ABSTRACT

Source and formation of high-molecular-weight dissolved organic matter (HMW-DOM) and its interaction with other organic components such as particulate organic matter (POM) and low-molecular-weight DOM (LMW-DOM) have been poorly understood so far. Molecular and isotopic compositions of HMW-DOM are generally far different from those of POM and LMW-DOM. In this study, we further investigate large proportions of 24-ethylcholesta-5,22-dien-3β-ol (stigmasterol) in the HMW-DOM (but not in POM and LMW-DOM) during summer in three types of lakes: freshwater Lakes Kizaki and Suwa and a brackish-water Lake Suigetsu, in Japan. Moreover, from the carbon and hydrogen isotope analysis, we reveal autochthonous algae (perhaps some minor, specific phytoplankton) as a major source of the abundant stigmasterol in the lakes. These results suggest that HMW-DOM in summer is derived from autochthonous phytoplankton rather than allochthonous terrestrial plants and that HMW-DOM likely has few interaction (e.g., exchange of components) with POM and DOM in the lakes.