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Presented at the NABS Annual meeting, Anchorage, Alaska, 2006
in Hyporheic Processes
Implications of biophysicochemical process coupling and hyporheic structure for modeling nitrogen dynamics in rivers
A.I. Packman1, J.D. Newbold2, S. Arnon1, and K.A. Gray1.1Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL, 60201, USA, 2Stroud Water Research Center, Avondale, PA, 19311, USA
In standard models for nutrient dynamics in rivers, nutrient consumption rates are normally assumed to be independent of hydrodynamic transport rates. However, this is not generally true. Because most bacterial activity in rivers occurs in the sediments (benthic and hyporheic regions), nutrient dynamics will often be coupled with hydrodynamic stream-subsurface exchange processes. This is particularly important for nitrogen because it can be used as an electron acceptor by denitrifying bacteria in addition to being assimilated, and its catabolic use is suppressed in the presence of oxygen. Oxygen delivery to sediments is mediated by hydrodynamic transport, with oxygen found at greater concentrations and greater depths under faster transport conditions. As a result, local nitrate consumption rates will normally vary with stream flow conditions. This coupling mechanism has been confirmed through controlled experiments with direct measurements of flow conditions, oxygen profiles, and bulk nitrate consumption. Here, we will reevaluate parameterization of nutrient dynamics models and demonstrate the effects of co-variance of hydrodynamic transport conditions and local denitrification rates on nitrogen dynamics. We will specifically show how this stream-subsurface, biophysicochemical process coupling influences integration of average process rates to the whole-system scale, which is critical for interpreting the results of whole-stream nutrient injection experiments.
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