yields of dissolved organic c, n, and p. from three high...

YIELDS OF DISSOLVED ORGANIC C, N, AND P. FROM THREEHIGH-ELEVATION CATCHMENTS, COLORADO FRONT RANGE,

U.S.A.

ERAN W. HOOD∗, MARK W. WILLIAMS and NEL CAINEDepartment of Geography and Institute for Arctic and Alpine Research, University of Colorado,

Boulder, CO, U.S.A.(∗ author for correspondence, e-mail: [email protected], fax: 303 492 6388)

(Accepted 12 February 2001)

Abstract. Ecosystem dynamics in high-elevation watersheds are extremely sensitive to changes inchemical, energy, and water fluxes. Here we report information on yields of dissolved organic C, N,and P for the 1999 snowmelt runoff season from three high-elevation catchments in the ColoradoFront Range, U.S.A.: Green Lake 4 (GL4) and Albion townsite (ALB) on North Boulder Creekand the Saddle Stream (SS), a tributary catchment dominated by alpine tundra. Dissolved organiccarbon (DOC) concentrations in stream waters ranged from <1 to 10 mg C L−1, with the highestvalues occurring at the SS site. Dissolved organic nitrogen (DON) concentrations ranged from belowdetection limits to 0.28 mg N L−1 and were again highest at the tundra-dominated site. Dissolvedorganic phosphorus (DOP) concentrations were at or near detection limits throughout the season inall three catchments indicating a strong terrestrial retention of P. Only DOC showed a significantrelationship to discharge. Yields of DOC in the three catchments ranged from 10.6 to 11.8 kg C ha−1

while yields of DON and DOP ranged from 0.32 to 0.41 and 0.02 to 0.08 kg ha−1, respectively.The relatively high yield of organic N and P relative to C from the highest elevation site (GL4) wassomewhat surprising and points to either: (1) a source of dissolved organic material (DOM) in theupper reaches of the catchment that is enriched in these nutrients or (2) the selective uptake andprocessing of organic N and P downstream of the sampling site. Additionally, seasonal changes inthe relative importance of DOM precursor materials appear to result in changes in the N content ofDOM at both the GL4 and ALB sites.

Keywords: alpine watershed, Colorado Front Range, DOC, DOM, DON, DOP, surface waters, yields

1. Introduction

Dissolved organic matter is an important component of the both the energy andnutrient balances of headwater catchments. Dissolved organic carbon is the majorsource of energy for non-photosynthetic biological activity (Wetzel, 1992). Dis-solved organic nitrogen appears to be a bioavailable source of N for plants (e.g.Kielland, 1994) and an important vector for N export (Sollins and McCorison,1981; Hedin et al., 1995) from these ecosystems. Dissolved organic phosphoroushas been suggested as an indicator of ecosystem P status, with P-limited sys-tems being conservative in terms of both the export of DOP and reactive formsof inorganic P (Neff et al., 2000).

Water, Air, and Soil Pollution: Focus 2: 165–180, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

166 E. W. HOOD ET AL.

Despite the importance of organic nutrients in ecosystems dynamics, there havebeen relatively few studies documenting yields of DOC, DON, and DOP fromheadwater catchments. The Hubbard Brook experimental watershed in the NEUnited States was the site of pioneering studies on fluxes of organic nutrients inforested watersheds (Likens et al., 1977; Meyer et al., 1981). At a small fores-ted catchment in the Colorado Front Range, Lewis and Grant (1979) found thatDOC yields increased with increasing discharge, while yields of DON and DOPremained unchanged. To our knowledge, little work has been done to documentyields of dissolved organic C, N, and P for unforested high-elevation catchments.

