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The carbonate chemistry of seawater from the Ria Formosa lagoon was experimentally manipulated, by diffusing pure CO₂, to attain two reduced pH levels, by −0.3 and −0.6 pH units, relative to unmanipulated seawater. After 84days of exposure, no differences were detected in terms of growth (somatic or shell) or mortality of juvenile mussels Mytilus galloprovincialis. The naturally elevated total alkalinity of the seawater (≈3550μmolkg⁻¹) prevented under-saturation of CaCO₃, even under pCO₂ values exceeding 4000μatm, attenuating the detrimental effects on the carbonate supply-side. Even so, variations in shell weight showed that net calcification was reduced under elevated CO₂ and reduced pH, although the magnitude and significance of this effect varied among size-classes. Most of the loss of shell material probably occurred as post-deposition dissolution in the internal aragonitic nacre layer. Our results …