Sediment bacterial biogeography across reservoirs in the Hanjiang river basin, southern China: the predominant influence of eutrophication-induced carbon enrichment

Abstract

A fundamental goal of reservoir ecosystem management is to understand bacterial biogeographic patterns and the mechanisms shaping them at a regional scale. However, little is known about how eutrophication, a major water quality challenge in reservoirs, influences sediment bacterial biogeographic patterns in subtropical regions. In this study, sediment bacterial communities were sampled from 21 subtropical reservoirs in the Hanjiang river basin, southern China, and spanning trophic states from oligotrophic to eutrophic. Our findings demonstrated that eutrophication-driven changes in total carbon (TC) significantly shaped the regional biogeographic patterns of sediment bacterial communities, weakening the "distance-decay" relationships that typically link bacterial community similarity to geographical distance. TC content exceeding a threshold of 13.2 g·kg^-1 resulted in substantial shifts in bacterial community structure. Specifically, high TC levels promoted the dominance of copiotrophic bacteria such as Syntrophales (Deltaproteobacteria), Clostridiaceae (Firmicutes), and VadinHA17 (Bacteroidetes), while oligotrophic taxa like Anaerolineaceae (Chloroflexi) and Nitrospirota were prevalent in low TC sediments. Additionally, higher TC content was associated with increased regional heterogeneity in bacterial community composition. Reservoirs with elevated TC levels exhibited more complex bacterial interaction networks, characterized by stronger niche segregation and higher competition compared to low TC networks. Overall, these findings underscore the pivotal role of sediment TC in shaping bacterial biogeography at a regional scale. They provide valuable insights for predicting ecosystem responses to eutrophication and offer guidance for mitigating the impacts of anthropogenic activities on freshwater ecosystems.

Keywords: biogeographic pattern; homogeneous selection; niche breadth; sediment bacteria; subtropical reservoirs.

Figure 1

Location of sampling sites in 21 reservoirs in the Hanjiang river basin, southern China. CT, Changtan reservoir; DB, Duobao reservoir; DFH, Dongfanghong reservoir; FH, Fenghuang reservoir; FS, Fushi reservoir; FX, Fengxi reservoir; GS, Gangshan reservoir; GT, Guitian reservoir; HS, Heshui reservoir; HSY, Heshanyan reservoir; HT, Huangtian reservoir; HZP, Huangzhuping reservoir; MHT, Mianhuatan reservoir; MX, Meixi reservoir; PX, Pengxi reservoir; QLS, Qingliangshan reservoir; QX, Qingxi reservoir; SB, Shibi reservoir; WG, Wengong reservoir; YQ, Yanqian reservoir; YT, Yitang reservoir.

Figure 2

Heatmap displaying the changes in sediment and water environment parameters of the studied reservoirs in the Hanjiang river basin, southern China. The color scale for each environmental parameter varies between the minimum and maximum values. TC, sediment total carbon; SWC, sediment water content; TS, sediment total sulfur; TH, sediment total hydrogen; Ca²⁺, sediment calcium; Fe³⁺, sediment iron; Cu²⁺, sediment copper; Mn²⁺, sediment manganese; TP, sediment total phosphorus; TN, sediment total nitrogen; TSI, water trophic state index; Chla, water chlorophyll a. The locations and the full names of the studied reservoirs are given in Supplementary Table S2.

Figure 3

Relationships among environmental factors and their correlations with the relative abundance of the dominant (sub) phyla of bacterial communities (A). The relative abundances of Bacteroidetes, Chloroflexi, Firmicutes, Nitrospirota, Planctomycetota, and Deltaproteobacteria along the TC gradient (B). The relative abundance of Chloroflexi and Nitrospirota decreases linearly with increasing TC content, whereas Bacteroidetes, Firmicutes, Planctomycetota, and Deltaproteobacteria show positive correlations with TC content. TC, sediment total carbon; SWC, sediment water content; TS, sediment total sulfur; TH, sediment total hydrogen; Ca²⁺, sediment calcium; Fe³⁺, sediment iron; Cu²⁺, sediment copper; Mn²⁺, sediment manganese; TP, sediment total phosphorus; TN, sediment total nitrogen; TSI, water trophic state index; Chla, water chlorophyll a.

Figure 4

Threshold indicator taxon analysis of sediment bacterial communities in responses to TC content (A). Red and blue symbols and areas indicate the magnitude of the summed z scores of increasing (z+) or decreasing (z–) taxa with an increasing TC content. The peak indicates the points where the sediment bacterial community structure gets large change along the total carbon gradient (the TC at the Z+ peak is approximately 13.2 g·kg⁻¹). Multiple regression tree (MRT) with bacterial community composition as the response variable and environmental factors as the explanatory variables [(B), n = 63].

Figure 5

Non-metric Multidimensional Scaling (NMDS) shows sediment bacterial community structure arranging to sites (A) or to total carbon groups (B). LC, low carbon content (< 13.2 g·kg⁻¹) and HC, high carbon content (≥ 13.2 g·kg⁻¹). Differences in bacterial community composition with geographical distance (C) as well as the beta diversity of bacterial communities in different total carbon groups (D).

Figure 6

Relative importance of bacterial community assembly processes across all reservoirs (A) and within reservoirs with high (HC) and low (LC) total carbon content (B).

Figure 7

Correlation networks of the bacterial community in both high-carbon (A) and low-carbon (B) groups. The color of the nodes represents different modularity types. The within-module degree (Zi) and the participation coefficients (Pi) of each node can be used to identify their roles in high-carbon (C) and low-carbon (D) networks, respectively. The taxonomy of network nodes was showed at the phylum level (E). The color represents the sum of phylum relative abundance, while the degree represents the sum of the phylum degrees in the network.