-1Whole-system estimation of denitrification in a plainsriver: a comparison of two methodsALENA L. PRIBYL, JAMES H. MCCUTCHAN Jr.*, WILLIAM M.LEWIS Jr., and JAMES F. SAUNDERS IIICenter for Limnology, Cooperative Institute for Research in Environmental Sciences and Departmentof Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0216, USA;*Author for correspondence (e-mail: mccutchj@cires.colorado.edu; phone: 303-492-5191; fax: 303-492-0928)Received 26 January 2004; accepted in revised form 8 June 2004Key words: Denitrification, Mass balance, Membrane-inlet mass spectrometry, Nitrogen, Open-channel N 2 method, South Platte RiverAbstract. Whole-system denitrification in the South Platte River was measured over a 13-monthperiod using an open-channel N 2 method and mass-balance measurements. Concentrations ofdissolved N 2 were measured with high precision by membrane-inlet mass spectrometry and esti-mates of denitrification were based on the mass flux of N 2 , after correction for reaeration andgroundwater flux. Open-channel estimates of denitrification ranged from 0 to 3.08 g N m ?2 d ?1and the mean annual rate was 1.62 g N m ?2 d ?1 , which corresponds to removal of approximately34% of the nitrate transported by the river over a distance of 18.5 km. Over the same period oftime, estimates of denitrification based on mass-balance measurements ranged from 0.29 to5.25 g N m ?2 d ?1 and the mean annual rate was 2.11 g N m ?2 d ?1 . The two methods revealedsimilar seasonal patterns of denitrification the highest rates were measured from late April toAugust and the lowest rates were in winter. Both methods provide whole-system estimates ofdenitrification in running waters; where reaeration rate coefficients are low and flux of groundwateris well quantified, the open-channel method has fewer sources of uncertainty and is easier toimplement.IntroductionAnthropogenic sources now dominate the supply of fixed nitrogen to manyaquatic ecosystems, and concentrations of dissolved forms of fixed nitrogen areincreasing in both surface waters (Howarth et al. 1996; Vitousek et al. 1997;Lovett et al. 2000) and groundwaters (Nolan et al. 1997). Because nitrogen canlimit primary production in both freshwater and marine systems (Morris andLewis 1988; Bergmann et al. 2002; Tank and Dodds 2003) and high concen-trations of nitrate in drinking water pose health risks to humans (Nolan et al.1997), it has become critically important to understand the biogeochemicalprocesses that comprise the nitrogen cycle.Mass-balance studies of the Mississippi River drainage (Alexander et al.2000) have shown that small streams are generally more retentive of nitrogenthan are large streams. Additionally,15 N-tracer studies have shown that ratesof uptake and transformation for N tend to be highest in small streamsBiogeochemistry (2005) 73: 439–455 ? Springer 2005DOI 10.1007/s10533-004-0565-4
