Factorial field manipulation reveals CO2 and temperature effects on a critical habitat-forming shellfish.

Ocean acidification and warming could have substantial negative impacts on marine organisms, particularly shell-building species. These environmental drivers may operate independently or interactively, amplifying or mitigating their impacts. Previous results have primarily come from lab studies, yet these climate drivers co-occur within naturally dynamic systems with high abiotic and biotic variability. Within intertidal habitats, the impacts of these drivers in situ remain poorly understood. We conducted a 6-month field manipulation to determine the effects of ocean acidification and warming on a habitat-forming shellfish, the Pacific blue mussel (Mytilus trossulus), in a dynamic intertidal system. Fourteen tide pools containing mussels were manipulated, including ambient (unmanipulated control), CO2 added, warmed, and combined CO2 added and warmed treatments. We measured mussel shell thickness, strength, and corrosion at 0, 3, and 6 months of exposure to treatment conditions. CO2 addition led to decreases in shell thickness and strength and increases in shell corrosion. However, we also detected increases in shell strength compared to controls for mussels exposed to both CO2 addition and warming. These findings indicate that ocean acidification negatively impacted shellfish overall, and the effects of acidification on shell strength might be mitigated under concurrent exposure to moderate warming, leading to an interactive effect of acidification and warming on this critical habitat-forming shellfish.