Sediment-associated processes drive spatial variation in ecosystem respiration in the Yakima River basin

Hyporheic zones (HZ) contribute substantially to total stream ecosystem respiration (ERtot). We rely on process-based models to understand HZ respiration at watershed scales and field-based estimates of ERtot at reach scales. These emphasize different but related processes that have not been jointly explored. Here we evaluate the degree to which spatial variation in HZ respiration predicted via process-based models can explain spatial variation in field-estimates of ERtot across 33 sites in the Yakima River basin (YRB). We found that HZ respiration predicted by process-based models did not explain spatial variation in field estimates of ERtot. Our results indicate that sediment-associated processes hydrologically connected to the active channel explained most of the spatial variation of ERtot in the YRB. 88% of sites showed that sediment-associated respiration contributed to 50% or more of ERtot, while a few sites showed as little as 0% contribution. We propose that ERtot is influenced by processes that integrate contributions from sediment components beyond the HZ, such as benthic algae and submerged macrophytes, which are key primary producers in rivers. We encourage conceptual and process-based models of ERtot to integrate shallow HZ exchange with sediment-associated primary production as these provide more complete accounting of processes influencing ERtot.