河流在连接陆地、湖泊与海洋生态系统中起着重要的作用，也提供重要的生态系统服务，例如提供生活生产用水，为水力发电提供能量，为航运提供运输渠道等。在水环境中，浮游细菌是营养元素的转运者和能量流动的主要贡献者。以往的研究者已经在土壤、湖泊等生态环境中，对环境中细菌群落的组成和多样性及其与环境因子的关系等方面进行了较为详尽的研究，而对于开放流动水体中的浮游细菌群落的了解还非常少。浮游细菌作为随水体流动的细菌类群，其组成和多样性都会受到沿途水环境因子的影响，例如无机营养盐因子、水的溶解性有机碳等可能作为细菌的营养来源而对不同营养类型的细菌的增殖直接产生影响，而水温、pH、流速等水文和理化因子则会通过改变栖息地的条件，间接地影响水体中优势类群的种类与组成。此外，河岸带土地利用因子作为反映人类活动的重要指标，一定程度上反映了非点源污染的强度，在环境保护和土地管理等方面占据重要地位，可能也会对河流中浮游微生物群落产生直接或间接的影响。过去由于遥感数据质量和地面物体识别的限制，其研究主要集中在某些特定类别的土地利用上，如农田或城市化地区，而忽略了集水区土地利用组成对细菌群落的综合影响。滦河位于中国北方，是典型的温带河流，同时也是“引滦入津”工程的水源地，其水质与下游的生产生活息息相关。本研究在春季沿滦河干流中上游进行采样，基于二代测序技术获得了21个样点的浮游细菌群落数据，分析地理距离因子，水质因子和土地利用因子对河流浮游细菌群落组成的影响，比较不同土地利用类型组之间浮游细菌群落组成、特征类群以及群落功能的差异，结果如下：

（1）对水体样本中的浮游细菌进行16S rDNA的高通量测序获得了有效序列3,414,503条，以97%的阈值对操作分类单元（OTU，Operational Taxonomic Unit）进行划分后，共获得15,856个OTU，分属细菌的28个门。选取至少在一个样本中相对丰度大于1%的优势类群对浮游细菌的组成进行分析，共有11个优势细菌门（纲）。在优势门（纲）中，除拟杆菌门、β-变形菌纲、放线菌门、γ-变形菌纲外，其余各门（纲）的相对丰度几乎均与距源头距离、氨氮、硝酸盐氮、总氮、电导率、总悬浮固体、河岸带缓冲区建设用地占比存在显著的负相关关系（P < 0.05），拟杆菌门和β-变形菌纲的相对丰度与上述环境因子均存在显著的正相关关系（P < 0.05），而放线菌门和γ-变形菌纲的相对丰度与大多数环境指标均不存在显著的相关关系。

（2）在所有样本中出现频率大于90%的OTU构成了核心群落。核心群落的相对丰度与距源头距离存在显著的正相关关系（P < 0.05），沿河流流向，样点核心群落相对丰度占比由~10%增加到~80%，群落组成逐渐简单化，最终形成与核心群落组成相似的，以变形菌门、放线菌门、拟杆菌门为主的群落。通过淡水细菌数据库对物种进行注释，发现距源头距离与非典型淡水分类群浮游细菌的相对丰度存在显著的负相关关系。

（3）对于α-多样性指数，OTU数，Chao1丰富度指数与香农多样性指数均与距源头距离存在显著的负相关关系，Chao1丰富度指数在河流中段的反常升高暗示了外源性偶见种的大量涌入现象，而导致这一结果的原因可能是土地利用结构的改变。

（4）对于β-多样性，通过Mantel检验考察细菌群落Bray-Curtis距离矩阵与地理空间因子、水质因子与土地利用因子欧式距离矩阵的相关性，以研究各样点浮游细菌群落组成差异与各水环境因子差异之间的关系。地理空间因子（距源头距离）是与细菌群落的Bray-Curtis距离矩阵相关性最高的变量。在以距源头距离为控制变量的情况下，只有总氮，硝酸盐氮，氨氮这三类与氮营养盐因子与细菌群落的Bray-Curtis距离矩阵相关（P < 0.05）。在土地利用因子（森林，草地，耕地，建设用地）中，无论是否以距源头距离作为控制变量，均只有建设用地与细菌群落的Bray-Curtis距离矩阵显著相关（P < 0.05），而将水质因子中的硝酸盐氮，电导率，总氮，氨氮，pH，叶绿素a和流速作为控制变量后，建设用地占比与细菌群落的Bray-Curtis距离矩阵不再存在显著的相关关系。这表明距源头距离代表的滞水时间和氮营养盐对细菌群落的门类组成存在显著影响；而土地利用因子，尤其是建设用地，其对细菌群落门类组成的影响可能是主要通过对河流理化条件的综合影响实现的。

