Fluorescent dissolved organic matter (FDOM) is a key indicator of the sources and transformation trends of organic matter in aquatic environments. However, in natural water, the composition of FDOM is often affected by external inputs, which complicates the isolation of environmental heterogeneity effects. To elucidate the intrinsic fluorescent characteristics of DOM with minimal external interference, we selected the Middle Route of the South-to-North Water Diversion Project as an ideal model system, given its artificially regulated hydrology and low external input. Using parallel factor analysis (PARAFAC), two-dimensional correlation spectroscopy (2D-COS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated the seasonal dynamics of FDOM molecular composition and its reactive sequence. The results demonstrated that seasonal variation significantly affects both the composition and transformation sequence of FDOM. Protein-like and microbial-derived humic-like components were strongly influenced by latitude, whereas the reactivity of terrestrially humic components was primarily governed by water level, discharge, and velocity (explaining 51 % of the variance). Thermodynamic analysis suggests that under stable hydrological conditions and minimal external inputs, an algal abundance of approximately 2.63 × 106 cells/L may alter the reactivity of protein-like components. This threshold underscores the ecological significance of algal biomass in regulating the bioavailability of dissolved organic matter.

中文翻译：

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对水文孤立水生连续体中荧光溶解有机物季节性变化的分子见解

荧光溶解有机物 （FDOM） 是水生环境中有机物来源和转化趋势的关键指标。然而，在天然水中，FDOM 的组成经常受到外部输入的影响，这使得环境异质性效应的隔离变得复杂。为了阐明 DOM 在最小外部干扰下固有的荧光特性，鉴于其人工调控的水文和较低的外部输入，我们选择了南水北调工程的中间路线作为理想的模型系统。使用平行因子分析 （PARAFAC） 、二维相关光谱 （2D-COS） 和傅里叶变换离子回旋共振质谱 （FT-ICR MS），我们研究了 FDOM 分子组成的季节性动态及其反应序列。结果表明，季节性变化显著影响 FDOM 的组成和转化序列。蛋白质样和微生物衍生的腐殖质成分受纬度的强烈影响，而陆生腐殖质成分的反应性主要受水位、流量和速度控制（解释了 51% 的方差）。热力学分析表明，在稳定的水文条件和最少的外部输入下，大约 2.63 × 106 个细胞/L 的藻类丰度可能会改变蛋白质样成分的反应性。该阈值强调了藻类生物量在调节溶解有机物的生物利用度方面的生态意义。