『Abstract
Highly resolved time series data are useful to accurately identify
the timing, rate, and magnitude of solute transport in streams
during hydrologically dynamic periods such as snowmelt. We used
in situ optical sensors for nitrate (NO3-)
and chromophoric dissolved organic matter fluorescence (FDOM)
to measure surface water concentrations at 30 min intervals over
the snowmelt period (March 21-May 13, 2009) at a 40.5 hectare
forested watershed at Sleepers River, Vermont. We also collected
discrete samples for laboratory absorbance and fluorescence as
well as δ18O-NO3- isotopes
to help interpret the drivers of variable NO3-
and FDOM concentrations measured in situ. In situ data revealed
seasonal, event and diurnal patterns associated with hydrological
and biogeochemical processes regulating stream NO3-
and FDOM concentrations. An observed decrease in NO3-
concentrations after peak snowmelt runoff and muted response to
spring rainfall was consistent with the flushing of a limited
supply of NO3- (mainly from nitrification)
from source areas in surficial soils. Stream FDOM concentrations
were coupled with flow throughout the study period, suggesting
a strong hydrologic control on DOM concentrations in the stream.
However, higher FDOM concentrations per unit streamflow after
snowmelt likely reflected a greater hydraulic connectivity of
the stream to leachable DOM sources in upland soils. We also observed
diurnal NO3- variability of 1-2μmol
l-1 after snowpack ablation, presumably due to in-stream
uptake prior to leafout. A comparison of NO3-
and dissolved organic carbon yields (DOC, measured by FDOM proxy)
calculated from weekly discrete samples and in situ data sub-sampled
daily resulted in small to moderate differences over the entire
study period (-4 to 1% for NO3-
and -3 to -14% for DOC), but resulted in much larger differences
for daily yields (-66 to +27% for NO3-
and -88 to +47% for DOC, respectively). Despite challenges inherent
in in situ sensor deployments in harsh seasonal conditions, these
data provide important insights into processes controlling NO3- and FDOM in streams, and will be
critical for evaluating the effects of climate change on snowmelt
delivery to downstream ecosystems.
Keywords: Nitrate; FDOM; Snowmelt; Forested; Diurnal』
Introduction
Methods
Site description
In situ optical measurements
Data handling and processing
Discrete measurements
Nitrate source apportionment
nitrate and DOC yields
Results
Discussion
NO3- and FDOM dynamics during
snowmelt
Diurnal NO3- variability
Snowmelt NO3- and DOC yields
Conclusions
Acknowledgments
References