Nutrient and mercury deposition and storage in an alpine snowpack of the Sierra Nevada , 1 USA 2

29 Bi-weekly snowpack core samples were collected at s even sites along two elevation 30 gradients in the Tahoe Basin during two consecutive snow years to evaluate total wintertime 31 snowpack accumulation of nutrients and pollutants i a high elevation watershed of the Sierra 32 Nevada. Additional sampling of wet deposition and d etailed snow pit profiles were conducted 33 the following year to compare wet deposition to sno wpack storage and assess the vertical 34 dynamics of snowpack nitrogen, phosphorus, and merc ury. Results show that on average organic 35 N comprised 48% of all snowpack N, while nitrate (N O3 -N) and TAN (total ammonia nitrogen) 36 made up 25% and 27%, respectively. Snowpack NO 3 concentrations were relatively uniform 37 across sampling sites over the sampling seasons and howed little difference between seasonal 38 wet deposition and integrated snow pit concentratio ns. These patterns are in agreement with 39 previous studies that identify wet deposition as th e dominant source of wintertime NO 3 -N 40 deposition. However, vertical snow pit profiles sho wed highly variable concentrations of NO 3 -N 41 within the snowpack indicative of additional deposi ti n and in snowpack dynamics. Unlike NO 3 42 -N, snowpack TAN doubled towards the end of winter, which we attribute to a strong dry 43 deposition component which was particularly pronoun ced in late winter and spring. Organic N 44 concentrations in the snowpack were highly variable (from 35% to 70%) and showed no clear 45 temporal, spatial, or vertical trends throughout th e season. Integrated snowpack organic N 46 concentrations were up to 2.5 times higher than sea sonal wet deposition, likely due to microbial 47 immobilization of inorganic N as evident by coincid ing increases of organic N and decreases of 48 inorganic N, in deeper, aged snow. Spatial and temp oral deposition patterns of snowpack P were 49 consistent with particulate-bound dry deposition in puts and strong impacts from in-basin sources 50 causing up to 6 times enrichment at urban locations c mpared to remote sites. Snowpack Hg 51

towards the end of winter and in addition to wet deposition, had a strong dry deposition component.Organic N concentrations in snowpack were highly variable (from 35 to 70 %) and showed no clear temporal or spatial dependence throughout the season.Integrated snowpack organic N concentrations were up to 2.5 times higher than seasonal wet deposition, likely due to microbial immobilization of inorganic N as evident by coinciding increases of organic N and decreases of inorganic N, in deeper, aged snowpack.Spatial and temporal deposition patterns of snowpack P were consistent with particulate-bound dry deposition inputs and strong impacts from in-basin sources causing up to 6 times enrichment at urban locations compared to remote sites.Snowpack Hg showed little temporal variability and was dominated by particulate-bound forms (78 % on average).Dissolved Hg concentrations were consistently lower in snowpack than in wet deposition which we attribute to photochemical-driven gaseous remission.In agreement with this pattern is a significant positive relationship between

Introduction
Atmospheric deposition accounts for significant nutrient and pollutant input to high elevation watersheds such as the Sierra Nevada (Dolislager, 2006;Fain et al., 2011;Mc-Daniel, 2013;Sickman et al., 2003;TERC, 2011;Vicars and Sickman, 2011;Williams and Melack, 1991a, b).Sierra Nevada snowpack supplies the majority of water to downstream communities as well as to some of the nation's largest agricultural areas.Quantifying atmospheric deposition in alpine watersheds is challenging because of large spatial variability in deposition rates caused by complex terrain, precipitation gradients, and varied origins of atmospheric constituents (i.e.local vs.regional and global, natural vs. anthropogenic; Jassby et al., 1994;Rohrbough et al., 2003).Singlesite measurements, therefore, do not allow for accurate extrapolation of nutrient or pollutant deposition in alpine regions and broader temporal and spatial data is needed to assess the mass and dynamics of atmospheric inputs.
In this study, we used multiple and repeated sampling of full depth snowpack cores (integrated snowpack sampling) across the Lake Tahoe basin to quantify atmospheric deposition loads and patterns from the first snowfall until the end of melting.Snowpack acts as an integrating reservoir for water, solutes, and particulates that deposit throughout winter and spring (Turk et al., 2001).Wet deposition, in the form of snowfall and rain, directly accumulates in developing snowpack throughout the snow season (Kuhn, 2001).Additionally, during storm-free periods, snowpack also collects dry de-Figures Back Close Full position which often is complicated to quantify since dry deposition samplers can be biased due to different collection efficiencies compared to natural surfaces (Jassby et al., 1994).Representing a natural surface that covers the ground for several months of the year, snowpack sampling thereby can provide accurate on-the-ground measurements of total (bulk: wet and dry) deposition occurring in mountainous areas.
While snowpack integrates wintertime atmospheric deposition input, it also records chemical and physical transformations that occur during storage such as elution during melt events, chemical transformations, and volatilization.For example, ionic pulses of anions and cations occur upon snowpack melt whereby ions are thought to be mobilized in the following order: SO  (Berg, 1992;Brooks and Williams, 1999;Kuhn, 2001;Stottlemyer and Rutkowski, 1990;Williams and Melack, 1991b).In addition, pollutants such as Hg and persistent organic pollutants (POPs) as well as nutrients can undergo photochemical transformations and be subject to substantial gaseous re-emission to the atmosphere (Fain et al., 2011;Halsall, 2004;Lalonde et al., 2002;Poulain et al., 2007).Specific examples include photochemical reduction and remission of mercury (Hg) during snowpack storage as well as photolysis and emission of nitrate (NO − 3 ) from polar snow (Galbavy et al., 2007;Jacobi and Hilker, 2007;Rothlisberger et al., 2002).In addition, microbial activity in and under seasonal snowpack can play an important role in snowpack N dynamics (Brooks et al., 1996;Williams et al., 1996); even in Artic environments with low temperatures and minimal water content (Larose et al., 2013).Therefore, snowpack sampling yields relevant temporal atmospheric deposition patterns in conjunction with post-depositional chemical losses or conversions.
Spatially, snowpack sampling can be an elegant tool to quantify gradients in atmospheric deposition that are difficult to assess with other methods; for example, the Sierra Nevada show strong orographic precipitation effects, with the leeward side receiving significantly less precipitation than the windward side (O'Hara et al., 2009).Such different precipitation patterns can cause large differences in wet deposition Introduction

