Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado

Journal of Hydrology, 140 (1992) 179-208 Elsevier Science Publishers B.V., A m s t e r d a m

179

[21

Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado R o b e r t S t o t t l e m y e r a a n d C . A . T r o e n d l e b'l

"Great Lakes Area Resource Studies Unit, Department of Biological Sciences, Michigan Technological University, Houghton, MI 49931, USA bU.S. Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526, USA (Received 29 February 1992; revision accepted 3 May 1992)

ABSTRACT Stottlemyer, R. and Troendle, C.A., 1992. Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado, J. Hydrol., 140: 179-208. Streamwater samples were collected during 1987-1988 from two adjacent gauged watersheds, the subalpine-alpine East St. Louis and the Fool Creek Alpine, in the Fraser Experimental Forest, Colorado. The study objective was to compare the relationships between streamwater discharge and ion concentration in alpine and alpine-subalpine watersheds at a site receiving low inputs of atmospheric contaminants. Streamwater discharge accounts for much of the variation in ion concentration. Trajectories of time, discharge, and ion concentration suggest that patterns of nutrient flux are controlled primarily by the magnitude of streamwater discharge, and seasonal differences in the relative contributions of snowmelt and soil water. In the subalpine catchment, increased streamwater discharge accounted for most of the decline in concentration of ions, with high concentrations in soil water relative to precipitation. This relationship was not seen in the alpine catchment, probably because of the influence of large diurnal variation in the ratio of snowmelt to soil water. In both catchments, ions with comparatively high concentrations in precipitation and the snowpack relative to soil water showed less concentration decline with increased streamwater discharge. The recurring nature of the trajectories, especially in the subalpine catchment, suggests that the time, discharge, and ion concentration patterns may represent a general characteristic in moderate-sized, undisturbed Rocky Mountain catchments which do not receive high inputs of airborne contaminants.

INTRODUCTION

In watershed ecosystems, the relationship of surface water chemistry to C o r r e s p o n d e n c e to: R. Stottlemyer, G r e a t Lakes Area Resource Studies Unit, D e p a r t m e n t of Biological Sciences, Michigan Technological University, H o u g h t o n , M I 49931, USA. t Present address: W a t e r Resources Division, N a t i o n a l Park Service, 240 W. Prospect Street, Fort Collins, C O 80526, USA.

0022-1694/92/$05.00

© 1992 - - Elsevier Science Publishers B.V. All rights reserved

180

R. STOTTLEMYER AND C.A. TROENDLE

streamwater discharge, seasonal variation in streamwater ionic concentration, ion input-output budgets, and trajectories of time, discharge, and ion concentration all have potential as indicators of terrestrial ecosystem function and change (Bond, 1979; Lewis and Grant, 1979a,b; Stednick, 1989). However, complicating these relationships are long-term changes in surface water chemistry which can arise because of both natural factors, such as on-site succession allocation (Vitousek and Reiners, 1975), and anthropic factors such as air pollution inputs (Driscoll and Newton, 1985; Driscoll et al., 1988). Watersheds with simple annual discharge patterns (Bond, 1979) and low atmospheric contaminant inputs (Driscoll et al., 1988) are especially good sites in which such relationships may be observed. In subalpine forested watersheds of the Fraser Experimental Forest, in the Central Rocky Mountains (Fig. 1), annual streamwater discharge is dominated by snowmelt. At the forest up to 97% of annual discharge may ultimately be attributable to snow inputs (Troendle and King, 1986; Troendle and Kaufmann, 1987). Because of terrestrial moisture deficits in summer, little rainfall produces streamwater discharge, but rather is lost through evapotranspiration. In addition, the state of Colorado has among the lowest wet precipitation deposition values of H + , NH + , N O [ and SO] in the USA (Table 1). The precipitation chemistry at the forest is among the least altered in Colorado (Stottlemyer, 1987) despite the high elevation of the calibrated watersheds which can cause higher precipitation amounts and contaminant concentrations (Lewis et al., 1984). There has been some study of surface water chemistry in watershed ecosystems of the Central Rockies, but the emphasis has been on the potential susceptibility of Rocky Mountain alpine lakes, catchments, and first-order streams to atmospheric contaminant input (Lewis and Grant, 1979a; Lewis, 1982; Turk and Adams, 1983; Baron et al., 1986; Turk and Campbell, 1987; Bales et al., 1990). Other studies have quantified alpine and subalpine ion budgets in both the short and long term (Stottlemyer and Troendle, 1987; Stednick, 1989). Precipitation inputs to such ecosystems are dominated by snowfall. Studies in other sections of the country show that during snowmelt stream hydrology and chemistry may exhibit the most pronounced variation of the hydrologic year (Cadle et al., 1984; Goodison et al., 1986; Lynch et al., 1986; Stottlemyer and Toczydlowski, 1990, 1991). However, the comparison of chemical change in streamwater during snowmelt and possible differences in meltwater solute pathways to streams in small alpine and subalpine catchments has received little attention in Central Rocky Mountain ecosystems (Lewis and Grant, 1979a). Such analyses are necessary first steps before variation in streamwater chemistry can be used as a possible indicator

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

DEADHORSE~

181

2725m/,¢/j EXPERIMENTAL FO EST HEADQUARTERS

3515m

j 2875m Roads

boundaries

,~

-~ Streamgauge • Eventcollector •

Recordingraingauge

I

Canopyplot! interception

0 o

2878m~ /

CREEK

3180m i ALPINE I / ~~" L173480Ui " Sm~5

1Mi

"~f--~ f3695 m

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DENVER FRASER o • EXPERIMENTAL FOREST

COLORADO Fig. 1. Principal watersheds under study at Fraser Experimental Forest.

of terrestrial ecosystem function and change. The objectives of this study were to (1) compare the relationship of streamwater ion concentration with streamwater discharge during spring runoff in two undisturbed, adjacent alpine and alpine-subalpine watersheds in the Fraser Experimental Forest, and (2) identify some of the possible sources of variation observed in streamwater chemistry of these two catchments. SITE D E S C R I P T I O N

The Fraser Experimental Forest is 137km west of Denver, CO. Both the East St. Louis Creek (803 ha, stream gauge elevation of 2880m) and Fool Creek Alpine (67ha, stream elevation of 3180m) watersheds have northnorthwestern aspects (Fig. 1). The long-term mean annual precipitation at

182

R. STOTTLEMYER AND C.A. TROENDLE

TABLE 1 Mean annual precipitation ion input (equiv. ha J) for 1984-1986 at the Fraser Experimental Forest and National Atmospheric Deposition Program (NADP) stations in Colorado Ion

