[Effects of Reservoir Water Depth on Different Plankton Communities and Keystone Species of Network Interaction].

In order to understand the impacts of the reservoir construction on the diversity and ecological network of different microbial communities, seven sampling sites were set up in the Hengshan Reservoir in 2021. Water samples were collected from the surface and bottom of the reservoir. After filtering and extracting total DNA samples, high-throughput sequencing was carried out based on 16S and 18S rDNA to investigate the response of community structure, molecular ecological network, and keystone species of different microbial groups to water environment changes. The results showed that the Richness, Simpson, Shannon, and Pielou's Evenness indices of bacterial community in the surface and bottom layers were higher than those in the eukaryote community. The dominant community of bacteria included Proteobacteria, Actinobacteria, and Bacteroidetes, and the eukaryote community included Arthropoda, Ciliophora, Ochrophyta, etc. Moreover, the density and average clustering coefficient of the microbial networks in the surface waters of different phytoplankton communities were higher than those in the bottom waters. It was also observed that the microbial ecological networks in the surface waters were more closely related, and the number of nodes and edges, as well as the number of keystone species, of bacterial communities in the surface and bottom layers were significantly higher than those in the eukaryote microbial communities, indicating that the bacterial community network was larger, and the cooperative relationship and network connectivity between species were stronger. The interaction between bacterial community and eukaryote community in different water depths was dominated by positive correlation, and the negative correlation of the two groups in the bottom layer was slightly greater than that in the surface, indicating that the competition between bottom-layer species was greater than that between surface-layer species. In addition, the environmental impact factors of all species and keystone species of the community in surface water were basically the same, but they differed greatly in deep water, indicating that the influence mechanism of water depth change on keystone species was not the same as that of all species. The results further revealed the effects of reservoir construction on the stability and interspecific interactions of different microbial communities and provided a theoretical basis for predicting variations in microbial community and material cycling in reservoirs.

[Effects of Damming and Impoundment on Planktonic Microbial Community Structure and Interspecific Interaction Patterns in Different Water Depths].

Microbial communities play an important role in ecological processes through direct or indirect interactions. However, the interspecific interaction pattern of reservoir microorganisms has still received little attention. The Hengshan Reservoir in Jiangsu province was selected as the research area. Surface and deep water samples were collected to respectively explore the response of planktonic microbial community structure and the molecular ecological network to water depth changes, as well as to identify the key species of planktonic microbial communities in different water depths. The results showed that the richness and evenness of planktonic microorganisms in the surface layer were higher than those in the deep layer, and there were significant differences in the community structure of planktonic microorganisms in different depths. Water depth indirectly affected the planktonic microbial community by changing the environmental heterogeneity, among which the indicators such as TOC, NO3-, NH4+, and PO43- had the most significant impact on the planktonic microbial community structure. In addition, the number of nodes and edges of the deep planktonic microbial molecular ecological network were significantly higher than those on the surface, and the type and number of key species in the deep planktonic microbial community were significantly higher than those on the surface, indicating that the network scale of the deep planktonic microbial community was larger; however, the interspecific cooperative relationship and network connectivity of the surface planktonic microbial community were stronger. These results will provide a scientific basis for the study of interspecific interactions of reservoir microbial communities and are of great significance for predicting the ecological law of microorganisms in reservoir ecosystems and the impact of global material cycles.

[Application Effect of Four Typical Submerged Macrophytes on Removing Cadmium from Polluted Sediment].

