its broad distribution along strong temperature and pH gradients and strong barriers to dispersal, the potential for local adaptation to cli- mate change variables among kelp populations remains poorly understood. We assessed this potential by ex- posing giant kelp early life stages from genetically disparate populations in Chile and California to current and projected temperature and pH levels in common garden experiments. We observed high resistance at the haploid life stage to elevated temperatures with developmental failure appearing at the egg and sporophyte production stages among Chilean and high-latitude California populations, suggesting a greater vulnerability to climate- or ENSO-driven warming events. Additionally, populations that experience low pH events via strong upwelling, internal waves, or estuarine processes, produced more eggs per female under experimental low-pH conditions, which could increase fertilization success. These results enhance our ability to predict population extinctions and ecosystem range shifts under projected declines in ocean pH and increases in ocean temperature. 1. Introduction Understanding the resistance and resilience of ecological commu- nities to anthropogenic global change is among the most pressing questions in ecology today (Buckley and Kingsolver, 2012; Chapin III et al., 2000). Adequately predicting whether a species or population will adapt, migrate, or go extinct in the face of changing conditions is complicated by several factors: the degree of change is not uniform across land- and seascapes; species currently exist within and are adapted to a mosaic of environmental conditions; and connectivity among populations may shift with changing biogeographic barriers (Dawson et al., 2011; González et al., 2013; Vargas et al., 2017). In- creasing observations of shifting species ranges and local extirpations highlight the urgency of understanding the pre-existing mosaic of local adaption (Chen et al., 2011; Harley et al., 2006). This urgency is especially acute in species that form habitat via trophic support and structure, known as foundation species (sensu Dayton, 1972). The loss of foundation species can have cascading effects across ecological communities and can result in complete phase changes from one eco- system type to another (Ellison et al., 2005; Hughes, 1994; Ling et al., 2015). Therefore, while all species in a community are impacted by changing environmental conditions, understanding the effects of change on foundation species and how these effects differ across a species' range is pivotal to understanding the resistance of entire com- munities to environmental change (Sunday et al., 2017). Climate change and anthropogenic emissions present unique stres- sors in nearshore marine systems. Since the onset of industrialization, the ocean has absorbed approximately one-third of anthropogenic carbon emitted into the atmosphere, resulting in an average ~0.1 unit decrease in ocean pH (Sabine et al., 2004). Concurrently, the heat content of the ocean has increased with the greatest change occurring in the past 30 years (Cheng et al., 2017). Importantly for marine species, these changes have not been uniform across the global seascape. Nearshore upwelling systems, where wind drives the advection of deep water onto the continental shelf, are experiencing decreases in pH more rapidly than any other marine ecosystem (Feely et al., 2008). While upwelled waters are naturally low in pH due to the accumulation of respiratory carbon below the photic zone, the uptake of anthropogenic CO 2 is already resulting in sea surface water chemistry that is corrosive to shell-building organisms in regions of intense upwelling (Feely et al., https://doi.org/10.1016/j.jembe.2019.151247 Received 22 May 2019; Received in revised form 7 October 2019; Accepted 7 October 2019 ⁎ Corresponding author at: 2099 Westshore Rd., Bodega Bay, CA 94923, USA. E-mail addresses: jahollarsmith@ucdavis.edu (J.A. Hollarsmith), abuschma@ulagos.cl (A.H. Buschmann), carolina.camus@ulagos.cl (C. Camus), tedgrosholz@ucdavis.edu (E.D. Grosholz). Journal of Experimental Marine Biology and Ecology 522 (2020) 151247 0022-0981/ © 2019 Elsevier B.V. All rights reserved. T