Fraser

Mesa Verde

Sand Spring

Manitou

Rocky Mountains, Lock Vale

Rocky Mountains, Beaver Meadow

Ca 2+ Mg2. K* Na + NH4+ H~ NO; SO] CI

104(103) 35(49) 32(37) 60(75) 40(35) 62(56) 82(48) 113(81) 54(47)

258(613) 43(55) 8(12) 40(64) 48(56) 80(105) 114(98) 195(196) 32(41)

128(245) 39(80) 7(15) 40(100) 54(62) 39(50) 104(128) 140(184) 30(54)

96(135) 29(40) 8(13) 28(38) 48(72) 47(47) 108(121) 116(118) 22(31

177(188) 57(70) 14(19) 64(91) 102(146) 109(109) 194(190) 261(232) 51(48)

80(84) 26(30) 9(19) 33(47) 62(76) 48(65) 91(72) 113(97) 25(29)

Cm ~

67(25)

(ppt) nb

81

54(9) 135

43(3) 144

40(7) 128

117(12) 127

44(2) 142

~Total centimeters of precipitation (number in parentheses is equivalent to one standard deviation). ~Number of samples.

Forest Headquarters (2725 m) is 58 cm. Annual precipitation over the entire forest averages 74cm (Alexander et al., 1985), about 70% of which falls as snow. Sharp increases in snowfall amount occur with elevation (Meimam 1987). The bedrock of both watersheds is dominated by gneiss and schist. This resistant bedrock and extensive glaciation of the area account for the rugged terrain and low inherent soil fertility (Retzer, 1962; R. Stottlemyer, unpublished data, 1987). Soils of both watersheds are dominated by gravelly sandy loams with alluvial soils near the streams (Retzer, 1962). The headwaters within the East St. Louis drainage originate in a cirque, and cross two terminal moraines before reaching the stream gauge. The soils are mostly Inceptisols with surface soil cation exchange capacities (CEC) averaging about 20 mmol c mequiv. 100g ~ and pH (CaC12) ranging from 4.5 to 6.1 (R. Stottlemyer, unpublished data, 1988). With base saturation about 11 mmol c 100 g ~, such soils are considered moderately sensitive to atmospheric contaminant inputs (Binkley et al., 1989). The Fool Creek Alpine watershed is vegetated primarily by alpine meadow, krummholtz, and some non-commercial forest at its lowest elevation. In the

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

183

subalpine-alpine East St. Louis watershed, lodgepole pine (Pinus contorta Dougl.) dominates lower and mid-elevation southern and eastern aspects. Upper elevations are vegetated by Engelmann spruce (Picea engelmannii Parry )-subalpine fir (Abies lasiocarpa (Hook.) Nutt.). Alpine tundra occurs above 3350 m elevation. METHODS This study is a component of the long-term research on surface water quality and hydrology under way at the forest (Stottlemyer and Troendle, 1987). Streams were sampled weekly, just above the gauging station and stilling pond, from 1 May to 2 November 1987, and 30 April to 29 October 1988. On occasion they have been sampled throughout winter, when stream discharge is very low and unchanging, to see whether stream chemistry also is unchanged. During snowmelt and rapid change in the hydrograph (June and July; Fig. 2), streams were sampled daily. Water samples were collected in amber 500 ml Nalgene polyethylene bottles from the center of the stream in an area of turbulence. Samples were brought up to room temperature for pH and alkalinity determinations (American Public Health Association, 17th ed., p. 2.36) at the forest field laboratory. A Fisher (Pittsburgh, PA) AC meter was used to measure pH and alkalinity. Alkalinity was determined by titration with 0.02 N-H 2SO4 to a pH 4.5 endpoint. All analyses were completed within 8 h following sample collection. Filtered (0.45 #m) subsamples were analyzed for macro ions ( C a : + , Mg 2+ , Na + , K + , NH4~, PO 3 , C1-, NO3, and SO] ) on an automated Dionex (Sunnyvale, CA) Model 2020 ion chromatograph (IC). Laboratory quality assurance procedures included splitting weekly National Atmospheric Deposition Program (NADP) samples (Michigan Station MI99), participation in the Environmental Protection Agency's National Acid Precipitation Assessment program (NAPAP) quality assurance (QA) program, routine use of National Bureau of Standards inorganic standards, and the long-term comparison of paired cation results from atomic adsorption (AA) and IC (Stottlemyer et al., 1989). To assess any change in precipitation quality with elevation, Aerochem Metrics (Miami, FL) precipitation collectors ('event collector', Fig. 1) are located at two elevations (2730 and 3350 m) near the Forest Headquarters and near the top of Lexen Creek watershed. These collectors are sampled weekly. Samples were processed in the same manner as stream samples except that alkalinity was determined using double endpoint potentiometric titration (American Public Health Association, 17th ed., p. 2.38). Daily precipitation quantity at these stations was recorded with Belfort recording rain gauges. The mean precipitation input from these two stations was used as an estimate

184

R. STOTTLEMYER AND C.A. TROENDLE

140

,20

~ ii

100

'"

EAST ST. LOUIS

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MAY 1

JUN 1

JUL 1

AUG 1

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DATE / TIME Fig. 2. May to September 1987 [988 stream discharge from East St. Louis and Fool Creek Alpine drainages.

of precipitation input to the East St. Louis watershed. Precipitation input to the Fool Creek Alpine catchment was measured with a recording rain gauge at mid-elevation in the watershed. We have not yet detected a significant trend in precipitation ion chemistry with time or with elevation at the forest (Stottlemyer, 1987). Therefore, ionic inputs to the catchments were estimated by using 1987-1988 ion chemistry from the 2730 m elevation Aerochem Metrics

