Temporal Variability in the Concentration and Stable Carbon Isotope Composition of Dissolved Inorganic and Organic Carbon in Two Montana,USA Rivers |
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Authors: | Stephen R Parker Simon R Poulson M Garrett Smith Charmaine L Weyer Kenneth M Bates |
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Institution: | (1) Department of Chemistry and Geochemistry, Montana Tech of The University of Montana, 1300 W. Park St., Butte, MT 59701, USA;(2) Department of Geological Sciences and Engineering, University of Nevada-Reno, 1664 N. Virginia St., Reno, NV 89557-0138, USA |
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Abstract: | Here we report diel (24 h) and seasonal differences in the concentration and stable carbon isotope composition of dissolved
inorganic (DIC) and organic carbon (DOC) in the Clark Fork (CFR) and Big Hole (BHR) Rivers of southwestern Montana, USA. In
the CFR, DIC concentration decreased during the daytime and increased at night while DOC showed an inverse temporal relationship;
increasing in the daytime most likely due to release of organic photosynthates and decreasing overnight due to heterotrophic
consumption. The stable isotope composition of DIC (δ13C-DIC) became enriched during the day and depleted over night and the δ13C-DOC displayed the inverse temporal pattern. Additionally, the night time molar rate of decrease in the concentration of
DOC was up to two orders of magnitude smaller than the rate of increase in the concentration of DIC indicating that oxidation
of DOC was responsible for only a small part of the increase in inorganic carbon. In the BHR, in two successive years (late
summer 2006 & 2007), the DIC displayed little diel concentration change, however, the δ13C-DIC did show a more typical diel pattern characteristic of the influences of photosynthesis and respiration indicating that
the isotopic composition of DIC can change while the concentration stays relatively constant. During 2006, a sharp night time
increase in DOC was measured; opposite to the result observed in the CFR and may be related to the night time increase in
flow and pH also observed in that year. This night time increase in DOC, flow, and pH was not observed 1 year later at approximately
the same time of year. An in-stream mesocosm chamber used during 2006 showed that the night time increase in pH and DOC did
not occur in water that was isolated from upstream or hyporheic contributions. This result suggests that a “pulse” of high
DOC and pH water was advected to the sampling site in the BHR in 2006 and a model is proposed to explain this temporal pattern. |
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