Seasonal variations in the concentrations of methyl and ethyl nitrate in a shallow

freshwater lake

Claire Hughes,* Anthony J. Kettle, Godwin A. Unazi, Keith Weston, Matthew R. Jones, andMartin T. Johnson

Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich,UK

Abstract

Between February and October 2007, a time series of alkyl nitrate concentrations in the water column of ashallow freshwater lake, University of East Anglia Broad, located in southeastern England was carried out todetermine whether methyl and ethyl nitrate are present in freshwater systems and to improve understanding of alkylnitrate production mechanisms in aquatic environments. Concentrations ranged from 4.7 (6 0.5) to 53.7 (64.36) pmol L21 methyl nitrate, and 2.5 (6 0.3) to 11.1 (6 0.4) pmol L21 ethyl nitrate and were within the range ofthose measured previously in seawater. Peaks in the concentrations of methyl and ethyl nitrate at 4 m were observedin 9 and 6 of the 18 depth profiles measured, respectively. Gradients in concentrations within the hypolimnionsuggest that the alkyl nitrates are produced in the bottom waters or sediments, are transported to the lake ingroundwater, or both. Colored dissolved organic matter absorbance data suggests that the penetration of ultravioletlight was limited in the lake, so the deep maxima must be due to a non-photochemical alkyl nitrate source.

Methyl and ethyl nitrate (CH3ONO2 and C2H5ONO2)are nitrogen-containing volatile organic compounds thatare known to occur naturally in seawater (Chuck et al.2002; Moore and Blough 2002; Dahl et al. 2003). Measure-ments have shown that seawater concentrations of the alkylnitrates are high enough to drive a significant sea-to-airflux of these compounds (Chuck et al. 2002; Dahl et al.2005). This is of interest because sea-to-air fluxes of methyland ethyl nitrate potentially represent a major source ofoxidized nitrogen to the remote marine atmosphere (Atlaset al. 1993). The oxidized nitrogen budget of the atmo-sphere is crucial in that it influences one of the main path-ways for ozone formation in the troposphere (Jones et al.1999; Talbot et al. 2000). Understanding the mechanismsby which methyl and ethyl nitrates are formed in seawateris important for developing predictive tools that canproduce accurate sea-to-air flux estimates and, hence,establish the importance of these compounds.

To date, only one mechanism for the formation ofmethyl and ethyl nitrates in seawater has been confirmed.In a series of laboratory experiments, Dahl et al. (2003)showed that both of these compounds can be producedfrom photochemically formed peroxy (ROO2) and nitricoxide (NO2) radicals. In their experiments, ROO2 isformed via photochemical reactions involving colored dis-solved organic matter (CDOM; Blough and Del Velcchio2002), and NO2 is produced by the photolysis of nitrite(Zafiriou and True 1981). A number of studies, includingDahl et al. (2003), propose that additional non-photo-chemical alkyl nitrate sources exist in seawater. Forexample, Chuck et al. (2002) speculate that the high ethylnitrate concentrations in the water column of the SouthAtlantic subtropical gyre are suggestive of a biogenicsource. Additionally, Moore and Blough (2002) propose

that photochemistry cannot solely be responsible for thedeep (175 m) methyl nitrate maxima they observe in theNorth Atlantic because of the rapid attenuation ofultraviolet (UV) light in the upper water column. Infor-mation presented in the literature suggests that theprecursors for alkyl nitrate formation identified by Dahlet al. (2003) can be produced via non-photochemicalmechanisms. For example, NO2 can be formed biologicallyduring oxidative stress (Yasuko et al. 2002) and duringnitrification and denitrification (Stuven et al. 1992; Stuvenand Bock 2001). Consequently, it is likely that photochem-istry is not the only alkyl nitrate source in the marineenvironment.

Resolving trace gas production mechanisms in oceanwaters is often complicated because the chemical, biolog-ical, and physical history of a parcel of water is difficult toestablish. Here, we present a time series of methyl and ethylnitrate concentrations measured in a shallow, freshwaterlake (the University of East Anglia Broad) betweenFebruary and October 2007. Alkyl nitrate fluxes fromlakes located in urban areas such as the one studied hereare unlikely to have a major effect on atmosphericchemistry because there is a large source of oxidizednitrogen from industrial pollution (Derwent and Stewart1973). The measurements made in this study in a simplelake system can, however, be used to suggest possibleproduction mechanisms and help improve our understand-ing of alkyl nitrate production in the marine environment,where sea-to-air fluxes of these compounds are believed toinfluence the ozone budget of the atmosphere.

