While the immediate impacts of global climate change are of serious concern, the outcomes of these environmental changes for populations will ultimately play out over multiple generations. Despite this, our understanding of the evolutionary impacts of climate-related environmental change is still in its early stages, particularly in the marine realm. Furthermore, evolutionary processes can act over short, ecological timescales, such that they may play a key role in both the short-term resistance and long-term resilience of natural populations. Therefore, the objective of this dissertation is to better incorporate evolutionary processes into the developing understanding of the ecological effects of global change. In particular, I focus my investigations on the relationships between genetic diversity, local adaptation, and environmental change to better understand the evolutionary factors that contribute to a population’s adaptive capacity and resilience. A population’s genetic diversity may translate to response (phenotypic) diversity, the level of which will determine the likelihood of evolutionary rescue via the portfolio effect. Adaptation of subpopulations to their local conditions (local adaptation) has the potential to enhance broad-scale genetic diversity within a species, potentially increasing resilience in the face of environmental change, but can simultaneously reduce local diversity, increasing the risk of extinction for subpopulations if gene flow is low. In this dissertation, I use a pair of marine fish species with unique life-histories (Embiotoca jacksoni and Brachyistius frenatus; family Embiotocidae) to 1) test the molecular impact of environmental change on an important temperate fish group, 2) investigate the scale of genetic diversity and admixture along the Pacific coast of North America, 3) provide evidence of local adaptation among subpopulations, and 4) associate genomic differences between subpopulations with regional environmental differences to better understand the physiological pressures imposed by climate variables and form hypotheses for the genetic mechanisms that may underlie ongoing adaptation to climate change.

Natural and anthropogenic environmental changes are impacting marine species worldwide. However, our understanding of how changes to multiple environmental drivers impact the physiology and ecology of organisms is still largely unknown. Kelp forest ecosystems along the coast of California present a unique system to assess how environmental variability (both natural and human-induced) impacts key species of concern. In particular, these biodiverse ecosystems reside within the California Current System, which is characterized by dynamic oceanographic conditions that vary across latitude largely due to differences in the strength and intensity of coastal upwelling. Furthermore, environmental conditions are predicted to change especially rapidly in this region due to accelerated acidification, deoxygenation, and warming. In this study I first use ecologically and economically important grazer taxa to understanding how current and future environmental changes impact the physiology and ecology of marine organisms. Secondly, I conduct a synthesis of multiple driver experiments across ecosystems to assess the generality in interactive effects of warming and ocean acidification. In chapter one, I use insitu monitoring of pH, temperature, and dissolved oxygen conditions within a central California kelp forest to further understanding of the relationships between environmental conditions across seasons. I then use these environmental relationships to undertake a manipulative laboratory mesocosm experiment to assess how upwelling impacts the physiology and ecology of the gastropod, Promartynia pulligo, and the echinoderm, Mesocentrotus franciscancus. In chapter two, I expand monitoring of pH, temperature, and dissolved oxygen to northern and southern California kelp forests to better understand differences in the coupling of environmental drivers across regions that experience strong versus weak upwelling. I then conduct a laboratory mesocosm experiment to look for signs of local adaptation of red sea urchins, Mesocentrotus franciscanus, across regions and compare responses of M. franciscanus to region-specific coupled future changes in pH, temperature, and dissolved oxygen. In my final chapter, I zoom out to think more broadly about how multiple environmental drivers interact to alter species responses to warming and ocean acidification. I use a meta-analysis to calculate the frequency of interaction types across seven response variables and eight broad taxonomic groupings. I then assess the relationship between the magnitude of the predicted cumulative effect and the measured cumulative effect to determine when the magnitude of the effect drives the interaction. In this dissertation I show that kelp forest grazers responses to environmental changes vary across species and populations and that broad patterns in how species respond to multiple environmental drivers are likely to be less pronounced than predicted.

As the oceans absorb excess heat and CO2 from the atmosphere, marine primary producers face significant changes to their abiotic environments and their biotic interactions with other species. Understanding the bottom-up consequences of these effects on marine food webs is essential to informing adaptive management plans that can sustain ecosystem and cultural services. In response to this need, this dissertation provides an in-depth consideration of the effects of global change on foundational macroalgal (seaweed) species in a poorly studied, yet highly productive region of our world’s oceans. To explore how seaweeds within seasonally dynamic giant kelp forest ecosystems will respond to ocean warming and acidification, I employ a variety of methods: year-round environmental monitoring using an in situ sensor array, monthly subtidal community surveys, and a series of manipulative experiments. I find that a complementary phenology of macroalgal production currently characterizes these communities, providing complex habitat and a nutritionally diverse energy supply to support higher trophic levels throughout the year. I also find that future ocean warming and acidification will lead to substantial shifts in the phenology, quantity and quality of macroalgal production in these systems. My results suggest that the giant kelp Macrocystis pyrifera may be relatively resilient to the effects of global change in future winter and summer seasons at high latitudes. In contrast, the calcifying coralline algae Bossiella orbigniana and Crusticorallina spp. and the understory kelps Hedophyllum nigripes and Neoagarum fimbriatum will experience a suite of negative impacts, especially in future winter conditions. The resulting indirect effects on macroalgal-supported coastal food webs will be profound, with projected reductions in habitat and seasonal food supply on rocky reefs. Coming at a time of heightened interest in seaweed production potential at high latitudes, this dissertation provides a comprehensive evaluation of the future of these foundational organisms in a changing environment.