Variation Of NO Research Articles

Regional space monitoring of nitrogen dioxide in the troposphere

Satellite instruments for the routine global monitoring of NO2 in the atmosphere—the Global Ozone Monitoring Experiment (GOME) on the ERS-2 satellite, the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) on the ENVISAT satellite, the Ozone Monitoring Instrument (OMI) on the AURA satellite, and the GOME-2 on the MetOp satellite—are briefly described. It is shown that the error of measuring the NO2 total column amount (∼10% for the background conditions in the troposphere) substantially increases in regions subject to anthropogenic pollution. Examples of practically using multiyear satellite measurements for the regional monitoring of NO2 in the troposphere are presented, including mapping the tropospheric NO2 in Russia, identifying the weekly and annual cycles in tropospheric NO2 variations for megalopolises (St. Petersburg, Moscow, Paris), and estimating the long-term linear trend in 1995–2007.

Latitudinal structure of variations and trends in stratospheric NO2

Using data from ground-based spectrometric measurements of stratospheric column nitrogen dioxide (NO2) within the Network for the Detection of Atmospheric Composition Change (NDACC), analyses are made of the diurnal and annual variations of NO2, the NO2 decrease relating to the Pinatubo eruption, the NO2 change during the 11-year cycle of solar activity (SA), and the linear trends in NO2. Latitudinal dependences of these variations and trends are revealed. In particular, the annual trend estimates of column NO2 are predominantly positive in the middle latitudes of the Southern Hemisphere (SH) whereas negative NO2 trends are observed in the middle Northern Hemisphere (NH) latitudes. Column NO2 content in the middle latitudes of the two hemispheres is generally larger during the SA minimum than during the SA maximum (negative sign of the SA effect).

Assessment of groundwater contamination by NO3 − using geographical information system in the Zhangye Basin, Northwest China

Geographical information system (GIS) technology was used to investigate NO3− concentration of groundwater in the Zhangye Basin, Northwest China. Thematic information and NO3− data of groundwater from the Zhangye Basin were analyzed in a GIS environment to study the extent and variation of NO3− concentration and to establish spatial relationships with corresponding land use types. About 38.8% of groundwater samples showed NO3− concentration above the human affected value (13 mg/l NO3−), while more than 10.2% exceeded the maximum acceptable level (50 mg/l NO3−) according to WHO regulations. The spatial distribution of NO3− of groundwater in the Zhangye Basin showed an obvious character of zonation; the high NO3− concentration was mainly located in the city seat areas, and Zhangye City was the high-value center. The probability of over 90% for NO3− concentration exceeding 13 mg/l was found in urban seat areas. The spatial analyses indicated that groundwater contamination by NO3− was closely related to one specific land use class, the urban. The NO3− concentration of groundwater under urban was significantly higher than that under irrigation land or sand dune. Most of the unacceptable NO3− levels were found in groundwater samples assigned to urban area, but a few were also found allotted to irrigation land class. Therefore, urban source was considered the principal source of NO3− contamination of groundwater in the Zhangye Basin, while irrigation land source was still a possible source of NO3− contamination.

Mechanisms of transient nitric oxide and nitrous oxide production in a complex biofilm

Nitric oxide (NO) and nitrous oxide (N(2)O) are formed during N-cycling in complex microbial communities in response to fluctuating molecular oxygen (O(2)) and nitrite (NO(2)(-)) concentrations. Until now, the formation of NO and N(2)O in microbial communities has been measured with low spatial and temporal resolution, which hampered elucidation of the turnover pathways and their regulation. In this study, we combined microsensor measurements with metabolic modeling to investigate the functional response of a complex biofilm with nitrifying and denitrifying activity to variations in O(2) and NO(2)(-). In steady state, NO and N(2)O formation was detected if ammonium (NH(4)(+)) was present under oxic conditions and if NO(2)(-) was present under anoxic conditions. Thus, NO and N(2)O are produced by ammonia-oxidizing bacteria (AOB) under oxic conditions and by heterotrophic denitrifiers under anoxic conditions. NO and N(2)O formation by AOB occurred at fully oxic conditions if NO(2)(-) concentrations were high. Modeling showed that steady-state NO concentrations are controlled by the affinity of NO-consuming processes to NO. Transient accumulation of NO and N(2)O occurred upon O(2) removal from, or NO(2)(-) addition to, the medium only if NH(4)(+) was present under oxic conditions or if NO(2)(-) was already present under anoxic conditions. This showed that AOB and heterotrophic denitrifiers need to be metabolically active to respond with instantaneous NO and N(2)O production upon perturbations. Transiently accumulated NO and N(2)O decreased rapidly after their formation, indicating a direct effect of NO on the metabolism. By fitting model results to measurements, the kinetic relationships in the model were extended with dynamic parameters to predict transient NO release from perturbed ecosystems.

Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake

We measured uptake length of 15NO3− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban‐urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3− concentrations, showing a loss in removal efficiency in streams with high NO3− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3− removal. The fraction of catchment area as agriculture and suburban‐urban land use weakly predicted NO3− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3− uptake lengths via directly increasing both gross primary production and NO3− concentration. Gross primary production shortened SWtot, while increasing NO3− lengthened SWtot resulting in no net effect of land use on NO3− removal.

Comparison of weekly cycle of NO2 satellite retrievals and NOx emission inventories for the continental United States

Spatially resolved weekly NO2 variations are obtained from 2003 to 2005 Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) tropospheric NO2 columns for three different types of regions: urban, rural, and rural‐point (rural with significant electricity generation unit (EGU) emissions). Regions are compared for magnitudes and weekly profiles. Rural regions do not show any weekly pattern, whereas urban areas show a distinct decrease on the weekends. Rural regions with EGUs show a slight decrease on Sundays. When compared with estimated mobile and stationary nitrogen oxides (NOx) emissions from the year 2004 for seven cities, the satellite data have greater variation during weekdays (Monday–Friday). Overall comparisons show that SCIAMACHY‐derived NO2 correlate well with estimated NOx emissions for urban and rural but less for rural‐point regions.

Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

Denitrification is an important net sink for NO3 − 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 NO3 − 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 NO3 − 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 μmol N m−2 h−1), 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 μmol N m−2 h−1) and stream NO3 − concentration (~20–1,100 μmol L−1) representing seasonal variation from spring high flow (high NO3 −) to late summer low flow (low NO3 −). The temporal dependence of U denit on NO3 − was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U denit ≈26 * [NO3 −]0.44 or U denit ≈640 * [NO3 −]/[180 + NO3 −]; for a partially weighted dataset, U denit ≈14 * [NO3 −]0.54 or U denit ≈700 * [NO3 −]/[320 + NO3 −]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO3 − concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO3 − concentration. Hypothetical models based on our results illustrate: (1) U denit was inversely related to denitrification rate constant (k1denit, in day−1) and vertical transfer velocity (v f,denit, in m day−1) at seasonal and possibly event time scales; (2) although k1denit was relatively large at low flow (low NO3 −), 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 NO3 − assimilation and denitrification were linked through production of organic reactants, rates of NO3 − loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1denit and v f,denit are linked implicitly with stream depth, NO3 − concentration, and(or) NO3 − load, estimates of U denit may be related more directly to field factors (including NO3 − 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 NO3 − concentration and accounting for temporal variation.

Spatial Variability of Ambient Nitrogen Dioxide and Sulfur Dioxide in Sarnia, “Chemical Valley,” Ontario, Canada

This study aimed at developing models to predict nitrogen dioxide (NO2) and sulfur dioxide (SO2) concentrations in Sarnia, “Chemical Valley”, Ontario, Canada, and model the intra-urban variation of ambient NO2 and SO2 in the city for a community health study. NO2 and SO2 samples were monitored with Ogawa passive samplers at 39 locations across the city for 2 wk during the fall of 2005. The final land use regression models were constructed to generate independent variables that might best predict NO2 and SO2 concentrations. The coefficients of determinations for the final NO2 and SO2 models were .79 and .66, respectively. The explanatory variables in the final NO2 model were: proximity to the industrial core, industrial areas within 1600 m, highways within 400 m, and dwelling counts within 2400 m. The variables in the final SO2 model were: proximity to the industrial core, industrial areas within 1200 m, and major roads within 100 m. The spatial variations captured in these analyses are being used to estimate ambient pollution concentrations for a large health study.

Spatial and temporal variability of nitrate concentration in seepage water under a mature Norway spruce [Picea abies (L.) Karst] stand before and after clear cut

