DIEL CHANGES IN THE CONCENTRATION AND STABLE CARBON ISOTOPE COMPOSITION OF DISSOLVED INORGANIC AND ORGANIC CARBON IN TWO MONTANA, USA RIVERS

DOC represents a significant pool of reduced carbon in most aquatic ecosystems that is readily available to heterotrophic microorganisms as an energy source. It has also been suggested that the DOC pool may be the largest source of carbon for microbial activity. The types and concentration of the DOC can have a significant influence on the chemical composition of surface waters; such as the bioavailability of metal ions and the absorption of light in the visible and UV ranges. In this report we describe diel and seasonal differences in the concentration and stable carbon isotope composition of dissolved organic (DOC) and inorganic carbon (DIC) in the Clark Fork (CFR) and Big Hole (BHR) Rivers of SW Montana, USA.

In the CFR, DIC concentration decreased during the daytime and increased at night while DOC showed the inverse temporal relationship. This DOC increase in the daytime is most likely due to the “leakage” of organic photosynthates from phytoplankton and macrophytes. The DOC concentration decreased overnight due to heterotrophic consumption. The δ¹³C-DIC became enriched during the day and depleted over night and the δ¹³C-DOC displayed the inverse temporal pattern. The molar rate of the nighttime DOC concentration decrease was up to two orders of magnitude smaller than the rate of increase in the DIC concentration.

In the BHR, in two successive years, 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 nighttime increase in DOC was measured which may be related to the nighttime increase in flow and pH also observed in that year. This nighttime increase in DOC, flow and pH was not observed one year later at approximately the same time of year. An in-stream mesocosm chamber used during 2006 showed that the nighttime 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 which is connected to the hydrologic flow cycle that was present in the BHR during 2006.