Tributary Inflows to a Regulated River Influence Bacterial Communities and Increase Bacterial Carbon Assimilation

Inflows from unregulated tributaries change the physical, chemical, and biotic conditions in receiving regulated rivers, impacting microbial community structure and metabolic function. Understanding how tributary inflows affect bacterial carbon production (BCP) is integral to understanding energy transfer in riverine ecosystems. To investigate the role of tributary inflows on bacterial community composition and BCP, a similar to 90th percentile natural flow event was sampled over 5 days along the Lachlan River and its tributaries within the Murray-Darling Basin of eastern Australia. Increased tributary inflows after rainfall corresponded with a significantly different and more diverse bacterial community in the regulated mainstem. The major contributor to this difference was an increase in relative abundance of bacterial groups with a potential metabolic preference for humic substances (Burkholderiaceae Polynucleobacter, Alcaligenaceae GKS98 freshwater group, Saccharimonadia) and a significant decrease in Spirosomaceae Pseudarcicella, known to metabolise algal exudates. Increases in orthophosphate and river discharge explained 31% of community change, suggesting a combination of resource delivery and microbial community coalescence as major drivers. BCP initially decreased significantly with tributary inflows, but the total load of carbon assimilated by bacteria increased by up to 20 times with flow due to increased water volume. The significant drivers of BCP were dissolved organic carbon, water temperature, and conductivity. Notably, BCP was not correlated with bacterial diversity or community composition. Tributary inflows were shown to alter mainstem bacterial community structure and metabolic function to take advantage of fresh terrestrial dissolved organic material, resulting in substantial changes to riverine carbon assimilation over small times scales.

Automated summary

Inflows from unregulated tributaries have a significant impact on microbial community structure and function in regulated rivers. This study investigated the effects of tributary inflows on bacterial community composition and bacterial carbon production (BCP) in the Lachlan River of eastern Australia. The results showed that increased inflows led to a more diverse bacterial community in the regulated mainstem, with a shift towards bacterial groups that prefer humic substances and a decrease in a group that metabolizes algal exudates. Orthophosphate and river discharge were identified as major drivers for the community change. BCP initially decreased with tributary inflows, but increased significantly due to increased water volume. Dissolved organic carbon, water temperature, and conductivity were the key factors driving BCP. Notably, BCP was not influenced by bacterial diversity or community composition. These findings highlight the importance of tributary inflows in shaping the bacterial community and carbon assimilation in riverine ecosystems.