2025, Vol. 46
2. 广西大学资源环境与材料学院, 南宁 530004
2. School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China
溶解性有机质（dissolved organic matter, DOM）在自然水环境中分布广泛, 约占内陆碳库的50%, 是由环境中的氨基酸、腐植酸和芳香烃聚合物等组成的非均相的高度复杂的一类水溶性有机物[1,2]. 作为全球最大的活性和可交换的有机碳库, DOM不仅对全球碳的生物化学循环具有重要作用, 还对水体中的重金属、难降解性有机物等污染物的环境行为及其生态风险具有显著影响[3,4]. 水体DOM组分特征的改变会引起重金属离子、全氟化合物、氯化后消毒副产物等污染物的环境地球化学行为发生改变, 进一步影响其对环境的生态风险[4 ~ 7].
长江是我国第一大河, 其流域面积约180万km2, 覆盖我国19个省市及自治区, 不同区域的气候水文和地理条件具有明显差异, 由此导致的流域内DOM组分特征的变化极为复杂, 已经得到广泛关注, 这种变化引起污染物生态风险的改变也愈发得到重视[8,9]. 目前, 对长江流域内水体DOM的研究主要关注极端天气、全球变暖和人类活动等条件下DOM组分特征的时空演变, 及其对重金属、消毒副产物和新兴污染物等污染物的环境地球化学行为的改变的相关研究[4,8,10 ~ 12]. 随着傅里叶变换离子回旋共振质谱（Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR MS）等先进技术手段的应用, 通过从分子水平精确解析DOM的化学组成, 极大提高了对长江流域DOM的时空演变、碳循环以及对其他污染物的环境地球化学行为和影响机制等方面的认识[13,14].
通过Web of Science和中国知网数据库, 分别以“TS=[（Yangtze OR Yangtze basin）AND（River OR Lake OR reservoir OR Estuary）] AND TS=（dissolved organic matters）”和“TKA=[（长江+长江流域）AND（河+湖+水库+长江口）]AND TKA=（溶解性有机质）”作为检索公式进行检索, 共计检索出相关文献753篇. 对检索结果进行逐一筛选, 去除不相关文献, 最终共计确定297篇有效文献. 基于297篇有效文献中可获取的数据进行处理, 处理方式如下：①长江干流点位经纬度与文献基本一致, 若同一点位存在多个数据或存在邻近点位数据则取平均值；②对长江流域内支流和湖泊等水体, 则对其整体取平均值.
本文围绕长江流域干支流、湖泊和三峡水库中DOM的组分特征的时空演变规律、影响因素等研究内容进行了梳理和归纳. 在此基础上, 提出了未来该研究领域需要重点关注的研究方向, 以期为流域尺度下污染物环境行为的研究和水质水生态安全提供理论依据.
1 长江干支流水体及邻近海域DOM组分特征 1.1 长江干支流水体DOM组分特征从上游到下游的长距离输水过程中, 长江干流水体DOM组分特征具有显著的空间异质性. 总体而言, 长江干流从上游到河口段DOC浓度沿程增加（图 1）, 且下游ρ（DOC）[（2.04±0.82）mg·L-1]显著高于上游ρ（DOC）[（1.26±0.23）mg·L-1]和中游ρ（DOC）[（1.54±0.18）mg·L-1]（P < 0.05）. 长江流域DOM主要由类腐殖质和类蛋白质组成, 流域内干支流DOM的类腐殖质与类蛋白质丰度比（humoid to proteoid abundance ratio, H/P）为（2.37±1.22）, 说明整体上流域内干支流水体DOM以类腐殖质为主. 长江干流上游与中游H/P均高于下游；而类蛋白质丰度则与之相反（图 2）[11,15]. 腐殖化指数（humification index, HIX）能够表征DOM腐殖化程度, HIX越高说明相对分子质量越高, 结构越稳定；而与254 nm波长下的DOC吸光度与浓度的比值（carbon-specific absorption at 254 nm, SUVA254）则可表征评估芳香族化合物含量即芳香性[8,16]. 长江干流中游的SUVA254[（5.17±2.03）L·mg-1·m-1]和HIX（7.95±3.43）高于上游和下游（图 3和图 4）, 表明长江干流中游腐殖化程度和芳香性比上下游更高.
