It has been suggested that the incorporation of strontium into biogenic aragonite may be influenced by the degree of carbonate saturation in aquatic environments. We measured the Sr/Ca ratios of river water and the aragonitic shells of freshwater bivalves (Margaritifera margaritifera) from two sites with different carbonate saturation states in the River Ehen, Cumbria, UK. Shell Sr/Ca and river water Sr/Ca are 0.1 mmol/mol and 0.42 mmol/mol lower, respectively, at the high carbonate saturation site. However, the distribution coefficients of strontium into aragonite (KD) are the same (~0.28) at both sites. These analyses show that Sr uptake into aragonite bivalve shells is not affected by the degree of carbonate saturation of the water, perhaps reflecting a strong biological control on trace element incorporation.