High-elevation catchments which extend above treeline are highly sensitive toeven small environmental changes relative to forested catchments (Baron, 1992).For example, small changes in energy, chemical and water fluxes to these catch-ments may translate into large changes in ecosystem dynamics which, in turn, mayaffect the quality and quantity of DOM leached to surface waters. Although N andP are minor constituents of organic material, these elements can significantly in-fluence aquatic biogeochemical systems. In the last decade, several high-elevationwatersheds in the Colorado Front Range have been the subject of intensive researchon hydrochemistry and nutrient cycling because of concerns about increased levelsof inorganic N deposition (e.g. Williams et al., 1996a; Campbell et al., 2000a).In the Green Lakes Valley, our study site, total N deposition in wetfall for theperiod 1996 to 1998 ranged from 3.9 to 5.6 kg ha−1 yr−1 (Williams et al., 2001)which is typical of high-elevation catchments along the Front Range (Williamsand Tonnesson, 2000). It is our hope that by monitoring yields of dissolved organicnutrients for the Green Lakes Valley, we will be better able to assess how futurechanges in chemical inputs may affect the DOM load in surface waters in thissensitive high-elevation area.

Here we report fluxes of DOC, DON, and DOP for the 1999 snowmelt sea-son from three high-elevation catchments in the Colorado Front Range. We havechosen catchments draining different landscape types in an effort to understand theinteraction between DOM cycling and landscape type. Specific hypotheses we testinclude:

(1) Dissolved organic N and P will be a significant portion of the yield of totaldissolved N (TDN) and total dissolved P (TDP) from alpine ecosystems.

(2) Nutrient limitation in terrestrial alpine ecosystems will be reflected in the nu-trient load of aquatic streams.

(3) Dissolved nutrient yields will vary by landscape type with vegetated tundraand forest landscapes having higher yields than talus landscapes.

(4) Organic C, N, and P will show a similar relationship to discharge.(5) Seasonal changes in nutrient yields will provide insights into ecological con-

trols on retention and export of dissolved organic nutrients at the watershedlevel.

ORGANIC CNP FLUXES AT HIGH ELEVATIONS 167

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TABLE I

Characteristics of the 3 sampling sites

Site Abbreviation Elevation Area Landscape type

(m) (ha)

Green Lake 4 GL4 3550 221 Talus

Albion ALB 3250 709 Subalpine Forest

Saddle Stream SS 3350 23 Alpine Tundra

2. Materials and Methods

2.1. STUDY SITE

Green Lakes Valley is an east-facing headwater catchment located 40 km NE of theDenver-Boulder metropolitan area in the Colorado Front Range. The catchmentranges in elevation from 3200 to greater than 4000 m at the Continental Divide(Figure 1). Green Lakes Valley is owned by the city of Boulder and public accessis prohibited. Consequently it does not have the recreational impacts of other high-elevation catchments in the Front Range. The catchment is a linear cascade offive lakes, with Green Lake 1 tributary to the sequence. Annual precipitation isapproximately 1000 mm, 80% of which falls as snow (Williams et al., 1996a).Stream flows are markedly seasonal varying from less than 5000 m3 day−1 in thewinter to greater than 100 000 m3 day−1 during spring runoff on high flow years(Caine, 1996).

Despite the large seasonal range in discharge, sediment yields in the GreenLakes Valley are quite low. Nine years of instantaneous measurements collectedapproximately weekly in the lightly vegetated Martinelli sub-catchment (Figure 1)show that suspended sediment concentrations rarely exceed 1 mg L−1 (Caine,1992). Further, the surface sediments subject to erosion and subsequent transport inthe Martinelli stream channel have an organic material content of ∼3% (Caine andSwanson, 1989). These findings indicate that losses of C, N, and P in the particulateform are likely to be extremely low in Green Lakes Valley. This assertion is sup-ported by work in the nearby Loch Vale watershed showing that suspended organiccarbon concentrations are an order of magnitude lower than dissolved organic car-bon concentrations (McKnight, 1993). As a result, hydrologic losses of organic C,N, and P from the watersheds we are considering should occur primarily in thedissolved form.

