4.2 Inferences from finer-scale variation
Using sensors, we were able to observe finer-scale temporal variation in
DOC and NO3-N C-Q dynamics than are apparent from grab
samples. Of particular note, higher Q generated by ROS or other
precipitation events resulted in marked enrichment of DOC and
NO3-N (Figure 3c-h). Notably, this enrichment response
required rainfall (or rapidly melting snowfall). Diel variation in Q
during melting of the snowpack, though of similar or often even greater
magnitude, does not generate the same concentration response as even a
small increase in Q due to new precipitation. Furthermore, it occurs
during periods when DOC and NO3-N concentrations have
otherwise shown signs of being depleted. This suggests that recent
precipitation-derived increases in Q, as opposed to snowpack derived,
are somehow able to mobilize additional solute sources, or short circuit
retention along regular transport pathways. The temporal lag between Q
and concentration, and the counter-clockwise hysteresis it produces
(Figure 3a,b), suggests it is the later arriving water that is more
enriched (Evans and Davies 1998). One potential explanation is that
stores of carbon and nitrogen in more distal regions of the catchment,
being further from the “drain”, are less depleted (e.g., Hood et al.,
2006). Rainfall events are able to temporarily establish great
connectivity with these regions, and their distance is responsible for
the temporal lag. The temporal alignment of high Q and high
concentrations makes ROS and other precipitation-driven events “hot
moments” for solute transport, generating some of the highest
instantaneous flux values observed in the record. However, their
relatively infrequent occurrence and short duration mean they are still
a much smaller contributor to annual flux budgets than the seasonal melt
pulse.
High-frequency measurements also enabled us to observe diel variation.
During the melt pulse (Figure 4a), diel Q variation was consistent with
greater afternoon melting (Caine, 1992; Lundquist and Cayan, 2002;
Kirchner et al., 2020). Following the melt pulse, the timing of peak
daily Q shifted to earlier in the day, consistent with effects of
riparian ET (Gribovszki et al., 2010; Kirchner et al., 2020). We also
observed small, but noticeable diel variation in DOC and
NO3-N concentrations. Signal timing was often consistent
with daytime production of DOC through gross primary production (GPP),
and the coupled assimilatory uptake of NO3-N (Heffernan
and Cohen 2010). However, we were confounded in our attempt to estimate
rates of GPP using the streamMetabolizer R model (Appling et al., 2018),
because the diel DO signal exhibited daytime minima instead of expected
maxima (Figure 4c & 5c). To some extent this was the result of effects
of diurnal temperature variation on solubility. However, even after
accounting for this, the percent saturation still exhibited consistent
daytime minima (not shown). One possible explanation is that ecosystem
respiration (ER) is stimulated by temperature, and daytime rates of
stream ER have been found to be considerably higher than nighttime rates
(Tobias et al., 2007; Hotchkiss and Hall 2014). In a narrow, forested
stream such as Como Creek, light inhibition by the canopy may limit GPP
to a level where daytime enhancement of ER by warmer daytime water
temperatures, and the resulting consumption of DO, may become the
dominant process. Rates of denitrification are also sensitive to
temperature (Rusjan and Mikos, 2010), and this could also potentially
explain the diel NO3-N signal absent assimilatory
uptake. It is quite interesting to note however, that some of the
largest diel variation in NO3-N (Figure 4b) occurred
prior to the melt pulse when the stream was still mostly snow covered,
there was almost no diel variation in DO, and temperatures were very
near freezing. An alternative explanation is that diel solute variation
(including DO) is not primarily biological, but potentially an
expression of diel Q variation (Nimick et al., 2011; Hensley et al.,
2017). This conclusion is supported by the large diel variation in
conservative SpC during much of the year. Unfortunately, parameters
which could be used to more definitively constrain biologic versus
hydrologic influence (e.x. C and N isotopes, concentrations of other
dissolved gases), are not collected at the sub-daily resolution
required.