[Effects of Landscape Structures on Bacterioplankton Communities at Multi-spatial Scales in the Yuanhe River].

The aim of this study was to examine the relationships between land use and bacterioplankton communities at different spatial scales and the mechanisms underlying the effects of land use on bacterioplankton communities. Here, surface water samples were collected in 14 tributaries of the Yuanhe River in August 2019 (wet season) and January 2020 (dry season), and high-throughput sequencing technology was used to determine the characteristics of the bacterioplankton communities. Statistical methods such as Bioenv and variance partitioning analysis (VPA) were used to explore the relationships among landscape structure (i.e., landscape compositions and landscape configurations), water chemistry, and bacterioplankton communities. Furthermore, metacommunity theory was employed to explain the mechanisms by which land use and water chemistry affect bacterial communities. The results showed that:① in general, the effects of landscape configuration on bacterial communities were weak, whereas the effects of landscape composition on bacterial communities were significant and greater at the buffer scale than that at the sub-basin scale. ② There was no distinct distance-decay pattern for the effects of landscape composition on bacterial communities from the near-distance (100 m) to the long-distance (1000 m) buffer zones, with the maximal effects occurring in the 500 m circular buffer (wet season) and 300 m riparian buffer (dry season), respectively. ③ Land use influenced the bacterioplankton communities both directly through exogenous inputs (i.e., "mass effect" process) and indirectly by affecting water chemistry (i.e., "species sorting" process). VPA showed that the total explanation of bacterial community variations by water chemistry and the intersection of water chemistry and land use (13.5% in the wet season and 11.7% in the dry season) was higher than that of land use alone (2.7% in the wet season and 6.9% in the dry season). These results suggest that mass effects and species sorting jointly shaped bacterial community assembly but that the effects of species sorting outweighed those of mass effects. This study revealed the variability of landscape structure at different spatial scales on bacterial communities, and its results will help to determine the optimal spatial scale affecting bacterial communities and provide a reference basis for watershed land-use management.