How bacterioplankton community can go with cascade damming in the highly regulated Lancang–Mekong River Basin

Abstract

Rivers make vital contributions to the transport of water, sediment and nutrients from terrestrial to marine ecosystems. However, many large rivers worldwide are suffering from dam regulation. Increasing attention has been paid to bacterioplankton communities since they are highly responsive to river alterations and may influence biogeochemical processes. Here, a comprehensive study was conducted in the highly regulated Lancang-Mekong River Basin to address the question of how bacterioplankton communities respond to cascade damming. The results showed that dam constructions increased nutrient concentrations and threatened water quality in cascade reservoirs. Bacterioplankton cell abundance was reduced by damming, and α-diversity was inhibited in cascade reservoirs. Fortunately, however, river ecosystems were resilient after the remarkable disturbance caused by damming. Moreover, bacterioplankton community composition was significantly altered by cascade dams, including a shift in the dominant phylum from r-strategists to k-strategists. Meanwhile, according to GeoChip analysis, the functional composition of bacterioplankton was less affected than taxonomic composition. In addition, geographic and environmental features both followed a distance-decay relationship with community and functional composition, but the local environment condition was the dominant driver in the Lancang River. Therefore, the impoundments of cascade dams had significant impacts on bacterioplankton communities and more attention should be paid to the potential ecological consequences of river regulation.

Sedimentary microeukaryotes reveal more dispersal limitation and form networks with less connectivity than planktonic microeukaryotes in a highly regulated river

“…Taxonomic composition of microeukaryotes in the Lancang River are markedly different from those in estuaries (Chen et al., 2017) and oceans (Wu et al., 2017), probably due to the distinct differences in salinity and hydraulic conditions. Similar to the composition patterns of dominant bacterial phyla in rivers (Chen et al., 2020; Wang et al., 2018), the relative abundances of dominant microeukaryotic phyla varied between different river sections. Notably, Diatomea, as obligate phototrophs present in all types of aquatic ecosystems (Cermeno & Falkowski, 2009), had higher relative abundance in the upstream than in the downstream, for both planktonic and sedimentary microeukaryotes.…”

“…(2009) found that sedimentary bacteria in certain reaches of the Yellow River had slightly lower diversity than the bacterioplankton, presumably due to the high concentration of suspended sediment. Although bacterial diversity was reported to alter from headwaters to the river mouth (Savio et al., 2015; Wang et al., 2018), the change in microeukaryotic diversity in different river sections has rarely been investigated. For the planktonic microeukaryotic community, we observed lower species richness in the cascade reservoir section (from Mwei.A to NZD.4), especially in dam‐affected sites, suggesting a negative effect of dams on microeukaryotic diversity.…”

“…The TN concentration in water and sediment gradually increased from upstream to downstream in the Lancang River, probably owing to the enhanced retention of nutrients in reservoirs (Ruiz‐Gonzalez et al., 2013). Since most of the downstream sites are in densely populated regions (Wang et al., 2018), anthropogenic discharges may also increase N levels. In contrast, the slow‐moving water bodies in reservoirs increase sedimentation of suspended particles (Timpe & Kaplan, 2017), leading to decreased TOC concentration in water.…”

“…Understanding the mechanisms that underpin the assembly of microbial community and their biogeographic patterns is a hotspot in microbial ecology (Mo et al., 2018; Xue et al., 2018). Recent studies have increasingly emphasised the potential contribution of both environmental selection and dispersal limitation processes upon microbial community composition (Langenheder & Székely, 2011; Liao et al., 2017; Lindström and Langenheder 2012; Wang et al., 2018). However, there is an ongoing debate regarding the dominant process driving spatial variation in microbial community in aquatic ecosystems.…”

Fungal community demonstrates stronger dispersal limitation and less network connectivity than bacterial community in sediments along a large river

“…It is known that microbial diversity is significantly altered by varying geographical and environmental factors along a large river (Velimirov et al ., ; Tian et al ., ). Spatial variability of microbial composition from headwaters to the river mouth has been well demonstrated (Savio et al ., ; Chen et al ., ; Wang et al ., ), but most previous studies only focused on one taxonomic group. Regarding bacteria and fungi in large rivers, one would expect that the spatial patterns of their diversities differed substantially among different river sections.…”