Anthropogenic carbon emissions are predicted to alter marine ecosystems. One such change is the decline in ocean pH, known as ocean acidification. Model predictions of ocean acidification have guided biological experiments for more than a decade. Many studies predict negative consequences of future ocean pH on marine species. To understand how species will respond to future conditions, however, knowledge of present-day pH exposures is necessary and often limited. In this dissertation, I described present-day pH variability in three coastal regions and used the data to design laboratory experiments assessing the physiological response of two organisms, sea urchins and mussels, to changing ocean conditions. As recorded by autonomous pH sensors, I found three unique patterns of coastal pH variability. Near-shore Antarctica was characterized by a steep seasonal increase in pH and pH variability during summer phytoplankton blooms.

The northern Channel Islands, California, exhibited event-scale and diurnal pH variability due to primary production of phytoplankton and fixed vegetation. Only mild effects from upwelling were detected at the islands, suggesting that this region may become a spatial refuge from extreme low pH in the future. Finally, Oregon was characterized by event-scale decreases in pH due to periodic upwelling events. The results from this research show that many coastal species experience short-term changes in pH that are within the same magnitude of change predicted for ocean acidification by the end of the century. While such present-day exposures to pH variability may promote tolerance of future pH change, these near-shore regions are also characterized by unique patterns of thermal stress. I conducted two studies to investigate the interactive effects of pH and temperature on organismal physiology.

First, Antarctic sea urchin, Sterechinus neumayeri, early developmental stages (EDSs) currently experience < 2 &deg;C seasonal warming and may only experience a few degrees of ocean warming over the next 100 years. Despite development under pH and temperatures outside of current exposures, S. neumayeri EDSs exhibited high tolerance of a one-hour heat stress test, suggesting this species may be more resilient to ocean change than previously thought. Second, unlike Antarctic species, intertidal species at mid-latitudes experience daily temperature fluctuations that can exceed four times end-century predictions of ocean warming, due to tidal cycles. In Oregon, upwelling events enhance this temperature range by periodically delivering cold, low pH water to the intertidal zone.

Depending on sea water conditions simulating an upwelling event (cold, low pH) or wind relaxation (warm, high pH), the intertidal mussel Mytilus californianus generated different transcriptomic signatures of the cellular heat shock response, following exposure to aerial heat stress. This suggests that future changes in seawater conditions may alter the heat stress tolerance of M. californianus during low tides. The results from this dissertation highlight the importance of designing experiments that reflect species' present-day and future multi-stressor environment, in order to generate ecologically relevant conclusions. As anthropogenic stressors continue to take hold of coastal seas, understanding the biological consequences is critical for management and conservation efforts.