Managed environmental flows and tributary inflows differently stimulate bacterial production and change microeukaryote communities in a regulated river

Globally, river regulation has greatly impacted the natural flow regime of many major rivers, resulting in detrimental changes to water chemistry and ecology. Environmental flows (e-flows) are legislated in many countries to improve the ecological health of regulated rivers. However, their impacts on the structure and function of the microbial food web and subsequent capture and transfer of terrestrial dissolved organic carbon (DOC) into riverine food webs, are poorly understood. To assess the impact of e-flows on instream bacterial carbon production (BCP) and microbial community composition, the Macquarie-Wambuul River in the Murray-Darling Basin of eastern Australia was sampled frequently during both a dam release e-flow and a high tributary inflow event. Although the e-flow did not increase measured DOC or nutrient concentrations in the water column, BCP increased immediately and there was a significant increase in the relative abundance and ITS gene count of the ciliate Tintinnidium fluviatile on the sixth day of the e-flow. In contrast, high tributary inflows significantly increased DOC and nitrite concentrations and stimulated BCP to rates four times higher than occurred during the e-flow as well as significantly increasing bacterial respiration and bacterial growth efficiency compared to base and e-flows. Changes in microbial communities during the tributary inflow indicated suppressed photosynthesis and increased heterotrophy and mixotrophy. These findings represent the first evidence that e-flows can stimulate instream BCP and demonstrate that e-flows and tributary inflows may provide food web support to regulated rivers via different heterotrophic energy pathways.