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

185

collector at Forest Headquarters (Fig. 1) weighted by the amount of precipitation which fell in the catchment. Streamwater discharge (1 s i) from the Fool Creek Alpine watershed was measured with a trapezoidal flume, and with a 2.5 m Cipolletti weir for the subalpine-alpine East St. Louis watershed. Annual catchment ion output was estimated as follows. For those ions where the relationship (r 2) between concentration and streamwater discharge was less than 0.50, there was little reason to assume that the concentration observed in a given week was related to streamwater discharge for that week. In this case, output for the AprilOctober period was obtained by multiplying the mean unweighted weekly ion concentration by the total streamwater discharge. The long-term hydrologic record for the watersheds shows that less than 10% of the annual runoff" occurs during the November-April period (mean less than 0.02Is ~ha i). For ion discharge during this period, the mean ion concentration for late October and early in the following April was multiplied by the total winter discharge. The sum of these two time periods made up the annual ion discharge. The estimate of output error was obtained by multiplying the standard deviation of the mean ion concentration by the total discharge for the two periods. When r 2 was greater than 0.50 and there was reason to assume a relationship between ion concentration and strearnwater discharge, weekly output was determined by multiplying the mean concentration between successive weekly samples by the streamwater discharge for the week, which was calculated from mean daily discharge. A similar computation was made for the November-April period. Annual output was again the sum of these two periods. The estimate of error was calculated by using the standard error St..... from the regression relationship between concentration and discharge multiplied by the total discharge for the period. I n p u t - o u t p u t calculations assumed that the watersheds were tight, and the following observations appear to support this assumption. Long-term annual streamwater discharge from these watersheds is about 50% of annual precipitation input. Summer precipitation appears to be lost on site with little contribution to stream flow (less than 4% return) (Troendle and King, 1986). Soil moisture depletion (Troendle, 1987), and winter evaporative losses (Troendle et al., 1989), can also be well defined. Independent estimates of the annual water balance are consistent with evapotranspiration as estimated from precipitation minus runoff'; this finding implies that the stream gauges provide a relatively good index of runoff`.

186

R. STOTTLEMYER A N D C.A. T R O E N D L E

TABLE 2 Comparisons of linear regressions of log streamwater discharge vs. log ionic concentration, subalpine East St. Louis Creek; the regression for all data (both limbs) is compared with that for data up to peak discharge (rising limb) and after peak discharge (falling limb) Ion

Year

Concentration mean (l~equiv. I - i )

% Error accounted for by regression

Significance of

Slope o f regression

difference between

Both limbs

Rising limb

Falling limb

slope of rising and falling limbs

Both limbs

Rising limb

Falling limb

Ca 2+

87 88

176(61) a 242(43)

44 ***b 72***

83*** 63***

68*** 97***

< 0.0001 < 0.0001

-0.18 -0.08

0.30 -0.07

0.22 0.10

Mg2 *

87 88

47(28) 91(20)

40*** 2***

36 65***

56*** 96***

< 0.0001 ns c

0.29 -- 0.10

- 0.20 0.09

- 0.40 0.11

Na +

87 88

76(10) 88(16)

38*** 39***

63** 36*

80*** 74***

< 0.0001 <0.05

-0.07 0.06

-0.11 -0.06

-0.08 0.09

K ~

87 88

15(2) 16(4)

3 13

69** 14

65*** 5

<0.0001 ns

-0.02 - 0.05

0.09 0.04

0.05 0.04

H~

87 88

0.14(0.06) 0.26(0.20)

1 5*

40* 35*

7 67***

ns ns

0.001 0.04

0.04 0.05

0.01 0.08

NH +

87 88

1.20(I.7) 1.10(2.2)

6 1

I 79

30 20

ns ns

0.16 0.02

0.01 0.20

-0.22 0.12

NO~

87 88

2.0(1.1) 2.1(I.9)

2 2

34 < 1

I 3

ns ns

0.07 0.08

0.24 0.01

0.04 -0.14

SO~

87 88

CI

87 88

HCO~

87 88

c/a a

87 88

36(6) 45(11) 7.3(6.5) 3.8(I.4) 263(49) 290(51)

"

10" 52***

60*** 49***

74*** 78***

< 0.0001 ns

0.05 -0.09

-0.15 -0.08

-0.08 -0.10

26** 10

30 1

47*** 29*

<0.01 ns

-0.26 0.06

-0.10 0.01

-0.41 0.13

39*** 37***

55** 40**

33*** 45***

< 0.0001 < 0.0001

0.10 -0.06

0.17 -0.06

-0.08 0.10

1.02(0.2) 1.30(0.16)

aStandard deviation given in parentheses. bSignificance of regression: no asterisk, not significant; * P < 0.05; **P < 0.01; ***P < 0.001. Cns, Not significant dc/a, Ratio of sum of cations to sum o f anions.

RESULTS AND DISCUSSION Stream ion concentration

Both streams had similar ionic concentrations (Tables 2 and 3). The exceptions were the higher concentrations of Ca 2+ , N O r , S O ] - , and C1 in the subalpine catchment. The relatively high concentration of base cations in

N U T R I E N T C O N C E N T R A T I O N S IN ALPINE A N D SUB ALPINE STREAMS

187

TABLE 3 Comparisons of linear regressions of log streamwater discharge vs. log ionic concentration, Fool Creek Alpine catchment; the regression for all data (both limbs) is compared with that for data up to peak discharge (rising limb) and after peak discharge (falling limb) Ion

Ca 2 ~ Mg 2 + Na ~ K+ H+ NH4 ~ NO 3 SO~ CI HCO 3

c/ad

Year

87 87 87 87 87 87 87 87 87 87 1.02

, Concentration mean (l~equiv. I i)

151(39) a 48(23) 95(1 I) 16(2,0) 0.17(0.I) 1.60(3) 1.00(1.3) 28(3) 5.1(3) 260(51 )

% Error accounted for by regression Both limbs

Rising limb

Falling limb

I 1 47 ***b 19" 3 2 24 12 25* 6

37 56 44 90** 25 I 71 92** 75 33

1 1 65*** 2 10 5 15 I 7 1

Significance o f difference between slope o f rising and falling limbs

ns c ns < 0.0001 < 0.0001 ns ns ns <0.0001 ns < 0.01

Slope o f regression Both limbs

Rising limb

Falling limb

0.08 0.05

0.06 -0.11

0.10 0.01

0.9

-0.1

0.01

- 0.08

0.02

0,03

~Standard deviation given in parentheses. bSignificance o f regression: no asterisk, not significant: *P < 0.05; **P < 0.01; ***P < 0.001. Cns, Not significant. de~a, Ratio of cations to sum o f anions.

