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).