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图 1 长江流域不同位置水体DOC浓度 Fig. 1 DOC of water body at different locations in the Yangtze River Basin |
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图 2 长江流域干支流H/P Fig. 2 H/P of the mainstream and tributaries of the Yangtze River Basin |
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图 3 长江流域干支流SUVA254 Fig. 3 SUVA254 of the mainstream and tributaries of the Yangtze River Basin |
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图 4 长江流域干支流HIX Fig. 4 HIX of the mainstream and tributaries of the Yangtze River Basin |
DOM常见的表征技术手段主要包括紫外-可见光光谱法、三维荧光光谱法和色谱-质谱法等. 其中紫外-可见光谱法和三维荧光光谱法能够快速且无损地对DOM进行表征, 并能通过计算一系列光学参数的方式表征DOM的芳香性（SUVA254）、腐殖化程度（HIX）和来源（荧光指数和生物指数）等特征[17]. 此外, 三维荧光光谱法还能通过平行因子分析区分不同荧光团, 对DOM组分进行判别, 如类腐殖质和类蛋白质等[18,19]. 色谱-质谱法是常见的分析复杂混合有机物的方法, 能够将DOM按照分子尺寸大小或极性大小进行分离, 从而获得DOM的分子组成信息[18,19]. 紫外可见光光谱法和三维荧光光谱法受限于自身原理, 仅能对DOM的宏观性质进行表征和对DOM进行简单分类, 难以获取DOM结构和分子信息[14,18], 而色谱法由于DOM分子组成过于复杂, 很难将各类DOM进行有效分离[14]. FT-ICR MS是一项超高分辨率质谱技术, 能够克服传统技术的局限性, 获取详细的DOM分子组成数据, 准确鉴定单个DOM分子的化合式, 使得从分子水平探究和理解DOM对气候变化、人类活动和土地利用类型变化的响应成为可能[13,18,20].
长江干流分子式数量特别是独特分子式数量沿程显著增加, 其中长江干流元素组成以CHO最高, 而中下游由于耕地、废水排放增多的原因导致CHON和CHOS等杂原子明显高于上游, 这说明了长江干流DOM分子多样性沿程呈增加趋势[8,21,22].
长江干支流中DOM来源主要包括两类：①外源, 主要包括源于陆地土壤、腐烂植物的陆源DOM以及人为输入的DOM；②内源, 主要为水中微生物、浮游生物生命活动所产生[23 ~ 27]. 水体DOM来源可通过荧光指数（fluorescence index, FI）与生物指数（biological index, BIX）进行推断：① FI < 1.4表示主要为陆源, FI > 1.8表示主要为本地源；② BIX > 1表示DOM主要来自内源, 0.6 < BIX < 0.7表示主要来源于陆源输入[4,8]. 长江干流FI和BIX分别为（1.49±0.15）和（0.67±0.01）, 长江干流DOM主要来源于流域内植物腐烂和土壤侵蚀即陆源性DOM[4,8]（图 5和图 6）. 但需要注意的是, 长江中下游特别是经济发达的城市河段, 尽管类腐殖质占比相对较高, 但由于废水的排入导致这些河段内含有更为丰富的人为输入的类蛋白质[8,15].
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图 5 长江流域干支流FI Fig. 5 FI of the mainstream and tributaries of the Yangtze River Basin |
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图 6 长江流域干支流BIX Fig. 6 BIX of the mainstream and tributaries of the Yangtze River Basin |
长江支流水体DOM同样具有显著的空间异质性. 长江支流水体DOC浓度在空间上的变化趋势与干流相似, 即DOC浓度为：上游支流 < 中游支流 < 下游支流（图 1）并且下游支流显著高于上游支流（P < 0.05）[11,28]. 长江下游的支流水体中H/P、HIX和SUVA254低于其上游或中游的支流, 说明下游存在更多的类蛋白质, 且DOM芳香性和腐殖化程度较上游和中游更低（图 2、图 3和图 4）. 与长江干流相比, 长江支流的FI（1.08~1.87）和BIX（0.63~1.42）的范围更大, 这说明了不同支流间DOM来源差异更明显（图 5和图 6）.
长江流域干流DOM组分特征存在显著的季节性变化. 总体上, 长江干流丰水期DOC浓度略低于枯水期, 其DOM的类腐殖质比例、芳香性及其组成多样性（分子式多样性、独特分子式和不饱和化合物相对丰度）均高于枯水期[8,9,28]. 其中, 独特分子式主要以木质素降解产物为主, 具有较强的抗生物降解和光降解的能力, 导致长江干流向东海输入更多难降解的DOM[9]. 另外, 长江干流丰水期的不稳定脂肪族类DOM和类蛋白质DOM尽管相对丰度低于枯水期, 但浓度相对长江干流枯水期更高[8,9].
