Nitrate Content and Potential Microbial Signature of Rock Glacier Outflow, Colorado Front Range

Here we characterize the nutrient content in the outflow of the Green Lake 5 rock glacier, located in the Green Lakes Valley of the Colorado Front Range. Dissolved organic carbon (DOC) was present in all samples with a mean concentration of 0·85 mg L. A one-way analysis of variance test shows no statistical difference in DOC amounts among surface waters (p = 0·42). Average nitrate concentrations were 69 μmoles L− in the outflow of the rock glacier, compared to 7 μmoles L in snow and 25 μmoles L in rain. Nitrate concentrations from the rock glacier generally increased with time, with maximum concentrations of 135 μmoles L− in October, among the highest nitrate concentrations reported for highelevation surface waters. These high nitrate concentrations appear to be characteristic of rock glacier outflow in the Rocky Mountains, as a paired-difference t-test shows that nitrate concentrations from the outflow of 7 additional rock glaciers were significantly greater compared to their reference streams ( p = 0·003). End-member mixing analysis suggest that snow was the dominant source of nitrate in June, ‘soil’ solution was the dominant nitrate source in July, and base flow was the dominant source in September. Fluoresence index values and PARAFAC analyses of dissolved organic matter (DOM) are also consistent with a switch from terrestrial DOM in the summer time period to an increasing aquatic-like microbial source during the autumn months. Copyright © 2006 John Wiley & Sons, Ltd.