Comparing the Dissolved and Colloidal Trace Element Chemistry of Two Major Rivers: the Mississippi and Yukon Rivers

We have been examining the temporal variability of dissolved and colloidal trace element chemistry in two contrasting major river systems: the Mississippi and Yukon Rivers. These two systems are similar in that they both have high suspended loads and moderately alkaline pH's. However, they differ greatly in climate and the extent of human engineering. For instance, in the lower Mississippi the effect of the long residence time of water in upstream impoundments would be expected to provide a major signature. In contrast, the Yukon River has faster flow-through and greater inputs of fresh terrestrial organic matter. Observationally, in the lower Mississippi River seasonal variability in many dissolved metals follows a seasonal "redox pump." This pump is caused by inputs from up-stream seasonally stratified lakes and reservoirs as well as by the temperature dependance of in-stream microbial manganese oxidation. In the Yukon River there is additionally evidence of a spring pulse of trace elements associated with early season soil flushing. The spring pulse has been observed in certain headwaters of the Mississippi River system, so it's absence in the lower river may reflect the effects of impoundments in damping out this signal. We also observe some effects of the greater amount of fresh terrestrial DOC in the Yukon which, for example, results in greater amounts of colloidal Fe in the Yukon relative to the Mississippi. This colloidal Fe could represent a significant pool of reactive Fe delivered to the coastal system. Despite these differences, many dissolved trace elements have similar concentration ranges in the two systems.