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.

The diversity of phytoplankton is fundamental to the health of aquatic ecosystems. Those commonly used diversity indices (e.g., Shannon-weiner diversity index, etc.) were assumed that a uniform distribution was the optimal status of phytoplankton taxa in aquatic system during the assessment of these indices. We discovered that the relative abundance of phytoplankton within an aquatic ecosystem can be fitted with exponential distribution, suggesting that the exponential distribution was the intrinsic and inherent distribution of phytoplankton communities. Through mathematical derivation, taxa capacity (N), competition coefficient (k), critical relative abundance of extinction (a), degree of monopoly, and the residual sum of squares of the phytoplankton community can be obtained to provide a greater insight into phytoplankton diversity. Our results highlight a new methodology based on the above indices has been developed to provide a better insight into the phytoplankton community diversity. It will also help to evaluate the aquatic system ecological health.

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.