Seasonal variation in sources of dissolved organic carbon in a lacustrine environment revealed by paired isotopic measurements (Δ14C and δ13C)

To investigate the sources and cycling of dissolved organic carbon (DOC) in a lacustrine environment, isotopic measurements of C-14 and C-13 in DOC were carried out for Lake Kasumigaura-which is famous as a very eutrophic and shallow (mean depth 4.0 m) lake in central Japan-and its tributary rivers. Lake and river samples were collected in the spring and autumn (May and September) of 2003. The Delta C-14 measurements of DOC were performed using the accelerator mass spectrometer at the National Institute for Environmental Studies (NIES-TERRA), Japan. In September, the C-14 values of DOC were light (around -200%) and did not differ significantly between lake and river water samples, indicating that DOC in Lake Kasumigaura and its tributary rivers yields older C-14 ages than the age expected from the lake-water residence time (average 200 days). This result suggests that terrestrial sources are important contributors to DOC in Lake Kasumigaura. Nevertheless, delta C-13 values indicated that during spring, DOC in the lake is mainly autochthonous. Thus, sources and cycling of DOC in Lake Kasumigaura may vary seasonally.