Introduction:
The hyporheic zone (HZ), which forms the interface between surface water
and groundwater in streams and rivers, is a spatially heterogeneous and
temporally dynamic biogeochemical zone (McClain et al. 2003; Gomez-Velez
et al. 2014; Bernhardt et al. 2017; Lee-Cullin et al. 2018). Because the
HZ regulates nutrient exchange/processing and provides habitat to
diverse biological communities, it serves as a biogeochemical reactor
for aquatic metabolism across river corridors (Gomez-Velez and Harvey
2014; Sackett et al. 2019). In fact, the hyporheic zone accounts for the
majority of ecosystem metabolism in some aquatic systems (e.g., Naegeli
and Uehlinger 1997; Fulton et al. 2024). Although characterizing
hyporheic metabolism is key for understanding river corridor
biogeochemistry, high spatiotemporal heterogeneity and interacting
environmental drivers in the HZ makes it difficult to develop predictive
relationships for hyporheic metabolism at reach-to-basin scales
(Buser-Young et al. 2023; Stegen et al. 2023; Tureţcaia et al. 2023).
Allometry, or a power-law relationship between function and size, is a
central theory of metabolism in ecology (e.g., Brown et al. 2004), and
has been applied to understand how biogeochemical properties scale
across freshwater and estuarine environments (Bertuzzo et al. 2017;
Nidzieko 2018). A recently introduced theoretical framework for an
idealized watershed suggests ecosystem metabolism scales allometrically,
meaning that cumulative metabolism of entire river networks relates
predictably to cumulative watershed area via a power law (Wollheim et
al. 2022). However, it remains unknown how, or even if this relationship
is transferable to non-idealized cases (real watersheds), or if
cumulative hyporheic metabolism exhibits equivalent allometric scaling
behavior. Additionally, although we know that watershed characteristics
relate to hyporheic metabolic processes, (Son et al. 2022a; Buser-Young
et al. 2023), it is unknown how differences in watershed characteristics
within or between watersheds might impact such scaling relationships.
We addressed this knowledge gap by asking two questions: 1) Does
hyporheic respiration follow an allometric relationship to watershed
area across reach-to-basin scales?, and 2) What ecosystem properties
relate to any observed differences in allometry? We used previously
modeled reach-scale hyporheic respiration for more than 16,000
individual reaches in two river basins to calculate allometric
relationships between cumulative hyporheic aerobic respiration and
cumulative watershed area. We then explored allometric relationships in
the context of watershed characteristics to understand common patterns
between our study basins. Finally, we identified next steps for
developing a more generalizable understanding of the controls on
hyporheic respiration allometry across basins.