The spatial and temporal variability of the nitrate (NO3−) concentration in seepage water below the main rooting zone of a mature spruce forest was investigated using 121 suction cups which were implemented in a 2 × 2 m grid at 40 cm depth in the mineral soil. Seepage was collected at least monthly during the vegetation period in 2005. In January 2006 a clear cut was performed on the plot and monthly seepage water was collected with 118 suction cups. Conventional and geostatistical methods were used to investigate the spatial and temporal variability in NO3−. We tried to explain the observed variability with multiple classification analysis (MCA) and multiple linear regression models (MLR) based on vegetation, stand and soil parameters, which could be measured without disturbance of the ecosystem. The NO3− concentration in the mature stand reached mean values up to 35.2 mg l−1 per sampling date and the distributions were positively skewed. The temporal variability was much lower than the spatial variability. NO3− in seepage water showed clear structural patterns over the whole vegetation period. Spatial autocorrelation ranged between 16 and 19 m and structural variance was between 65% and 80% of the whole model semivariance. However, for practical purposes it should be sufficient to consider an autocorrelation range of about 12 m as the last 5–6.5 m only explained 5% of the total structural variance of NO3− in seepage water. Vegetation and stand parameters such as distance and size of the trees surrounding the measuring points explained about 40% of total variability in the MCA and MLR models. After clear cut, concentration means per sampling date were below 10 mg l−1 in spring but increased to more then 150 mg l−1 until December 2006. The distribution was nearly normal. The patterns of spatial and temporal variability were reversed compared to the mature stand. Structuring was less pronounced with autocorrelation ranges decreasing from 10 m in April 2006 to values below 3 m during the following vegetation period. The structuring variance was below 30% at most sampling dates. Thickness of the humus layer and amount of water in the sampling bottles, which were not considered in the mature stand, contributed significantly to the explanation of variability after the clear cut. Together with vegetation coverage and stand parameters 30% of the total NO3− variability could be explained in the MCA and MLR models. The consequences for future seepage water investigations are discussed.

Measurement of SO2and NO2applying ToTaL-DOAS from a remote site

The concentration and diurnal variation of SO2 and NO2 were remotely measured in a main street of Montevideo using a recently proposed experimental procedure called topographic target light scattering differential optical absorption spectroscopy (ToTaL-DOAS). Our instrument was placed on the roof of a building 3 km away from the measured site. We give a general description of the procedure and present the first results of the 2006 campaign at Montevideo.

Simultaneous Measurements of Gaseous Nitrous Acid and Particulate Nitrite Using Diffusion Scrubber/Steam Chamber/Luminol Chemiluminescence

An instrument was developed for the simultaneous determination of gas- and aerosol-phase nitrous acid (HONO). Gaseous HONO (HONO(g)) was sampled by a diffusion scrubber and particulate nitrite (NO2 − (p)) was collected by a particle growth chamber. The collected samples were analyzed in time-sharing manner, based on the peroxynitrite-induced luminol chemiluminescence. The automated system was found to be sensitive with 13 pptv of detection limit, fast with 4 min. of sampling frequency, and simple and affordable to construct and operate. The system was optimized by adjusting the experimental parameters. The system was applied to the field measurement of gas- and particle-phase HONO during the springtime of 2004 in Gwangju, South Korea. HONO(g) concentrations varied diurnally from 200 pptv around 3 P.M. to 800 pptv at 5 A.M. The variation of NO2 − (p) was not significant with the maximum of 240 pptv at 11 P.M. and the minimum of 170 pptv at 4 P.M., not displaying distinct characteristics.

Effect of air pollution on diabetes and cardiovascular diseases in São Paulo, Brazil

Type 2 diabetes increases the risk of cardiovascular mortality and these patients, even without previous myocardial infarction, run the risk of fatal coronary heart disease similar to non-diabetic patients surviving myocardial infarction. There is evidence showing that particulate matter air pollution is associated with increases in cardiopulmonary morbidity and mortality. The present study was carried out to evaluate the effect of diabetes mellitus on the association of air pollution with cardiovascular emergency room visits in a tertiary referral hospital in the city of São Paulo. Using a time-series approach, and adopting generalized linear Poisson regression models, we assessed the effect of daily variations in PM10, CO, NO2, SO2, and O3 on the daily number of emergency room visits for cardiovascular diseases in diabetic and non-diabetic patients from 2001 to 2003. A semi-parametric smoother (natural spline) was adopted to control long-term trends, linear term seasonal usage and weather variables. In this period, 45,000 cardiovascular emergency room visits were registered. The observed increase in interquartile range within the 2-day moving average of 8.0 microg/m(3) SO2 was associated with 7.0% (95%CI: 4.0-11.0) and 20.0% (95%CI: 5.0-44.0) increases in cardiovascular disease emergency room visits by non-diabetic and diabetic groups, respectively. These data indicate that air pollution causes an increase of cardiovascular emergency room visits, and that diabetic patients are extremely susceptible to the adverse effects of air pollution on their health conditions.