1.2 长江口及其邻近海域DOM组分特征长江口及其邻近海域指长江径流与潮流, 淡水与咸水相互作用的区域, 根据浊度和盐度可分为淡水区、浑浊区和沿海区[29,30]. 有研究表明, 从河口向外海的输移过程中, DOM会发生显著改变, 因此不同区域中的DOM组分特征具有显著差异[29]. 整体上, 长江口及其邻近海域DOC浓度[图 7（a）]芳香性和类腐殖质相对丰度[图 7（b）]沿程呈近海高, 外海低的特点, 即：淡水区 > 浑浊区 > 沿海区[29,31,32], 类蛋白质相对丰度则呈现相反的特征[图 7（b）]. Zhao等[33]对长江口水域DOM分子大小变化的研究表明, DOM从淡水区向沿海区迁移的过程中, 整体上分子大小呈下降趋势. Zhou等[29]和Zhao等[34]的研究发现, 淡水区DOM分子式数量（包括总量、CHO、CHOS、CHON和CHONS）显著高于浑浊区和沿海区. 而Chen等[35]则进一步发现, CHON的相对丰度则为：沿海区 > 浑浊区 > 淡水区. 长江口及其邻近海域DOM依据来源可分为三类：长江径流输入的陆源DOM、浮游生物生命活动产生的自生源性DOM和废水排放等人为输入性DOM[32,36,37]. 长江口及其邻近海洋中, DOM的主要来源为自生源；其中, 浮游生物生命活动产生的自生源性DOM丰度整体上处于同一水平, 无显著性差异；而陆源性DOM和人为输入性DOM的丰度则为：淡水区 > 浑浊区 > 沿海区[34,36].
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（a）长江口及其邻近海域ρ（DOC）, （b）长江口及其邻近海域水体类腐殖质和类蛋白质的相对丰度 图 7 长江口及其邻近海域DOC及类腐殖质和类蛋白质相对丰度 Fig. 7 Relative abundance of DOC, humus and proteinoid in the Yangtze River Estuary and its adjacent waters |
长江口及其邻近海域DOM组分特征的区域性差异主要受长江径流输入、盐淡水混合过程的稀释作用、人类活动、浮游生物生命活动和悬浮颗粒物吸附与絮凝等因素控制[29,31,33,38]. 长江径流将大量类腐殖质和芳香族化合物输入淡水区, 并且长江口的高度城市化也导致大量含有脂质、类蛋白质等DOM的废水排入长江口, 最终导致淡水区含有最高的DOC浓度和DOM丰度（包括芳香族化合物、脂质、杂原子化合物、类腐殖质和类蛋白质等）[29,36,37,39]. 浑浊区是淡水和海水混合的重要区域, 淡水区输入的DOM在此区域受到物理稀释, 使得浑浊区DOC浓度和DOM丰度均出现显著下降, 尤其是类腐殖质和人为输入性DOM丰度[33,36,37,40,41]. 淡水和海水混合增强了沉积物再悬浮, 促进了沉积物和孔隙水中DOM的释放, 增加了浑浊区CHON的相对丰度[35,42]. 除此之外, 浑浊区悬浮颗粒物浓度较高, 通过絮凝作用导致芳香族化合物被从水体中去除, 进一步降低浑浊区芳香族化合物的丰度[29,36,37]. 沿海区DOM主要为海洋浮游生物生命活动产生的类蛋白质[32,41,42]. 值得注意的是, 尽管经过浑浊区稀释作用和复杂的生物地球化学的影响, 沿海区仍然具有比浑浊区更高的脂肪族化合物, 分子式中CHON的数量与浑浊区处于同一水平, 但相对丰度显著高于浑浊区[42]. 这一方面是因为沿海区距离陆地较远, 陆源DOM和人为输入性DOM在输移过程中, 一部分DOM已通过光降解、生物降解和悬浮颗粒物吸附絮凝等方式被去除, 导致人类活动和径流贡献较低[32,35]；另一方面, 相较于浑浊区, 沿海区具有更高的水体透明度, 再加上营养盐的输入, 浮游生物更加活跃, 提升了该区域浮游生物的初级生产力, 产生大量分子组成为CHON的DOM, 提升CHON的相对丰度占比[35,39,43,44].
