Application of Orbitrap mass spectrometry to investigate seasonal variations of dissolved organic matter composition in a eutrophic lake in Japan†
Rapid urbanization induces organic pollution in closed water bodies and accelerates eutrophication. Dissolved organic matter (DOM), a major fraction of organic matter, causes various problems in water use. DOM sources in lakes are diverse. Moreover, DOM properties can be altered biologically and physicochemically during retention time. However, DOM in lakes consists of complex unknown substances. In the present study, high-resolution Orbitrap mass spectrometry (Orbitrap MS) was employed to determine the molecular-level composition of low-molecular-weight DOM in Lake Inba, the most polluted eutrophic lake in Japan. One-year monitoring demonstrated that the lake's DOM consisted of core components that were detected year-around and season-specific components. Although most DOM components extracted by solid phase extraction in the lake came from inflow rivers, lake-specific components were present, which could be associated with algal growth in the lake. Permanganate pretreatment was integrated with Orbitrap MS to screen for components contributing to dissolved chemical oxygen demand (DCODMn), an environmental parameter used in Japan to define the water quality of lakes and reservoirs. Results indicated that only minor and highly unsaturated components were responsible for DCODMn. With this approach, we were able to shed light on how the DOM composition in a eutrophic lake was shaped at molecular level, which cannot be achieved by conventional analysis.