Over centuries, shellfish populations have directly and indirectly benefitted humans living in coastal communities by providing fisheries and ecosystem services. The naturally dynamic estuarine environment, home to many economically important shellfish populations is, however, also commonly subjected to anthropogenic pressure from exploitation, pollution, and the acceleration of climate change. Climate change alters the rate and direction of long-term biogeochemical change in the ocean, but also, in combination with large-scale climate oscillations and other factors, can modulate the frequency, persistence, and/or magnitude of extreme coastal events including estuarine heatwaves, coastal hypoxia, and coastal acidification. This chapter explores the dynamic variability of the estuarine environment and assesses the impacts of climate stressors in isolation and in combination with other climatic/anthropogenic stressors on estuarine shellfish species. Individually, warming temperatures can alter the rates of physiological processes and can result in changes in growth and reproduction, while extremes in temperature can elicit physiological stress, mortality, or even local extinctions. Range contractions or expansions resulting from shifts in temperature or salinity can have cascading effects on ecosystem functioning, as important functional roles associated with shellfish (i.e., suspension-feeding, habitat engineering, bioturbation, predation) are gained or lost. Since nearly all shellfish species produce calcified structures exposed to the external environment, increasing CO2 concentrations and extremes in CO2 can have negative consequences on calcification that may vary by life stage and may have fitness-related consequences. Low oxygen extremes, which may become more persistent or severe under warming temperatures, consistently yield negative effects on the growth, development, metabolism, reproduction, survival, and/or abundance of mollusks and crustaceans and, thus, can have disproportionate impacts on ecosystem functioning.

Estuaries commonly host co-occurring extremes (e.g., hypoxia and acidification), forcing organisms to cope with multiple stressors. Multi-stressors, an emerging field of research, can have a range of additive, synergistic, and antagonistic effects on shellfish species, with additional stressors typically yielding more negative outcomes than single stressors. Still, there are many unknowns regarding the potential effects of climate change syndromes on coastal shellfish, particularly in dynamic estuarine environments, and examinations of the combined impacts of warming/hypoxia/acidification and/or harmful algal blooms have only just begun. Autonomous observing platforms and high-frequency sensor arrays are essential to generating long-term and fine-scale time series datasets to characterize the shifting biogeochemical patterns under climate change. It will also be critical to scale up physiological studies to assess impacts on populations, communities, and ecosystems. Finally, to protect and/or restore shellfish resources, continued collaboration between communities and researchers on adaptive strategies that mitigate harm to shellfish populations experiencing extremes in future change will be vital.