We report streamflow and chemistry data from three sampling sites: the outflowof Green Lake 4 (GL4), the Albion townsite (ALB), and the Saddle Stream (SS)(Figure 1). Green Lake 4 is on the main stem of North Boulder Creek in the upperreach of the catchment. The 221 ha of the catchment above GL4 is unforested


and alpine in nature (Table I). Steep rock walls and talus slopes are the dominantlandforms and vegetation is sparse. The ALB site is located below treeline 3 kmdownstream of Green Lake 4 on North Boulder Creek. The Saddle Stream is a trib-utary stream to North Boulder Creek which drains alpine tundra on the southeastflank of Niwot Ridge. In contrast to the other two sites, there are no lakes in the SSdrainage.

2.2. DISCHARGE MEASUREMENTS

Discharge is measured with a pressure transducer at all three sites. During 1999,water levels were recorded on hourly (ALB) and 10 min (GL4 and SS) intervalsand calibrated by field measurements every 2 to 5 days. The resulting water levelrecords were converted to volumetric discharges by empirical ratings that were val-idated by gauging at different flow levels for each season. These ratings remainedconsistent through the period of study.

2.3. STREAM CHEMISTRY

Stream samples were collected in 1999 as grab samples following the protocol ofWilliams et al. (1996b). Samples were collected weekly at all three sites throughpeak snowmelt runoff and bi-weekly to monthly during baseflow in the late summerand fall. Polyethylene bottles were soaked with deionized water overnight and thenrinsed copiously; bottles were further rinsed three times with sample water at thetime of collection. Samples were transported the same day as collection to our wetchemistry laboratory where subsamples were immediately filtered through glassfiber filters with a 1.0 µm pore size and stored in the dark at 4 ◦C for subsequentanalysis. Samples for DOC analysis were collected in precombusted amber glassbottles. DOC samples were filtered through precombusted Gelman A/E glass fiberfilters with an approximate pore size of 0.7 µm at the time of collection.

All water samples were analyzed for pH, acid neutralizing capacity (ANC),conductance, and major ions at the Kiowa Environmental Chemistry Laboratorylocated approximately 6 km from North Boulder Creek. Nitrate (NO−

3 ) was de-termined using a Dionex DX 500 ion chromatograph with an IonPac AS4A-SCAnalytical Column. The detection limit was 0.42 µg L−1. Ammonium was determ-ined on a Lachat QuikChem 4000 Flow Injection Analyzer using a method basedon the Berthelot reaction. The detection limit was 4.62 µg L−1. Total dissolvednitrogen (TDN) on filtered samples was determined by a potassium persulfate di-gestion to oxidize all dissolved forms of dissolved N into NO−

3 which was thenmeasured on a Lachat QuikChem 4000 Flow Injection Analyzer. The detectionlimit for TDN Was 9.8 µg L−1. DON was calculated by subtracting measuredinorganic N (NO−

3 + NH+4 ) from TDN.

Soluble reactive phosphorous (SRP) was measured as the orthophosphate ion(PO3−

4 ) using a Lachat QuikChem 8000 employing spectrophotometric detection.The detection limit for SRP was 1.3 µg L−1. Total dissolved phosphorous was


Figure 2. April to November 1999 stream discharge at the three sampling sites. All values arethree-day moving averages reported in m3 day−1; note the difference in scale for the SS discharge.Recharge of Lake Albion in the June 19–29 interval accounts for low flows at ALB at that time. Peakstreamflow at all three sites occurred between late May and early July.

calculated by using potassium persulfate and 3.75 N NaOH to digest and oxidizea filtered sample. The sample was then analyzed for PO3−

4 as described above.Dissolved organic phosphorous was calculated by subtracting SRP from TDP.

DOC was analyzed at the Institute of Arctic and Alpine Research in Boulderusing a Dohrman high temperature combustion instrument. Three replicate DOCanalyses were done for each sample. The standard deviation for these analyses wastypically 0.04 mg C L−1 with a range of 0.01 to 0.08 mg C L−1L.

2.4. YIELD CALCULATIONS

The export rates of DOC, DON, and DOP from the three drainage basins haveall been estimated as the product of measured concentrations and the accumulatedwater discharge for intervals centered on the day of sampling. Calculated yields areweighted by drainage basin area above the sampling site and expressed as kg ha−1.