2 长江流域湖库水体DOM组分特征 2.1 流域内湖泊水体DOM组分特征长江流域湖泊水体DOM组分特征具有空间异质性, 且与长江干支流具有显著差异. 长江流域湖泊ρ（DOC）为（5.09±3.04）mg·L-1. 在空间上DOC的变化趋势与干支流相反, 即：上游湖泊 > 中游湖泊 > 下游湖泊（图 1）. 流域内湖泊DOC分布极不均匀, 其中巢湖的ρ（DOC）最高（15.18 mg·L-1）, 洞庭湖的最低（1.77 mg·L-1）（图 1）. 湖泊水体DOM腐殖化程度显著低于长江干支流水体, 长江流域湖泊水体类蛋白质相对丰度显著高于长江干支流（P < 0.05）, 整体上以类蛋白质为主（图 8、图 9和图 10）. 流域内湖泊水体FI（1.64±0.26）和BIX（1.32±0.40）均显著高于长江干支流（P < 0.05）, 其中仅有滆湖FI < 1.4, 红枫湖、百花湖、长寿湖和邵伯湖BIX < 1（图 5和图 6）. 因此, 整体上流域内湖泊DOM主要来源于水体微生物生命活动. 流域内湖泊DOM的空间异质性主要归因于气候、人类活动强度、营养状态和湖泊水体自身流动性[45 ~ 47].
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图 8 长江流域湖泊H/P Fig. 8 H/P of lakes of the Yangtze River Basin |
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图 9 长江流域湖泊SUVA254 Fig. 9 SUVA254 of lakes of the Yangtze River Basin |
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图 10 长江流域湖泊HIX Fig. 10 HIX of lakes of the Yangtze River Basin |
三峡水库DOM组分特征存在空间异质性. 库区干流DOC浓度与HIX较支流更高, 说明支流DOM浓度与腐殖化程度低于干流；与之相反, 支流FI与BIX更高, 说明库区支流DOM的微生物等内源贡献高于干流[4,48 ~ 50]. Wang等[51]以香溪河为例进行研究, 发现库区支流人为输入量低于干流, 类色氨酸、杂原子等从香溪河到库区干流呈上升趋势, 说明支流的人为输入沿程增加.
三峡水库的DOM组分特征存在显著的季节性变化. 丰水期库区水体中SUVA254、HIX和类腐殖质荧光强度均高于枯水期, 而水体中DOC、BIX、FI和类蛋白质荧光强度均低于丰水期[52,53]. 通过FT-ICR MS对两个水期库区支流DOM的研究表明, 香溪河和大宁河等支流枯水期DOM的分子式总量、独有分子式数量、异构体复杂度和高度不饱和化合物均高于丰水期[54,55]. 这些研究表明丰水期库区支流水体DOM具有更明显的陆源特征和相对简单的成分构成. 而枯水期水体DOM浓度更高, 其来源特征主要为更高的内源占比和更多的人为输入[54,55]. 异重流是库区DOM组分特征出现季节性差异的关键原因之一. 三峡水库在丰水期处于低水位运行期, 形成的水体异重流导致库区干流水体倒灌至支流水体, 从而向支流水体中引入更多陆源DOM[48,54].
三峡水库支流DOM在水体剖面上存在着显著的分层现象. 例如, 香溪河表层水体的DOC浓度、内源DOM丰度和N、S等杂原子丰度均高于中下层水体；而中下层水体的陆源DOM丰度及DOM相对分子质量显著高于表层水体[55]. 导致支流水体分层现象出现的原因是由微生物、水体热分层、沉积物释放及异重流等多种因素共同影响的. 水库表层水体微生物较为活跃, 对腐殖质进行生物降解的同时产生大量内源DOM, 并且充足的光照使得腐殖质更易发生光降解[55 ~ 58]. 随着深度的增加, 光强、水温、溶解氧等逐渐下降, 这导致微生物活性及数量随之下降, 减少了内源DOM的产生以及腐殖质的光降解和生物降解[56 ~ 59]. 水库形成的热分层会影响水体垂向结构的能量交换和物质迁移, 导致水库DOM出现垂向分层[56,60 ~ 62]. 除此之外, 下层水体的缺氧环境可能会导致沉积物向水体释放具有结构复杂、相对分子质量高和难降解等特点的腐殖质[63,64]. 丰水期库区支流异重流的出现将干流类腐殖质引入支流, 不仅提高了中层水体陆源DOM丰度, 还阻碍了水体垂向上的物质交换[55]. 因此库区表层中多肽、类蛋白质、不饱和脂肪族化合物等内源DOM丰度较中下层更高, 并且具有多杂原子、低芳香性和相对分子质量低的特点；中下层水体DOM主要为陆源性类腐殖质, 其组成具有难降解、高芳香性和相对分子质量高的特点[55,57,58].