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Full across mountain ranges (Fain et al., 2011;NADP, 2012).Assessing spatial deposition patterns using snowpack sampling at multiple locations across a watershed should allow for better characterization of basin-wide deposition patterns as well as assessment of impacts of nearby urban areas vs. regional and global sources of atmospheric deposition (Brown et al., 2011;Kuhn, 2001;Morales-Baquero et al., 2006;Vicars and Sickman, 2011).
The main goal of this study was to quantify N, P, and Hg concentrations and loads in Sierra Nevada snowpack in order to characterize the magnitude, origin, and fate of atmospheric deposition of nutrients and pollutants that accumulate throughout the winter and spring in this mountain range.We quantified chemical loading at seven sites in the Lake Tahoe basin, along two elevation transects, throughout the duration of two full snow seasons.Sampling included bi-weekly snowpack cores (full profile; integrated snowpack samples) representing an integrated load of constituents in the developing snowpack collected throughout the 2011-2012 and 2012-2013 snow years.In addition, volume-weighted wet deposition measured at two sites in 2013-2014 was compared to snowpack accumulation and detailed vertical snow pit profiles in that year to compare snowpack accumulation to wet deposition and to further study in-snowpack chemical dynamics.Finally, basin-wide loading estimates (mass area −1 ) were calculated by spatially extrapolating nutrient and pollutant measurements across the basin combined with a satellite-based snowpack reconstruction model.

Study site
The Lake Tahoe watershed lies in the northern portion of the Sierra Nevada range along the border of Nevada and California.Renowned for its intense blue color and water clarity, this lake has become a national landmark and tourism hotspot.Lake clarity measurements have decreased, however, from approximately 30.5 to 21.3 m since the Introduction

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Full 1960s due to eutrophication from increased input of N and P, as well as additional input of light scattering particulates (TERC, 2011).Directly west and upwind of the basin lies the central valley of California and cities of Sacramento and San Francisco, California, which are suspected of contributing significant amounts of nutrients and pollutants to the basin through agricultural and industrial emissions.
Including all drainages, the Lake Tahoe watershed has an area of 1310 km 2 (Fig. 1).The lake is 19 km wide and 35 km long with a total surface area of 495 km 2 .The lake lies at 1897 m a.s.l. and is on average 300 m deep.Surrounding the lake on all sides are mountains up to elevations of 3068 m.At the lake's surface, summer temperatures on average reach 27 • C and wintertime lows reach −9 • C. Precipitation patterns in the watershed are highly dependent on elevation with an average annual precipitation of 0.76 m at lake level and an average of 2.03 m falling at higher elevations in the surrounding mountains (Fram, 2011).Extreme snow events in this area are common and often produce snowpack depths greater than 4.5 m at high elevations.Rain shadow effects typically lead to decreased snow loading on the downwind, eastern side of the basin.Approximately two-thirds of Lake Tahoe basin parent material is granitic and onethird is volcanic (LTTMDL, 2008).Vegetation, consisting of mixed coniferous forest and montane-subalpine species, cover approximately 80 % of the basin (LTTMDL, 2010).Areas of dense urban development occur along the shoreline at South Lake Tahoe, Tahoe City, and Incline Village.Large portions of the northern and western shores are occupied by seasonal cabins, while much of the eastern shore is undeveloped.April in 2012-2013.The seven sites were distributed along eastern and western elevation transects (Fig. 1).and soil contact and contamination were avoided in order to capture only constituents stored within the snowpack.While sampling, the first core taken at each site was discarded in order to avoid carryover from previous sampling.Between each sampling campaign, the Federal Sampler was cleaned with Milli-Q deionized water (< 18.2 M Ω) and a chelating soap in accordance with trace metal sampling procedures (EPA, 2002).

Sample collection
Field blanks were measured by rinsing the sampler with Milli-Q water prior to each sampling campaign.

Wet deposition sampling and snow pit collection: 2013-2014 snow year
In order to differentiate between snowpack storage and wet deposition and further asses dynamics in snowpack, additional sampling of full snow pit profiles and wet Full