3. Results

3.1. HYDROLOGY

Snowfall in 1999 was within 5% of the long-term average. Maximum snow waterequivalent (SWE) at a SNOTEL site 1 km NE of the Green Lakes Valley was363 mm in 1999 as compared to a fifteen year average of 358 mm.

At ALB and GL4, streamwater discharge began a rapid, steady increase theweek of 20 May (Figure 2). At the SS site, a consistent rise in discharge didnot occur until the week of 3 June. Because of the south-facing orientation and


Figure 3. Concentrations of dissolved organic C, N and P at the three sampling sites as a function oftime. DOC and DON are in mg L−1, DOP is in µg L−1.

relatively small size of the SS catchment, peak daily discharge of 4600 m3 day−1

occurred relatively early on 17 June. Daily discharge at GL4 peaked at greaterthan 48 000 m3 day−1 on 9 June coincident with the breaching of an ice damjust upstream in Green Lake 5. Discharge at GL4 then decreased rapidly beforeincreasing to a second peak of 35 500 m3 day−1 on 23 June. Streamflow at ALBwas delayed because of longer transit times and the recharge of Lake Albion from19 June to 29 June. As a result, streamflow at ALB peaked at 69 600 m3 day−1 on3 July.

3.2. ORGANIC C, N, AND P CONCENTRATIONS

Seasonal DOC concentrations peaked first at the SS site in late May and later atthe GL4 and ALB sites in early June and late June, respectively. At all three sites,seasonal maximum concentrations occurred before peak runoff on the ascending


limb of the hydrograph (Figure 3a). This pattern is consistent with the early sea-son flushing of soluble organic C from near surface soil horizons by infiltratingsnowmelt water (Hornberger et al., 1994). The highest DOC concentrations in thecatchment were recorded at the tundra-dominated site (SS). At SS, the highestmeasured DOC concentration was nearly 10 mg C L−1 on 22 May, although theseasonal peak could well have occurred earlier, before sampling was initiated. AfterMay 22, DOC concentrations at SS declined steadily to < 1 mg C L−1 in the latesummer and early fall. At the treeline ALB site, DOC peaked at 3.2 mg C L−1 inlate June and gradually declined to baseflow values ranging from 0.8–1.5 mg CL−1 late in the season. In the upper reaches of the catchment, the talus dominatedGL4 site consistently had the lowest DOC concentrations with a peak of 2.6 mg CL−1 in early June and baseflow values of <0.8 mg C L−1.

DON concentrations were lower and somewhat more variable than DOC con-centrations at all three sites (Figure 2b). After 5 June, DON concentrations didnot exceed 0.1 mg N L−1 at any of the three sites. At the SS site, DON peaked atnearly 0.3 mg N L−1 in early May and declined rapidly to less than 0.05 mg N L−1

by early July. DON concentrations at SS were below detection limits for much ofthe late summer and fall. In contrast to the SS site, DON concentrations at bothALB and GL4 did not show a distinct early season peak and remained at or below0.08 mg N L−1 throughout the runoff season. After mid-July, DON concentrationsat the ALB site were generally higher than at the other two sites, however DONconcentrations at GL4 did exceed those at ALB in late October and November.

DOP concentrations were extremely low throughout the runoff season at allthree sites (Figure 2c). At the SS site, DOP showed a small peak of 14 µg P L−1 inearly May but was consistently below 6 µg P L−1 for the rest of the measurementperiod. For the season, DOP concentrations at ALB fluctuated between below de-tection limits to 6 µg P L−1. Similarly, DOP concentrations at GL4 ranged fromdetection limits to 9 µg P L−1. These results appear to indicate strong ecosystemretention of P regardless of landscape type.

At the SS site, DOC concentrations explained a significant amount of the vari-ability in DON concentrations (r2 = 0.85, p < 0.001). In contrast, DOC and DONconcentrations were not significantly correlated at either the GL4 (r2 = 0.19, p >0.1) or ALB (r2 = 0.14, p > 0.1) sites. There was not a significant relationshipbetween DOC and DOP at any of the sites.

