Coupled effects of landscape structures and water chemistry on bacterioplankton communities at multi-spatial scales.

Bacterioplankton communities in rivers are strongly influenced by the surrounding landscape, yet the relationships between land use and bacterioplankton communities at multi-spatial scales and the mechanisms that shape bacterioplankton communities remain unclear. Here, we collected surface water samples from 14 tributaries of the Yuan River, a secondary tributary of the Yangtze River, which has been heavily impacted by human activities. We characterized the bacterioplankton communities by high-throughput sequencing techniques, and managed to identify the mechanisms governing bacterioplankton community assembly. The results showed that, in general, both landscape compositions and landscape configurations had significant effects on bacterial communities, and the effects were greater at the buffer scale than at the sub-basin scale. Additionally, there was no distinct distance-decay pattern for the effects of landscape structures on bacterial communities from the near-distance (100 m) to the long-distance (1000 m) buffer zones, with the maximal effects occurring in the 1000 m circular buffer (wet season) and 500 m riparian buffer (dry season) zone, respectively. Land use influenced the bacterioplankton community both directly through exogenous inputs (mass effect) and indirectly by affecting water chemistry (species sorting). Variance partitioning analyses showed that the total explanations of bacterial community variations by water chemistry and the intersections of water chemistry and land use (56.2% in wet season and 50.4% in dry season) were higher than that of land use alone (6.1% in wet season and 25.4% in dry season). These suggest that mass effects and species sorting jointly shaped bacterial community assembly, but that the effects of species sorting outweighed those of mass effects. Nevertheless, more biotic and abiotic factors need to be considered to better understand the microbial assembly mechanisms in anthropogenically influenced riverine ecosystems.