Coastal marine organisms experience chemical variability in their external environment, which has direct consequences for the growth of individuals. Of notable import are human-derived perturbations to the carbonate system, resulting in reductions in seawater pH (termed 'ocean acidification') and changes coinciding with decreased salinity due to freshwater inputs. In particular, step-like shifts in salinity can disrupt seawater total alkalinity in estuarine and coastal habitats, persisting from hours to weeks, whereas ocean acidification does not alter total alkalinity. Both low alkalinity and reduced salinity, alongside low pH from ocean acidification, pose potential challenges to the growth and survival of shell-building species. In contrast, rivers that elevate coastal seawater alkalinity may offer intermittent periods of chemical refuge, provided osmotic stress from reduced salinity remains tolerable. Changes of this nature have significant import for ecologically and economically valued species that are unable to flee from suboptimal conditions. For example, along many coastlines, mussels and oysters form expansive habitats for numerous other species. In addition, they are harvested recreationally and commercially grown for human consumption in many coastal communities. Understanding potential consequences of perturbed seawater conditions on these bivalves is crucial, given the profound connection between their growth and both ecological and economic performance. Biological responses and abiotic shell dissolution jointly contribute to the overall effect, necessitating focused attention to both, over variable exposure durations. This dissertation investigates how three coastal bivalve species respond to varying levels of ocean acidification and freshwater inputs, focusing on characteristics likely to influence their response to altered conditions, in addition to investigating how responses change as a function of time. Specifically, Chapter 2 explores the potential role of the periostracum in protecting the shell of a coastal mussel species, revealing that mussels with higher periostracum coverage experience greater protection from low pH shell dissolution. Moreover, mussels from shady and low tide environments in the field exhibited higher periostracum cover, suggesting natural agents of removal may vary across microhabitats. In Chapter 3, I examine the effect of altered seawater conditions, specifically simulating those attributed to freshwater influence, on physiological responses in an open-coast and estuarine congener mussel species. Results indicate that the consequences of low salinity on shell-building rates are lessened with high alkalinity, in ranges of salinity where oxygen consumption was unimpacted, while only salinity affected oxygen consumption. Individual responses were influenced by body condition, emphasizing the importance of considering multiple performance responses when assessing how mussels and coastal species respond to chemical changes in seawater. Chapter 4 expands from mussels to estuarine oysters, exploring shell growth responses to altered seawater carbonate conditions and salinity over time. Distinct growth patterns, in response to total alkalinity, were exhibited during different exposure windows. However, total alkalinity had no overall effect on growth. Oyster shell thickness and condition index remain uninhibited by the experiment's end, highlighting the need to distinguish growth variability due to biological cycles from the effects of altered conditions. As oceans continue to change, comprehending organism responses to these changes becomes increasingly critical.