How dam construction affects the activity of alkaline phosphatases in reservoir sediments: A study of two highly regulated rivers

Dam construction causes phosphorus (P) accumulation in reservoir sediments and significantly affects the generation of available P. However, the effect of dam construction on the activity of sediment alkaline phosphatase (ALP), which is encoded by the bacterial phoD gene and participates in P mineralization, in river sediments remains unclear. Here, we investigated the ALP activities in 78 sediment samples collected from the cascade reservoir regions located in the Lancang River and the Jinsha River, two highly regulated rivers in southwestern China. The abundance and community composition of phoD-harboring bacteria were determined based on the phoD gene using quantitative real-time PCR and MiSeq sequencing, respectively. Comparison of control and affected sites indicated that dam construction significantly increased sediment ALP activity in both rivers. The abundances of phoD-harboring bacteria increased and their community compositions varied in response to dam construction; the relative abundances of the dominant genera Methylobacterium and Bradyrhizobium were particularly higher in affected site than control site. Co-occurrence network analyses revealed much higher network connectivity and relative abundances of keystone species in affected sites. Some microbial factors including phoD-harboring bacterial abundances, network clustering coefficients, and relative abundance of keystone species were positively correlated with ALP activity. The relative abundance of keystone species was identified as the most important microbial factor contributing to variation in ALP activity based on structural equation modeling analysis. These findings enhance our understanding of how dam construction affects the functions of phoD-harboring bacteria and their role in the P biogeochemical cycle in highly regulated rivers.