PNNL

Abstract

The parafluvial hyporheic zone combines the heightened biogeochemical and microbial interactions indicative of a hyporheic region with direct atmospheric/terrestrial inputs and the effects of wet/dry cycles. Therefore, understanding biogeochemical cycling and microbial interactions in this ecotone is fundamental to understanding carbon cycling at the aquatic-terrestrial interface and to creating robust hydrobiogeochemical models. We aimed to (i) characterize biogeochemical and microbial differences in the parafluvial hyporheic zone across a small spatial domain (6 lateral meters) that integrates a breadth of inundation histories and (ii) examine how parafluvial hyporheic sediments respond to laboratory-simulated reinundation. Surface sediment for assays (organic matter (OM), C/N, moisture content, grain size, 16S sequencing, ITS sequencing, and qPCR) and forced inundation laboratory incubations (destructively sampled at 0.5 hours and 25 hours; assays: respiration rate (RR) via headspace CO2, cations, anions, organic matter, organic acids and sugars, and dissolved non-purgeable organic carbon) was collected at four elevations along three transects (12 sites total) perpendicular to flow of the Columbia River, eastern WA, USA, encompassing 1 meter of elevation change. The sampling elevations were inundated by the river 0 days, 13 days, 127 days, and 398 days prior to sampling. Spatial distributions of environmental variables (OM, moisture, nitrate, glucose, % C, % N) and microbial communities (PERMANOVA, bacteria: P0.1), indicating that inundation history influences the ability of parafluvial hyporheic microbial communities to respond to re-inundation by the river. The spatial distribution of RR does not follow previous studies that observed high CO2 production when dry soils were wetted. Biogeochemical dynamics in the parafluvial hyporheic zone need to be further studied in order to fully understand aquatic-terrestrial carbon cycling and inform reactive transport models, particularly in dam-controlled watersheds where river water elevation can fluctuate meters in a single day.