Abstract:

The objective of this study was to investigate the composition, origin, and transformation mechanisms of dissolved organic matter (DOM) in various water bodies of tropical islands. The research focused on the Nandu River basin, estuary, adjacent sea area, and Hongcheng Lake in Haikou City. A total of 15 sampling points were established: four sites within the Nandu River basin (LY-1 to LY-4), four sites in the estuarine region (HK-1 to HK-4), four sites offshore (HY-1 to HY-4), and three sites at Hongcheng Lake (HP-1 to HP-3). Surface water samples of 3L each were collected using clean glass bottles, filtered through a 0.45 μm microporous filter membrane, and stored at 4 ℃ in darkness to ensure that all analytical tests were conducted within 48 hours. The DOM was analyzed utilizing a combination of UV-visible absorption spectroscopy, three-dimensional fluorescence spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that DOM composition showed obvious spatial differences along the land-sea continuum, with the highest DOC concentration at HP-2 site in Hongchenghu Lake (58.77 mg∙L⁻¹, calculated by C) and the lowest at HK-3 site in the estuary (3.37 mg∙L⁻¹, by C). The SUVA₂₅₄ values of all samples were higher than the typical values of open ocean water (<2.0 L∙mg⁻¹∙m⁻¹), indicating that the DOM in the study area was generally affected by strong territorial input. The aromaticity of river and reservoir DOM was higher (SUVA₂₅₄: 13.02‒18.88 L∙mg⁻¹∙m⁻¹), while the aromaticity of estuary DOM was abnormally increased (SUVA₂₅₄:228.31 L∙mg⁻¹∙m⁻¹) and protein-like components increased, which revealed the co-shaping effect of land-sea mixing and biological activities. The offshore DOM showed low aromaticity (SUVA₂₅₄: 10.53 L∙mg⁻¹∙m⁻¹) and high autogeny (FI: 1.30), which were consistent with the characteristics of photodegradation and microbial degradation. 3D fluorescence spectra further revealed the strong influence of human activities: the combination of abnormally low FI values (0.07‒0.08) and high HIX values in Hongcheng Lake samples, which seriously deviated from the typical freshwater range. The BIX value of HK-3 samples in estuary was abnormally high (8.16), which was far beyond the typical threshold. At the same time, the fluorescence intensity of HY-4 (−8.0×10⁶ R.U.) in the ocean samples adjacent to the harbor is 2‒8 times that of other samples. In addition, the lack of a significant negative correlation between the spectral slopes S_275‒295 and SUVA₂₅₄ suggests that the highly aromatic DOM in this region may have a unique molecular structure different from that of conventional humus. FT-ICR MS analysis systematically reveals the spatial differentiation characteristics of DOM composition and its influencing factors from the molecular level. van Krevelen plot analysis showed that the molecular composition of all samples was mainly concentrated in lignin/CRAM-like structures and protein regions, showing common features of terrigenous inputs and microbial metabolites. The results of principal component analysis showed that there were significant differences in DOM molecular composition among different water types, and the estuary samples were different from other samples in PC1 axis (contribution rate of 58.9%) due to the influence of aromaticity index and double bond equivalent. The relative abundance of nitrogen compounds (CHON) in estuarine samples was 27.71%, significantly higher than 17.90% in river samples, reflecting the unique nitrogen cycling process in estuarine regions. At the same time, sulfur compounds also showed significant differences among different water types (p < 0.05), and the relative abundance of CHOS compounds was the highest in the estuary area (4.01%), indicating that there may be an active sulfur cycling process in this area. In addition, the analysis of molecular structure parameters showed that the relative abundance of the lake samples in the high condensation region with DBE>10 was 22.6%‒28.4%, which was significantly higher than that of the ocean samples (12.4%‒18.9%), reflecting the significant difference in the condensation degree of DOM molecules in different water environments. In addition, the study found human activity markers. The combination of low FI value (0.10) and high SUVA₂₅₄ at the Nandu River basin site (LY-1) corresponds to the molecular formula of the identified polycyclic aromatic structure, indicating that there may be signs of polycyclic aromatic hydrocarbon contamination. At the same time, benzothiazoles were detected in the estuary area, and were enriched synchronously with CHOS compounds (relative abundance of 4.01%), indicating the presence of sewage input. The combination of very low FI (0.07 to 0.08) and high HIX in Hongcheng Lake indicates that the DOM composition of Hongcheng Lake is strongly affected by urban activities, and also indicates from the molecular level that human activities play an important role in the formation of DOM composition characteristics in the study area. Molecular transformation pathway analysis showed that methylation and demethylation reactions were dominant in all water bodies, and the reaction proportion was stable between 12.39% and 13.72%. The dominant reaction pathways of different water bodies showed significant differences: reducing reactions (such as hydrogenation/dehydration) were dominant in lake environment, while oxidation reactions accounted for a higher proportion in Marine environment. The estuary region showed the most complex transformation characteristics, and its molecular transformation network had the highest number of nodes (2045) and edges (12832), and the network density (0.0041) was significantly higher than that of other water bodies. For example, for compound C15H9O8, multiple transformation pathways such as dehydrogenesis, oxidation and reduction were simultaneously present in the estuary region, while only a single methylation pathway was observed in the lake. Sulfur-containing compound C13H17O5S forms a sulfur-methylation cycle with both methylation and demethylation in the estuary samples, which is related to the active sulfur cycling process in the estuary region. Molecular network analysis further showed that there were multiple biogeochemical processes such as aerobic and anoxic microenvironments, photochemical and microbial processes, and the synergistic effects of these processes jointly shaped the unique DOM transformation pattern in the estuary. The results reveal the different characteristics of DOM transformation pathways in different water environments at the molecular level. This study demonstrates that the characteristics of DOM in tropical island water bodies result from both their sources and transformation processes. The integration of optical indicators with molecular fingerprints effectively identifies DOM sources while distinguishing human activity influences. These findings provide a scientific foundation for understanding carbon cycling and managing water environments in this region.