值得注意的是, 以往关于水库DOM垂向分层的研究主要围绕热分层进行[56]. 但有研究表明DOM分层界面与水库热分层界面并不一致, 而且即使在未有明显热分层的冬季水体中, DOM的混合也不完全, 仍存在垂向分层现象[56 ~ 62]. 这表明了以往研究的局限性, 对深大水库DOM垂向分层机制及时空演变规律的认识并不全面, 有待继续深入.
3 长江流域水体DOM组分特征的影响因素 3.1 气候对长江流域水体DOM组分特征的影响有研究表明, 降雨量、气温及光照等气候变化对长江流域水体DOM组分特征具有显著影响[8,9,65]. 长江干流丰水期降雨量较枯水期更高, 土壤受到的侵蚀作用更强烈, 更多高芳香性和生物可利用性DOM通过淋溶和地表径流进入长江干流, 增加了长江干流DOC通量, 提高了芳香性和类腐殖质相对丰度, 因此长江干流在丰水期具有更明显的陆源特征 [8,9,66]. 除此之外, 与枯水期相比, 丰水期更高的光照强度与温度增强了浮游动植物等水体中微生物的生命活动, 导致不稳定的脂肪族和类蛋白质的释放高于枯水期[8,9].
全球变暖正在改变长江流域水体DOM组分特征. 青藏高原平均气温在26 a内上升了1.17℃[67], 加剧了冻土带和冰川的融化, 导致DOM从陆地向水生生态系统的快速迁移转化[68,69]. Zhou等[70]的研究表明, 冰川融水的大量流入, 导致了青藏高原河流湖泊中脂肪族和类蛋白质相对丰度的增加. 青藏高原平均气温的迅速上升, 不仅使得流域内冻土带湖泊DOM分子组成中CHO的比例逐渐下降, CHON逐步上升；还导致DOM矿化速率增加, 引入更多多酚类DOM[71]. 除此之外, 全球变暖带来的气候变化和冰川融水的增加正在改变河流湖泊等水体微生物群落组成, 这将导致水体由微生物生命活动产生的内源性DOM组分特征发生改变[72 ~ 75]. 目前, 全球变暖对水体DOM的影响的研究局限于源区及部分典型湖泊, 缺少流域尺度的相关研究, 应当予以重视.
随着全球变暖的加剧, 极端天气出现频次和影响范围显著增加, 这对长江流域水体DOM组分特征的影响同样不容忽视[76 ~ 78]. 极端降雨极大增强了地表径流对土壤的侵蚀作用和淋溶作用, 显著提高了陆源DOM的输入, 改变了河流[8,9]、湖泊[66]、水库[10,79]和长江口及其邻近海域[34,80]等水体DOM的组分特征. 除极端降雨外, 极端高温通过引发山火同样能够影响水体DOM组分特征[81 ~ 83]. 植物在燃烧后会产生大量DOC、DON、颗粒物和碎屑等, 这些物质通过大气沉降或地表径流等方式输入水体, 并能通过改变微生物群落, 间接影响输入水体DOM的组分特征[81,82]. 例如, 2022年重庆市涪陵区山林发生了森林火灾, Xu等[82]对火灾后长江干流及嘉陵江水体DOM组分特征的变化进行了研究, 结果表明, 大量热原性DOM被输入干流和嘉陵江, 其中缩合芳烃、蛋白质和不饱和碳氢化合物相较于其他区域显著增加, CHON、CHOS和CHONS化合物同样显著高于其他区域. 热原性DOM的输入可能改变河流微生物群落的结构和功能, 从而影响内源性DOM的组分特征[81,82]. 但目前热原性DOM对流域内水体DOM的长期影响未见报道. 目前相关研究主要以模拟实验为主, 热原性DOM对河流湖泊DOM组分特征的长期的、具体的影响仍不清晰. 全球变暖加剧, 极热引发的山火频次逐年上升, 应当尽快开展热原性DOM对水体DOM组分特征长期影响的研究.