Methods

Study site—The site is the University of East Anglia(UEA) Broad, which is a manmade, freshwater lake locatedin the southeast of England at 52u389N, 01u189E. A map of* Corresponding author: claire.hughes@uea.ac.uk

Limnol. Oceanogr., 55(1), 2010, 305–314

E 2010, by the American Society of Limnology and Oceanography, Inc.

305

the site is given in Fig. 1. The UEA Broad has a maximumdepth of 6.5 m and a mean depth of 3.0 m and isapproximately 700 m in length. No direct inputs flow tothe lake from any river or stream systems, aside fromperiodic storm runoff. Buoys were placed at one of thedeepest sections of the lake (5.2 m) to mark the samplingsite. Between 14 February, day of year (DOY) 45, and 03October 2007 (DOY 276) the site was visited in a small boat18 times to collect water samples and water columnprofiles. Weather conditions and sampling requirementsdid not permit regular sampling, so the time betweensampling events ranged from 5 to 44 d. However, on onlythree occasions did the gap between samplings exceed 14 d.

Ancillary parameters—Meteorological parameters: Rain-fall, wind speed, and atmospheric temperature weremeasured by a roof-top weather station (Davis) locatedapproximately 200 m from the study site. Measurementswere made at intervals of 10–15 min with this instrumentthroughout the study period.

Water column parameters: Profiles of water columntemperature and dissolved oxygen saturation were made ateach sampling event with the use of a Yellow SpringsInstruments (YSI) Environmental Model 556 MPS (Multi-Probe System). The dissolved oxygen data were calibratedbefore each sampling event by equilibration with theatmosphere and periodic use of Winkler titration (Carpen-ter 1965). Levels of photosynthetically active radiation(PAR; 400–700 nm) were measured at 10-min intervalsthroughout the study period with Hobo Pendant dataloggers moored at five depths in the water column (0.5, 1.5,2.5, 3.5, and 4.5 m). These data were used to calculate thedownward diffuse attenuation coefficient (KdPAR) accord-ing to Eq. 1,

Kd~1

z2{z1ln

I1

I2

� �ð1Þ

where I1 is the light intensity (lux) at the upper depth z1,

and I2 is the light intensity (lux) at the lower depth z2. Thedata were processed to calculate an average daily KdPAR

value from 11:00 h to 13:00 h Greenwich Mean Time.Mixed-layer depth is defined as the position of themaximum rate of change in temperature with depth withinthe water column.

Samples were collected for measurements of chlorophylla (Chl a), nitrate (NO {

3 ), nitrite (NO {2 ), and CDOM

absorbance from five depths in the water column (0, 1, 2, 3,and 4 m) with the use of a Niskin bottle hand-winched todepth. For return to our laboratory at the University ofEast Anglia, water samples for Chl a and nitrate werestored in 2-liter polycarbonate bottles, and those forCDOM analysis were placed in 250-mL glass amberbottles. All samples were stored in the dark duringtransport. For Chl a, 300-mL samples were gently filtered(0.7 mm, GF/F, Whatman) in duplicate, and the filters werestored at 280uC until analysis 1–6 mo after collection.After extraction in 100% methanol, Chl a samples wereanalyzed with a Turner fluorometer according to themethod described in Yentsch and Menzel (1963). ForNO {

3 and NO {2 , 50-mL samples were gently filtered across

ashed (4 h at 450uC) filters (0.7 mm, GF/F, Whatman), andthe resulting filtrate was stored at 220uC until analysis.NO {

3 and NO {2 were then determined with the use of a

Skalar autoanalyzer and the spectrophotometric methoddescribed by Kirkwood (1994). CDOM absorbance at350 nm (A350) was measured on filtered (0.2 mm, Nylon)samples with a Perkin Elmer UV and visible lightspectrophotometer. The CDOM absorption coefficient at350 nm (a350) was then calculated according to Eq. 2

a350~2:303A350|d{1 ð2Þ

where d is the optical path length (m). All samples for Chla, NO {

3 , NO {2 , and CDOM were processed and stored

within 1–2 h of return to UEA.