the Fool Creek Alpine stream water compared with concentrations in precipitation suggest that alpine meltwaters were mixed with significant amounts of higher concentration soil water. This can occur even in relatively small catchments as a result of sustained snowpack meltwaters exerting a piston action on soil solution, increasing its relative contribution to streamwater discharge, especially later in the snowmelt period (Driscoll et al., 1987; Stottlemyer and Toczydlowski, 1990, 1991). The low concentrations of H +, N H +, N O 3 , and SOl- in Fool C r e e k Alpine stream water were further evidence of the relatively low levels of anthropic species in precipitation (Stottlemyer and Troendle, 1987) and the snowpack (Stottlemyer, 1987, Tables 1 and 4). Concentrations of H + , N O 3 , and SO ] were lower than or similar to those for headwater catchments receiving very low levels of atmospheric inputs of these species, such as Jamieson Creek in British Columbia (Driscoll et al., 1988). Conversely, in areas with high precipitation c o n t a m i n a n t concentrations of H + , N O 3 , and SO42 , as at H u b b a r d Brook, NH, headwater chemistry can reflect that of the precipitation. For example, H + , N O 3 , and SO]- concentrations in precipitation at H u b b a r d Brook are about 100/~equiv. 1 i , 20/~equiv. l-E, and 50/~equiv. 1-~, respectively ( N A D P (National Atmospheric Deposition

188

R. STOTTLEMYER AND C.A. TROENDLE

TABLE 4 Snowpack water equivalent (SWE) and ion load (equiv. ha ~) at canopy interception research site 1 km from Lexen Creek (Fig. 1), late March 1987 and 1988 Amount (cm H:O)

Ca-" ~ Mg 2~ Na* K+ NH4~ H+ NO3 SO] C1 c/a ~

Snowpack content (equiv. ha ~) Downwind 15.8 + 3.9cm H:O

Treatment 26.8 _+ 2cm H,O

Upwind 17.5 + 4.5cm H20

30.0 12.0 7.6 20.0 2.4 8.5 7.3 17.4 6.6 1.06

50.6 10.4 13.7 12.3 4.8 15.3 20.4 25.2 6.7 0.9

44.3 12.6 7.4 13.5 4.0 8.9 4.6 18.6 6.6 1.08

'Downwind', forested plot; ~Treatment'. clear-cut plot; 'Upwind', control forested plot. %/a, cations/anions.

Program), 1988), with very similar catchment headwater concentrations of H +, N O 3 , and SO] (Driscoll et al., 1988). Biological incorporation, transformation, or soil exchange could also be partly responsible for the reduced low levels of H + , NH 2 , and N O ; found in the Fool Creek Alpine catchment. In studies of other small watersheds, such processes have been found particularly important during and following the period of peak snowmelt (Stottlemyer and Toczydlowski, 1990, 1991). The higher SO ]- concentration in the subalpine streamwater was thought to be a result of S-bearing mineral or organic matter weathering, and an overall increase in watershed evapotranspiration relative to the Fool Creek Alpine catchment, but we have no on-site data to confirm this. Relationships between stream ion concentration and discharge

The concentration of ions in stream water at Fraser varied considerably over short time periods. A major source of this variation, and that observed in other studies, was stream discharge (Bond, 1979; Lewis and Grant, 1979b; Stottlemyer, 1987; Stottlemyer and Troendle, 1987; Stednick, 1989) (Fig. 2). However, variation in ionic concentration generally was not proportional to change in streamwater discharge, but appears to have a logarithmic relationship to it. If one plots ion concentration against stream discharge and

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

189

400

Solid 300

I

~

line

Dashed

- 1987

line

1988

-

100

0

,

0

,

,

I

20

i

i

i

I

40

i

=

i

I

l

60

i

i

I

80

,

,

'

I

100

Discharge (L/s) Fig. 3. Time, stream discharge, and C a 2÷ concentration trajectory for the rising and falling limbs of the 1987-1988 East St. Louis (subalpine) hydrographs. For this and following trajectories, ion concentration was plotted against stream discharge at time of sampling, with the points joined by increasing date beginning in late April and ending in August: peak discharge was on or about I0 June.

connects the points representing successive dates, a trajectory is produced showing the pattern of ionic variation with time and discharge (Fig. 3) (Stottlemyer, 1987). Such trajectories will serve here to explain, in part, the relative importance of mechanisms that may account for variation in surface water chemistry of alpine and subalpine catchments at the Fraser Experimental Forest. Calcium and magnesium The subalpine Ca 2+ trajectories were clockwise with a sharp decline in concentration observed in 1987 with increasing discharge (Fig. 3). In 1988, a year with higher peak and total annual streamwater and Ca 2+ discharge, the decline in concentration with discharge was considerably reduced (Table 2). Stream Ca 2+ concentrations began to increase immediately after peak snowmelt. However, stream concentrations did not return to premelt levels until late in autumn. Discharge accounted for almost all variation in Ca 2+ concentration for both years combined (not shown), annually, and during the rising and falling trajectory limbs of 1987 and 1988 respectively. The slope of the regression was always inverse to increasing discharge, and was steepest during the falling (declining discharge) limb of the trajectory. The slopes of the

190

R, STOTTLEMYER AND C.A. TROENDLE

rising and falling limbs were significantly different from one another in both years. The relatively high Ca R+ concentration at peak discharge in 1988 largely accounted for the higher annual volume-weighted concentrations and watershed output from the subalpine catchment compared with 1987. The 1987 hydrographs (Fig. 2) indicate that this was a year of very rapid initial melting and runoff. Based upon other soil hydrologic study at the forest (Troendle and Nilles, 1987), it is possible that input exceeded soil hydrologic capacity, resulting in a larger fraction of meltwater quickly passing through near-surface soil macropores to the stream. The rapid increase in dilute runoff to the stream mixed with only limited soil water contributions caused the rapid decline in streamwater Ca 2+ concentration. The rapid meltwater movement was probably greater through near-surface macropores than by overland flow, owing to the porous nature of the soils (Retzer, 1962). Except within the riparian zone, we have not observed overland flow at the forest during snowmelt. The elevation gradient would further reduce meltwater residence times and minimize contact with near-surface soil exchange sites. Conversely, in 1988 the increase in discharge was more gradual and uniform, as reflected in the slopes of the rising and falling trajectory limbs. The larger peak snowpack water equivalent in 1988 coupled with the more gradual melt period probably resulted in a more uniform mixing of meltwater and soil water contributions to the stream. The sustained snowmelt period may also have increased the percentage of meltwater entering mineral soils, resulting in a piston action which increased the relative proportion of soil water in streamftow (Driscoll et al., 1987; Stottlemyer and Toczydlowski, 1990). In 1988, the overall shape of the rising limb of the hydrograph was very similar to the declining limb, indicating that the proportion of soil water to meltwater was about the same along both limbs. The subalpine Mg 2+ trajectories (not shown) also were clockwise, showing a decline in concentration with increased streamwater discharge. The regression slopes were similar to those observed for Ca 2+. Discharge accounted for much of the variation in Mg 2+ concentration, especially in 1988 (Table 2). The slope of the regression was less steep in 1988 when peak streamwater discharge was about twice that observed in 1987. However, as seen with Ca 2+ , the mean annual concentration and catchment outputs were much higher in 1988. The slopes of the rising and falling trajectory limbs were significantly different in 1987 but not in 1988. In the Fool Creek Alpine catchment, Mg 2+ concentration was similar to that in the subalpine watershed whereas Ca 2+ concentration was somewhat lower (Table 3). This probably reflected the significant contributions made to streamflow by soil water, as also indicated by the seeps and springs in this

