SOURCES AND AGES OF DISSOLVED ORGANIC MATTER IN THE MISSOURI RIVER

Chemical and isotopic data were used to delineate the source of dissolved organic carbon (DOC) in the Missouri River. The d¹³C values of DOC in the Missouri River varied approximately 2.0 per mil over a one-year period and approximately followed mean water temperature. This suggested contribution of algae components into the DOC pool. Dissolved hydrophobic acids, which constituted approximately 50% of the DOC showed little variation and were consistent with terrestrial plant-derived material (-25 to –28 per mil). Daily mean stream flow measurements at St. Josephs and at Hermann were used to compute the fraction of water derived upstream of the State of Missouri versus that derived within State. The concentration and radiocarbon (¹⁴C) content of DOC sampled in St. Louis exhibited strong linear correlation with fraction of discharge and was related to the marked regional climate differences between upstream (approx. 20 in rain/yr) and downstream (approx. 40 in rain/yr) of this large river system. The DOC concentration varied from 3.0 mg/L for upstream-dominated discharge to 7.0 mg/L for downstream-dominant. The dissolved hydrophobic acids in upstream discharge had ¹⁴C contents as low as 87 pmc, whereas in downstream discharge it was near 100 pmc.

To better understand the ¹⁴C variability, the ¹⁴C content of several dissolved hydrophobic acids isolated from rivers and lakes representing various climates were measured. The ¹⁴C data ranged from 77 to 130 pmc and showed a strong positive correlation with the mean annual precipitation rate (10 in/yr to 80 in/yr; proxy for net primary productivity) of the sample area. The range in calculated apparent ¹⁴C ages is at least a factor of five higher than the range represented in precipitation rates, suggesting soil retention likely has a secondary role in controlling the ¹⁴C variability of hydrophobic acids in water.