Methyl and ethyl nitrate analysis—Alkyl nitrate analysiswas carried out with a Markes thermal desorption unit andAgilent gas chromatograph–mass spectrometer (GC-MS).Samples were collected from five depths in the watercolumn (0, 1, 2, 3, and 4 m) with the use of a Niskin bottle.From each depth, 3 3 100-mL glass syringes were filledthrough a section of Tygon tubing directly from the samplebottle. All samples were stored in the dark and returned toour laboratory at the University of East Anglia andanalyzed within 1–3 h of collection. Before analysis, thesamples were filtered (0.7 mm, GF/F Whatman) into asecond 100-mL glass syringe with the use of an in-linefiltration unit. During sampling and filtration, care wastaken to not introduce any bubbles into the samples. Afterfiltration, 40-mL samples were injected into a glass purgevessel and purged for 10 min to extract the alkyl nitrateswith the use of oxygen-free nitrogen (OFN) at a flow rate of95 mL min21. The extracted gases were trapped andconcentrated on three-bed stainless steel sorbent tubes(Markes) containing Tenax, Carbograph, and Carboxen. AMarkes Unity thermal desorption unit and an UltrAautosampler (Markes) were used to desorb the alkyl

Fig. 1. Map showing the location of the study site inEngland (white circle).

306 Hughes et al.

nitrates from the sorbent tubes and introduce them in tothe GC. Specific details of the purge-and-trap system andGC-MS trace gas analysis are given in Hughes et al. (2008).System calibrations were carried out with the use of liquidstandards gravimetrically prepared in methanol and aMainz air standard as in Chuck et al. (2002). A smallvolume (2 mL) of an internal standard mixture containingdeuterated methyl iodide and 2-iodopropane prepared inmethanol was injected into each sample to check forvariability in system sensitivity throughout the samplingperiod, as in Hughes et al. (2008).

Results

Ancillary parameters—Information on the backgroundmeteorology and physical and chemical features of theUEA Broad observed during the study period are given inFig. 2. It can be seen from Fig. 2a that the watertemperature at 1 m followed the seasonal increase in airtemperature. The maximum temperature at this depth(20.4uC) was measured on DOY 219 and coincided witha peak air temperature of 27.1uC. The lowest watertemperature (5.5uC) was recorded on DOY 45. Themixed-layer depth was found to be highly variablethroughout the study period (Fig. 2b), ranging from 0.5to 5.0 m. The shallowest mixed layers (, 2 m) wereobserved during periods of thermal stratification inducedby increased surface heating and reduced wind speeds.During thermally stratified periods, a deep hypolimnion orrelatively cold layer formed below the mixed layer(Fig. 2a). Figure 2c shows that the observed variability inthe mixed layer had a significant effect on dissolved oxygensaturation at depth in the Broad. At the start of the timeseries (DOY 45), the water column was well mixed and hada uniform oxygen saturation of approximately 100%.However, when the mixed layer shallowed, the saturationlevels in the hypolimnion decreased, reaching 0% on someoccasions. The decreased saturation levels in the hypolim-nion are likely due to a high biological oxygen demandfrom processes such as respiration of sedimented phyto-plankton biomass and nitrification (Volkmar and Dahlgren2006) and reduced exchange with the atmosphere. The highChl a concentrations measured in the surface waters of theBroad (13.9 to 205.5 mg L21, data not shown) throughoutthe study period show that the lake is highly productive,which would maintain a continuous flux of organic

Fig. 2. Time series of (a) temperatures in the air and at 1 and4 m in the water column; (b) mixed layer depth and wind speed;(c) rainfall, nitrate (NO {

3 ), and nitrite (NO {2 ); (d) dissolved

oxygen saturation; and (e) diffuse attenuation coefficient forphotosynthetically active radiation (KdPAR) and the coloreddissolved organic matter (CDOM) absorption coefficient (a350)collected in the University of East Anglia Broad between

r

February and October 2007. (a) The gray line shows airtemperature, the solid black line shows the water temperature at1 m, and the dashed line shows the water temperature at 4 m. (b)The gray line shows wind speed, and the black lines indicate themixed-layer depth. (c) The open and filled circles show thedissolved oxygen saturation at 1 and 4 m, respectively. (d) Theopen and filled circles show NO {

3 , and the open and filledtriangles show NO {

2 concentrations at 1 and 4 m, respectively.The gray line shows rainfall. (e) The black line shows KdPAR andthe open circles a350 at 1 m. (a–e) The vertical dashed lines showthe period during which the depth profiles given in Fig. 3were measured.

Methyl and ethyl nitrate in a lake 307

material to the hypolimnion and sediments. The highoxygen saturation levels (up to 204%) that were measuredat 1 m depth are typical of productive lake systems. Forexample, Yacobi et al. (1993) previously observed similaroxygen supersaturations (up to 207%) in the surface watersof Lake Kinneret, Israel, and concluded that these highvalues were due to a high phytoplankton standing stockand low wind-driven turbulence.