Laboratory analysis
Samples were analyzed for nitrite (NO − 2 -N), nitrate (NO − 3 -N), total ammonia nitrogen (TAN; NH 3 + NH + 4 ), total Kjeldhal nitrogen (TKN), orthophosphate (o-PO 4 ), total phosphorus (TP), total Hg (THg, no filtration), and dissolved Hg (DHg, filtration).Prior to analysis, all samples were removed from the freezer and placed in a dark cabinet at room temperature for approximately 18 h to melt.Once fully melted, the samples were thoroughly mixed and dispensed into various aliquots for each analysis.Subsamples of NO Ortho-phosphate and TP were measured according to EPA Standard Method (SM) 365.1 and SM 365.1/USGSI-4600-85, respectively (EPA, 1993;USGS, 1985).Method detection limits (MDL) for these techniques were 0.60 and 0.63 µg L −1 , respectively.
Both techniques employed colorimetric measurement with ascorbic acid.Prior to measurement of TP, samples were digested with persulfate.Absorbance was then measured through flow injection analysis (FIA; Rapid Flow Analyzer 300 equipped with an Astoria-Pacific 305D high sensitivity photometer detector; Alpkem, College Station, TX).Nitrite, NO − 3 -N, and TAN analyses followed EPA SM 353.2 andSM 353.1 (EPA, 1979, 1993).Nitrite and NO Total Hg and DHg were measured using a water analyzer (Model 2600; Tekran Inc., Toronto, Canada) according to EPA method 1631 revision E (EPA, 2002).For DHg samples, approximately 50 mL of sample were filtered through a 0.45 µm filter (Acrodisc syringe filter with Supor ® Membrane; Pall Corporation, Port Washington, NY) while for THg, 50 mL of sample were poured directly into a vial for analysis.Laboratory filter blanks were below the detection limit (DL) of the system (< 0.3 ng L −1 ).Samples were preserved with 10 % bromine chloride (BrCl) solution for storage until analysis the next day.Before analysis, excess BrCl was neutralized with pre-purified hydroxylamine hydrochloride.During analysis, samples were automatically mixed with stannous chloride (SnCl 2 ) in a phase separator; reducing oxidized Hg to elemental Hg.Elemental Hg is then loaded onto two sequential gold traps by an argon carrier gas.The Hg is then released through thermal desorption and detected using atomic fluorescence spectrometry.The Tekran Model 2600 was calibrated using a NIST SRM-3133 Hg standard (with concentrations of 0, 0.5, 1.0, 5.0, 10.0, 25.0, and 50.0 ng L −1 Hg).System reliability was checked using ongoing precision recovery injections of 5 ng L −1 throughout each run and ranged between 87 to 112 % recovery.Reagent blanks measured regularly throughout each run ensured no contamination of the system.DLs calculated as three times the SD of the calibration blanks, averaged 0.3 ng L −1 for all runs.Particulate Hg was calculated as the difference between THg and DHg.Introduction

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Statistics
We performed analysis of variance (ANOVA) for all chemical species using the following independent variables: (i) year (n = 2, 2011-2012 and 2012-2013), (ii) site elevation (n = 3; low, mid-and high elevation site), (iii) location (n = 2; eastern and western basin); and season (n = 2; early season [December through February] and late season [March and April]).ANOVAs attribute variance of dependent variables to these various independent variables and test their significance against the residual variance.
All relationships were considered statistically significant when p values were ≤ 0.05.Integrated snowpack concentrations were calculated by weighting each 10 cm snow pit layer by its density.Seasonal wet deposition was calculated by weighting all wet deposition samples by their volume up to the date of sampling.Linear regression analyses were performed to test for correlations between snowpack chemical concentrations, SWE, and elevation.All error bars in figures represent standard error.

Basin-wide modeling with SWE reconstruction
Basin-wide loads and distribution were assessed using chemical concentrations and loads measured throughout the 2011-2012 and 2012-2013 snow seasons as well as basin-wide mean peak SWE estimates from SWE reconstruction for the Sierra Nevada from 2000 to 2011 (Rittger, 2012).SWE reconstruction uses estimates of snow energy balance with areal snow cover depletion from MODIS Snow Covered Area and Grain size (MODSCAG) (Rittger, 2011).MODSCAG calculates fractional snow cover area and grain size from Moderate Resolution Imaging Spectroradiometer (MODIS) data (Painter et al., 2009).Compared with previous methods, MODSCAG has proven to give reliable depletion rates throughout the spring season when snowmelt is highest (Rittger et al., 2013).Finally, the spatially refined MODSCAG data set was combined with energy balance and temperature data to give accurate reconstructed estimates of SWE throughout the Sierra Nevada, and specifically the Lake Tahoe Basin.At the time of our study, SWE reconstruction data were only available for 2000 to 2011, with Introduction

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Full no information from our sampling seasons, 2011-2012 or 2012-2013.The 2000 to 2011 data set includes both high and low accumulation snow years and gives a reasonable representation of average snowpack accumulation in the Lake Tahoe basin.In order to give an estimate of average annual snowpack chemical storage, we applied the decadal average peak SWE for 2000 to 2011 to our data (Fig. 3a).Estimates made during this study were to establish relationships to previous estimates of the Lake Tahoe nutrient budget and were not meant to represent a completely accurate distribution or load stored within the basin's snowpack each year.Snowpack sampling throughout the Lake Tahoe basin during 2011-2012 and 2012-2013 allowed for assessment of spatial and temporal chemical deposition patterns.
Specifically, relationships to wet or dry deposition, in-basin or out-of-basin sources, and early or late season increases were identified.These deposition and source controls were then related to orographic characteristics to estimate chemical concentrations throughout the basin in unknown areas.A GIS land-use layer of the Tahoe Basin (LTTMDL, 2010) was applied in order to separate urban and non-urban locations with similar orographic characteristics for urban influenced species (i.e.TP).These scaled concentrations were then applied to SWE reconstruction estimates to determine total snowpack chemical loading throughout the entire basin.
Snowpack sampling occurred in open areas free of canopy coverage, but it is possible that tree and plant particulate matter still were incorporated in the snowpack.
Litterfall contributions represent a form of chemical recycling and will cause an overestimate of atmospheric contributions made during this study.Visual inspection of snow samples, however, showed low contributions of plant detritus in samples, and due to consistent forest types present across the basin we would expect any additional plantderived inputs to be random and unbiased across sites.Introduction

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Full 3 Results and discussion

