Sediment‑associated processes dominate spatial variation in stream ecosystem respiration

Abstract Hyporheic zones (HZ) can contribute substantially to total stream ecosystem respiration (ER tot ). HZ-focused process-based models may, therefore, effectively predict ER tot across sites, yet this remains untested under variable environmental conditions. Here, we evaluate whether spatial variation in HZ respiration predicted via a process-based model explains spatial variation in field-estimates of ER tot across 33 sites in the Yakima River basin in Washington State, USA. We found that HZ respiration predictions did not explain spatial variation in field estimates of ER tot . To investigate further, we partitioned ER tot contributions into water-column respiration (ER wc ) and sediment-associated respiration (ER sed ). ER sed contributed >50% of ER tot at 88% of sites, though relative contributions varied substantially. Despite this dominance, modeled HZ respiration explained neither spatial variation in ER tot nor in ER sed , suggesting that the HZ model alone does not capture the drivers of sediment-associated respiration across these sites. Instead, ER sed spatial variation was primarily explained by gross primary production, stream slope, velocity, and total dissolved nitrogen rather than median grain size, a primary control of HZ respiration predicted by the process-based model. Consistent with recent studies, our results indicate that improving basin-scale ER tot predictions requires integrating hydrologic and biogeochemical processes across hyporheic, benthic, and water-column zones.