Study on ambient air quality in Beijing for the summer 2008 Olympic Games

With the coming/approaching of the Olympic Games in 2008, air pollution in Beijing attracts the attention of government and people. The objective of this study is to define the air quality during the Olympic Games; we conducted the observation of SO2, NO, CO, NO2, O3, and PM10 from August 7 to September 30 in 2007 in Beijing. The results showed that the average daily concentrations of SO2, NO2, CO, and PM10 during observation were 0.024, 0.072, 2.25, and 0.19 mg m−3, respectively. Compared with the National Ambient Air Quality Standard II, the concentrations of SO2 and CO in the observation were low, the concentration of NO2 basically satisfied the National Ambient Air Quality Standard II, and the concentrations of O3 and PM10 were much higher than the values of the standard. The characteristics of diurnal variation of NO, NO2, CO, and PM10 were similar, and the lower concentrations of these pollutants were observed by day and the higher concentrations at night. The concentration of SO2 in the daytime was a little higher than that at night. The highest 1-h concentration of O3 occurred at 14:00 local time.

Significance of transient luminous events to neutral chemistry: Experimental measurements

For many years it has been suggested that transient luminous events (TLE) occurring over thunderstorms may produce significant modifications to neutral atmospheric chemistry. Some have speculated that large ionisation increases from red sprites, one type of TLE, could result in enhancements of odd nitrogen. In this study we make use of nighttime NO2 observations by the GOMOS instrument to test whether TLE are producing significant NOx enhancements in the middle atmosphere on a regional scale. Comparing regional variations of NO2 with 2–3 order of magnitude variations in lightning activity, we show that there is no significant impact of red sprites, giant jets or blue jets upon NOx levels in the stratosphere and mesosphere (20–70 km), within the detection levels of the instrument. While individual TLE may cause a local variation in NOx, these do not appear to be significant on regional scales (or beyond).

Cardiac arrhythmia emergency room visits and environmental air pollution in São Paulo, Brazil

Objectives:Air-pollution exposure has been associated with increased cardiovascular hospital admissions and mortality in time-series studies. We evaluated the relation between air pollutants and emergency room (ER) visits because of cardiac...

The trend, seasonal cycle, and sources of tropospheric NO2 over China during 1997–2006 based on satellite measurement

The characteristics of spatial and temporal distribution of tropospheric NO2 column density concentration over China are presented, on the basis of measurements from the satellite instruments GOME and SCIAMACHY. From these observations, monthly averaged tropospheric NO2 variations are determined for the period of 1997 to 2006. The trend and seasonal cycle are also investigated. The possible source of tropospheric NO2 over megacity area is discussed in this paper. The results show a large growth of tropospheric NO2 over eastern China, especially above the industrial areas with a fast economical growth, such as, Yangtze Rive Delta region and Pearl River Delta region because of the prominent anthropogenic activity. There is a rapid increase of tropospheric NO2 over megacities in China. For instance, Shanghai had a linear significant increase in NO2 columns of ∼20% per year (reference year 1997) in the period of 1997–2006, which is the rapidest increase among all the selected cities. The seasonal pattern of the NO2 concentration shows a difference between the east and west in China. In the eastern part of China, an expected winter maximum in seasonal cycle is found because of the prominent anthropogenic activity and meteorological conditions. In the western part this cycle shows a NO2 maximum in summer time, which is attributed to natural emissions, especially soil emissions and lightning. A quickly increasing vehicle population may contribute to the increase of tropospheric NO2 over megacities in China for the remarkable correlation for vehicle population with tropospheric NO2.

Vertical profiles in NO3 and N2O5 measured from an aircraft: Results from the NOAA P‐3 and surface platforms during the New England Air Quality Study 2004

The nocturnal nitrogen oxides, NO3 and N2O5, are important to the chemical transformation and transport of NOx, O3 and VOC. Their concentrations, sources and sinks are known to be vertically stratified in the nighttime atmosphere. In this paper, we report vertical profiles for NO3 and N2O5 measured from an aircraft (the NOAA P‐3) as part of the New England Air Quality Study in July and August 2004. The aircraft data are compared to surface measurements made in situ from a ship and by long‐path DOAS. Consistent with previous, vertically resolved studies of NO3 and N2O5, the aircraft measurements show that these species occur at larger concentrations and are longer lived aloft than they are at the surface. The array of in situ measurements available on the P‐3 allows for investigation of the mechanisms that give rise to the observed vertical gradients. Selected vertical profiles from this campaign illustrate the role of biogenic VOC, particularly isoprene and dimethyl sulfide, both within and above the nocturnal and/or marine boundary layer. Gradients in relative humidity and aerosol surface may also create a vertical gradient in the rate of N2O5 hydrolysis. Low‐altitude intercepts of power plant plumes showed strong vertical stratification, with plume depths of 80 m. The efficiency of N2O5 hydrolysis within these plumes was an important factor determining the low‐level NOx and O3 transport or loss at night. Averages of nocturnal O3, NO2, NO3 and N2O5 binned according to altitude were consistent with the trends from individual profiles. While production rates of NO3 peaked near the surface, lifetimes of NO3 and N2O5 were maximum aloft, particularly in the free troposphere. Variability in NO3 and N2O5 was large and exceeded that of NO2 or O3 because of inhomogeneous distribution of NOx emissions and NO3 and N2O5 sinks.