In this study, four typical submerged macrophytes, namely Hydrilla verticillata, Elodea canadensis, Potamogeton crispus, and Ceratophyllum demersum, were tested for their chlorophyll content and antioxidant enzyme activity, to analyze their cadmium tolerance. The biota-sediment accumulation factor (BSAF) and plant transport factor (TF) were utilized to understand the accumulation capacity of the plants. Finally, the distribution of cadmium in submerged macrophytes was revealed through phytohistochemical methods, to provide theoretical support for the practical application of submerged macrophytes. The results showed that three of the plants, excluding C. demersum, exhibited varying tolerances to cadmium pollution in sediments, among which the tolerance of H. verticillata and E. canadensis were the strongest. P. crispus had the strongest accumulation capacity (BSAF was 2.32) at relatively low pollution levels (≤20 mg·kg-1). In this study, because of its weak root system, the BSAF of C. demersum was less than 1.0, indicating that macrophyte roots play an important role in phytoremediation of Cd-contaminated sediments. Comparing the TF of different plants, it can be found that the roots of H. verticillata had the strongest above-ground cadmium transport capacity, while P. crispus mainly accumulated cadmium in its roots. At the same time, when the cadmium concentration was 50 mg·kg-1, cadmium was found to be evenly distributed in the stem organelles of P. crispus, in contrast to the other plants. Based on the cadmium tolerance and accumulation mechanism of the plants, and in consideration of real-world factors, H. verticillata and P. crispus were selected as ideal plants for repairing cadmium-containing sediments. Meanwhile, based on the different transport capabilities of plants, it is recommended that only the above-ground part of H. verticillate should be removed, while P. crispus should be uprooted regularly.

[Biogeographic Distribution Patterns of Diatoms in Lancang River and Their Key Drivers].

As an important primary producer, diatoms play a key role in aquatic ecosystems. However, little is known about the geographical distribution characteristics and driving factors of diatoms in large rivers. In this study, based on a high-throughput sequencing dataset of microeukaryotes, we analyzed the diversities and community compositions of planktonic and sedimentary diatoms in the 1200 km mainstream of Lancang River, a typical large river in southwestern China. The results showed that the diversities of planktonic and sedimentary diatoms in Lancang River were higher in the upstream natural section, and the community compositions of both groups were significantly different among different river sections. Dam construction had a significant effect on the dominant genera. Variance partitioning analysis showed that dispersal limitation was a major driving factor for the distribution pattern of planktonic and sedimentary diatoms, with explanation proportions of 16.7% and 29.8%. Co-occurrence network analyses showed that the interspecific competition relationship and network connectivity of the planktonic diatom network were stronger than the sedimentary ones. The network connectivity of planktonic and sedimentary diatoms in the cascade reservoir section was higher than that of the upstream natural section. This study will help to better understand the biogeographical distribution of diatoms in large rivers and provide useful information for ecological responses of diatoms to dam construction in rivers.

[Contaminant Characteristics and Ecological Risk Assessments of Heavy Metals from River Networks in the Western Area of the Wangyu River].

To study the pollution characteristics and ecological risks of heavy metals in the western area of the Wangyu River, water and surface sediment samples of five rivers were investigated. The concentration and chemical fractions of eight heavy metals (Ni, Cu, Cr, Zn, As, Cd, Pb, and Hg) were analyzed, the pollution sources of heavy metals were studied using multivariate statistical analysis, and various ecological risk assessments were applied to identify the level of heavy metal contaminants. The results showed that the concentration of heavy metals in water was low, except for Hg, which was lower than the Class I standard of Surface Water Environmental Quality Standard. The content of heavy metals in surface sediments was high, except for Hg, which was significantly higher than the environmental background values. The partition coefficient of heavy metals in water-sediments showed that Cd, As, and Hg had strong re-emission potential. The Igeo indicated that Cu, Zn, and Cd were in a high pollution state as a whole. The RI showed that Cd was the main ecological risk factor in the study area. The RRSP indicated that the degree of pollution by Ni, Zn, and As was low because of its high residual fraction ratio, which was difficult to release under natural conditions. However, the degree of pollution by Cd was high because it had the highest content of weak acid extractable fraction among all the heavy metals. Therefore, it is necessary to pay more attention to Cd contamination. The source analysis of heavy metals showed that the western area of the Wangyu River was mainly affected by the pollution emissions from the surrounding machinery factories.