Ervilia castanea (Mollusca, Bivalvia) populations adversely affected at CO2 seeps in the North Atlantic

Highlights: • The bivalve Ervilia castanea was studied at volcanic CO₂ seeps and reference sites. • Abundance, size and net-calcification were inversely related to CO₂ levels. • Large individuals were scarce or absent at high CO₂ sites. • Recruitment of this bivalve was highest at the CO₂ seeps. • Abundance and size of E. castanea were positively correlated with Chl-a in sediment. Sites with naturally high CO₂ conditions provide unique opportunities to forecast the vulnerability of coastal ecosystems to ocean acidification, by studying the biological responses and potential adaptations to this increased environmental variability. In this study, we investigated the bivalve Ervilia castanea in coastal sandy sediments at reference sites and at volcanic CO₂ seeps off the Azores, where the pH of bottom waters ranged from average oceanic levels of 8.2, along gradients, down to 6.81, in carbonated seawater at the seeps. The bivalve population structure changed markedly at the seeps. Large individuals became less abundant as seawater CO₂ levels rose and were completely absent from the most acidified sites. In contrast, small bivalves were most abundant at the CO₂ seeps. We propose that larvae can settle and initially live in high abundances under elevated CO₂ levels, but that high rates of post-settlement dispersal and/or mortality occur. Ervilia castanea were susceptible to elevated CO₂ levels and these effects were consistently associated with lower food supplies. This raises concerns about the effects of ocean acidification on the brood stock of this species and other bivalve molluscs with similar life history traits.