The concentrations of NO {3 and NO {

2 were highlyvariable throughout the water column of the Broad duringthe study period. Figure 2d shows that at the start of thetime series, the concentrations of NO {

3 were relatively high(. 200 mmol L21) at both 1 and 4 m. A gradual decline inNO {

3 was then seen at all depths, presumably because ofbiological uptake. For example, at 1 m the NO {

3 concen-tration was 253.1 mmol L21 on DOY 45, but this haddecreased to 6.4 mmol L21 by DOY 129. Following thisdecline, spikes in this nutrient are evident at both 1 and4 m. Nitrite (NO {

2 ) concentrations ranged from 0.2 to2.5 mmol L21 at 1 m and 0.2 to 3.4 mmol L21 at 4 m.Increases in NO {

3 and NO {2 concentrations were associ-

ated with or occurred shortly after periods of rainfall(Fig. 2d), suggesting inputs from surface runoff or ground-water inflow (Brock et al. 1982; Sharpley et al. 1983;Simmons and Lyons 1994). The magnitude of the spikes inNO {

3 and NO {2 do not correlate with rainfall levels, but

the concentrations of these nutrients in surface runoff andgroundwater can vary depending on the conditions of thecatchment area (Sharpley et al. 1983). Another source ofNO {

3 in lake systems that could have contributed to theobserved variability in this nutrient is nitrification (Hall etal. 1984; Senga et al. 2001, 2002) which is the bacteriallymediated oxidation of ammonium (NH z

4 ). High NH z4

production in the hypolimnion is expected because of theremineralization of particulate organic matter, as shown bythe high biological oxygen demand. NO {

2 is additionallyproduced as an intermediate in both nitrification anddenitrification (Tomaszek and Czerwieniec 2003).

High levels of light attenuation were maintained in theBroad throughout the study period (Fig. 2e). Direct mea-surements of KdPAR gave results ranging from 1.1 to5.6 m21. These values are within the range of thoseobserved previously in productive lake systems. (Morriset al. 1995; Erm et al. 2001; Belzile et al. 2002). Forexample, Erm et al. (2001) observed a range of KdPAR of 1.1to 3.1 m21 in Lake Ulemiste (Estonia). An indirect measureof the attenuation of UV light was made by determiningCDOM absorbance at 350 nm. Figure 2e also shows theCDOM absorbance coefficients (a350) determined forsamples collected at 1 m depth in the UEA Broad, whichranged from 4.5 to 11.6 m21. Again, these values are withinthe range of those measured previously in freshwater lakes(Morris et al. 1995).

Methyl and ethyl nitrate concentrations—Both methyland ethyl nitrate were found to be present throughout thewater column of the UEA Broad. As has reportedpreviously for the marine environment (Chuck et al.2002; Dahl et al. 2005; Hughes et al. 2008), the concen-trations of methyl nitrate were found to be consistently

higher than those of ethyl nitrate. Observed ranges inconcentrations (61 SD) were 4.7 (6 0.5) to 53.7 (64.36) pmol L21 for methyl nitrate and 2.5 (6 0.3) to 11.1(6 0.4) pmol L21 for ethyl nitrate. Figure 3 shows fourdepth profiles of methyl and ethyl nitrate measuredbetween DOY 163 and 192. These plots show that duringperiods when the mixed layer is shallow (DOY 163–172),peaks in the concentrations of both alkyl nitrates wereobserved at depth. For example, on DOY 163 when themixed-layer depth is at 1 m, the methyl nitrate concentra-tion is 9.1 (6 0.8) pmol L21 at 1 m and 45.7 (61.8) pmol L21 at 4 m. The gradients in methyl and ethylnitrate observed within the hypolimnion from 4 m to thebottom of the mixed layer (DOY 163–172) suggest that thealkyl nitrate source is from the bottom of the Broad. Asexpected, the depth profile measured on DOY 185 showsthat when the mixed layer deepens to $ 4 m, the alkylnitrate concentrations become more homogeneous in thewater column. The observed ratio of methyl to ethyl nitratein the lake ranged from 1 : 1 to 6 : 1. Figure 4 shows that thehighest methyl to ethyl nitrate ratios (i.e., . 4 : 1) wereobserved at 4 m, and the lowest water column ratios (,2 : 1) were seen at the start of the time series when the watercolumn was well mixed throughout. The observed correla-tion coefficient (R2) between methyl and ethyl nitratecalculated from least squares regression analysis of the data(Fig. 5) was 0.65 (p , 0.001, n 5 18) at 4 m, 0.49 (p ,0.001, n 5 18) at 3 m, 0.15 (p . 0.01, n 5 18) at 2 m, 0.30 (p, 0.001, n 5 17) at 1 m, and 0.46 (p , 0.001, n 5 14) at0 m.

