Diel Stream Temperature Effects on Nitrogen Cycling in Hyporheic Zones.

Stream temperature often varies diurnally and seasonally, and these variations propagate into hyporheic zones (HZs), forming dynamic and heterogeneous thermal patterns. The complex thermal distribution creates potential biogeochemical hotspots and hot moments. Yet, how diel temperature variations affect HZ nitrogen cycling is unknown. We thus conducted a series of multiphysics numerical simulations of nonisothermal fluid flow and multicomponent reactive solute transport to investigate this problem. We imposed a sinusoidally varying stream temperature representing diel warming and cooling and studied the effects of different temperature means and amplitudes on HZ nitrate removal efficiency inside a streambed with a dune. The results showed that the time‐variable nitrification, denitrification, and nitrate removal efficiency responded differently to the diel stream temperature signal. The temporal variation of spatially averaged nitrification rate tracks the stream temperature signal, whereas the spatially averaged denitrification variation pattern has a more complex connection to temperature. We observed a persistent hotspot where significant denitrification rates are present over the 24‐hr period. We further evaluated and estimated the bulk nitrate removal efficiency calculated by time integration of spatially averaged reaction rates over a day. For denitrification‐dominant systems, the bulk nitrate removal efficiency for cases with dynamic stream temperature was effectively the same as those with an equivalent constant mean temperature. Therefore, the bulk removal efficiency of a thermally dynamic diel system may be represented by an equivalent isothermal system given stable flow conditions. However, since large instantaneous variations in various rates were observed, the results imply that randomly timed field measurements are unlikely to be representative. This has to be considered for both past and future synoptic observational studies. Plain Language Summary: Stream temperature naturally varies over daily and seasonal periods. This could cause temperature‐dependent chemical and biological processes to vary simultaneously. Within the permeable streambed are areas called hyporheic zones where stream water circulates through in and out of the bed. The heat carried by this hyporheic flow that originates from the stream forms dynamic and complex temperature distributions in the hyporheic zone. The thermal patterns within the streambed could determine the presence of denitrification hotspots and hot moments since the hyporheic zone is an important site for many chemical reactions including nitrogen cycling. Through modeling the flow and reaction transport processes within the hyporheic zone, we found that denitrification hotspots persisted despite dynamic and complex hyporheic zone thermal patterns. Our simulation results can help explain previous observations of warmer stream temperature corresponding to lower in‐stream nitrate concentrations. The results also imply that instantaneous snapshots through synoptic chemical sampling or reaction rate quantification may not be representative since the results and interpretations from such sampling strategies will depend on the time the samples were taken. Despite exploring a large range of modeled instantaneous variations of stream temperature, the results show that the steady, mean temperature can be used to estimate the bulk nitrate removal efficiency of the hyporheic zone. Key Points: A multiphysics flow and transport model is used to examine the diel dynamics of temperature‐dependent C and N cycling in hyporheic zonesBiogeochemical hotspots persisted despite dynamic and complex hyporheic zone thermal patternsBulk daily nitrate removal efficiency can be estimated assuming steady temperatures despite large instantaneous variations under dynamic temperature conditions [ABSTRACT FROM AUTHOR]