Spatial and temporal trends of snow accumulation and SWE
In the Lake Tahoe basin, approximately 70 % of annual precipitation falls during the winter and spring as snow (Fram, 2011).The 2011The -2012The , 2012The -2013, and , and 2012, 2012-2013, and 2013-2014, respectively.These values were comparable with previous measurements at the Emerald Lake Watershed, a remote watershed in the southern Sierra Nevada (Williams et al., 1995).During 2011-2012 and 2012-2013 (i.e. the two years with detailed spatial and temporal sampling), no distinguishable temporal or spatial pattern was observed in either snowpack NO − 3 -N concentrations or loads (Fig. 4).ANOVA results confirmed that snowpack NO − 3 -N concentrations were not statistically affected by elevation, location (i.e.east/west), or early vs. late season sampling (Table 1).Comparisons of seasonal wet deposition and integrated average snow pit concentrations during the 2013-2014 snow year showed that snowpack NO − 3 -N concentrations were similar to volume-weighted wet deposition up to the date of snowpack sampling (Fig. 5).This result is similar to patterns observed by Clow et al. (2002) and Williams and Melack (1991a) and may be indicative of wet deposition as the main source of NO − 3 -N deposition.For example, wintertime deposition of NO − 3 -N in the Rockies was found to be highly correlated to precipitation with little difference between snowpack and NADP precipitation volume-weighted mean concentrations suggesting mainly wet deposition inputs (Clow et al., 2002).Similarly, a study at the Emerald Lake Watershed identified that dry deposition of NO − 3 was not an important contributor of total NO − 3 load in winter snowpack (Williams and Melack, 1991a).Our study revealed that increased precipitation on the west side of the Tahoe Basin However, vertical snow pit profile patterns show large variability in NO − 3 -N concentrations with depth, e.g.decreasing concentrations below the top 30-40 cm (Fig. 6).This variability suggests pronounced in snowpack dynamics possibly driven by conversion, vertical transport, or elution.In addition, several studies have shown significant wintertime dry deposition of NO − 3 -N, in particular close to highways and urban areas (Cape et al., 2004;Dasch and Cadle, 1986;Kirchner et al., 2005).Therefore, the fact that wet deposition concentrations were very similar to snowpack concentrations could be merely a coincidence and we do not feel confident to include or exclude significant dry deposition processes.Finally, previous studies have observed parallel concentration declines of SO 2− 4 and NO − 3 -N during snowpack melt events due to similar early elution characteristics (Stottlemyer and Rutkowski, 1990;Williams and Melack, 1991b).Comparing volume-weighted seasonal wet deposition concentrations of SO 2− 4 to snowpack concentrations showed no elution losses through our sampling period and that SO 2− 4 was also not subject to additional increases (Fig. 5).Our results therefore suggest that Sierra Nevada snowpack is subject to multiple inputs and complex in snowpack processes.

Total ammonia nitrogen (TAN)
Snowpack concentrations of TAN ranged from 16 to 104 µg L −1 (n = 49 cores), 10 to 77 µg L −1 (n = 56 cores), and 28 to 85 µg L −1 (n = 3 integrated snow pits) during 2011-2012, 2012-2013, and 2013-2014, respectively.Snowpack TAN concentrations are within the range of previous measurements made in the Emerald Lake Watershed of California, where the amount of TAN deposited within the seasonal snowpack accounted for approximately 90 % of annual loading (Williams et al., 1995).Unlike NO − 3 -N, TAN is known to deposit through both wet and dry pathways during winter (Clow et al., 2002;Ingersoll et al., 2008).In our study, strong evidence for an important role of TAN dry deposition can be inferred from the fact that snowpack TAN concentrations doubled from early (December-February) to the late (March-April) sea-Introduction

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Full son in both 2011-2012 and 2012-2013 (Fig. 4).ANOVA results confirmed significant differences in snowpack TAN concentrations between early and late season snowpack sampling (Table 1, p = 0.01).Increased late season TAN concentration in snowpack is consistent with similar observations in the Rocky Mountains and the Stubai Alps (Bowman, 1992;Kuhn, 2001).These increases were attributed to the onset of agricultural production in upwind valleys, as well as increased dry deposition due to decreased atmospheric stability and increased convection.Importantly, the late season increase in snowpack TAN occurred in both years, even though no significant late season snowfall occurred in 2012-2013 (Fig. 4).The patterns of increasing TAN concentration in late season snowpack with no significant snowfall agree with previous research showing dry deposition as the significant source of TAN deposition in the Sierra Nevada (Bytnerowicz and Fenn, 1996).Large increases in NH 3 emissions from winter to spring have been measured upwind of the Sierra Nevada in the San Joaquin Valley, CA and were attributed to increased agricultural and livestock activities (Battye et al., 2003).Further support of snowpack TAN sourcing in the San Joaquin Valley, was higher concentrations at west basin sites than east basin sites during both 2011-2012 and 2012-2013.ANOVA results revealed a significant difference between the east and west basin snowpack TAN concentrations (Table 1, p = 0.03).This increase is likely due to the west basin sites being closer in proximity to San Joaquin Valley agricultural activity allowing for increased transport and deposition.
During the 2013-2014 snow year, TAN concentrations were consistently higher (up to a factor of 3) in volume-weighted wet deposition than integrated snow pit samples (Fig. 5; p = 0.08, note low replicate of n = 3).This increase of TAN further emphasizes the importance of dry deposition of TAN to Tahoe Basin snowpack.During snowpack storage, TAN is known to elute relatively late during melt events (Kuhn, 2001); however, other transformations such as microbial conversion can lead to decreases and losses throughout the season.Snow pit depth profile sampling shows a decrease in TAN concentrations with depth and therefore age (Fig. 6).This decrease coincides Introduction

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Full with increases in organic N suggesting microbial conversion of inorganic N to organic N. Despite these possible losses; the increase we observe between wet deposition and snow pit concentrations indicates that the additional input of TAN from dry deposition is large enough to exceed transformations that occur during snowpack storage.Late season deposition doubled TAN snowpack loads prior to end-of-season melt.
The fate of snowpack TAN has been studied extensively through both watershed mass balance and tracer-based research.For example, less than 1 % of TAN stored in snowpack at Emerald Lake, California reached the lake as TAN during melt and runoff (Williams and Melack, 1991b).During a later study, however, snowmelt with isotopically labeled NH 3 was retained in the soils during melt making it a possible contributor to future NO − 3 stream pulses after nitrification (Williams et al., 1996).Current predictions show an increase in total N emissions during the next half-century in the western United States due to large increases in agricultural and livestock NH 3 emissions (Fenn et al., 2003).Such increased emissions could result in significant additional deposition loads of TAN to snowpack in the Sierra Nevada with the potential to alter ecosystem nutrient dynamics.4).ANOVA results supported this finding with no significant effects of location, elevation, or early/late season on organic N concentrations (Table 1).A previous study found large variation in wintertime deposition of organic N throughout the Rocky Mountain Range; accounting for 40, 3, and 50 % of total N in wet deposition during January, February, and March, respectively (Benedict et al., 2013).Deposition rates and patterns of organic N are difficult to quantify due to the large number of compounds -including gaseous, particulate, and dissolved phases -origi-Introduction