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

191

300

0

I

0

I

I

I

2

I

I

I

I

4

1

I

,

6

1

I

8

,

,

I

10

Discharge (L/s) Fig. 4. Time, stream discharge, and Ca 2+ concentration trajectory for the rising and falling limbs of the 1987 Fool Creek Alpine hydrograph.

small catchment (Troendle and Kaufmann, 1987). Soil water from the alpine catchment makes up most of the summer baseflow from the larger Fool Creek drainage (297ha) within which the alpine catchment contains only the headwaters. The Fool Creek Alpine trajectories (not shown) were computed for 1987 only. In 1988, the rising limb of the hydrograph increased so rapidly that we were able to collect only a few samples along it, and the results are not reported. For Ca 2+ and Mg 2+ , alpine streamwater discharge accounted for less than 1% of the variation (Table 3). Relative to the subalpine catchment, the alpine catchment had a highly variable discharge (Fig. 2) dominated by major diurnal fluctuation during snowmelt. The poor relationship between Ca 2+ (Fig. 4) and Mg 2+ concentration and streamwater discharge can be attributed to the rapid daily variation in the ratio of snowmelt to soil water contributions, and the reduced time and distance for meltwater and soil water to become uniformly mixed before reaching the weir. Sodium

The trajectories for Na + in the subalpine (Fig. 5) and alpine (Fig. 6) were very similar to those for Mg 2+ in the subalpine catchment. The regression accounted for a significant fraction of the variation in ion concentration, and there were significant differences in slope of the rising and falling trajectory

192

R. STOTTLEMYER AND C.A. TROENDLE

200

150

Solid line = 1987

,.:

100

Dashed line - 1 9 8 8

L

~.

[~¢~."~-'-:::::..::.~. ......... 50

O

.

0

I

20

i

,

I

40

,

,

,

I

60

,

,

I

80

,

I

100

Discharge (L/s) Fig. 5. Time, stream discharge, and Na + concentration trajectory for the rising and falling limbs of the 1987-1988 East St. Louis (subalpine) hydrographs.

200

150

50

0

2

4

6

8

10

Discharge (L/s) Fig. 6. Time, stream discharge, and Na + concentration trajectory for the rising and falling limbs of the 1987 Fool Creek Alpine hydrograph.

NutrIent

c O N c F N T r A T I O N s iN A L p I n e AND SUB ALPINE STREAMS

193

limbs. The alpine mean Na ÷ concentration slightly exceeded the subalpine stream concentration. Of the base cations, Na ÷ has the highest concentration in precipitation and the snowpack relative to its streamwater and soil water concentrations (Stottlemyer and Troendle, 1987). This probably explains why the regression accounted for more of the Na + concentration variation than for Ca 2+ or Mg 2+ in the alpine catchment. Variation in Ca 2~ , M f ÷ , and Na ÷ concentration accounted for by the regression following peak discharge (falling trajectory limb) was generally as great or greater than during the rising limb. As previously mentioned, this suggests that the proportion of soil water contributions during the rising and falling hydrograph limbs are fairly uniform within a given year. To further test this assumption, regressions were run on only those samples collected while the hydrograph was actively driven by snowmelt runoffduring 1987, eliminating all samples collected after 30 June when factors other than change in streamwater discharge (i.e. summer evapotranspiration, biological productivity, and decomposition) could have significantly influenced stream chemistry. Except for NO3, this did not change the overall results to any appreciable degree, which suggests that such factors have relatively little influence on streamwater inorganic chemistry in these catchments. Potassium

In the subalpine catchment, stream discharge accounted for much less of the variation in K+concentration than observed for the other base cations. In 1987, there was a significant difference in the rising and falling limbs of the clockwise trajectory, but not in 1988 (Fig. 7). Alpine streamwater K + concentration (Fig. 8) equalled that of the subalpine, and streamwater discharge accounted for a significant portion of its variation. In both drainages, discharge accounted for a major portion of the variation in K + concentration during the rising trajectory limb in 1987. Of the base cations, K + concentration decreased the least with increased stream water discharge. These results probably reflect the relatively high concentration and loading of K ÷ in precipitation (snowpack) compared with streamwater, the leaching of this highly mobile ion from organic material in the snowpack or forest floor (Stottlemyer and Toczydlowski, 1990), and the relatively high exchangeable K + levels in the top 3cm of mineral soil at the forest (R. Stottlemyer, unpublished data, 1989). The rather uniform decline of K + concentration in both catchments during the rising hydrograph limb in 1987 appears to support these assumptions. On an annual basis, Fraser ecosystems conserve K ~ in sharp contrast to other base cations (Stottlemyer and Troendle, 1987).

194

R. STOTTLEMYER AND C A . TROENDt.E

40

30 Solid line

A ~

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Discharge (L/s) Fig. 7. Time, stream discharge, and K + concentration trajectory for the rising and falling limbs of the 1987-1988 East St. Louis (subalpine) hydrographs.

40

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Discharge (L/s) Fig. 8. Time, stream discharge, and K + concentration trajectory for the rising and falling limbs of the 1987 Fool Creek Alpine hydrograph.

