Characterization of Advantageous Traits Across Heterogeneous Environments and Their Implications for Mussel Persistence in a Changing Ocean

Anthropogenic stressors have intensified the need to better understand linked cellular, evolutionary, and ecosystem processes to predict how systems will respond to climate induced changes. Although theoretical and empirical evidence demonstrates that ecological systems have the capacity to respond to climate change, the mechanisms and tempo of these responses are less understood, particularly in natural environments. Understanding how species respond to environmental stress is especially important in marine systems, where the combined effects of increasing sea surface temperatures and ocean acidification are altering organismal physiology, reshaping patterns of selection, and driving changes in community structure at an alarming rate. To address this, my thesis investigates adaptive responses across biological scales using the foundational intertidal bivalve Mytilus californianus as a study system. Coastal marine systems are characterized by pronounced and dynamic environmental variability, where strong diel cycles in photosynthesis and respiration drive fluctuations in seawater chemistry. For example, in coastal habitats pH can range almost a single unit (an order of magnitude), from 'normal' pH values of ~8.1 during the day to low values of 7.3 during darkness. Spatial gradients can further magnify these fluctuations as individuals positioned higher on the intertidal experience greater environmental stress due to longer emergence times during low tidal cycles. Beyond local-scale heterogeneity, environmental variation across broader geographic scales also plays a critical role in shaping the genetic backgrounds of individuals, directly influencing their capacity to respond to future environmental change. Despite growing recognition of these multiscale stressors, the extent to which plastic and evolutionary responses mitigate their effects remain unclear. Accordingly, this thesis addresses three primary questions: (i) how fine-scale environmental gradients shape phenotypic plasticity and local adaptation within populations; (ii) how spatial environmental heterogeneity and gene flow interact to structure genetic variation and influence adaptive potential across species ranges; and (iii) how organismal and evolutionary processes scale up to influence community stability under long-term environmental change. Together, this work advances our understanding of how adaptive responses emerge across natural populations distributed throughout heterogeneous environments and provides a mechanistic framework for predicting the resilience of marine organisms and communities to ongoing climate changes.