The present medium term (21 d) study was performed to evaluate the effects of HCl or CO2-induced acidified seawater (pH 7.7, 7.1 or 6.5; control: pH 8.1) on the physiological responses of the blue mussel, Mytilus edulis, at different levels of biological organization. The results demonstrate that: (1) either HCl or CO2 enrichment had significant impacts on physiological changes in M. edulis: the mortality increased while condition index (CI) decreased steadily as the pH decreased, those indexes indicate the metabolic activities (e.g. filtering rate, oxygen consumption rate, etc.) underwent similar changes; moreover, the decrease of calcification rate and carbonic anhydrase activity indicate that the carbon sink ability of the mussels was significantly affected. We hypothesize that acidification induced intracellular energy crisis and a decrease in enzyme activities could be a potential explanation for our findings. (2) Comparatively, CO2 enrichment had more severe effects on mortality but caused less stress to the metabolic and carbon sink indexes than HCl adjustment at the same pH level. Apoptosis caused by the 'intracellular acidification' in the CO2 group and difference in cytoplasmic Ca(2+) concentration between two groups are suggested to be responsible for these results. (3) An integrated biomarker response (IBR) was set up on the basis of the estimated indexes; it was determined that the IBR decreased steadily with the decrease of pH, and a positive relationship was observed between them, inferring that the IBR might be a potential biological monitoring method in evaluating the effects of seawater acidification.