The hyporheic zone (HZ) is a critical component of riverine ecosystems—responsible for up to 90% of ecosystem respiration—that represents an interface among terrestrial, surface water, and groundwater ecosystems. The HZ is characterized by groundwater-surface water mixing and in many systems the HZ has a relatively small spatial extent, but in larger riverine systems groundwater-surface water mixing can occur 100s of meters beyond the surface water shoreline. We refer to this broader domain as the ‘subsurface interaction zone’ (SIZ), which includes and goes beyond the traditional HZ. While the SIZ plays a key role in ecosystem function, little is known about spatiotemporal linkages among hydrology, biogeochemistry and associated microbial communities in the SIZ.
Here—by coupling 4-D geophysics, ecological null modeling, and ultrahigh resolution carbon profiling—we show that (i) river stage fluctuations cause intrusion of surface water into the SIZ that stimulates heterotrophic respiration, leading to substantial losses of dissolved organic carbon, (ii) despite spatiotemporal variation in groundwater-surface water mixing, microbial community composition across much of the SIZ is maintained through time due to homogeneous ecological selection, and (iii) changes in organic carbon composition—from CHO-containing lignin-like compounds to CHON-containing amino-sugar-like compounds—across the river/HZ boundary are associated with variable ecological selection and significant shifts in microbial community composition. These results suggest that changes in surface water hydrology—in response to altered climate—will strongly influence carbon cycling across terrestrial-aquatic interfaces by altering the amount of organic carbon processed through the SIZ and that concomitant changes in organic carbon composition will likely drive shifts in HZ microbial composition; predictive hydro-biogeochemical models will need to include processes underlying these impacts.