3.3. CONCENTRATION/DISCHARGE RELATIONSHIPS

Concentrations of dissolved organic C, N and P all show considerable variationover short time periods. An important source of this variation in headwater catch-ments is stream discharge (Lewis and Grant, 1979; Stottlemyer and Troendle, 1992).At all three sites, plots of organic C, N and P concentrations with time and dis-charge have a counterclockwise trajectory. This clockwise hysteresis is consist-ent with the flushing of soil pore water that is concentrated in dissolved organic


Figure 4. Time, stream discharge, and dissolved organic C, N and P concentration trajectories for theALB (treeline) sampling site in 1999. DOC and DON are in mg L−1, DOP is in µg L−1. Trajectoriesat the SS and GL4 sites were similar.

nutrients on the ascending limb of the hydrograph. Here we highlight concentra-tion/discharge plots from the ALB site (Figure 4). A similar relationship betweendissolved organics and discharge was seen at the GL4 and SS sites.

The relationship between DOC and discharge was the best developed trajectorywith a sharp decrease in concentration beginning just before peak discharge (Fig-ure 4a). The trajectories for DON and DOP were similarly in a clockwise direction,however persistent short term fluctuations in concentration resulted in less clearclockwise trajectories (Figures 4b and c). Simple regression analyses comparingthe concentrations of dissolved organic species at the ALB site with dischargeover the whole season, as well as with the ascending and descending limbs of thehydrograph separately, showed that only DOC was significantly correlated withdischarge (Table II). For the season, discharge explained 40% of the variation in


TABLE II

Relationship between nutrient concentrations and discharge at the ALB site forthe 1999 snowmelt runoff season

Mean Compared with

(mg L−1) All discharge Ascending limb Descending limb

(r2) (r2) (r2)

DOC 1.90 0.40a 0.52a 0.85a

DON 0.05 0.03 <0.01 0.11

DOP 0.002 0.05 0.08 0.02

a Correlation significant at p = 0.05 level.

TABLE III

Yields (kg ha−1) of dissolved organic N, P and C from three sample sites in Green Lakes Valley.Atomic ratios for annual nutrient export are shown for all three sites. The Organic atomic ratiois the ratio of DOC, DON, and DOP yields. The total dissolved atomic ratio includes inorganicN (NO−

3 + NH+4 ) and inorganic P (SRP) export. C:N ratios are shown for all atomic ratios

Site DOC DON DOP Atomic ratio:Organic Atomic ratio:Total dissolved

(kg ha−1) C:N:P C:N C:N:P C:N

GL4 10.6 0.41 0.08 133:5:1 27:1 133:38:1 3.5:1

ALB 10.9 0.34 0.02 545:17:1 32:1 545:63:1 8.5:1

SS 11.8 0.32 0.02 590:16:1 37:1 590:27:1 22:1

DOC concentrations. The correlation between DOC and discharge was particularlystrong on the descending limb of the hydrograph with discharge explaining 85%of the variation in DOC concentrations. This close relationship was the result ofthe marked decline in discharge at the ALB site being matched by continuousdecreases in DOC concentrations.

3.4. YIELDS AND ATOMIC RATIOS OF ORGANIC C, N, AND P

The mass yield of organic carbon was highest at the SS site, which exported 11.8 kgC ha−1 (Table III). Export of organic C was lowest from the lightly vegetated GL4catchment (10.6 kg C ha−1) and intermediate at the treeline ALB site (10.9 kg Cha−1). Area weighted export of organic N showed exactly the opposite trend withthe highest export at GL4 (0.41 kg N ha−1) and the lowest export at SS (0.32 kgN ha−1). ALB, with an organic N yield of 0.34 kg N ha−1, was again intermediateto the other two sites. Seasonal export of organic P export was low at all sites. As


with organic N, the yield of organic P was higher at GL4 (0.08 kg P ha−1) than ateither ALB or SS (both 0.02 kg P ha−1).

