Fluid-flow effects on denitrification hot spot activity in streams

The main objective of this research was to examine the effects of fluid-motion on denitrification in streams. Field investigations were performed in agricultural streams of southern Minnesota (nitrogen-rich) and pristine forested streams of northern California (nitrogenlimited). Measurements of denitrification rates and quantification of environmental conditions were made using micro- to reach-scale techniques in order to address the scalecomplexity of the problem. Results from southern Minnesota demonstrated that while denitrification occurs within agricultural streams with high nitrate concentrations, its effect on annual nitrate loadings to receiving water bodies is minimal. The largest obstacle for in-stream denitrification in agricultural watersheds is the lack of hydrologic residence time generated by extensive man-made drainage systems. Data collected from northern California were used to assess the spatial variability of denitrification occurring within stream reaches. Denitrification rates correlated with the quantity of denitrifying bacteria, and hot spots for denitrification were defined according to parameters of a lognormal probability distribution. Dimensional analysis was used to develop a power law relationship between denitrification rates and controlling environmental factors. This scaling relationship was used to assess spatial variability of denitrification within stream reaches using data describing the heterogeneity in turbulence levels, carbon and nitrogen amounts, and dissolved oxygen flux. Laboratory experiments were performed to examine how fluid-flow affects dissolved oxygen flux and its resulting inhibition of denitrification in stream sediments. Quasi-periodic motions in a turbulent flow were shown to control the mass transfer of dissolved oxygen to sediments. Quantification of these sweep and eject motions was incorporated into a similarity model that described dissolved oxygen flux according to the surface renewal theory. Micro-scale examination of nitrate and dissolved oxygen concentrations in sediments revealed that denitrification was confined to a small region just below the oxic sediment layer. Increasing turbulence levels increased the penetration of dissolved oxygen into sediments, which forced the denitrification zone deeper into the sediments, as well as decreased denitrification rates.