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Full nating from local, regional, and global sources and subject to biological and chemical transformations (Cape et al., 2011;Neff et al., 2002).Overall, snowpack core samples collected during the 2011-2012 and 2012-2013 seasons showed very high fractions of organic N accounting for 49 ± 17 % of total snowpack N on average.Inorganic forms, TAN and NO − 3 -N, accounted for 21 ± 10 and 29 ± 10 %, respectively (Fig. 7).Research at a high elevation catchment in the Colorado Front Range identified organic N as an important component in both wintertime wet deposition and stream export (Williams et al., 2001), while data from a fourteen year study (WY1985-1998 in the Southern Sierra Nevada reports that on average dissolved organic nitrogen (DON) accounted for 35 % of total N (NH in winter precipitation (Sickman et al., 2001).Comparison of volume-weighted wet deposition and integrated snow pit concentrations showed higher concentrations (up to a factor of 2.5) of organic N levels in snowpack (Fig. 5).Two possible sources could cause higher concentrations of organic N in snowpack compared to wet deposition: snowpack microbial conversion of inorganic N to organic N and dry deposition of organic N during storm-free periods (Clement et al., 2012;Jones, 1999;Williams et al., 2001).Our data does not allow for differentiation between the two possible sources of snowpack organic N; however snow pit profile sampling shows coinciding decreases of inorganic N and increases in organic N with snow pit depth and therefore age (Fig. 6).One Artic snowpack study found that microbial-based N cycling was a dominant process explaining N species availability at the base of the snowpack (Larose et al., 2013).We suggest that microbial uptake of inorganic N may be a primary driver of the increasing snowpack organic N levels during storage.Overall, we observed that the dominant form of N in Sierra snowpack during our study was organic N, and propose that this large representation warrants detailed studies in regard to the sources, cycling, and fate of organic N in the Sierra Nevada.
Concentrations and loads of total N in snowpack are apparently dependent on contributions of both inorganic and organic forms; with respective differences in deposition pathways (wet vs. dry deposition), potential conversion processes (e.g., from in-Introduction

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Phosphorus
Snowpack TP concentrations ranged from 3 to 109 µg L −1 in 2011-2012 (n = 49 cores), 3 to 59 µg L −1 in 2012-2013 (n = 56 cores), and 10 to 41 µg L −1 in 2013-2014 (n = 3 integrated snow pits).Figure 8 shows that the urban site in Incline Village at lake level had by far the highest snowpack TP concentrations, ranging up to six times higher than any other snowpack concentration at similar elevation (i.e.lake level).In comparison, the Thunderbird site also at lake level, located in a very remote setting just 10 km from Incline, had much lower P concentrations.Sources such as fugitive dust from plowing, forest and agriculture biomass burning, and diesel engine combustion have been identified as major sources of particulate-phase atmospheric P in California (Alexis, 2001).Specifically in the Lake Tahoe basin, road dust has been identified as a primary contributor of P input into Lake Tahoe (Dolislager et al., 2012), while another study found significant P emissions from urban biomass burning (Zhang et al., 2013).Our patterns suggest that urban areas in the Lake Tahoe basin are a major source area for P deposition to snowpack during winter and spring.Local and regional emissions are also relevant at larger scales, as evident in 2011-2012, where remote sites at eastern locations in the basin showed higher TP concentrations than western sites.We propose that the large concentration of urban source sites at lake level combined with the dominant west to east wind pattern led to increased deposition on the east side of the basin.During 2012-2013, no west-to-east increase in TP concentration was observed; however, the strong influence of urban activity remained.It is unlikely that sources of P in the basin changed between 2011-Introduction

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Full 2012 and 2012-2013, and it is more likely that different deposition patterns due to differences in snow accumulation timing and storm track direction caused this change.Even though there was significantly higher P deposition on the east side of the basin from urban influence, the relatively remote west basin snowpack still had TP concentrations of 11.8 µg L −1 on average.Diffuse regional P sources to the Tahoe Basin include both dust and aerosol inputs.Particulate matter particles smaller than 10 µm in diameter (PM 10 ) are capable of long-range transport, while larger particles have higher deposition velocities and decreased transport (Vicars et al., 2010).Specifically, dustderived inputs originate from geologic sources and erosion from both agricultural and urban activity, while burning from both forest and domestic fires contributes additional particulate matter in the form of ash and soot (Raison et al., 1985).Differences in P deposition rates between the dry and wet seasons as well as spatial patterns associated with wind direction and soil erosion vulnerability have been observed in the southern Sierra; Ontario, Canada; and the Mediterranean (Brown et al., 2011;Morales-Baquero et al., 2006;Vicars and Sickman, 2011).
Comparison of volume-weighted wet deposition and integrated snow pit concentrations showed higher levels of TP (up to a factor of 5.8) in snowpack than wet deposition (Fig. 5).This increase further supports dry deposition as a primary input of snowpack P. Finally, snowpack o-PO 4 , the most bioavailable form of P (Dodds, 2003), accounted for 34 ± 15 % of snowpack TP; similar to previous work in the Lake Tahoe region that estimated approximately 40 % of TP in atmospheric deposition was in a bioavailable form (LTTMDL, 2010).
Low P levels in parent material make high elevation watersheds of the Sierra Nevada, sensitive to the effects of external nutrient inputs (Melack and Stoddard, 1991;White et al., 1999).Further research, however, has shown that extractable P levels of parent material strongly influence P adsorption.The very high extractable P levels in granitic soils in the Sierra Nevada lead to low P adsorption potentials, while the low extractable P levels and sesquioxide content of volcanic soils in the Sierra increase adsorption (Johnson et al., 1997).Approximately two-thirds of the Lake Tahoe basin parent ma-Introduction

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Full terial is granitic and one-third is volcanic (LTTMDL, 2008), making soil adsorption potentials of atmospherically deposited P throughout the watershed highly variable with location.Along with N, P levels directly control algal production within aquatic ecosystems, and algal production is a key reason for declining clarity in Lake Tahoe (Dolislager, 2006).In particular, the high snowpack concentrations at urban locations near the lake may cause a significant influx of P into Lake Tahoe during melt.