（5）根据河岸带土地利用组成的差异，可将样点分为三类：草地占主导的近自然组样点，林地、耕地和建设用地混合分布的过渡型组样点，以及建设用地占主导的城镇化组样点。PERMANOVA结果表明三类样点之间群落组成存在显著差异（P < 0.05）。三类样点之间，OTU数和香农多样性指数均存在显著差异（P < 0.05）。对于Chao1丰富度指数来说，近自然组与过渡型组, 近自然组与城镇化组样点存在显著差异（P < 0.05），而过渡型组和城镇化组样点之间不存在显著差异。三类样点之间，核心群落相对丰度和非典型淡水分类群相对丰度均存在显著差异（P < 0.05）。

（6）通过LEfSe分析，对三类样点的特征类群进行了鉴别。近自然组的特征类群有7个，均与总悬浮固体，总氮，硝酸盐氮，氨氮，电导率存在显著的负相关关系（P < 0.05）。过渡型组样点的特征类群有1个，与总悬浮固体和总磷存在显著的正相关关系。城镇化组样点的特征类群有9个，几乎都与总氮，硝酸盐氮，氨氮和电导率存在显著的正相关关系。

（7）使用Faprotax对整体的细菌群落功能进行了评估，并结合多重比较寻找不同土地利用分组间具有差异性的判别群落功能。其中近自然组样点的判别群落功能主要为As, S, Fe的代谢，硝化与反硝化作用，氨氧化作用，芳香化合物与木质素的分解等；而过渡型组样点则只有芳香族碳水化合物的分解；城镇化组样点的判别群落功能主要为动植物病原体与肠道微生物，塑料降解，甲基营养型等。

Rivers play an important role in connecting terrestrial and Marine ecosystems and  providing ecosystem services, such as water for life and production, energy for hydropower generation, and transportation channels for shipping. In water environment, bacterioplankton are major contributors to nutrient transport and energy flow. Previous researchers have conducted detailed studies on the composition and diversity of bacterioplankton and their relationship with environmental factors in soil, lakes and other ecological environments. However, little is known about the bacterioplankton groups in open lotic water. Bacterioplankton, as bacteria groups flowing with water flow, the composition and diversity would be influenced by the water environmental factors along the way. For example, inorganic nutrient and dissolved organic carbon may directly affect the proliferation of bacteria of different nutrient types as the nutrient source, while hydrologic and physicochemical factors such as water temperature, pH and flow rate may indirectly affect the species and composition of dominant taxa in water by changing the habitat conditions. Besides, as an key indicator of anthropogenic activities, land use in riparian zone reflects the intensity of non-point pollution to some extent, plays an important role in environmental protection and land management, and may also have direct or indirect impact on the phytoplankton groups in the river. Due to the limitation of the quality of remote sensing data and ground object recognition, previous studies mainly focused on one certain type of land use, such as farmland or urbanized land, while the comprehensive impact of land use composition of watershed on phytoplankton groups was neglected. Located in the north of China, the Luanhe River basin is a typical temperate river and the water source for Luanhe-Tianjin water diversion project, whose water quality is closely related to the production and life of the downstream residents. In this study, samples were taken in the spring along the middle and upper reaches of the Luanhe River mainstream. Based on the next generation sequencing technology, 21 samples of bacterioplankton community data were obtained. This study analyzed the effects of geomorphological factor, water quality factor and land use factor on the composition of bacterioplankton communities in the river, and compared the differences in the composition, characteristic taxa (discriminant taxa) and community function potential of them among different type of land use groups. Based on the laboratory test, the water quality factor data were obtained. Based on the resolution of 30 m Landsat thermal Mapper image, the data of land use were obtained. The influence of geographical distance factor, water quality factor and land use factor on the composition of river bacterioplankton community were analyzed. Among the sample points of different land use composition groups, the differences of bacterioplankton community composition, predominant discriminant taxa and comunity function were revealed. The results are as follows:

(1) High-throughput 16S rDNA sequencing of bacterioplankton in water samples yielded 3,414,503 high-quality sequences. After dividing the OTUs (Operational Taxonomic Unit) with 97% threshold value, a total of 15,856 OTUs were obtained, which could be classified into 28 phyla of bacteria. The composition of bacterioplankton was analyzed after selecting dominant groups with relative abundance greater than 1% in at least one sample, and 11 dominant phyla (class) were found. In those bacterioplankton, except Bacteroides, β-Proteobacteria, Actinobacteria and γ-Proteobacteria, the relative abundance of almost all dominant groups were negatively correlated with the distance from headwater, ammonia nitrogen, nitrate nitrogen, total nitrogen, electroconductibility, total suspended solids, construction land proportion of the riparian buffer (P < 0.05). The relative abundance of Bacteroides and β-Proteobacteria were positively correlated with those environmental factors. The relative abundance of Actinobacteria and γ-Proteobacteria were not significantly correlated with almost all environmental factors.

