Brood Chamber Carbonate Chemistry in the oyster Ostrea edulis Is Dynamically Shaped by Warming, Acidification, and Ventilation

Brooding organisms can modify the chemical environment experienced by developing offspring, potentially altering their exposure to climate change. In marine systems, however, the carbonate chemistry of brood chambers, and the mechanisms governing it, remain poorly resolved. Here, we combine high‐resolution pH and CO 2 measurements of the empty brood chamber (i.e., mantle cavity above gills) in the European flat oyster ( Ostrea edulis ) while also tracking valve gape to quantify how warming, ocean acidification, and ventilation interact to shape the internal environment experienced by larvae during incubation. Brood chamber pH was consistently lower than that of the surrounding seawater and responded to external conditions in a state‐dependent manner governed by ventilation and respiration. When oysters were actively ventilating, brood chamber chemistry tracked ambient seawater with a persistent offset, whereas valve closure led to rapid CO 2 accumulation and pronounced declines in pH. These dynamics generated highly variable and behaviorally mediated exposure regimes, in which metabolic processes distorted ambient chemical signals over short timescales. Paired pH and CO 2 measurements provided preliminary constraints on aragonite saturation state (Ωar), revealing that brood chambers can remain undersaturated (Ωar < 1) for prolonged periods even in ventilating oysters and while overlying seawater remains favorable. Together, these results demonstrate that brood chambers function as dynamic microenvironments in which larval exposure to ocean acidification is governed not only by external conditions, but also by behavior. Because warming enhances metabolic CO 2 production and intensifies acidification within the chamber, future climate change is likely to amplify both the magnitude and variability of larval exposure. These findings help explain the apparent resilience of brooding species to ocean acidification, while also highlighting potential limits as environmental conditions move beyond historical bounds.