Figure 6 shows the observed temporal variations inmethyl and ethyl nitrate concentrations in samples collectedat 1 and 4 m during the study period. It can be seen clearlyfrom this plot that methyl and ethyl nitrate concentrationsat 4 m exceeded those at 1 m on a number of occasions. Intotal, 18 alkyl nitrate depth profiles were collected duringthe study period, and the methyl nitrate concentration at4 m exceeded that at 1 m on nine occasions (DOY 122, 129,144, 152, 163, 172, 185, 220, and 235). The ethyl nitrateconcentration at 4 m exceeded that at 1 m on six occasions(DOY 129, 144, 152, 163, 172, and 220). The highestincreases in the concentrations of methyl and ethyl nitrateobserved at 4 m between sampling events were seenbetween DOY 144–152 and 192–220. For example, betweenthese events, the concentrations of methyl nitrate increasedfrom 12.7 (6 0.6) to 36.5 (6 1.5) pmol L21 and 17.7 (61.1) to 53.7 (6 4.4) pmol L21. No significant correlations(R2 , 0.3) were observed between the alkyl nitrateconcentrations and ancillary parameters (dissolved oxygensaturation, rainfall, NO {

3 , or NO {2 ) measured at 4 m.

However, in general, all peaks in the concentrations of thealkyl nitrates were observed when the mixed layer wasshallow and dissolved oxygen saturation was , 31%.

Discussion

Alkyl nitrate concentrations—Methyl and ethyl nitrateconcentrations were measured in the water column of ashallow (6 m) freshwater lake (University of East AngliaBroad) located in southeastern England between February

308 Hughes et al.

and October 2007. Both compounds were found to bepresent in the lake at concentrations ranging from 4.7 (60.5) to 53.7 (6 4.36) pmol L21 methyl nitrate and 2.5 (60.3) to 11.1 (6 0.4) pmol L21 ethyl nitrate. Because theseare, to our knowledge, the first alkyl nitrate determinationsmade in a freshwater system, it is not possible to comparethese results to data collected previously in other lakes.However, the alkyl nitrate concentrations observed in theUEA Broad are within the range of those measured inother studies in the marine environment. For example,Chuck et al. (2002) observed concentrations ranging from0.1 to 227.1 pmol L21 of methyl nitrate and from 0.8 to33.5 pmol L21 of ethyl nitrate in the Atlantic Ocean. Also,Hughes et al. (2008) reported methyl and ethyl nitrateconcentrations in Southern Ocean waters ranging from 3.1to 194.9 and from 0.3 to 71.8 pmol L21, respectively.Although studies carried out in marine waters have shownstrong correlations between the concentrations of methyland ethyl nitrate (e.g., R2 5 0.81, Hughes et al. 2008), thoseobserved in this study were relatively lower (R2 5 0.15–0.65). The highest correlation between the alkyl nitrates (R2

5 0.65, p , 0.001, n 5 18) was observed at 4 m, with theleast significant observed at 2 m (R2 5 0.15, p . 0.01, n 5

18). These results suggest that the factors influencing theconcentrations of methyl and ethyl nitrate were similar at4 m but were different for the two gases at shallowerdepths. Potential controls on alkyl nitrate production in thewater column of the UEA Broad are discussed.

Deep maxima and a bottom alkyl nitrate source—Maximain the concentrations of methyl and ethyl nitrate at 4 mwere observed in 9 and 6 of the 18 depth profiles measuredin the UEA Broad, respectively. For methyl nitrate, thesewere distinct maxima with up to 10 times higher concentra-tions of this compound at 4 m compared with 1 m. Thedifference in concentrations was less pronounced for ethylnitrate, with the maximum disparity between the twodepths found to be a factor of 3. This is not the first timethat deep maxima in the concentrations of methyl and ethylnitrate have been observed. Both Chuck et al. (2002) andMoore and Blough (2002) report increased concentrationsat depth in the marine environment. The deep maxima inalkyl nitrate concentrations were observed in the Broadwhen the mixed-layer depth was , 4 m and methyl andethyl nitrate within the hypolimnion would be isolatedfrom losses to the atmosphere. The removal of gas exchange

Fig. 3. Depth profiles of methyl and ethyl nitrate concentrations measured on DOY 163, 172, 185, and 192 in the University of EastAnglia Broad. The dashed lines show the depth of the mixed layer. On DOY 185, the water column was mixed to 5 m. Error bars arestandard deviations, and where they cannot be seen, they are smaller than the size of the symbols.