Mercury
Snowpack THg concentrations ranged from 0.81 to 7.  -2012, 2012-2013 and 2013-2014, respectively.The large percentage of particulate Hg in snowpack agrees with previous findings from a study in Canada that saw a post-depositional increase in particulate associated Hg from approximately 50 to 70 % (Poulain et al., 2007).This study attributed particulate throughfall and photochemical induced emission as the main causes of the speciation shift and also noted strong differences in snowpack Hg concentrations between open and forested areas which were attributed to throughfall contributions from tree canopies as well as shading reducing photochemical evasion.Snowpack coring revealed no dominant temporal or spatial patterns in THg or DHg deposition with ANOVA results showing no significant effects of season (i.e., early vs. late) or location (i.e., east vs. west; Table 1).The lack of either temporal or spatial trends suggests that global background atmospheric pollution, rather than specific Introduction

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Full point sources such as urban areas, as the main source of snowpack Hg in the Lake Tahoe basin.Mercury's long atmospheric lifetime and global circulation allow for diffuse deposition to this relatively remote mountain region (Fain et al., 2011;Schroeder and Munthe, 1998); and the majority of large snowfall events in the Sierra Nevada originate as large-scale convection cells in the eastern Pacific and travel hundreds of kilometers before reaching the Tahoe Basin (O'Hara et al., 2009).To our knowledge, few point sources for Hg emission exist within the Lake Tahoe basin, although one study with in the basin reported that significant amounts of particulate Hg are emitted from wildfires (Zhang et al., 2013) and found increased levels of particulate Hg in urban areas of the Lake Tahoe basin.
Both THg and DHg concentrations in snowpack significantly increased with elevation in the basin (Table 1; p < 0.05).This finding is in contrast to an expected "washout effect" which causes declines in Hg precipitation concentrations with storm duration and magnitude (Poissant and Pilote, 1998).King and Simpson (2001) observed that approximately 85 % of photochemical reactions occur in the top 10 cm of the snowpack.Therefore, we attribute the increase in Hg concentration with elevation to decreased light penetration relative to snowpack depth and reduced photochemical re-emission, as increased elevation leads to the formation of a deeper, denser snowpack.In support of this notion is a significant positive correlation between snowpack THg concentration and total SWE (slope: 0.002 [ng L −1 SWE (mm) −1 ]; p value: < 0.05), as well as strong elevation gradients in total snowpack Hg pools.In agreement, total snowpack Hg loading was significantly higher in 2012-2013 than in 2011-2012 (Table 1; p < 0.01) in parallel with higher overall SWE.The combination of strong precipitation gradients and increased THg concentration with SWE lead to large spatial variability in the total snowpack Hg pools in mountainous areas.A previous study noted relationships between soil Hg content and elevation (Gunda and Scanlon, 2013), possibly attributable to precipitation gradients, while another study found that soil Hg storage was positively correlated to total precipitation across multiple study sites (albeit not related to snow; Obrist et al., 2009Obrist et al., , 2011)).Introduction

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Full Further support of photochemical reemission of Hg during snowpack storage can be inferred from the consistently lower DHg concentrations (up to a factor of 4.5) in integrated snow pit samples than volume-weighted wet deposition samples (Fig. 5).Photochemical reduction and volatile reemission of gaseous Hg during snowpack storage has been widely studied and is known to account for losses of up to 50 % from that measured in initial deposition (Fain et al., 2007(Fain et al., , 2011;;Lalonde et al., 2002;Mann et al., 2011;Poulain et al., 2007).In addition to the declines of DHg during storage, increase in particulate Hg was observed in two of the three comparisons of snow pit and wet deposition samples (Fig. 5).This increase in particulate Hg in snow pit samples is likely due to gaseous-dry deposition and particulate throughfall during storm free periods.
After photochemical losses, there is still a substantial amount of Hg left in the snowpack that will be subject to melt and infiltration into the watershed.The study at the nearby Sagehen Creek, California watershed quantified that only 4 % of total annual Hg wet deposition was exported from the watershed in stream water and identified soil uptake and storage as well as photochemical re-emission as the major sinks of atmospherically deposited Hg (Fain et al., 2011).While soil uptake serves as a buffer delaying the transport of upland wet deposition to streams, sediment core analyses still showed that upland watershed contributions (i.e., through soil erosion and sediment flux) are significant contributors of Hg input to lakes even under relatively low watershed to lake area ratios as in the Lake Tahoe basin (extrapolated to 42 % contributions when using relationships presented by Lorey and Driscoll, 1999).Snowpack-based Hg input to the watershed, therefore, is expected to contribute to lake water quality through erosion and sediment-based influx, albeit delayed in time and closely linked to soil Hg pools and mobilization.