3.2 人类活动对长江流域水体DOM组分特征的影响有研究表明, 人类活动是塑造长江流域水体DOM的组分特征的决定性因素之一, 且比气候变化更加显著[45,84,85]. Zhou等[86]的研究表明, 人类活动强度越高, 对区域内水体DOM组分特征的影响就越显著. 人类活动主要通过两种方式塑造长江流域水体DOM组分特征：①直接输入, 如污水排放、农业活动和湖泊复垦等[8,87]；②改变水体营养状态, 影响微生物生命活动, 间接影响, 如促进微生物原位生产、抑制DOM腐殖化进程等[88]. 人类在进行经济生产的过程中, 会产生大量人为源DOM, 这些DOM具有高丰度N、S等杂原子的特点, 并通过污水排放、地表径流和土壤淋溶等方式向水体输入, 直接影响流域内水体DOM组分特征[8,86,89]. 农业活动[89]、工业生产[90]和城市生活[91]等不同人类活动输入的DOM具有明显差异. 这种差异与土地利用类型紧密相关, 并塑造了长江干流上、中、下游DOM的组分特征.
人类活动在对水体输入DOM的同时, 大量营养盐也随之输入, 促进水体富营养化. Liu等[46]的研究表明, 与贫营养和中营养湖泊相比, 富营养化湖泊不仅具有更高的脂肪族、多肽类和类蛋白质的相对丰度, 分子组成也更为复杂, 含有更多的含硫DOM. 这主要是因为丰富的营养物质不仅抑制了微生物对DOM的腐殖化进程, 而且加速了浮游植物的生长, 增加了外源DOM的降解和内源DOM的释放, 促使DOM向更复杂的结构转变[46,88,92]. 长江干支流水体营养状态整体上以贫营养和中营养为主, 而流域内80%以上的湖泊处于轻富营养化到中富营养化状态, 并且湖泊水体流动性相对河流较差, 外源DOM滞留时间长, 陆源DOM更易被浮游植物转化为内源DOM, 因此人类活动对流域内湖泊的直接和间接影响均高于河流, 尤其是太湖、巢湖等中下游距离城市较近的湖泊[93,94].
3.3 土地利用类型变化对长江干流DOM组分特征的影响不同土地利用类型的空间分布是导致长江流域水体DOM组分特征存在显著差异的主要原因之一. 其中, 林地土壤中DOM主要是以木质素、单宁等不饱和芳族化合物为代表的腐殖质, 其元素组成以CHO为主, 具有结构简单且相对分子质量低的特点[95,96]. 与林地土壤相比, 由于化肥和农药的广泛使用, 耕地土壤DOM相对分子质量和结构复杂度相对较高, 元素组成中含有更多的N、S等杂原子[97,98]. 与林地和耕地相比, 城市建设用地输入DOM的主要成分不仅含有大量脂质和类蛋白质, 而且由CHON、CHOS和CHONS等含杂原子的组成丰度高于林地和耕地[8,99,100]. 不同土壤类型DOM输入水体方式不同, 林地土壤和耕地土壤中DOM主要以土壤侵蚀为主, 而城市建设用地主要以废水排放和地表径流为主. 长江流域水体中DOM沿程变化存在如下特点：随着上游至下游林地逐渐减少, 耕地和城市用地逐渐增加, 上游水体DOM以木质素、单宁等不饱和芳香族化合物为主, 呈现出较低的DOC浓度、腐殖化程度以及分子式多样性的组分特征；而下游由于沿程耕地和城市用地的增加, 以及上游部分陆源DOM的沿程降解, 其水体中DOM的相对分子质量和杂原子丰度更高, 且结构更为复杂, 分子多样性更为丰富的特点[8,21,101 ~ 103]. 因此, 长江流域水体DOM组分呈现显著的空间异质性.
