Lay Abstract The exchange of gasses between water and air is important to the budgets of carbon, nutrients, and pollutants. This exchange is driven, in part, by the turbulent energy at the air–water interface. Turbulent energy at the air–water interface scales with the gas transfer velocity (k), which can be measured in streams through various methods. We performed a metadata analysis of studies that have measured k in streams using direct gas tracer releases. We evaluated models that predict k based on stream morphology. We found that models that use slope and velocity to predict k perform reasonably well and are consistent with general theory. We also used the data set to provide new stream hydraulic equations that predict stream morphology (width, depth, velocity) based on discharge.

Denitrification is an important net sink for NO 3 -in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO 3 -concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO 3 -concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U denit , in), results of different methods for a given set of conditions commonly were in agreement within a factor of 2-3. At approximately constant temperature (*20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (*0-4,000 lmol N m -2 h -1 ) and stream NO 3 -concentration (*20-1,100 lmol L -1 ) representing seasonal variation from spring high flow (high NO 3 -) to late summer low flow (low NO 3 -). The temporal dependence of U denit on NO 3-was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, -008-9282-8 in m day -1 ) at seasonal and possibly event time scales; (2) although k1 denit was relatively large at low flow (low NO 3 -), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U denit ; and (3) although NO 3 -assimilation and denitrification were linked through production of organic reactants, rates of NO 3 -loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1 denit and v f,denit are linked implicitly with stream depth, NO 3 -concentration, and(or) NO 3 -load, estimates of U denit may be related more directly to field factors (including NO 3 -concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U denit and stream NO 3 -concentration and accounting for temporal variation.

1. Denitrification, net oxygen consumption and net nitrous oxide flux to the atmosphere were measured in three small rivers (discharge approximately 2-27 m 3 s )1 ) at the whole reach scale during Spring and Summer, 2002. Two of these rivers (Iroquois River and Sugar Creek in north-west Indiana -north-east Illinois, U.S.A.) drained agricultural catchments and the other (Millstone River in central New Jersey, U.S.A.) drained a mixed suburban-agricultural catchment. 2. Denitrification, oxygen consumption and N 2 O flux were measured based on net changes in dissolved gas concentrations (N 2 , O 2 , and N 2 O) during riverine transport, correcting for atmospheric exchange. On each date, measurements were made during both light and dark periods. 3. Denitrification rates in these rivers ranged from 0.31 to 15.91 mmol N m )2 h )1 , and rates within each river reach were consistently higher during the day than during the night. This diurnal pattern could be related to cyclic patterns of nitrification driven by diurnal variations in water column pH and temperature. 4. Oxygen consumption ranged from 2.56 to 241 mmol O 2 m )2 h )1 . In contrast to denitrification, net oxygen consumption was generally higher during the night than during the day. 5. River water was consistently supersaturated with N 2 O, ranging from 102 to 209% saturated. Net flux of N 2 O to the atmosphere ranged from 0.4 to 60 lmol N m )2 h )1 . Net flux of N 2 O was generally higher at night than during the day. The high flux of N 2 O from these rivers strengthens the argument that rivers are an important contributor to anthropogenic emissions of this greenhouse gas.

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A model-based approach was recently introduced for measuring riverine denitrification based on measured changes in dissolved N 2 concentration during riverine transport (Laursen & Seitzinger, 2002a). Inputs to the model, including water temperature, channel depth, wind velocity, and time-of-travel between sampling locations, vary greatly among natural systems. Simulations were run by varying the values of these inputs and determining rates of N 2 accumulation in river water and the detection limits for measuring denitrification using this method. Dinitrogen was found to accumulate most rapidly in streams that were shallow, particularly under conditions of low wind velocity. Dissolved N 2 concentrations, modeled in rivers with a diurnal temperature variation of 5°C and under conditions of no denitrification or 1 mmol N m )2 h )1 , showed that sensitivity of the method can vary as temperatures change. Under low wind conditions and in rivers <1m in depth, this method is capable of detecting denitrification rates as low as 30-100 lmol N m )2 h )1 . This limit of detection should be adequate to measure in situ rates in many North American streams, particularly in agricultural watersheds. In deeper rivers N 2 accumulated more slowly and the method became less sensitive. The results of this study should guide decisions regarding the application of this method based on the specific characteristics of a study reach (channel geometry) and the physical conditions (i.e. wind velocity and water temperature) under which measurements are to be made.

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