Basin-wide loading estimates
Declines in Lake Tahoe water quality have been observed during the last 50 years (Sahoo et al., 2010;Schuster and Grismer, 2004).Specifically, secchi depths, a measure of lake transparency, have decreased from approximately 30.5 to 21.3 m since the 1960s Introduction

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Full ( TERC, 2011).Eutrophication from atmospheric and terrestrial nitrogen (N) and phosphorus (P) inputs as well as light scattering by particulate inputs are the main causes of this decline (Jassby et al., 2003;Swift et al., 2006).Most previous studies in the Lake Tahoe basin have focused on direct atmospheric deposition to the lake surface (Dolislager et al., 2012;NADP, 2012), and little information is available on snowpackbased loading for the surrounding upland watershed.The surrounding land surface covers 814 km 2 of the 1310 km 2 Lake Tahoe watershed.Direct atmospheric inputs to the lake surface are estimated to contribute 55 and 15 % of total N and P, respectively (TERC, 2011).Stream monitoring data show that upon snowmelt, Lake Tahoe receives large pulses of N and P (Goldman et al., 1989;Hatch et al., 1999), which together control algal production within the basin's aquatic ecosystems contributing to the decline in clarity in Lake Tahoe during the last 50 years (Dolislager, 2006).Although much of snowpack-based chemical loads may not directly enter Lake Tahoe upon melt, snowpack loads are important for terrestrial chemical budgets.For example, nutrient rich O-horizon runoff -measuring as high as 87.2 mg L −1 NH 4 -N, 95.4 mg L −1 NO 3 -N, and 24.4 mg L −1 PO 4 -P -has been observed in Lake Tahoe forests during snowmelt events due to leaching from the forest litter layer (Miller et al., 2005).In order to relate peak snowpack nutrient and pollutant loading to previous terrestrial and lake chemical budgets, we here estimate average peak basin-wide snowpack chemical storage using the peak SWE decadal average from 2000-2011 (Fig. 3a).

Nitrogen
Snowpack NO − 3 -N loading was highly dependent on snow accumulation, but concentrations showed little temporal or spatial trends throughout the Lake Tahoe basin (Table 1).To calculate basin-wide NO nates from runoff and terrestrial sources, annual snowpack N storage would replenish approximately 63 % of this flux.

Phosphorus
Snowpack P accumulation in the Lake Tahoe basin was strongly related to proximity to urban sources, as well as transport along the dominant westerly winds throughout the basin.This dependence caused the highest P concentration in snowpack to occur in developed areas and higher concentrations across east basin sites than remote west basin sites (Table 1).Applying different P concentrations based on degree of urbanization (see Sect. 2.5), highest P loading (up to approximately 0.4 kg ha −1 ) occurs, therefore, at high elevations with significant impacts of urban emissions (i.e., northeastern and southern locations influenced by Incline Village, Nevada and South Lake Tahoe, California).The basin-wide average TP storage estimated during this study of 0.11 kg ha −1 is over double the average snowpack storage reported for the ELW watershed (0.04 kg ha −1 ; Sickman et al., 2003) and reflects increased urbanization within the Tahoe Basin.Homyak et al. (2014) estimate that atmospheric deposition has contributed up to 31 % of P accumulation and loss in soils and runoff since deglaciation of the Emerald Lake Watershed.The higher snowpack loading rates estimated during this study indicate that atmospheric deposition could be the primary supplier of excess P input to the Tahoe Basin.Overall, we estimate a peak P load of approximately 9.3 t of P stored annually in Lake Tahoe basin snowpack (Fig. 3c).Previous pollutant loading studies for Lake Tahoe estimated that approximately 46 t of P enters the lake each year with approximately 39 t of the annual budget originating from land-based sources (LTTMDL, 2010).Annual snowpack TP storage estimates, therefore, could represent approximately 20 % of total P input into Lake Tahoe each year.Introduction

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Full

Mercury
Similar to NO − 3 -N, snowpack THg concentrations showed little temporal or east to west variation (Table 1).However, THg concentrations were positively related to total SWE (slope: 0.00201 [ng L −1 mm −1 ]; p value: 0.016).Applying this relationship to reconstructed SWE data produced the following THg distribution throughout the Lake Tahoe basin (Fig. 3d); THg loading throughout the basin followed strong elevation gradients, with the uppermost areas of the basin receiving the highest concentrations and total loading (up to approximately 125 mg ha −1 ) due to increased snow accumulation.Average annual snowpack THg concentration and loading for the Lake Tahoe watershed was 3.6 ng L −1 and 30 mg ha −1 , respectively, based on the decadal SWE accumulation average of 750 mm.We do not have any previous data on Hg deposition to this basin, but these values are comparable to the 3.3 ng L −1 average snowpack Hg concentration and 13 mg ha −1 peak snowpack loading from the Sagehen Creek watershed in 2009 when snowpack accumulation was approximately 400 mm (Fain et al., 2011).The basin-wide estimate of THg stored within the annual snowpack was 1166.2 g.Snowbased Hg fluxes estimated during this study fall within range of measurements (3.36 to 36 mg ha −1 yr −1 ) taken at seven national parks throughout western North America during the Western Airborne Contaminants Assessment Project (WACAP), which found fish Hg levels above the human consumption threshold even at sites with relatively low Hg deposition (Landers et al., 2008).

Conclusions
In summary, spatial and temporal pattern analyses suggest that out-of-basin sources were important for Hg and TAN, while in-basin sources controlled P deposition, with the highest concentrations measured near urban areas, exceeding remote location concentrations by up to a factor of 6. Snowpack NO mary input; however high variability in snow pit vertical concentrations suggests additional inputs and in snowpack transport and conversion processes.Second, increased NH 3 emissions from the San Joaquin Valley and increased atmospheric vertical mixing during the onset of spring likely led to dry deposition-based increases of snowpack TAN during March and April, effectively doubling snowpack TAN concentrations prior to melt.Third, chemical speciation showed that organic N in Lake Tahoe snowpack accounted for 48 % of total N on average with possible microbial conversion leading to higher enhanced organic N levels in deeper older snowpack.Fourth, particulate Hg was the dominant form of Hg (78 % on average) within Tahoe snowpack and concentrations of both THg and DHg increased with elevation and SWE likely due to decreased light penetration and reduced photochemical reemission in deeper snowpack.Finally, basin-wide modeling estimates indicated that Lake Tahoe basin snowpack acts as a substantial reservoir in which atmospheric nutrients and pollutants accumulated throughout winter and spring.Estimates of basin-wide annual snowpack mass loading showed accumulation of N, P, and Hg yielding 113 t of N, 9.3 t of P, and 1166.2 g of Hg.Further research should focus on quantifying the relationship between snowmelt processes and stream and groundwater input, and address the substantial amount of organic N stored within the basin's snowpack.Introduction

