Revisiting the application of open-channel estimates of denitrification

Development of an open-channel method for measurement of denitrification, without the use of expensive isotopic tracers, has generated considerable interest among researchers attempting to quantify 6 loss from lotic systems. Membrane inlet mass spectrometry allows measurement of small changes in 6 h concentrations, facilitating calculation of whole reach denitrification rates using an 6 h mass balance corrected for gas exchange. The method has been applied successfully within numerous rivers ranging widely in size and denitrification rate. Previous model-based analyses suggest that the method can be applied in a broader suite of ecosystems, and specifically, that it is well suited to shallow streams where denitrification rates as low as 30-100 mu mol N m(-2) h(-1) may be measurable. This coupled with increasing availability of necessary equipment, relatively low cost of measurements, and the ability to measure denitrification at environmentally relevant spatial scales suggests that broad adoption of the method is likely. In this paper, we revisit this model-based analysis using alternate models of gas exchange and demonstrate that benthic turbulence-induced gas exchange will restrict the suite of suitable study streams. Specifically, we note that within shallow streams and fast-flowing systems denitrification may be measurable only at moderate or high rates. To help facilitate further application of the method, we extend our discussion beyond site selection to discuss assumptions of the open-channel method, options for estimating the error in denitrification rates, and recommended practices for future studies.