The importance of dissolved organic N and P in terms of total ecosystems yieldsof dissolved N and P can be seen by comparing the organic N and P yields to thosefor inorganic N and P. For the same time period in 1999, the flux of dissolved inor-ganic nitrogen (DIN), defined as NH+

4 + NO−3 , showed a wide range from 0.25 kg

N ha−1 at SS to 0.83 kg N ha−1 at ALB and 2.63 kg N ha−1 at GL4. Consequently,organic N was only 13% of the total dissolved N load at GL4 but increased to 29%at ALB and 56% at SS. In contrast, levels of SRP remained below detection limitsthroughout the season at all three sites. As a result, the small measured yields oforganic P constituted the entire flux of dissolved P out of the system.

The annual dissolved organic C:N:P ratios in streamwater at all three sites wereelevated. (Table III). The C:N:P ratio at the GL4 site of 133:5:1 was similar to theRedfield ratio for protoplasm (106:16:1) for C and P but depleted in N (Redfield,1958). At ALB and SS, C:N:P ratios were greatly enriched in C relative to theRedfield ratio, suggesting a terrestrial source of DOM (e.g., Lewis, 1986). Theorganic matter exported from SS, the alpine tundra site, was the most enriched inC relative to P with a C:P ratio of 590:1. The C:P ratio for the ALB site (545:1)was similar to that at the SS site. The organic C:N was lowest at GL4, 26:1, andincreased to 32:1 downstream at ALB. As with organic C:P ratio, the SS site hadthe highest organic C:N ratio (37:1).

The N enrichment of nutrient export from the GL4 site is particularly apparentif inorganic nutrient export is included in the analysis. Including inorganic N, theratio of C:N export at GL4 decreased by nearly a factor of 8 to 3.5 when comparedto the organic C:N ratio (Table III). Similarly, the C:N ratios at ALB and SS alsodecreased when inorganic N was included. These results highlight important dif-ferences in N retention according to landscape type. The lightly vegetated alpinesite (GL4) shows little N retention. N retention increases near treeline at the ALBsite and is highest at the SS site which drains alpine tundra. The relatively highyield of organic N and P relative to C from GL4 was somewhat surprising andpoints to either: (1) a DOM source that is enriched in these nutrients in the upperreaches of the catchment or (2) the selective uptake and processing of organic Nand P between GL4 and ALB.

4. Discussion

4.1. CONCENTRATIONS AND ECOLOGICAL CONTROLS

Landscape type appears to be a strong control on the dissolved organic nutrientload in surface waters in the Green Lakes Valley. Early in the snowmelt season,organic C, N and P concentrations were highest in the stream draining alpine tun-dra (SS). After peak runoff, organic C and N concentrations were highest at the


treeline site (ALB). Organic P concentrations showed little relationship to land-scape type and were extremely low at all three sites throughout the season. TheDOC concentrations we report are similar to those reported by Boyer et al. (1997)(1–4 mg C L−1) for the subalpine Deer Creek drainage in central Colorado, butlower than those documented in forested catchments in the NE (Campbell et al.,2000) and the Great Lakes (Stottlemyer and Toczydlowski, 1991). Few, if any,studies report direct measurements of seasonal DON and DOP concentrations forhigh-elevation catchments. Coats and Goldman (2001) report discharge weightedaverage DON concentrations 4–5 times those we documented in Green Lakes Val-ley for 11 alpine/subalpine catchments in the Lake Tahoe basin in California. Ingeneral, the streamwater leaving the alpine reaches of the Green Lakes catchmenthad low concentrations of organics, however, the higher concentrations of C and Nin the stream draining alpine tundra indicate that high-elevation catchments dom-inated by tundra rather than talus could export higher concentrations of dissolvedorganic nutrients to downstream forested ecosystems.