Key words: dissolved organic matter, tropical island water bodies, fourier transform ion cyclotron resonance mass spectrometry, molecular composition, chemical transformation

摘要：

为厘清人类活动与自然过程对热带岛屿水体溶解性有机质（DOM）特征的影响，选取典型水体（河流、湖泊、河口及近海），结合紫外-可见吸收光谱、三维荧光光谱和傅里叶变换离子回旋共振质谱（FT-ICR MS），系统解析DOM的来源、组成及转化机制。结果表明，DOM组成在不同水体间具有差异：紫外光谱显示河流与湖泊DOM芳香性较高（SUVA₂₅₄为13.02－18.88 L∙mg⁻¹·m⁻¹），陆源特征显著；河口DOM芳香性异常升高（SUVA₂₅₄达148.29－228.31 L∙mg⁻¹·m⁻¹），三维荧光光谱显示类蛋白组分增多，体现陆海混合与污水输入的共同作用；近海DOM则以低芳香性（SUVA₂₅₄为7.85－14.63 L∙mg⁻¹·m⁻¹）、高自生源（FI为1.21－1.30）特征为主。而FT-ICR MS从分子层面表明：流域样品中异常荧光峰与高SUVA₂₅₄、低FI值（0.10）的组合，对应分子组成中高丰度稠环芳香结构，表明可能存在多环芳烃污染；河口区检出苯并噻唑等人工有机硫化物，且与CHOS类化合物同步富集，表明存在污水输入的迹象。分子转化路径分析进一步表明，河口DOM转化网络最为复杂，同时存在氧化、还原、甲基化等多种反应，体现该区域在陆海交互与人为干扰下的活跃转化特征。该研究使用光学指标与分子指纹相结合的方法，可有效揭示DOM组成规律并识别人类活动影响，结果可为热带岛屿水环境保护及碳循环研究提供了科学依据。

关键词: 溶解性有机质, 热带岛屿水体, 傅里叶变换离子回旋共振质谱, 分子组成, 化学转化