Lake Inba, which serves as a drinking water source in Chiba Prefecture, Japan, has not complied with the environmental standard for COD (3 mg/L) since the 1970s. The high level of dissolved organic matter (DOM) caused by eutrophication in the lake has risen the concern of water treatment. To understand the compositions and sources of DOM in Lake Inba, this study compared molecular compositions of low molecular weight DOM (LMW-DOM, <1 kDa) in the lake and a major river flowing to the lake. Water samples were collected from Lake Inba and Kashima River in June 2015. After solidphase extraction, LMW-DOM was analyzed by Orbitrap mass spectrometry coupled with electrospray ionization. Based on the accurate mass data, 1,263 and 1,393 molecular formulae were assigned for the lake and river samples, respectively. Among them, 1,193 formulae were shared between the lake and the river, indicating that Kashima River was the representative source of LMW-DOM in the lake at the sampling occasion. Elemental composition of molecular formula demonstrated that 61% of the shared formulae were composed of carbon, hydrogen and oxygen. Hydrogen to carbon ratio and oxygen to carbon ratio of these common molecules were similar to the fingerprints of lignin and tannin.

No abstract

In 2016, only 56.7% of the lakes and reservoirs in Japan complied with the standard of chemical oxygen demand determined by permanganate (COD Mn). The dissolved organic matter (DOM) contributing to COD Mn in lakes is heterogenous in nature; hence, DOM compositions in different lake waters are not well known. In this study, molecular components contributing to dissolved COD Mn (DCOD Mn) in lake water were identified via high-resolution Orbitrap mass spectrometry (Orbitrap MS). Water samples were collected from Lake Inba in September 2016. The filtered samples were oxidized by KMnO 4 according to the JIS method K0102 for different durations (ranging between 0-120 min). The DOM extracted via solid phase extraction was analyzed using Orbitrap MS, and molecular formulas were assigned. In all, 1,115 components were observed before oxidation. During time-course oxidation, components with higher unsaturation were selectively oxidized, suggesting that they contributed to the DCOD Mn in lake water. DCOD Mn components were screened for DOM in Lake Inba, and other DOM references were isolated from the Suwannee River and Lake Biwa. Only 14%-32% of the total components were regarded as DCOD Mn components, indicating the limited coverage of COD Mn. The compositions of DCOD Mn components were highly diverse, depending on the sources.

Orbitrap mass spectrometry can reveal the composition of DOM and the fraction responsible for COD_Mn in an eutrophic lake.