The fact that DOC concentrations are significantly correlated with DON con-centrations at the SS site but not at the other two sites highlights apparent differ-ences in the sources of organic matter between the three sites. The SS has littleinstream production and drains a relatively uniform landscape compared to theother two sites. Both a lack of instream processing or production and a seasonallyconstant DOM source (detrital plant matter) are consistent with the significantrelationship between DOC and DON concentrations at SS. The SS site behaveslike many other headwater catchments where condensed humic material from theterrestrial environment is the primary source of DOM (Hornberger et al., 1994;Boyer et al., 1997).

In contrast, the lack of a relationship between DOC and DON at GL4 and ALBindicates that the N content of DOM changes during the snowmelt runoff seasonat these sites. Because of the chain of alpine lakes in the upper valley, DOM at thetwo North Boulder Creek sites is likely derived from a range of sources including:leaf and litter inputs, soil and lake sediment leachates, and instream productionof phytoplankton (Baron et al., 1991; McKnight et al., 1997). Seasonal changesin the relative importance of these precursor materials appear to result in changesin the N content of DOM at both GL4 and ALB and precludes any significantrelationship between concentrations of DOC and DON at these sites. The lack ofa significant relationship between DOC and DOP at any of the three sites is mostlikely a function of the fact that DOP concentrations are near detection limits formost of the season.

4.2. YIELDS AND ECOLOGICAL CONTROLS

Surface water yields of DOM in Green Lakes Valley appear to reflect both thenutrient status of the surrounding terrestrial system and instream processing andproduction of organic C, N, and P. The influence of standing stocks of C, N and P in


ecosystem soils on DOM quality has been reported previously by Neff et al. (2000)who found that the ratio of DOC:DON in soil water was mediated primarily by soilorganic matter C:N ratios in a chronosequence of Hawaiian soils. Additionally,Neff et al. found that DOP fluxes were closely linked to inorganic P fluxes andresponded relatively quickly to changes in reactive P availability.

In Green Lakes Valley, areas of talus are C limited but have active microbialpopulations and large inputs of inorganic N from snowmelt (Williams et al., 1997).As a result, these areas appear to leach organic material that is enriched in N relativeto C. In contrast tundra and forest landscapes have an abundance of C as well as arelatively high ability to assimilate N. These vegetated areas of the catchment showa higher yield of C relative to N. These differences in nutrient availability betweenvegetated and unvegetated areas of high-elevation catchments are reflected in thenutrient capital of catchment soils. Past research has shown that soil C:N ratios areon the order of 15–20 in the alpine and increase to >25 in downstream forestedecosystems (Hood and Williams, 1999). These differences in soil organic matterstocks appear to be reflected in the nutrient yields from our three sampling sites.

It is possible that N deposition in Green Lakes Valley is altering the amountof DOM produced by both terrestrial and aquatic sources within our study catch-ments. Recent work indicates that inorganic N deposition in Green Lakes Valleymay be increasing instream DOM production from algal biomass (Waters et al., inreview). Similarly, it is possible that sustained inorganic N deposition could causean increase in the amount of DON produced by direct leaching of plant detritalmaterial. However, it is unlikely that deposition of organic N as DON is directlyincreasing DON yields from our study catchments. In Green Lakes Valley, DONdeposition in wetfall is typically 5 to 10 times lower than DIN deposition (Williamset al., 2001). For the years 1996 to 1998, DON inputs ranged from 0.15 to 0.54 kgha−1 yr−1, and in two of those years DON yields exceeded DON inputs at GreenLake 4 indicating that the catchment was a net source of DON. In contrast, annualyields of DIN were only 42 to 62% of annual DIN inputs over the same time periodindicating that the watershed is a net sink for NH+

4 and NO−3 (Williams et al., 2001).

The atomic ratios of organic material from SS, GL4, and ALB indicate thatall three sites are depleted in P relative to C. One explanation for this observationis persistent P-limitation in the terrestrial systems in the upper Green Lakes Val-ley. Even the talus-dominated GL4 catchment, which is relatively C-limited, hasa C:P ratio which exceeds the Redfield ratio. Evidence of terrestrial P-limitationhas been reported for both alpine tundra and high-elevation ecosystems in theColorado Front Range. Previous work at Niwot Ridge (Bowman et al., 1993) hasshown that alpine meadows, particularly in moist areas, are co-limited by both Nand P. Additionally, Williams et al. (1996c) report that high levels of inorganic Ndeposition have caused an increase in the foliar N:P ratios of Bristlecone pines athigh-elevation sites possibly indicating a switch from N to P limitation.