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Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | During the 2011-2012 and 2012-2013 water years, full snowpack bi-weekly core samples were collected at seven sites in the basin starting from the first measureable snowpack until the majority of spring melting occurred (2011-2012: n = 49; 2012-2013: n = 56).This included mid-January through mid-April in 2012 and December to early-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | o-PO 4 were filtered through 0.45um filters (Pall Supor © ) prior to analysis.Laboratory filter blanks were approximately < 2 µg L −1 for NO − 2 -N, 6 µg L −1 for NO − 3 -N, 5 µg L −1 for TAN, < 20 µg L −1 for SO 2− 4 , and 2 µg L −1 for o-PO 4 .
were measured by automated colorimetric analysis with cadmium reduction being applied for the nitrate samples.Each sample was then measured by FIA (Rapid Flow Analyzer 300 equipped with an Astoria-Pacific 305D high sensitivity photometer detector; Alpkem, College Station, TX).TAN samples were analyzed using automated phenate colorimetric techniques.Total Kjeldhal Nitrogen was analyzed using automated phenate block digestion according to EPA method 351.2.The MDL for TKN was 11.3 µg L −1 .Organic N (bulk) was calculated as the difference between Discussion Paper | Discussion Paper | Discussion Paper | TKN and TAN.All nitrogen species were reported as [µg L −1 ]-N with total N calculated as the sum of organic N, TAN, and NO − 3 -N.All snow sample NO − 2 concentrations were below the DL.Sulfate was determined using a chromatography system (ICS 2000 with Chromeleon Software version 6.6 and AS14A Column; Dionex Inc., Sunnyville, CA) by EPA Method 300.0(EPA, 1979).The MDL for Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Rose/Squaw Valley SNOTEL data).These early storms were followed by three dry months with very little accumulation for the rest of winter.The 2013-2014 snow year experienced the lowest snow accumulation of all three study years with minimal snowpack development occurring until late season storms in March and April brought peak SWE storage up to approximately 575 mm.Minimal snowpack development occurred at lower lake level elevations (e.g.Tahoe City SNOTEL data) throughout the entire 2013-2014 season.concentrations ranged from 20 to 138 µg L −1 (n = 49 cores), 14 to 98 µg L −1 (n = 56 cores), and 28 to 62 µg L −1 (n = 3 integrated snow pits) during 2011- Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | snowpack core organic N concentrations ranged from 30 to 280 µg L −1 in 2011-2012 (n = 49 cores), 30 to 180 µg L −1 in 2012-2013 (n = 56 cores), and 120 to 260 µg L −1 in 2013-2014 (n = 3 integrated snow pit).No dominant spatial or temporal patterns were observed in snowpack organic N concentration or load for either 2011-2012 or 2012-2013 (Fig.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | organic to organic forms), and different mobilization during elution sequences leading to large fluctuations in both the concentration and spatial-temporal patterns of snowpack total N throughout the season.Total N accumulation in Sierra Nevada snowpack shows strong interannual variability as well as different representation of various N species and tracing N speciation throughout the snowpack season may give insight into microbial-driven snowpack chemical cycling.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 58 ng L −1 in 2011-2012 (n = 49 cores), 0.97 to 5.96 ng L −1 in 2012-2013 (n = 56 cores), and 3.28 to 7.56 ng L −1 in 2013-2014 (n = 3 integrated snow pits).Tahoe Basin average snowpack core THg concentration for 2011-2012 and 2012-2013 was 2.56 ± 1.3 ng L −1 .Observed THg concentrations are slightly lower, but within range of the end-of-season average snowpack concentration measured during a watershed Hg balance study in 2009 at Sagehen Creek, CA (i.e.3.3 ng L −1 ; Fain et al., 2011), a remote watershed located only 32 km north of the Tahoe Basin.Particulate Hg was the dominant form of Hg within Tahoe snowpack accounting for 76.1 ± 8.7, 70.3 ± 13.4, and 87.1 ± 4.7 of THg on average during 2011 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

− 3 -
N loads, we therefore multiplied the two-year seasonal average concentration (47.1 µg L −1 ) by the decadal average reconstructed SWE.Basin-wide NO − 3 -N loading estimates (mass area −1 ) thus reflect snowpack accumulation patterns (i.e., SWE) with the highest loading occurring on the west-side of the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

− 3 -
N concentrations were relatively uniform throughout the basin indicating out-of-basin sourced wet deposition as a pri-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .Figure 2 .Figure 5 .Figure 8 .
Figure 1.Lake Tahoe watershed map with bi-weekly sampling sites located along east and west basin elevation gradients for spatial and temporal sampling campaigns in 2011-2012 and 2012-2013.Additional wet deposition and snow pit profile samples were collected near the Homewood High and Mt. Rose sites during the 2013-2014 snow year.
Three of the sites were located at lake level (one remote site; two sites in urban areas; elevation approximately 1900 m); two sites were at mid-

was inserted into the snow adjacent to the sample wall two to three times at each layer before sampling to avoid carry over. Duplicate samples were
collected at each height and analyzed separately.All samples were double bagged in in Whirl-pack © clean bags and weighed for density.Samples were then transferred to −20 • C storage at the Desert Research Institute in Reno, NV until analysis.Reported concentrations and densities are averages of the duplicate samples.Introduction deposition was completed during the 2013-2014 snow year.Bi-weekly wet deposition sample collection following National Atmospheric Deposition Program protocol (http://nadp.sws.uiuc.edu/)was conducted at the two high altitude sites by N-Con
d.f.p value p value p value p value p value p value p value a p value < 0.10.b p value < 0.05.Introduction