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

195

Hydrogen Streamwater H + concentrations were very low. In the subalpine catchment, stream discharge accounted for a significant portion of the variation in H + concentration during the rising trajectory limbs and the 1988 falling limb. There was an increase in concentration with increasing discharge in both years, and in 1988 the highest H + concentration (0.9#equiv. 1 l) occurred during peak stream discharge. In the alpine catchment, discharge accounted for little of the variation in H + concentration, and there was little if any change in concentration with increasing discharge. With initiation of snowmelt runoff, both catchments showed slightly elevated H + concentrations. The increase in H + concentration with discharge probably reflected organic acid inputs from winter decomposition in forest soils or possibly the acidification products from limited nitrification which may have occurred during winter. H ~ concentration in precipitation at the Fraser Experimental Forest is low (Stottlemyer and Troendle, 1987). However, of all ions H + and NO 3 concentrations in precipitation and the snowpack are the highest relative to streamwater concentrations. During initial thaw, the release of H + from the snowpack is more rapid than observed for most ions (Johannessen and Henriksen, 1978; Stottlemyer and Toczydlowski, 1990). The possible early H + release coupled with increasing runoff', which reduces ecosystem retention times and the opportunity for exchange within the soil, could also contribute to the slight seasonal increase in streamwater H + concentration. However, in an earlier study of soil solution before, during, and after snowmelt at the forest, H + was very rapidly exchanged in surface mineral soils (Troendle and Nilles, 1987). After peak snowmelt, it is likely that any H + released from the snowpack or forest organic layers would be adsorbed in surface mineral soils. An analysis of 5 years of watershed budget has shown that little H + (less than 0.5% of inputs) leaves the ecosystem (Stottlemyer and Troendle, 1987). Ammonium and nitrate There was no significant relationship between NH~ concentration and streamwater discharge. In the subalpine catchment, the initial increase in streamwater discharge showed a brief but pronounced increase in NH4~ concentration, and the trajectory showed an increase in NH~- concentration with increasing discharge in 1988. In the alpine catchment, the highest concentration was found in the initial runoff, with concentrations rapidly declining to below detection limits with increasing discharge. Detectable NH4~ concentrations in streamwater were not found again until late August. However, mean N H g concentrations in the alpine basin were higher than

196

r . STOTTLEMYER A N D C.A. TROENDLE

those in the subalpine, with the highest concentration nearly twice that observed in the subalpine. Of the ions analyzed, NO~- had the highest concentration in precipitation relative to that in streamwater. Change in streamwater discharge did not account for a significant portion of its variation in streamwater concentration for either catchment. However, although the trajectory was irregular, NO3 concentration generally increased or remained the same up to peak streamwater discharge. The initial increase in streamwater NO~ concentration with snowmelt probably indicated meltwaters displacing soil solution with relatively high NO~ concentrations. The high soil solution concentrations could be the result of low biological uptake, overwinter N mineralization, and perhaps some nitrification (Rascher et al., 1987; Stottlemyer and Toczydlowski, 1990, 1991). As streamwater discharge further increased, the relative contribution of shallow soil solution from meltwaters probably increased while its residence time decreased (Rascher et al., 1987). This improves the likelihood that later NO~ releases in snowmelt will pass into the stream, reducing or eliminating concentration declines during increased streamwater discharge. NO3 concentrations were generally lower in the alpine catchment relative to the subalpine. This probably reflected the smaller N pools for remineralization in the alpine catchment. If only those samples which were collected when stream discharge was actively driven by snowmelt, i.e. before 30 June 1987, are considered, the fraction of variation in NO£ concentration accounted for by the regression during both trajectory limbs was improved significantly.

Sulfate In the subalpine catchment, stream discharge accounted for a significant fraction of the variation in SO]- concentration (Fig. 9). Its decline in concentration with increasing streamwater discharge was not as pronounced as observed for Ca 2+ or Mg 2+ . Precipitation and snowpack SO]- concentrations are much closer to those observed in soil solution and streamwater (Stottlemyer, 1987; Troendle and Nilles, 1987). SO]- is another ion preferentially released during early snowpack thaw (Johannessen and Henriksen, 1978; Stottlemyer and Toczydlowski, 1990, 1991), and its snowmelt concentration at the forest, although not yet directly measured, is probably similar to that of soil solution and stream water. Snowmelt would then result in much less dilution of soil solution and streamwater SOl concentrations. The SOltrajectory was similar to that for NO3, again suggesting the importance of relatively high concentrations in precipitation coupled with the fact that SO] is not retained in these ecosystems (Stottlemyer and Troendle, 1987). In the alpine catchment, a similar relationship between SO 2- concentration

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

197

1OO

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and streamwater discharge was observed for all of 1987, and especially the rising limb of the trajectory (Fig. 10). However, the slope of the rising limb was much lower than observed for the subalpine catchment. This probably reflected the higher snowpack SO 2- loading in this high-elevation catchment (see the following section and Tables 4 and 5). Chloride Change in discharge also accounted for a significant portion of the observed variation in streamwater C1 concentration in the subalpine catchment during 1987 and in the falling trajectory limb in both years. Slope estimates showed a decline in concentration with increasing discharge, and much sharper rates of concentration increase during the declining discharge limb. In the alpine catchment, discharge also accounted for a significant portion of variation in concentration. Streamwater C1- concentrations at the end of the summer were always slightly higher (approaching 20%) than just before snowmelt. Bicarbonate In the subalpine catchment, streamwater discharge accounted for a significant fraction of the variation in HCO3 concentration. Concentrations declined with increasing discharge in a pattern similar to that observed for

198

R. STOTTLEMYER AND C A . TROENDLE

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Ca 2+ , Mg 2+ , and Na + , but the rate of decline in concentration was lower (Fig. 11). Unlike other ions, the HCO 3 trajectories were clockwise at or near peak streamwater discharge, but showed little separation of trajectory limbs during the remainder of the period. There were significant differences in the slope of the rising and falling trajectory limbs in both years. This rather close relationship between discharge and HCO~ - concentration was not observed in the alpine catchment. However, its trajectory was similar to that observed in the subalpine catchment (Fig. 12). The concentration of HCO 3 and its range also were very similar for both streams. The HCO~ results again suggest that mechanisms similar to those described for Ca 2+ and Mg 2+ account for much of the high variation seen in the alpine catchment relative to that observed in the subalpine catchment. The variation in the relative contributions of snowmelt and soil water becomes especially complex later in the snowmelt period when meltwaters are virtually free of solutes, and probably are showing greater diurnal variation in release because of the steadily increasing daytime temperatures. This daily input comes on top of increasingly saturated soils which probably accelerate meltwater passage, chiefly as near-surface lateral flow through large macropores, to the stream.

N U T R I E N T C O N C E N T R A T I O N S IN ALPINE A N D SUB ALPINE STREAMS

199

TABLE 5 Annual input and output (equiv. ha ~) for major ion species, East St. Louis (ESL) and Fool Creek Alpine (ALP) watersheds, Fraser Experimental Forest, Colorado Ion

Ca: + Mg2+ Na ~ K' H~ NH2 NO3 SO] CI HCOj

Cm~

Year

1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988

ESL

ALP

Input

Output

Input

92 90 20 20 35 34 17 17 121 119 79 77 107 106 111 109 23 23 86 84 65 64

475 871 127 327 182 317 40 58 0.4 0.9 3.2 4.0 5.4 7.6 97 162 20 14 710 1044 27 36

108 23 41 20 143 93 127 130 27 101 77 -

Output 606 191 381 64 0.7 6.2 3.8 112 20 1043 40 -

"Centimeters.

