New Insightinto the Source Contribution of Dissolvedand Particulate Organic Matter in a Typical Macrophyte-Dominated LakeUsing Optical Spectroscopy, Stable Isotopes, and FT-ICR MS

The source identification of organic matter is essential for understanding its biogeochemical fate and provides valuable guidance for lake management. However, the quantitative contribution considering both natural or anthropogenic and allochthonous or autochthonous and their contribution in shaping molecular composition of organic matter have rarely been reported. Here, the molecular composition of organic matter both in dissolved (DOM) and particulate form (POM) was systematically studied by a combination of stable isotopes, optical and high-resolution mass spectrum, and the source contributions were quantitatively estimated through a novel end-member mixing analysis (EMMA) model in a typical macrophyte-dominated lake, Caohai, China. We found that DOM was more aromatic, unsaturated, and oxygenated with greater lignin and tannin compounds, while POM contained more lignin and aliphatic substances with lower aromaticity and humification. The distinct molecular composition could be attributed to the distinct relative contributions from diverse sources. The EMMA results revealed that the averaged relative contribution from autochthonous and natural allochthonous inputs was 68.7 and 80.5% for DOM and POM, respectively. This suggested that nonanthropogenic sources attribute much to water quality in macrophyte-dominated lakes and distinct lake management strategies are essential for natural lakes in the future.