Impacts of Ocean Acidification and Elevated Alkalinity on the Growth and Survival of Juvenile Oysters

Ocean acidification (OA), driven by rising atmospheric CO₂ levels, threatens bivalve mollusks as their calcium carbonate shells are vulnerable to dissolution under acidic conditions. This study investigated the impacts of total alkalinity (TA) on the growth of juvenile Pacific oysters (Magallana gigas) and European flat oysters (Ostrea edulis), as well as the combined effects of OA and elevated TA on the growth and survival of juvenile M. gigas. Two 54-day experiments were conducted at the Estação Experimental de Moluscicultura de Tavira (EEMT) in Southern Portugal, a site with naturally elevated TA (~3200 μmol.kg^-1). The first experiment demonstrated that both oyster species showed higher growth under high TA conditions than normal TA (~2300 μmol.kg^-1). The second experiment, focusing on M. gigas, revealed increased shell length and height under elevated CO₂ and TA, while shell width growth was greater under control CO₂ conditions. These findings suggest that the naturally high TA at EEMT can buffer the negative impacts of elevated CO₂ on oyster growth, highlighting the importance of local seawater chemistry in bivalve responses to OA. The results also indicate that manipulating TA levels in aquaculture may improve shellfish resilience and productivity in the face of ocean acidification.