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In order to enhance the support for groundwater development and utilization, as well as pollution control and prevention in Fengtai District, Beijing, a comprehensive study was conducted based on long-term monitoring data of shallow groundwater in the eastern area of Yongding River during the dry season. The mathematical statistics, Piper diagram, Gibbs diagram, and ion ratio analysis and other methods were employed to explore the pattern of groundwater hydrochemical evolution, the formation mechanism, and sources of pollution in Fengtai District. The findings were as follows:① Overall, the current groundwater quality in the study area was poor. The average concentration of each index in groundwater increased and then decreased from 1976 to the present. The pollution range of Cl - , SO 4 2- , and TH generally expanded, whereas the pollution range of TDS and NO 3 - expanded before 2005 and then decreased with 2005 as the turning point. ② The hydrochemical types of groundwater samples displayed a complex regional variation each year, as well as along the groundwater direction. The dominant anion in groundwater was HCO 3 - , and the dominant cation was Ca 2+ each year. The number of groundwater hydrochemical types in 1976 was 8, in which the predominant type was HCO 3 ·SO 4 -Ca·Mg·Na, accounting for 40%. However, the number of groundwater hydrochemical types in 2021 was 17, in which the predominant type was HCO 3 ·Cl·SO 4 -Ca·Na·Mg, accounting for 23.88%. The groundwater hydrochemical type showed a complex trend within the region and upstream along the flow direction each year, whereas the migration characteristics of groundwater samples, as depicted on the Piper diagram, indicated that the hydrochemical components of groundwater were significantly affected by human activities during its evolution. ③ The groundwater chemistry in the study area was influenced by both rock weathering and evaporative crystallization processes, with evaporation playing a major role. The alternation of groundwater cations was relatively weak, and the dissolution of carbonate minerals served as the primary source of Ca 2+ and Mg 2+ . ④ The ion ratio analysis suggested that exogenous sources, mainly agricultural activities and urban sewage, contributed to the input of NO 3 - and Cl - . The pollution impact from agricultural activities was significant before 2005, which aligned with the historical presence of numerous seepage pits, seepage wells, and direct discharge of industrial and domestic sewage for irrigation purposes in the study area. These activities were closely associated with the high levels of pollution. However, pollution input from agricultural activities notably decreased in 2021, likely due to the effective implementation of water environmental protection programs and action plans in recent years.

Phytoplankton is frequently utilized in the assessment of water ecological health, and a great number of related studies have been conducted in China; however, most of them are limited in scope. A phytoplankton survey was carried out at the basin scale in this study. A total of 139 sampling sites were set up in crucial locations of the main stream, from the Yangtze River's source region to the estuary, as well as the eight primary tributaries and the Three Gorges' tributaries. In the Yangtze River Basin, phytoplankton was found in seven phyla and 82 taxa, with Cryptophyta, Cyanophyta, and Bacillariophyta being the dominant species. To begin, the composition of phytoplankton communities in various sections of the Yangtze River Basin was studied, and LEfSe was utilized to identify highly enriched species in different regions. The association between phytoplankton communities and environmental factors in different sections of the Yangtze River Basin was then investigated using CCA. The generalized linear model demonstrated that TN and TP were strongly positively linked with phytoplankton density at the basin scale, whereas TITAN analysis identified the environmental indicator species and their corresponding optimal growth threshold range. Finally, the study assessed each Yangtze River Basin Region in terms of biotic and abiotic factors. Although the results of the two aspects were incongruent, the analysis of all indicators using the random forest method can yield comprehensive and objective ecological evaluation results for each section of the Yangtze River Basin.

To explore the periphytic algae community structure in the Yangtze River basin, samples were collected from 130 sampling sites, including the source to the estuary along the mainstream of the Yangtze River, eight primary tributaries, and the tributary of the Three Gorges area. The periphytic algae densities of different areas in the mainstream of the Yangtze River ranked from high to low were the upstream area, source area, middle and lower area, and the Jinsha River. The high periphytic algae density in the upstream area was associated with the shift in nutrition level, and the high periphytic algae density in the source area was associated with human activity. The spatial pattern of the periphytic algae community in the whole main stream from west to east presented the alternating dominance of Bacillariophyta and Cyanophyta; the Bacillariophyta ( Navicula ) had a competitive advantage in the main stream, and the distribution of the periphytic algae community was driven by total nitrogen, total phosphorus, and pH. For the tributary of the Yangtze River, the periphytic algae density in the Three Gorges tributary area was far higher than those in the eight primary tributaries; the periphytic algae community was dominated by Cyanophyta ( Lyngbya ), which had a competitive advantage in the tributaries of the Yangtze River. The distribution of the periphytic community was driven by dissolved oxygen and pH. According to the diversity analysis and assessment, the periphytic algae community in the source area showed lower species richness and higher evenness, thus leading to a high α -diversity and good assessment result (mesosaprobic zone). The middle and lower reaches of the Yangtze River also showed the same assessment result, the mesosaprobic zone. However, the community evenness of the middle and lower reaches was significantly lower than that of the source area, thus making the middle and lower reaches of the Yangtze River have a significantly lower α -diversity than that of the source area. All areas of the Yangtze River showed good water quality assessment; however, different areas had different WQI index numbers, and the assessment results of the WQI index were inconsistent with the results of the aquatic assessment. Therefore, a comprehensive assessment of aquatic ecosystem health should use both aquatic assessments and water quality assessments.

To explore the bacterioplankton community structure in the Yangtze River basin, water samples were collected from 177 sampling sites, including the source to the estuary along the mainstream of the Yangtze River, eight primary tributaries, and several lakes and reservoirs. The 16S rRNA genes were used to explore the bacterioplankton communities based on single molecule real-time sequencing, with the aim to study the diversity and community characteristics in a border sampling area and higher species annotation accuracy. Based on α -diversity analysis, the river area had higher species richness than that of the lake/reservoir area, resulting in these two areas having different bacterioplankton community diversities. Based on the β diversity analysis, the bacterioplankton showed different community compositions between the river and lake/reservoir areas; temperature was the key environmental factor for the river area, and pH was the key environmental factor for the lake/reservoir area. In order to study the influence of different bacterioplankton communities, this study further investigated the species, function, and community differences between the river and lake/reservoir areas. The results were as follows:for the river area, the eutrophication level gradually increased from west to east along the mainstream of the Yangtze River, resulting in a gradually increased relative abundance of specific species. The lake/reservoir area had a higher risk of cyanobacteria bloom, and the opportunistic pathogen had a high relative abundance in the Danjiangkou Reservoir, indicating a higher ecological risk. For species composition, the river and lake/reservoir areas shared most OTUs (84%); however, some uncultured bacteria showed a high relative abundance in the Yangtze River, meaning the bacterioplankton of the Yangtze River basin still requires further study. In general, the river and lake/reservoir shared most species; however, the different bacterioplankton diversity, community composition, and enriched species made the river and lake/reservoir have different key environmental factors, and they also showed differences in ecological functions.