This data set presents data from further 1.5 month-long laboratory experiments conducted at GEOMAR in Kiel, Germany where calcium ion concentration in seawater was manipulated to investigate how seawater calcium availability controls calcification in Baltic Sea Mytilus edulis trossulus mussels. Calcification rates (mass of calcium carbonate produced per unit time) was measured in 4 replicate tanks across 3 salinities and 5 calcium ion concentrations to estimate the concentration at which calcium ions start to limit calcification in the Baltic Sea at low salinities.