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Diel Stream Temperature Effects on Nitrogen Cycling in Hyporheic Zones
Author(s) -
Zheng Lizhi,
Bayani Cardenas M.
Publication year - 2018
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1029/2018jg004412
Subject(s) - diel vertical migration , hyporheic zone , denitrification , nitrification , nitrate , biogeochemical cycle , environmental science , nitrogen cycle , soil science , cycling , tracer , nitrogen , atmospheric sciences , chemistry , environmental chemistry , hydrology (agriculture) , ecology , environmental engineering , geology , surface water , history , physics , geotechnical engineering , organic chemistry , archaeology , nuclear physics , biology
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.