PNNL

Abstract

Terrestrial and aquatic elemental cycles are tightly linked in upland fluvial networks. Mineral weathering, degradation of organic matter, and anthropogenic influences all result in the movement of solutes (e.g., carbon, metals, nutrients) through these catchments, with implications for downstream water quality. Within the river channel, the region of hyporheic mixing represents a hotspot of microbial activity, exerting significant control over solute cycling. To investigate how snowmelt-driven seasonal changes in river discharge affects the extent of hyporheic mixing, pore water biogeochemistry, and microbial community assembly, depth-resolved pore water samples were recovered from multiple locations around a representative meander on the East River near Crested Butte, CO, USA. Vertical temperature sensor arrays were also installed in the riverbed to enable seepage flux estimates. Snowmelt-driven high river discharge led to an expanding zone of vertical hyporheic mixing and introduced dissolved oxygen into the riverbed that stimulated microbial aerobic respiration. These physicochemical processes contributed to microbial communities undergoing homogenizing selection, in contrast to other ecosystems where lower permeability may limit the extent of mixing. Conversely, lower river discharge conditions led to a greater influence of upwelling groundwater within the riverbed and a decrease in microbial respiration rates. Associated with these processes, microbial communities throughout the riverbed exhibited increasing dissimilarity between each other, suggesting that the earlier onset of snowmelt and longer periods of baseflow may lead to changes in the composition (and associated function) of riverbed microbiomes, with subsequent implications for the processing and export of solutes from upland catchments.