Do stable carbon and nitrogen isotope values of Nitella flexilis differ between softwater and hardwater lakes?

The composition of stable carbon and nitrogen isotopes ( δ 13 C and δ 15 N) is widely used for tracking the origin of organic matter (OM) present in lacustrine sediments. These isotopes also define the evolution of OM in food web loops. Charophyte species Nitella flexilis C. Agardh, 1824 can be found in different aquatic environments where it contributes significantly to sediment formation and influences biota function. Therefore, it is crucial to study more about δ 13 C and δ 15 N in different lake types. Here, we present the results of the first comprehensive study of N . flexilis δ 13 C and δ 15 N, which add to the knowledge of the C and N isotope records of charophytes. We obtained the δ 13 C and δ 15 N records of N . flexilis OM from hardwater and softwater lakes and checked for differences between these records. We also analyzed the differences in physical and chemical parameters. Finally, we compared the δ 13 C and δ 15 N records with physical and chemical parameters to identify the variables that have the highest influence on N . flexilis δ 13 C and δ 15 N values. Our study showed that both δ 13 C and δ 15 N did not differ significantly in the two types of lakes, although the lakes had significant differences in several physical and chemical parameters (pH, Ca 2+ , dissolved inorganic carbon, total phosphorus, conductivity). However, we observed that δ 13 C values were influenced by light conditions (photosynthetic active radiation, depth, dissolved OM), while δ 15 N values were influenced by the total nitrogen concentration in water.