Different types of rivers have their own ecosystems. Based on the laden-starved sediment content in rivers, they can be classified as sediment-laden turbid (SLT) and sediment-starved clear (SSC) river ecosystems. Understanding the evolution process and driving factors of these two river ecosystems is very important for the protection of their ecology. Until now, however, there has been no comprehensive study that integrates biological and nonbiological factors in river ecosystems. Here, the self-organizing feature map (SOFM) model based on the maximum generalized entropy principle can integrate the two and conduct research on the evolution of river ecosystems. First, we can evaluate the stabilities of the SLT river ecosystem (i.e., the Weihe River mainstream) and the SSC river ecosystem (i.e., the Weihe River tributaries at the northern foot of the Qinling Mountains). Second, the degree of influence of various factors on stability was assessed. Finally, correlation analysis was added to study the driving relationship between the internal and external aspects of the ecosystem. The results showed that the stability of the SLT river ecosystem (mean value 3.371) was lower than that of the SSC type (mean value 5.343). However, the SSC river ecosystem was more susceptible to being affected because more factors could affect its stability. Further study indicated that although turbidity was the largest connection weight in the two river ecosystems, the reasons for its increase were different. This result was due to the increased sediment content in the SLT river ecosystem and the growth of algae in the SSC ecosystem. The main reason for this difference was that the SLT and SSC river ecosystems had different external driving forces. The SLT river ecosystem was mainly driven by land utilization, and the SSC was mainly driven by hydrological situations and climate factors. Although there are differences between the two ecosystems, they may transform into each other when the external driving force of the river ecosystem changes. This study emphasizes the importance of external background to the evolution trend of river ecosystems and the difference in the influence of internal factors on the stability of river ecosystems.

Microbiota play essential roles in nitrogen (N) cycling in freshwater river ecosystems. However, microbial functional groups associated with N cycling (especially denitrification) in freshwater rivers under anthropogenic disturbance are still poorly understood. Here, we studied the impacts of different land-use types on denitrification-related microbial communities in Weihe River, Hanjiang River, and their tributaries, in the Qinling Mountains, China. The major land-use types in the three river areas were divided into natural (forest, shrub, grassland, and open water) and anthropogenic (agricultural and urbanized land) types. A landscape survey of microbiota in the river water and sediment was carried out with extensive sample sources based on deep 16S rRNA gene sequencing, which yielded operational taxonomic units for predicting functional groups. With increases in proportions of agricultural and urbanized land areas, electrical conductivity, total N, ammonium-N, and nitrate-N all increased in water samples. Conversely, microbial diversity exhibited a decreasing trend in water samples, whereas the relative abundance of denitrification-related functional groups increased, with increases in the proportions of agricultural and urbanized land areas. The relative abundances of denitrification- and human-related microbial functional groups in sediment samples were distinctively higher in Weihe River (mainly under agriculture and urbanization), when compared with those of Hanjiang River and Qinling tributaries (dominated by forests). The results indicate that anthropogenic land-use types, such as agricultural and urbanized land, result in simple microbial community structure and stimulate microbe-mediated denitrification in freshwater rivers.

Mountain rivers exert critical ecological effects downstream by retaining or transmitting sediment and nutrients, providing habitat and refuge for diverse aquatic and riparian organisms, and creating migration corridors. River microhabitat heterogeneity (RMH), which plays a key role in ecological restoration and improvement, is sensitive to external disturbances in mountain rivers. However, the effects of RMH, induced by hydro-geomorphological processes, on local macroinvertebrates have not been quantitatively studied. To explore the ecological significance of RMH, we selected five debris flow-dominated mountain rivers (DMR) and five equilibrium sediment transport mountain rivers (EMR) as contrasting examples based on the richness of sediment supply. We measured water depth, flow velocity, and substrate composition in all rivers and proposed a new RMH index (RMHI) for quantitative evaluation of RMH. Macroinvertebrate standing stocks, taxonomic diversity, functional diversity, and functional traits were compared between DMR and EMR. Macroinvertebrate standing stocks in DMRs were about one-third, and α-diversity was half, of those in EMRs. The macroinvertebrate communities exhibited a turnover-dominated pattern in both DMRs and EMRs. Resource availability and utilization efficiency were also smaller in DMRs than in EMRs, which caused a macroinvertebrate community shift from R-strategy to K-strategy. Besides, RMH supported macroinvertebrate α-diversity and functional richness in both DMR and EMR, but debris flow weakened the ecological role of RMH in DMR. Our findings suggested that, in order to maintain the ecological health of mountain rivers, RMHI should be ≥ 8.0. According study results, rivers with greater macroinvertebrate species richness should be managed as a priority for biodiversity conservation by maintaining RMH above its threshold. This could be achieved through the addition of large stones to rivers, which would act to create a large range of riverbed sediment sizes and variable flow regimes, as well as increasing the space available to macroinvertebrates.