Inter-annual Variations of Ozone and Nitrogen Dioxide Over Europe During 1958–2003 Simulated with a Regional CTM

Inter-annual variability of surface ozone (O3) and nitrogen dioxide (NO2) over Europe has been studied over the period 1958–2003 using a three-dimensional Chemistry-Transport Model coupled to meteorological data from the ERA40 data set produced at the European Centre of Medium-range Weather Forecasts (ECMWF). Emissions and boundary conditions were kept at present levels throughout the simulation period. It was found that the annual mean NO2 concentration varies between ±50% and the summer mean O3 concentration varies between −10 and +20 percent (%) compared to the 46-year average over the model domain. There is also variation in ozone and NO2 over longer time scales. The last 22 years display high concentrations of ozone in central and south-western Europe and low concentrations in north-eastern Europe. The first 22 years display very high concentrations of NO2 over the North Sea. There is indication of trends in ozone and nitrogen dioxide but this has to be investigated further. Such information is one factor that should be taken into account when considering future control strategies.

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N2O, CO2 and CH4 soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N2O emission in these regions on a landscape level. The mean N2O emission at site PF was 6.0 ± 0.1 SE μg N m−2 h−1. Fluxes of N2O increased from <5 μg N m−2 h−1 during dry season conditions to up to 24.5 μg N m−2 h−1 with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N2O were measured in the SF and RP sites, where mean N2O emissions were 7.3 ± 0.7 SE μg N m−2 h−1 and 4.1 ± 0.5 SE μg N m−2 h−1, respectively. The dependency of N2O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N2O emission peaks, not the total amount of N2O emitted. For all sites, significant positive correlations existed between N2O emissions and both soil moisture and soil temperature. Mean CH4 uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m−2 h−1), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m−2 h−1) and lowest for the RP site (−5.7 ± 0.5 SE μg C m−2 h−1). At all sites, CH4 uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N2O emissions, CH4 uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO2 emissions at the PF site increased significantly from 50 mg C m−2 h−1 up to 100 mg C m−2 h−1 (mean value 68.8 ± 0.8 SE mg C m−2 h−1), whereas CO2 emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m−2 h−1 (SF) and 34.9 ± 1.1 SE mg C m−2 h−1 (RP). A dependency of soil CO2 emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO2 emissions as compared to control chambers. Correlation of CO2 emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.

Effects of light on NO3−uptake in small forested streams: diurnal and day-to-day variations

We investigated the effects of autotrophy on short-term variations in nutrient dynamics by measuring diurnal and day-to-day variations in light level, primary productivity, and NO3− uptake during early and late spring in 2 forested streams, the East and West Forks of Walker Branch in eastern Tennessee, USA. We predicted that diurnal and day-to-day variations in NO3− uptake rate would be larger in the West Fork than in the East Fork in early spring because of higher rates of primary productivity resulting from a more stable substratum in the West Fork. We also predicted minimal diurnal variations in both streams in late spring after forest leaf emergence when light levels and primary productivity are uniformly low. Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O2 change technique, and reach-scale rates of NO3− uptake were determined by tracer 15N-NO3− additions. In the West Fork, significant diurnal and day-to-day variations in NO3− uptake were related to variations in light level and primary productivity in early spring but not in late spring, consistent with our predictions. In early spring, West Fork NO3− uptake rates were 2 to 3× higher at midday than during predawn hours and 50% higher on 2 clear days than on an overcast day several days earlier. In the East Fork, early spring rates of GPP were 4 to 5× lower than in the West Fork and diurnal and day-to-day variations in NO3− uptake rates were <30%, considerably lower than in the West Fork. However, diurnal variations in NO3− uptake rates were greater in late spring in the East Fork, possibly because of diurnal variation in water temperature. Our results indicate the important role of autotrophs in nutrient uptake in some forested streams, particularly during seasons when forest vegetation is dormant and light levels are relatively high. Our results also have important implications for longer-term assessments of N cycling in streams that rely on daytime measurements or measurements only under limited weather conditions (i.e., clear days).