(2) In all samples, OTUs with a frequency of more than 90% constituted the core community. The relative abundance of the core community was positively correlated with the distance from upper headwater (P < 0.05). Along the flow direction of the river, the relative abundance of the core community increased from ~ 10% to ~ 80%. The community composition was gradually simplified, and finally the community with Proteobacteria, Actinobacteria and Bacteroides as the core community was formed.  By annotating the species in the database of freshwater bacteria, it was found that the distance from headwater was negatively correlated with the relative abundance of non-typical freshwater taxa.

(3) As for the α-diversity index, number of OTUs, Chao1 richness index and Shannon diversity index were negatively correlated with the distance from headwater. The abnormal increase of Chao1 richness index in the middle reach suggested a large influx of Allochthonous Rare Species, which may be caused by the change of land use structure.

(4) As for the β-diversity index, the correlation between the Bray-Curtis dissimilarity matrix of bacterial community and the Euclidean dissimilarity matrix of geographical factors, water quality factors and land use factors was investigated by Mantel test, so as to study the relationship between the difference of bacterioplankton community composition and the difference of various water environment factors at various points. The Geographical factor (distance from headwater) was the variable most correlated with the Bray-Curtis dissimilarity matrix of bacterial community. In the case that the distance from headwater was the control variable, only total nitrogen, nitrate nitrogen and ammonium nitrogen were correlated to the Bray-Curtis dissimilarity matrix of bacterial community (P < 0.05). Among the four land use factors (area ratio of woodland, grassland, farmland and urbanized land), only the area ratio of urbanized land was correlated with the Bray-Curtis dissimilarity matrix of bacterial community (P < 0.05), regardless of whether the distance from headwater was used as the control variable. However, after taking nitrate nitrogen, electroconductivity, total nitrogen, ammonia nitrogen, pH, chlorophyll-a and velocity of flow as control variables, there was no significant correlation between area ratio of urbanized land and Bray-Curtis dissimilarity matrix of bacterial community. The result of Mantel test indicated that the water retention time (represented by distance from headwater) and nitrogen nutrients had significant effects on the phyla composition of the bacterial community. While the influence of land use factors, especially the area ratio of urbanized land, on the phyla composition of bacterial community may be realized mainly through the comprehensive influence on the physical and chemical conditions of rivers.

(5) According to the difference of land use composition in the riparian zone, the sample points can be classified into three groups: near-natural sample points, whose land use composition were dominated by grassland; transitional sample points, whose land use composition were mixed with woodland, farmland and urbanized land; urbanized sample points, whose land use composition were dominated by urbanized land. The results of PERMANOVA indicated significant differences in community composition between three groups of sample points (P < 0.05). There were significant differences in number of OTUs and Shannon diversity index between the three groups of samples (P < 0.05). As for the Chao1 richness index, there were significant differences between near-natural group and transitional group, and between near-natural group and urbanized group (P < 0.05), while there was no significant difference between transitional group and urbanized group. The relative abundance of the core community and non-typical freshwater taxa were significantly different between the three groups of sample points (P < 0.05).

(6) Characteristic taxa of three groups of sample points were identified by LEfSe analysis. There were 7 characteristic taxa in the near-natural group, all of which were negatively correlated with total suspended solid, total nitrogen, nitrate nitrogen, ammonia nitrogen and electroconductivity (P < 0.05). There was only one characteristic taxa in transitional samples, which had a significant positive correlation with total suspended solids and total phosphorus. There were 9 characteristic taxa in urbanized group, almost all of which were significantly positively correlated with electroconductivity, total nitrogen, nitrate nitrogen, and ammonia nitrogen.

(7) Faprotax was used to evaluate the function of bacterial community, and Nonparametric multiple comparison was applied to find the discriminant function among groups with different composition of land use. Among them, the discriminant functions of near-natural group were mainly metabolism of As, S, Fe, nitrification and denitrification, ammoxidation, degradation of aromatic compounds and lignin, etc. The discriminant function of transitional group was only the degradation of aromatic carbohydrate. The discriminant functions of urbanized group were mainly animal and plant pathogens, gut microorganisms, plastic degradation, methylotrophy, etc.