Diel Stream Temperature Effects on Nitrogen Cycling in Hyporheic Zones

Abstract

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 che...

Impact of Flow Alteration and Temperature Variability on Hyporheic Exchange

“…The effects of the system properties represented by transport time scales are investigated in the hyporheic exchange processes. Compared to previous studies (Cardenas & Wilson, ; Marzadri et al, ; Zheng & Cardenas, ; Zheng et al, ), this work provides first insights into coupled transient groundwater flow and heat transport processes in hyporheic zones by using natural hydrologic driving forces, providing better understandings to the impact of flow alterations on the hyporheic exchange processes.…”

“…Aquatic species use a diverse array of thermal habitats to meet their specific temperature requirements for survival, growth, and reproduction (Olden & Naiman, ). Temperature variability also extensively modifies the nutrient cycling and the spatial distribution of biogeochemical reaction hot spots in hyporheic zones (Song et al, ; Zheng & Cardenas, ). However, the temperature sensitivity of ecological and biogeochemical processes is often examined under oversimplified constant flow boundary conditions.…”

“…The thermally induced fluid viscosity variations can extensively modify the hyporheic flux (Cardenas & Wilson, ) and consequently control biogeochemical activities. Nevertheless, most of the previous studies that couple the modeling of flow and heat exchange processes are either sequentially coupled and use precalculated flow fields as boundary conditions of the subsequent heat transport modeling (Hester et al, ; Marzadri et al, ) or couple heat and flow modeling by considering only steady discharge (Burkholder et al, ; Cardenas & Wilson, ; Marzadri et al, ; Zheng et al, ) or only diel temperature fluctuations (Gerecht et al, ; Zheng & Cardenas, ; Zheng et al, ). In the present study, we aim to explore complex interactions between river discharge and temperature for river systems with varying degrees of flow alteration across multiple transport time scales.…”

Shear Flows Accelerate Mixing Dynamics in Hyporheic Zones and Hillslopes

“…The insights gained from the idealized solute transport scenarios may help understanding the impact of hydraulic head gradients at the surface-subsurface interface on a range of mixing processes. This includes in particular the transient dynamics of solute mixing and heat transfer in the hyporheic zone (Hester et al, 2017;Kaufman et al, 2017;Marzadri et al, 2013;Zheng & Cardenas, 2018). Furthermore, acceleration of mixing rates by shear flows in hillslopes may influence the statistics of concentrations in rivers (e.g., Kirchner & Neal, 2013;Shuai et al, 2017) and concentration-discharge relationships (e.g., Godsey et al, 2009;Neilson et al, 2018).…”

“…The consequences of shear flow on the propagation of such signals can be understood by convolution of the response to an instantaneous injection derived here with a sinusoidal input. The analysis of sinusoidal signal also finds importance for convective heat transfer in the hyporheic zone (Marzadri et al, 2013;Song et al, 2018;Zheng & Cardenas, 2018) and at basin scale (Kooi, 2016). The latter is often approximated by analytical solutions that assume one-dimensional advection and diffusion, hence neglecting the effect of shear flows (Hatch et al, 2006;Stallman, 1965).…”