Methyl and ethyl nitrate in a lake 309

as a loss process would allow higher concentrations of thegases to accumulate at depth in the presence of an alkylnitrate source at the bottom of the Broad. On DOY 163(Fig. 3), the methyl nitrate concentration at 4 m was foundto be 45.7 (6 1.8) pmol L–1, but that just below the mixedlayer at 2 m was 13.3 (6 2.1) pmol L–1. This type of bottomgradient is typical of trace gases, such as nitrous oxide(N2O), which are known to be produced in the low-oxygenhypolimnion and sediments of lake systems (Senga et al.2001, 2002).

It is informative to examine the observed variability inthe ratio of methyl nitrate : ethyl nitrate over time and withdepth in the water column. The range of ratios observed inthe UEA Broad (1 : 1 to 6 : 1) is similar to that reportedpreviously for marine waters (1 : 1 to 7 : 1, Chuck et al.2002; Dahl et al. 2007; Hughes et al. 2008); however, this isthe first time that the temporal variability of this parameterhas been monitored. Figure 4 presents depth profiles of thisratio from five distinct points in the UEA Broad timeseries: (1) late winter, before the onset of stratification(DOY 45); (2) spring (DOY 122), during a mixing eventbreaking the weak stratification that had been set up duringthe preceding weeks; (3, 4) stratified early summerconditions (DOY 152 and 163, respectively); and (5) asummer mixing event (DOY 185) leading to a fully mixedwater column. We propose that the observed ratios can beexplained in terms of a bottom source of the alkyl nitrates,which is relatively methyl nitrate–rich, and surface concen-trations that tend toward equilibrium with the overlyingatmosphere.

The magnitude and direction of the flux of trace gasesbetween a water surface and the atmosphere depends onthe transfer velocity (the water phase transfer velocity, kw,in the case of the relatively insoluble alkyl nitrates) and the

magnitude of the Henry’s law–adjusted concentrationdifference between the two phases (Eq. 3; Liss and Slater1974)

F~kw Cl{Cg

�H

� �ð3Þ

where Cg is the gas phase (atmospheric) concentration andCl is the concentration in surface water. The transfervelocity, kw, is dependent on wind speed and is a key factorin determining the rate at which equilibration between thewater and overlying air is achieved. kw is on the order of4 cm h21 at a wind speed of 2 m s21 and of 20 cm h21 at10 m s21 (Nightingale et al. 2000). Thus, it can be seen thatthe , 5-m water column of the Broad will equilibrate withthe atmosphere on a timescale of approximately 1 d invigorous wind conditions and that the top 1 m of astratified water column will equilibrate over a similarperiod in gentle wind conditions. Thus, water–air exchangeis likely to be a key control on surface water concentrationsof the alkyl nitrates, assuming no strong net source or sinkin surface waters.

After a full-depth, wind-driven mixing event, the ratio ofmethyl to ethyl nitrate is likely to represent the equilibriumratio of these species in the atmosphere. Because the Henry’slaw constants of the two compounds are very similar(0.06 mol L21 kPa21 for methyl nitrate and 0.05 mol L21

kPa21 for ethyl nitrate; Kames and Schurath 1992), the watercolumn ratios in these cases will translate approximately tothe same ratio in the overlying atmosphere—that is, betweenapproximately 1 : 1 (DOY 45) and 2 : 1 (DOY 183). Assumingthat the ratio at depth during stratified periods (, 5 : 1) isrepresentative of the production ratio of the two compoundsat the source, the vertical profile of the ratio during theseperiods can be explained by the preferential water–air transferof methyl nitrate compared with ethyl nitrate in the mixedlayer because of its greater supersaturation relative to theatmosphere.

The only confirmed aquatic alkyl nitrate source is thereaction between photochemically formed ROO2 and NO2

(Moore and Blough 2002; Dahl et al. 2003; Dahl and

Fig. 4. Depth profiles of the methyl : ethyl nitrate measuredon DOY 45 (filled circles), DOY 122 (open squares), DOY 152(filled triangles), DOY 163 (open circles), and DOY 185 (crosses)in the University of East Anglia Broad.

Fig. 5. Plots of methyl vs. ethyl nitrate concentrations at 0 m(open squares), 1 m (crosses), 2 m (open triangles), 3 m (opencircles), and 4 m (filled circles). Error bars are standard deviations.

