Anthropogenic activities, such as the burning of fossil fuels, interact with the chemistry of the ocean surface and cause ocean acidification (OA). The current trend and projections for OA are well defined for open ocean waters, although less is known about its effects on nearshore ecosystems, where most of the affected organisms reside. In this work, we analyzed almost a decade of carbonate chemistry data for two reef time series at Cheeca Rocks (CR), Florida, and La Parguera (LP), Puerto Rico, to assess the physical and biological processes that dictate carbonate seasonal variability. The results showed that temperature is the dominant driver of seawater CO₂ changes at LP, while at CR, biological processes have major effects on CO₂ chemistry. Continued observations will advance future projections of OA in nearshore areas and provide coastal managers with appropriate tools to monitor OA.