W a t e r s h e d input and output I o n l o a d i n g at the F r a s e r E x p e r i m e n t a l F o r e s t is relatively large because o f the high p r e c i p i t a t i o n a m o u n t s . H o w e v e r , the ion c o n c e n t r a t i o n is similar to or, in the case o f a n t h r o p i c - d e r i v e d species, lower t h a n that observed t h r o u g h o u t C o l o r a d o ( S t o t t l e m y e r a n d T r o e n d l e , 1987, N A D P , 1988). W e have n o t detected a significant difference in ion c o n c e n t r a t i o n with gain in elevation as has been the finding o f o t h e r investigators in the region (Lewis et al., 1984). A t o u r p r i m a r y high-elevation station, the A e r o c h e m Metrics sampler in the Lexen Creek c a t c h m e n t , p r e c i p i t a t i o n was generally high in Ca 2+ a n d H C O 3 c o n c e n t r a t i o n , which p r o b a b l y indicates local inputs. T h e u p p e r p o r t i o n s o f this c a t c h m e n t c o n t a i n limestone r e m n a n t s . I n winter, wind scarping exposes soil on p o r t i o n s o f the alpine c a t c h m e n t a b o u t 1 k m f r o m the

200

R. STOTTLEMYER A N D C.A. T R O E N D L E

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Fig. 12. Time, stream discharge, and HCO( concentration trajectory for the rising and falling limbs of the 1987 Fool Creek Alpine hydrograph.

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

201

sampler, and this could account for these relatively high base cation and HCO3 concentrations. The presence of limestone and scarping apparently is not a problem in the Fool Creek Alpine catchment. Therefore, the use of the Lexen Creek precipitation chemistry data for the alpine catchment could overestimate precipitation inputs of Ca 2 ~ and HCO 3 and underestimate contributions to streamwater from weathering. Loss of the forest canopy may considerably increase precipitation input at a given elevation (Table 4). In a multi-year study of the effect of forest canopy on snow interception, we have found that canopy removal results in a marked increase in snowpack water equivalent and Ca 2+ , Na + , H + , N O £ , and SOlload (Stottlemyer, 1987). These inputs further increase with gain in elevation (Meiman, 1987), and help account for the larger unit-area outputs from the Fool Creek Alpine catchment (Table 5). For those ions with much lower concentrations in the snowpack than in streamwater and soil solution - - base cations and HCO3 - - outputs from the alpine catchment were higher relative to inputs (Table 5). For example, in 1987 base cation inputs to the alpine catchment were 15% of outputs whereas in the subalpine catchment inputs amounted to 20% of annual output. Some of the increased output can be attributed to the greater runoff per unit area. In 1987, stream discharge amounted to 52% of inputs to the alpine catchment but only 41% in the subalpine catchment. With greater sustained snowmelt, there may be increased piston action on soil solution and a relative increase in soil water contributions to annual output. Some evidence to support this was provided by comparing the 1987 and 1988 data for the subalpine catchment. The 1988 runoff, expressed as a percentage of inputs, was 33% higher than in 1987. However, the discharge of Ca 2+ , Mg 2+ , Na + , K + , and HCO3 relative to inputs increased 83, 157, 74, 45 and 47%, respectively (Table 5). This suggests that the longer period of sustained high discharge had a higher proportion of soil water mixed with it. For those ions with snowpack concentrations similar to or higher than those found in soil and s t r e a m w a t e r - - H + , N H ] , N O 3 , and SO] - - outputs relative to inputs were higher in the alpine catchment except for N O 3 , This indicates the likelihood that a shorter residence time, particularly during the early period of snowmelt when most solutes are released from the snowpack, promoted greater ion loss from this catchment. Some Rocky Mountain watershed ecosystems apparently retain SO~ (Lewis and Grant, 1979b). However, this does not appear to be the case at the Fraser Experimental Forest (Stottlemyer and Troendle, 1987). Data collected since 1982 show that stream SO 2 output from both disturbed and non-disturbed watersheds

202

R. STOTTLEMYER ANT) C.A TROENDLE

equals or exceeds inputs. A small portion of this output probably was contributed by weathering of sulfur-bearing minerals and some inputs from dry deposition. However, neither of these has been directly measured. Comparison watersheds'

of Fraser Experimental Forest results with those ./or other

Precipitation and stream ionic concentration Alpine streamwater concentrations of Ca ~+ , Na +, K +, and especially NO3and SO]- were higher than those observed at nearby (69 km to the NE) Como Creek, a 665ha watershed with similar elevation and vegetation but less topographic relief (Lewis and Grant, 1979b). The concentrations of HCO3, the dominant anion at both sites, were similar. At Fraser, streamwater concentrations of all ions except H + , NO3, and C1- exceeded those observed in and near Rocky Mountain National Park whereas SO] concentrations were similar (Baron and Bricker, 1987~ Stednick, 1989). These differences are largely attributable to the widespread presence of resistant granites in the vicinity of Rocky Mountain National Park and some less resistant sedimentary remnants over the gneiss and schist bedrock at Fraser. The relatively modest change in stream chemistry with lengthening hydrologic path to the stream associated with decreasing elevation has been observed in other studies (Hirsch et al., 1982). Such a trend was apparent when comparing Ca 2+, Mg 2+ , and SO] concentrations between the alpine and subalpine-alpine catchments in the Hourglass drainage north of Rocky Mountain National Park (Stednick, 1989). Such modest change in ion concentration with elevation is in sharp contrast to upstream-downstream concentration trends observed in streams where organic anions are more important or where atmospheric inputs of strong acid components (H + and SO]-) are greater (Driscoll et al., 1988; Lawrence et al., 1988). Evidence of the likely effect of precipitation quality on surface water chemistry can be found by comparing Fraser Experimental Forest streamwater chemistry with that of the Hubbard Brook, New Hampshire, control watershed, Falls Brook, which is underlain by bedrock very similar to that at Frazer. At Falls Brook, streamwater base cation concentration is nearly matched by the concentrations of H + and aluminium species, and SO] constitutes about 68% of the anion concentration (Driscoll et al., 1988). At Fraser, total streamwater ionic strength was about 60% higher than observed at Falls Brook, with more than 99% of the cation concentration as base cations, and about 75% of the anion concentration as HCO3. Alpine-subalpine catchments within the Hourglass drainage give nearly identical percentages to those found at Fraser (Stednick, 1989), whereas the Lock Vale

