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.