While the influence of surrounding terrestrial systems on aquatic DOM qualityis clear, autochonous production and instream processing of organic material also


appear to be important processes in Green Lakes Valley. McKnight et al. (1994)have shown that DOM derived from algal sources is enriched in N relative to DOMfrom terrestrial plant sources. Green Lake 4 has been shown to have blooms ofplanktonic and benthic algae in the late summer and fall (Waters, 1999). Thismicrobial production augments the aquatic DOM load and could be responsiblefor the variability and relative enrichment in organic N observed at the GL4 site.Additionally, the production of autochonous DOM in the chain of alpine lakes inthe upper valley has the potential to affect the aquatic DOM load at the downstreamALB site.

The N content of aquatic humic substances, which constitute the majority ofDOM in most natural waters, is between 0.5–4.5% (McKnight et al., 1994). Fulvicacids in alpine environments are on the low end of this scale. In nearby RockyMountain National Park (RMNP), McKnight et al. (1997) report a range of 24.3to 31.4 for C:N ratios in surface water fulvic acids with lower values occurring inalpine lakes compared to downstream forested sites. We found that the C:N ratiosof our organic material compared well with the C:N ratios of material isolatedfrom fulvic acids collected in RMNP by McKnight et al. (1997). The range of C:Pratios at our sites (132–560) was much lower than the range for fulvic acids inRMNP (1060–1667). This difference in C:P ratios suggests that much of the P inthe organic material at our sites is associated with non-humic fractions of DOMincluding inositol phosphates and phospholipids (Thurman, 1985).

A final ecological control on stream organic nutrient yields is instream pro-cessing and sorption reactions. Due to the continuous exchange of water betweenthe stream and its hyporheic zone, both surface reactions and microbial processescan alter DOM quality in stream ecosystems. In Green Lakes Valley, the presenceof 5 lakes in the upper valley (Figure 1) provides ample opportunity for instreammicrobial processing. Additionally, the quality and quantity of organic materialyields are continuously altered by the exchange of organic material between thewater column and both lake and stream sediments via adsorption/desorption reac-tions. These processes, taken together, determine the final chemical composition ofDOM in the stream. The relative importance of microbial uptake versus chemicalsorption has important consequences for stream ecosystem dynamics, and devel-oping a better understanding of the relative importance of these two process at oursites constitutes an important area for further research.

In Green Lakes Valley, the combination of variations in terrestrial precursormaterial and instream processing result in higher yields of N and P relative toC in the alpine upper valley as compared to lower elevation vegetated sites. In-stream processes appear to be most important at the lightly vegetated GL4 siteand least important at the tundra-dominated SS site. These results demonstrate thatmeasuring yields of organic material can provide important insights into ecosystemdynamics at the catchment scale. What is less clear is how continued deposition ofinorganic N to sensitive alpine ecosystems in the Colorado Front Range will alterthe processing and export of organic nutrients in both terrestrial and aquatic eco-


systems, and how these changes might be reflected in downstream aquatic nutrientyields.

Acknowledgements

We would like to thank Oliver Platts-Mills and Tim Bardsley for help with fieldsampling. Laboratory analyses were provided by Chris Seibold and the staff ofthe Kiowa Lab. Tom Davinroy provided the map for Figure 1. Support for thisresearch came from the National Park Service Air Resources Division and Na-tional Science Foundation grant DEB 9211776 to the NWT LTER. EWH receivedadditional support from a graduate research traineeship (GRT) grant in hydrologyfrom the National Science Foundation. We would also like to thank two anonymousreviewers whose comments improved the final version of the manuscript.

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yields of dissolved organic c, n, and p. from three high...

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