3.4 三峡大坝对长江干流DOM水平输移的拦截作用目前, 已有研究对三峡大坝对长江干流DOM输移及影响因素仍不清晰. Xu等[104]研究发现, 三峡大坝对长江流域DOM水平输移具有一定的拦截作用. 与坝下水体相比, 上游DOM的浓度更高, 但平均相对分子质量和芳香性更低；三峡水库上游DOM中含有丰富的微生物源DOM, 而下游则含有更多的陆源DOM[104]. 这主要是由于三峡大坝打断了河流的连续性, 增加了库区水力停留时间, 造成上游DOM被水库截留或被微生物降解并释放生物源DOM[48,104 ~ 107]. 除此之外, 三峡水库上游被拦截的DOM随着时间的推移, 将以有机碳的形式被埋藏[2,48]. 因此, 三峡大坝一定程度上阻碍了长江干流的DOM输移过程, 从而对长江流域碳的生物化学循环产生一定影响. 然而, Zhang等[108]对坝下更大范围（从坝下到长江口）的研究发现, 三峡大坝对DOM拦截作用并不明显. 这可能与长江中下游土地利用类型变化、人类活动影响等因素相关.
4 展望随着傅里叶变换离子回旋共振质谱等技术手段的广泛应用, 长江流域水体DOM组分特征的时空演变研究已经取得了一定程度的进展, 但目前长江流域DOM演变特征的科学认识仍然处于初级阶段. 因此, 未来对长江流域DOM组分特征演变规律的进一步解析和探究需要从以下4个方面加强：
（1）目前研究主要集中于地表水, 尤其是仍然缺乏长江源区多介质中DOM组分特征的研究, 亟待开展长江流域多介质中DOM组分特征长序列-大尺度的研究, 厘清不同历史时期、不同区域多介质DOM组分特征的时空演变规律.
（2）大坝打断河流连续性, 阻碍河流DOM的输移, 从而影响流域内碳的生物地球化学循环, 但三峡大坝对长江干流DOM水平输移过程的影响仍不清晰. 因此, 开展大坝建设对流域DOM水平输移的影响研究有助于深入理解长江碳足迹的过程.
（3）亟待开展流域尺度下DOM组分特征演变对污染物环境行为影响的研究. DOM组分特征对重金属有效态、难降解有机物等污染物的迁移转化具有显著影响. 探究长江流域多介质DOM组分特征变化对污染物环境行为的影响, 对保障未来长江及三峡水库水质安全具有重要意义.
（4）全球变暖增加了强降雨等极端天气出现的频率, 从而影响长江流域水文情势变化, 这势必导致DOM组分特征的演变过程变得更加复杂. 因此, 开展极端天气对DOM组分特征及其输移的影响, 可为更深刻地理解流域内碳循环提供理论依据.
5 结论（1）长江流域水体DOM组分具有显著的时空异质性, 具体表现为：①长江干流下游DOC与多样性显著高于上游和中游, 芳香性与腐殖化程度中游高于上游和下游. 丰水期干流DOC浓度略低于枯水期, 而类腐殖质比例、芳香性及其组成多样性均高于枯水期. 长江支流水体DOC浓度、芳香性和腐殖化程度均为上游支流 < 中游支流 < 下游支流. 干支流DOM主要为来源于植物腐烂和土壤侵蚀的类腐殖质. ②流域内湖泊水体整体上DOC浓度高于干支流, 具有较高的结构复杂性, 主要为微生物生命活动释放的脂类、多肽类和类蛋白质. ③三峡库区支流水体DOC浓度、类腐殖质相对丰度沿程下降, 类色氨酸沿程增加. 丰水期水体DOM芳香性、腐殖化程度和类腐殖质丰度高于枯水期, DOC浓度、类蛋白质丰度和组成多样性和复杂程度则相反. 库区支流水体DOM组分存在显著的垂向分层, 表层水体DOM结构相对简单, 偏向内源特征；中下层DOM结构相对复杂, 更偏向陆源特征. ④长江口水体DOM浓度、芳香性、腐殖化程度和组成多样性为：淡水区 > 浑浊区 > 沿海区, 陆源和人为输入贡献呈近海高, 外海低的特点.
（2）长江流域水体DOM组分特征受到多种因素共同影响, 主要包括气候水文、人类活动强度和土地利用类型变化. 长江口及其邻近海域还受到长江径流输入和盐淡水混合过程的稀释作用等因素的影响. 除此之外, 三峡水库水体热分层、沉积物释放及异重流等因素导致其支流DOM存在显著的垂向分层. 值得注意的是, 目前已有研究对三峡大坝对长江干流DOM输移及影响因素仍不清晰.
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