310 Hughes et al.

Saltzman 2008), but this cannot explain the peaks in methyland ethyl nitrate at 4 m observed in the UEA Broadbecause of the high levels of light attenuation (Fig. 2e).Specifically, UV light (280–400 nm) is required forphotochemical precursors for alkyl nitrate production tobe formed (Zafiriou and True 1981; Blough and DelVecchio 2002; Dahl et al. 2003), but CDOM measurements(Fig. 2e, a350) suggest that it is unlikely that light at thesewavelengths was able to penetrate to the deeper regions of

the Broad. Direct UV measurements were not made in theBroad during this time series. However, several studies(Morris et al. 1995; Vincent et al. 2001) have shown goodagreement between UV attenuation and CDOM absorp-tion in lake systems and have produced models to describethis relationship. Morris et al. (1995) present models toexplain the relationship between Kd and CDOM absorptioncoefficients at 305, 320, 340, and 360 nm. If we apply themodel of Morris et al. (1995) established for 340 nm to

Fig. 6. Time series of (a) methyl and (b) ethyl nitrate concentrations measured at 1 m (white bars) and 4 m (gray bars) in theUniversity of East Anglia Broad. Error bars are standard deviations.

Methyl and ethyl nitrate in a lake 311

CDOM absorption coefficient data determined for 350 nmin the Broad, Kd(350) values ranging from 7.7 to 20.4 m21

are obtained. Even at the lowest Kd(350) value, UV lightwould be reduced to , 1% of the surface value by , 1 mdepth. Because these calculations only take into accountabsorption by CDOM—although in reality, the attenuationof UV is also influenced by scattering (Smith et al. 1999)and absorption by particulates (Laurion et al. 2000)—theyare likely to overestimate the depth to which light of thesewavelengths can penetrate. Consequently, it is not possiblethat a UV-driven photochemical production mechanism,such as that identified by Dahl et al. (2003), was responsiblefor the peaks in methyl and ethyl nitrate observed at 4 m inthe UEA Broad and that, as has been suggested previously(Chuck et al. 2002; Moore and Blough 2002; Dahl et al.2003), another non-photochemical alkyl nitrate sourcemust exist.

Possible inputs to the hypolimnia of lakes are in situproduction in the water or sediments or groundwaterinflow (Simmons and Lyons 1994; Muhlherr and Hiscock1998). Correlations can be used to identify specific sourcesof trace gases in aquatic environments, but no significantlinks were observed between the alkyl nitrate concentra-tions and any of the ancillary parameters (e.g., dissolvedoxygen, NO {

3 ) measured as part of this study. This result isnot surprising in that the conditions in a shallow systemsuch as the UEA Broad can change over relatively shorttime scales and cause and effect is difficult to resolve. Anunderstanding of the general conditions in the Broad andother lake systems can be used to suggest possible alkylnitrate sources. Although macro- and microalgae have beenfound to be responsible for the production of other tracegases in aquatic environments (Moore et al. 1996;Nightingale et al. 1995), high attenuation of PAR (KdPAR)in the surface waters of the lake (Fig. 2e) means that the 1%light level would have been well above 4 m throughout thestudy period, so no active photosynthesis would have takenplace in the isolated bottom waters of the lake. Addition-ally, unpublished results discussed in Moore and Blough(2002) suggest that marine microalgae do not producemethyl nitrate. Two important bacterially mediated pro-cesses that are known to occur at the sediment–waterinterface of lakes and in groundwater (Muhlherr andHiscock 1998; Liikanen and Martikainen 2003; Tomaszekand Czerwieniec 2003) that could have led to alkyl nitrateformation are nitrification and denitrification. Nitrificationoccurs in oxic environments and is the oxidation of NH z

4to NO {

3 , and denitrification takes place under anaerobicconditions and is the reduction of NO {

3 to N2. It is possiblethat the formation of methyl and ethyl nitrate is linked toeither or both of these processes in that (1) bothnitrification and denitrification are known to produceanother trace gas, nitrous oxide (N2O), as a side product(Senga et al. 2001, 2002); (2) both processes are known tolead to the formation of NO2 (Stuven et al. 1992; Stuvenand Bock 2001), which is a known precursor for alkylnitrate formation (Dahl et al. 2003); (3) most of the peaksin the alkyl nitrates at 4 m were observed during a periodwhen NO {

3 concentrations were fluctuating betweensampling events (DOY 152–185), possibly indicating

coupled nitrification and denitrification (Jensen et al.1993); and (4) the highest concentrations of methyl andethyl nitrate at 4 m were measured when the oxygensaturation at this depth was , 31%, whereas Senga et al.(2002) have shown that the yield of N2O from nitrificationis highest between 10% and 30% oxygen saturation.Further research is required to confirm whether theformation of methyl and ethyl nitrate is associated withthese two important aspects of the nitrogen cycle. However,the identification of another potential alkyl nitrate sourcein aquatic environments is interesting in terms of advancingunderstanding of the mechanisms of production of thealkyl nitrates in remote marine regions.

