All of the methods briefly described below are presented in detail in
a 2008 publication in Freshwater Biology,
doi:10.1111/j.1365-2427.2007.01941.x
Sample preparation - DOC and ¹³C-DOC samples were filtered through
pre-combusted Whatman GF/F filters and the Br- samples were filtered
through 0.2 μm Gelman HT filters.
¹³C tracer generation - Tulip poplar seedlings were grown in a ¹³C-CO₂
atmosphere and dried leaves, stems, and roots were ground and
extracted in deionized water in the dark at 4°C as described in
Weigner, T. N., Kaplan, L.A., Newbold, J. D., and Ostrom, P. H. 2005.
Synthesis of ¹³C-labeled tracer for stream DOC, labeling tulip poplar
carbon with ¹³CO₂. Ecosystems 8:501-511. doi
10.1007/s10021-003-0043-1. The extracted leachate was processed to remove
particles, stabilized through Tyndallization, and stored at 4°C prior
to use.
Dissolved organic carbon (DOC) - Measurements were made by
Pt-catalyzed persulfate oxidation in an OI 1010 analyzer.
¹³C-DOC - Analyses were performed as described in Gandhi, H., Wiegner,
T. N., Ostrom, P. H., Kaplan, L. A., and Ostrom, N. E. 2004. Isotopic
(13C) analysis of dissolved organic carbon in stream water using an
elemental analyzer coupled to a stable isotope ratio mass
spectrometer. Rapid Communication in Mass Spectrometry 18:903-906. doi
10.1002/rcm.1426. Filtered stream samples were concentrated with
rotary evaporation, acidified, lyophilized, combusted, and the
CO₂ analyzed with an elemental analyzer (EA
3000, Eurovector) interfaced to an isotope ratio mass spectrometer (GV
Instruments).
Bromide - Bromide was measured as an anion with ion chromatography
(Dionex Model 500).
¹³C-DOC tracer calculations - The ¹³C enrichment
or fractional abundance in the stream water DOC following the tracer
addition was calculated from the measured delta-¹³C-DOC values and used, together with the respective fractional abundances of the background stream water and of the added leachate, in a mixing model to calculate the concentration of leachate-derived ¹³C ("C13add") in the stream during the addition.
¹³C-DOC uptake in White Clay Creek - The longitudinal loss rate curve
for the ¹³C-DOC tracer was approximated from the sum of two
first-order loss rate curves associated with labile and semi-labile
biodegradable DOC fractions. We used averaged, background-corrected,
bromide-normalized 13C-DOC plateau concentrations from the downstream
sampling sites in a nonlinear least squares analysis (PROC NLIN, SAS
Institute). From the longitudinal loss rate coefficients we calculated
uptake lengths and mass transfer coefficients for the two lability
classes.
¹³C-DOC tracer and stream water DOC lability profiling - White Clay
Creek stream water or stream water amended with the ¹³C-DOC tracer
were pumped into 8 bioreactors with empty bed contact times ranging
from 0.5 to 73.8 minutes. Influent and effluent waters from the
bioreactors were sampled and analyzed for DOC and ¹³C-DOC.
Biodegradable DOC concentrations were calculated from the differences
in concentrations between the influents and effluents to the
bioreactors.
Stream injection - The ¹³C-DOM leachate and bromide (as NaBr) were added steadily from separate reservoirs to the East Branch of White Clay Creek at a point (39.8503 N, 75.7871 W) 1400 meters upstream from the bridge at Spencer Road, for a duration of 1.90 hours beginning 2 October 2002 at 09:43 EDT. The total quantity of ¹³C added was 53.4 mmol and the total quantity of bromide added was 308 g, as Br-. Streamflow during the addition (calculated by bromide dilution) was 12.8 L/s at 15 m downstream from the addition, 12.8 L/s at 51 m, 14.3 L/s at 426 m, and 17.7 L/s at 1265 m. Stream width averaged 3.3 m (SD=1.39 m), ranging from 0.77 to 6.7 m among 20 transects, with no significant longitudinal trend.
