Carbon Isotopes in Freshwater Mussel Shells: Metabolic Versus Environmental

The carbon isotopic signature of dissolved inorganic carbon (δ¹³C_DIC) is a powerful tool for understanding biogeochemical cycling. Biological carbonates are a potential record of past δ¹³C_DIC, but are not always easy to decipher. Metabolic carbon (C_M) can be incorporated in the carbonate and interfere with the environmental signal. The amount of C_M is usually considered to be low, around 10%, but up to 37% has been reported. A strong ontogenic increase in C_M has also been noted, which is probably due to increasing absolute metabolism with increasing body size. We tested to see if this was also the case for freshwater bivalves. Four living individuals of Pyganodon cataracta ranging in size from 21 to 88 mm in height were collected from a 0.5 m² area of a small stream in November 2006. Tissue δ¹³C, the last year of shell carbonate δ¹³C and δ¹⁸O and δ¹³C_DIC were analyzed. From these data, the percent metabolic carbon (%C_M) in each shell was calculated. We provide a rough estimate that between 15 and 35% of shell carbon is metabolic in origin. The δ¹⁸O values were not significantly different between individuals illustrating that similar time is represented in each shell and that they utilized the same water source. δ¹³C on the other hand shows a clear ontogenic decrease, and a strong linear relationship between shell height (H in mm) and %C_M (R² = 0.96; %C_M = 0.23*H+13.33), with the older individuals having more negative values indicating a higher metabolic carbon incorporation into the shell.