Alkyl nitrate source to the upper water column—At 1 mdepth, the concentrations of methyl and ethyl nitrate werefound to range from 5.3 (6 0.5) to 18.9 (6 2.6) pmol L21 and3.1 (6 0.3) to 8.3 (6 0.7) pmol L21, respectively, suggesting asource of these compounds to the mixed layer. In situproduction by a photochemical mechanism such as thatidentified by Dahl et al. (2003, 2008) for marine waters islikely, given the availability of NO {

3 and NO {2 as potential

nitrogen precursors (Fig. 2d) and high CDOM absorbance(Fig. 2e). However, peaks in both compounds in the upperwater column are only observed during periods of deepmixing, suggesting that entrainment from the hypolimnion isthe major source of alkyl nitrates to the surface waters of theBroad. For example, the maximum concentration of methylnitrate at 1 m (18.9 6 2.6 pmol L21) was measured on DOY185 when the mixed layer was at 5 m. Additionally, totalwater column–integrated methyl nitrate (down to 5 m) was109.8 (6 4.6) pmol m22 on DOY 172 and 98.6 (6 4.6) pmolm22 on DOY 185. This suggests that the increase in themethyl nitrate concentration at 1 m between these samplingevents (from 13.2 6 0.5 to 18.9 6 2.6 pmol L21) was mainlydue to a redistribution of the water column burden of thiscompound and not production at the surface. The slightdecrease in total water column methyl nitrate observedbetween DOY 172 and 185 is likely due to losses because ofgas transfer across the water–air interface. Calculationssuggest that diffusion across the mixed layer during stratifiedperiods is slow (i.e., , 1025 pmol m22 s21), so this processwould not contribute significantly to the inventory of alkylnitrates in the mixed layer. The diffusion rates of methyl andethyl nitrate across the thermocline were calculated accordingto Fick’s Law (Eq. 4)

Fd~{Dw dc=dzð Þ ð4Þ

where Fd is the rate of diffusion, Dw is the diffusioncoefficient of methyl nitrate in water (calculated on the basisof molar volume according to the equations given in Haydukand Laudie [1974]), and dc/dz is the average concentrationgradient across the thermocline. Because no atmosphericmixing ratios were measured, it is not possible to calculatewater-to-air flux rates of the alkyl nitrates. However, deepmixing is a route by which the methyl and ethyl nitratesproduced or transported to the lake at depth can be broughtto the surface and made available for exchange with theatmosphere.

312 Hughes et al.

The enhanced methyl and ethyl nitrate concentrationsobserved at depth in this time series carried out in a shallowfreshwater lake confirm that a non-photochemical alkylnitrate source must exist in aquatic environments becauseof the limited UV penetration in the lake. The data suggestthat the compounds are produced in situ in the bottomwater or sediments, transported to the hypolimnion of thelake in groundwater inflow, or both. It is suggested that theformation of methyl and ethyl nitrate is associated withnitrification, denitrification, or both, which are known tooccur at the sediment–water interface of lakes (Senga 2001;Liikanen and Martikainen 2003; Tomasek and Czerwieniec2003) and in groundwater inflow (Muhlherr and Hiscock1998). Given that both nitrification and denitrification areimportant processes in the world’s oceans (Zehr and Ward2002; Yool et al. 2007), an examination of a possible linkbetween the nitrogen cycle and formation of methyl andethyl nitrates is warranted.

AcknowledgmentsWe acknowledge the help and support of the UEA field store

and teaching laboratories and the helpful suggestions of twoanonymous reviewers.

This work was supported by the following UK NaturalEnvironmental Research Council (NERC) grants; NER/G/S/2003/00024 to C.H., NER/I/S/2002/00678 to A.J.K., NER/G/S/2002/00024 to K.W., NER/S/A/2006/14115 to M.R.J., and NE/C515904/1 to M.T.J. G.A.U. was funded by the Ford Foundationand University of East Anglia (UEA) M.Sc. program.

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Associate editor: Mary I. Scranton

Received: 10 February 2009Accepted: 20 July 2009

Amended: 18 August 2009

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