Ontogenic increase of metabolic carbon in freshwater mussel shells (Pyganodon cataracta)

Incorporation of isotopically light metabolic carbon (CM) can obscure carbon isotope records of dissolved inorganic carbon from biogenic carbonates. Ontogenic increases of CM are common in marine bivalved mollusks, likely reflecting increasing absolute metabolism with increasing body size. Here we use shell aragonite from four specimens of Pyganodon cataracta to test if the same is true in freshwater bivalves. Annual δ18Oaragonite values were not different between individuals suggesting that the aragonite samples represent the same time interval. δ13Caragonite, however, showed an ontogenic decrease, with a strong linear relationship between shell height (H in mm) and %CM (R2 = 0.96; %CM = 0.23*H + 13.33), indicating that more CM is incorporated into larger shells. We estimate that between 15 and 35% of shell carbon is derived from metabolic CO2. Nevertheless, this vital effect does not exclude the use of δ13Caragonite records from freshwater shells as environmental proxies.