Biological and water chemistry controls on Sr/Ca, Ba/Ca, Mg/Ca and δ18O profiles in freshwater pearl mussel Hyriopsis sp.

We investigated oxygen isotope and Sr/Ca, Ba/Ca and Mg/Ca ratios in shells of a commercially cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) and ambient water. The first-order fluctuations of oxygen isotope ratios of the outer shell layer along the maximum growth axis recorded the musselsʹ annual growth from 2002 to 2007. Clear annual Sr/Ca cycles suggest that Sr was incorporated into the shells at a high rate during warm seasons. All three examined shells showed a clear decrease in annual average Sr/Ca with age. Inter- and intra-annual trends suggest that Sr incorporation was higher during periods of faster growth. The Ba concentration and the Ba/Ca ratio of ambient water showed neither an annual periodicity nor a seasonal trend. In contrast to the findings of an inorganic aragonite precipitation experiment, Ba/Ca was positively correlated with ambient temperature. In addition, Ba/Ca and Sr/Ca ratios were positively correlated with each other. These results suggest that not thermodynamic equilibrium partitioning but the shell growth rate controls Ba incorporation into Hyriopsis shells. The Mg/Ca profiles of the three shells suggest that complex factors control the incorporation of Mg into shell aragonite. Water temperature and annual growth rates did not significantly influence Mg incorporation, and substantial interspecimen differences suggested that physicochemical factors do not exert any systematic control on Mg/Ca ratios in Hyriopsis. Instead, the Mg content of shell aragonite might reflect biological contamination with organic material or adsorption phenomena during crystal precipitation rather than regulated substitution into the aragonite crystal lattice. Our results suggest that trace element profiles in aragonitic bivalve shells could be interpreted as the result of strong biological controls over-riding environmental influences on trace element incorporation.