Currently, many rivers are highly regulated for hydropower or water supply purposes, which generates major hydrological disturbances at the spatial scale . The river continuum concept (RCC) proposed by Vannote et al.  considered that biological communities can be characterized as establishing a temporal continuum of synchronized species replacements, and the physical variables within a stream system exhibit a continuous gradient. However, few riverine ecosystems remain free-flowing along their entire course. The serial discontinuity concept (SDC) provides another conceptual framework to describe longitudinal changes in abiotic and biotic variables in rivers under dam influence . Regulation by dams typically results in an alternating series of lentic and lotic reaches, and the disruption in continuous processes and nutrient cycles causes changes in the ecological processes of the entire river . The act of river damming and impounding alters the river continuity, the velocity decreases upon approaching the dam site, and the created reservoir becomes a lacustrine system, thus driving the transition of the aquatic ecosystem from lotic into lentic [4, 5]. Moreover, artificial operation of dams for various purposes change the hydraulic retention time and impede the flow of biogenic elements, including carbon, phosphorus, nitrogen and silicon, along with river networks, leading to enhanced biogeochemical cycling . In addition, increased nutrient retention via sedimentation or outgassing in reservoirs influences downstream environments . As a result, the upstream and downstream sections of the reservoirs tend to greatly differ in their physical and chemical environments, and the composition and structure of the microbial community correspondingly undergo tremendous changes [1, 7–9].
Planktonic bacterial and eukaryotic communities are essential members of aquatic ecosystems with an extremely high level of genetic diversity. Both are important participants in the global biogeochemical cycle [1, 10]. Planktonic bacteria can assimilate and remineralize inorganic nutrients, which are channeled to higher trophic levels via the predation of protists [8, 11, 12]. The spatial and temporal distributions of microbial processes in freshwater ecosystems may vary depending on environmental variables [13, 14], temperature , hydrological factors [16, 17], grazing pressure , and changes in land use , etc., and the microbial diversity and community structure respond to changes in these environmental variables. Planktonic microbial biogeographic patterns and assembly mechanisms have been widely studied in large rivers, such as the River Thames Basin , Danube River , Mississippi Rivers [21, 22] and Yangtze River [9, 23]. These publications mostly emphasized distance-decay relationships and microbial community changes on a large catchment scale . They considered that dam construction on a river often leads to notable changes in microbial communities at dam-affected sites [7, 24, 25]. In addition, the bacterial taxa in sediments downstream of a dam are drastically reduced due to severe riverbed scouring . Our previous results also found that dams would significantly reduce the α-diversity of planktonic bacterial communities on the large-scale catchment, and the microbial communities/species would be conducive to recovery in river habitats. River damming often causes a sharp rise/decline in physical or hydrological variables, finally causing natural biophysical gradient discontinuities in local environments . However, it remains unclear whether these discontinuities caused by damming would directly or indirectly alter microbial communities and how microbes respond in these local-scale changing habitats. There still remains a knowledge gap regarding the impacts of damming on the transformation of microbial communities downstream of large dams.
At present, correlation network analysis has been widely applied to understand the organization of microbial communities and the interaction between their components in aquatic ecosystems. According to the topological characteristics of the network, potential target species (keystone species) can often be identified at the central node of the network, and these species play a more important role in the network, while their appearance or disappearance can disturb a mature community [26–29]. A particular bacterium in the network may adhere to a particular ecological (or life) strategy . Life strategies represent sets of correlated traits attributed to physiological or evolutionary tradeoffs, and in-plant communities, tradeoffs in key fitness traits have been represented through the conceptual r- and K-selection theories [30–32]. The classification of life strategies has been applied in microbial systems. For example, studies in the Thames River Basin and Lancang-Mekong River Basin indicated that along with the river network from upstream to downstream, the dominant phylum of bacteria shifted from r-strategists (Bacteroidetes) to K-strategists (Actinobacteria) [19, 25]. In particular, the network interaction and ecological strategies can characterize the response mechanisms of keystone species in these dynamic environments.
It is noteworthy that that large dams are not absolute barriers that disconnect the upstream reservoir and downstream river reach. Waters traveling through dams for hydropower production or other functions will bring aquatic microbial community to downstream river reach. Distinctive habitats shape different community assembly mechanisms in the upstream and downstream of the dam, projecting the potential damming impacts on riverine aquatic ecosystems. Compared to the extensive studies on larger scales, we highlighted our study focusing on local scale, i.e. the nearest and most accessible sampling sites upstream and downstream of the dams. We hypothesized that there would be possible links of aquatic microbial community between the sampling sites upstream and downstream of the dam due to the dam operation, shaping the ecological mechanisms of microbial assembly in damming rivers. We tried to explore the different responses of eukaryotic and bacterial communities on such local scale. We believe our study would provide new insights of aquatic microbial ecology in damming rivers.
Here, we selected three large dams in the upper Yangtze River (Xiluodu Dam, Xiangjiaba Dam, and Three Gorges Dam) as the research area. Both 16S rRNA and 18S rRNA high-throughput gene sequencing techniques were applied to investigate the bacterial and eukaryotic communities at sampling sites nearest upstream and downstream of these three dams in May, July, and November 2019. This study aimed to 1) examine the changes in microbial communities and diversity right downstream of these large dams; 2) explore the keystone species and natural connectivity of co-occurrence networks; 3) evaluate the potential damming effect on the composition of planktonic bacterial and eukaryotic communities on local scale.