Ocean acidification (OA) will decrease shellfish growth and survival, with ecological and economic consequences for fisheries and aquaculture. However, the high variability of results among experiments, and the lack of long-term studies, make it difficult to predict the effect that OA will have on bivalve species. We tested the long-term effect of high CO2 on growth, calcification rates, and survival of juveniles of the commercial bivalve species Chamelea gallina from Southern Portugal. The local high alkalinity of seawater probably buffered the negative effect of the pH drop, and after 75 days juveniles increased their growth and calcification rates with CO2. However, after 217 days, the situation reversed, bivalves under control conditions had the highest growth and calcification rates, while individuals under high CO2 presented negative calcification rates. The biometric variable that responded first was the width of the individuals, followed by the height and length of the shells. Survival was unaffected except for a mortality peak of juveniles under control and intermediate conditions as a consequence of a temperature drop. In the short term, C. gallina will increase their calcification rates to compensate for OA. However, in the long term, the additional energy expended will be translated into growth losses with negative repercussions for the fisheries and aquaculture. The cultivation of shellfish on high alkaline seawater should be further explored as a bioremediation measure to mitigate the negative effect of OA on shellfish aquaculture.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2021) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2021-10-20.