Stable carbon isotope values in dissolved inorganic carbon of ambient waters and shell carbonate of the freshwater pearl mussel (Hyriopsis sp.)

Abstract

We investigated carbon sources and cycling in a canal connected to Lake Kasumigaura (Japan) using stable carbon isotope values (δ13C) in dissolved inorganic carbon (DIC) of ambient water and in shell carbonate of the commercially cultured freshwater pearl mussel (Hyriopsis sp., Unionidae). The δ13CDIC showed correlations with DIC concentrations, pH, monthly precipitation and pCO2. Thus, the carbon isotopic composition can be used as a proxy for environmental variables. The pCO2 ranged from 3,046 to 9,535 μatm, with a mean value of 6,035 μatm, and remained well above the atmospheric value throughout the years of study. Changes in CO2 solubility, outgassing and biological processes in the river had only minor influence on the carbon isotope composition. DIC concentration and δ13CDIC were controlled primarily by external inputs of soil and weathering-derived DIC associated with precipitation. The δ13C values in different growth zones of the Hyriopsis shells were similar, suggesting that given a constant δ13CDIC source throughout shell formation, the influence of size-related metabolic variations on shell δ13C was negligible. Comparison of δ13C values in DIC and Hyriopsis shell indicated that mussel ontogeny did not influence carbon isotope fractionation, but inter-specimen differences suggested that physiological factors exert systematic control on δ13C values in Hyriopsis. Results also suggest that if the annual mean δ13CDIC value is to be inferred for a fluvial system from shell δ13C values, multiple specimens should be sampled, and the annual mean value yields a temporally integrated measure of the stable isotopic composition of the river system. Given the carbon isotope fractionation between ambient DIC and shell is +2.0 ± 0.7 ‰ (2 se), we estimated that pCO2 averaged ~6,850 μatm during shell growth periods from 2002 to 2007. The pCO2 concentration was an order of magnitude greater than the atmospheric equilibrium value in all years, suggesting that CO2 was lost from surface water to the atmosphere or downstream to the lake. These losses must be accounted for when considering the regional redistribution of CO2. Our results extend the utility of δ13C measures in freshwater bivalves for inferring the δ13CDIC history of ambient waters. This tool provides information that is complementary to that gained from direct δ13CDIC measurement and should enable inference of long-term changes in critical variables such as pCO2.