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

Areas where groundwater and surface water mix (i.e., hyporheic zones, HZ) contribute substantially to stream ecosystem respiration (ERtot). We rely on reactive transport models to understand HZ respiration at large scales; however, model outputs have not been evaluated with field estimates of ERtot. Here we evaluate the degree to which spatial variation in model-predicted HZ respiration can explain spatial variation in field-estimated ERtot across 32 sites in the Yakima River basin (YRB). We find that predicted HZ respiration did not explain spatial variation in ERtot. We hypothesize that ERtot is influenced by processes that integrate contributions from sediments, such as benthic algae, submerged macrophytes, and shallow HZ. Our results indicate that sediment-associated processes hydrologically connected to the active channel are primary drivers of spatial variation in ERtot in the YRB. We encourage conceptual and physical models of stream ERtot to integrate shallow hyporheic exchange with sediment associated primary production.