2.3 Application of analytical representations on predicting
N-cycling.
Hyporheic zone denitrification ranges between 1-200% of denitrification
in the stream water column (Harvey et al., 2013). The Denitrification in
streambed sediments depends on the presence of nitrate, labile organic
carbon, and anaerobic zones within the sediment bed (Gomez-Velez et al.,
2015; Zarnetske et al., 2012). In the downwelling zone, the movement of
oxygen rich water from the stream into the sediments favors aerobic
conditions which are generally (with the exception of anaerobic
microzones (e.g., Zarnetske et al., 2011, 2012) unfavorable to
denitrification (Azizian et al., 2015). As water parcels continue their
journey through the hyporheic zone (i.e., at relatively long \(t^{*}\))
heterotrophic respiration of organic matter (associated with in-sediment
microbial biofilms) leads to a progressive depletion of oxygen and an
eventual shift to nitrate reducing (i.e., denitrifying) conditions
(Azizian et al., 2015; Harvey et al., 2013; Zarnetske et al., 2011,
2015).
The boundary between aerobic and anaerobic zones is delineated by the
Damköhler number for aerobic respiration (\(\text{Da}_{\text{rp}}\))
(Zarnetske et al., 2012). It is defined as the ratio between the median
residence time of all flow paths (\(t_{50}\)) and the characteristic
time necessary for oxygen consumption (\(\tau_{\text{rp}}\sim 1\) h;
Gomez et al., 2012; Gomez-Velez et al., 2015; Zarnetske et al., 2011).
At \(\text{Da}_{\text{rp}}\) < 1, aerobic conditions are
dominant and denitrification is inhibited, while at\(\text{Da}_{\text{rp}}\) >> 1 anaerobic
conditions prevail and denitrification is likely (i.e., provided that
the other two requirements, the presence of nitrate and labile organic
matter, is satisfied) (Zarnetske et al., 2012). When flow paths exhibit
residence times longer than a characteristic denitrification timescale
(\(\tau_{\text{dn}}\sim 10\) h; Gomez-Velez et al., 2015; Harvey et al.,
2013; Zarnetske et al., 2015), they tend to be fully anaerobic, and all
or most of the nitrate is removed
(\(\text{Da}_{\text{dn}}\ \)>> 1). Correctly
representing the residence time scales in the hyporheic zone, and
therefore the Damköhler Numbers for aerobic respiration and
denitrification, can lead to more accurate assessments of the link
between the hyporheic zone RTD and key ecological functions, such as
stream metabolism, generally, and denitrification, in particular
(Mulholland et al., 2009).
The hydraulic and morphological properties of the Embarras River (Table
2) located 15 km south of Urbana-Champaign, east-central Illinois, USA
(Sukhodolov et al., 2006) was considered in this study to demonstrate
how the analytical framework introduced in this paper can be applied for
predicting Da. This river was chosen due to the existence of
a dunes-like bedform as its predominant morphology.