4.1 Hydrologic exchange fluxes organize hyporheic respiration allometry
We found two consistent patterns in allometric relationships across the study basins where 1) scaling was consistently weakest (Uncertain) for lowest HEF quantiles, and 2) scaling was consistently strong (high R2) and Super-linear at highest HEF quantiles (Figure 2). Because HEFs regulate carbon and nutrient delivery to the hyporheic zone and residence times, which in turn influence respiration (Krause et al. 2022), consistency in the relationship between cumulative respiration and cumulative HEF is expected. Reaches with Uncertain scaling relationships were largely clustered in the lower elevation headwater portions of each watershed (Figure 3, Figure S1), which is likely linked to weaker vertical hydrologic connectivity between surface water and the hyporheic zone. Lower water velocities may result in higher heterogeneity in subsurface biogeochemistry, including redox (Briggs et al. 2013), resulting in more variable (and therefore weaker) allometric behavior. Conversely, strong, Super-linear scaling at highest cumulative HEFs is consistent with previous observations that HEFs control biogeochemical processes in the hyporheic zone, including denitrification (Son et al. 2022a).
We observed differences in scaling patterns between the YRB and WRB for middle HEF quantiles, which may indicate that basin-specific characteristics limit our ability to effectively predict respiration scaling behavior for these portions of the basin. Specifically, we suggest that, at lower HEF quantiles, hyporheic respiration is transport-limited where movement of carbon, nutrients, and oxygen between surface and subsurface is the limiting factor, but also highly variable. In contrast, at highest HEF quantiles, hyporheic respiration is reaction-limited because of high exchange fluxes between the surface and subsurface. This is consistent with observations from surface water biogeochemistry that watersheds are generally transport-limited for smaller watershed areas and transition to reaction-limited as watershed area grows (Liu et al. 2022), and likely also varies in time, where baseflow conditions are more transport-limited while quickflow conditions are more reaction-limited (Raymond et al. 2016; Wollheim et al. 2022).