NUTRIENT CONCENTRATIONS IN ALPINE AND SUB ALPINE STREAMS

203

catchment in the eastern portion of Rocky Mountain National Park and east of the continental divide shows an increased importance of H ÷ in cations (9%) and SO~ (more than 30%) in anions (Baron and Bricker, 1987). These differences are attributable to differences in soil quality and its susceptibility to weathering and, particularly for Hubbard Brook, to differences in the present and historical pattern of precipitation inputs. Watersheds at Hubbard Brook receive more than 2.5 times the precipitation inputs of NO3 and SO~ and l0 times the H ~ input observed at the Fraser Experimental Forest (Stottlemyer and Troendle, 1987; NADP, 1988). Use of the snowpack ion load at peak snowpack water equivalent as a measure of winter precipitation input at Fraser appears to underestimate cumulative winter precipitation inputs (Tables 3-5). Most precipitation input at Fraser occurs as snowfall, but the ion load in the snowpack was small compared with annual precipitation inputs. This suggests that by late March and early April, when peak snowpack water equivalent normally occurs, a significant fraction of the snowpack ion load has already been lost. Solutes within a snowpack can readily migrate at temperatures 3-5°C below freezing. In other studies on large snowpacks, we have found that more than 50% of the snowpack solute load is lost before significant spring melt occurs (Stottlemyer and Toczydlowski, 1990, 1991). Relationships between stream ion concentration and discharge The importance of streamwater discharge in accounting for variation in stream chemistry has been found in both more dilute and more concentrated surface waters (Bond, 1979; Lewis and Grant, 1979b; Stottlemyer, 1987; Stottlemyer and Troendle, 1987; Stednick, 1989). The relationship of stream discharge to concentrations of Ca 2+, Mg 2+, and Na + in the subalpine catchment was similar to the findings of Bond (1979) in a much larger, lower-elevation catchment in Utah where ionic concentrations were about 20-25 times higher than observed at Fraser. However, in contrast to the Utah study, at Fraser the concentration of streamwater base cations after peak discharge was even more closely tied to streamwater discharge. The poor relationship between NH4 ~ concentration and stream discharge, despite the relatively high loading of NH + in snowpacks, has been found in other similar studies. We have found in a Northern Michigan catchment that a much higher percentage of the NH + released during midwinter thaws reaches the stream than does NH~ released late in the snowmelt period despite the much larger load released in late season (Stottlemyer and Toczydlowski, 1990, 1991). This we attribute to late season biological uptake, and the transformation of N H ] to NO~ . Relative to other sites, at the Fraser Experimental Forest mineral

204

R. STOTTLEMYER AND C.A. TROENDLE

weathering appears to play a reduced role in the quick recovery of ion concentration in soil solution and streamwater after snowmelt. This was indicated by the prevalence of clockwise trajectories, the rather consistent occurrence of limb separation in trajectories, and the reduced post-melt streamwater ion concentrations which generally did not return to premelt levels until the following winter. SUMMARY In a watershed ecosystem context, determining those factors responsible for regulating ion concentration in streamwater improves our understanding of terrestrial ecosystem processes, and provides potential tools for detecting long-term change. The regulation of streamwater ionic concentration can be examined through intensive analyses of the relationship of ion concentration to streamwater discharge over time. This approach appears to be most effective in watersheds with simple hydrographs. At Fraser, annual stream hydrographs are dominated by spring snowmelt. There appeared to be a definite pattern of change in ion concentration with increasing streamwater discharge. The relationship between concentration and streamwater discharge for C a 2+ and M g 2+ w a s very different for the two catchments. In the subalpine catchment, the relationship between base cation concentration and streamwater discharge generally showed a significant difference in the rising and falling slopes of the ion concentration-discharge curve. The high amount of variation accounted for by the regression suggests a relatively uniform mixing of runoff and soil water during both trajectory limbs. This can be attributed to the comparatively long flow paths to the stream. On an annual basis, the concentration--discharge slopes did not differ between the 2 years for any base cations. For K +, the discharge-concentration relationship usually broke down during the declining limb. At lower elevations of the subalpine catchment biological activity is significant, especially during the later phases of upper-elevation snowmelt, and ecosystem conservation of K + probably results in more variation of its concentration in early summer stream discharge. In the alpine catchment, several factors could account for the poor relationship between divalent base cation concentration and stream discharge. These ions have very low concentrations in precipitation relative to stream water, and they rapidly leave the snowpack during early snowmelt. Their high concentration during increasing stream discharge indicates significant contributions of high-concentration soil water. During much of the runoff period rising daily air temperatures resulted in considerable diurnal fluctuation of snowmelt release and streamwater discharge. This probably resulted in daily

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205

change in the proportion of soil water and meltwater contributions to stream discharge. Later in the snowmelt period, meltwater falls on increasingly saturated soils; this increases near-surface mineral soil macropore flow and thus further reduces the time required to reach the stream. This would probably result in still more rapid change in the relative contributions of soil water and snowmelt, and could account for the high variation in concentration with discharge especially on the declining trajectory limb. Conversely, the greater amount of ions as K + in the snowpack, its high mobility, and reduced biological uptake probably explain why the regression accounts for more of its variation in concentration with discharge. Certain hydrograph characteristics appear to have some predictive capacity when defining the relationship between streamwater discharge and ion concentration: (1) A rapid increase in the rising hydrograph limb results in maximum dilution of ions with high concentrations in soil and streamwater. However, this hydrograph characteristic does not appear effective in predicting the occurrence of snowmelt ionic 'pulses' to aquatic ecosystems. (2) After large precipitation or snowmelt inputs, streamwater discharge usually declines more gradually than it increases. This is particularly true when soil water contributions are large relative to precipitation and snowmelt contributions. For those ion species with high soil and streamwater concentrations, this would result in a counter-clockwise trajectory, with higher streamwater ion concentrations after peak discharge. At the Fraser catchments, the rising and falling limbs of the hydrographs were of very similar shape, suggesting that soil water contributions were not greater after peak stream water discharge. This was supported by the absence of well-defined counterclockwise trajectories for those ions with high soil and soilwater concentrations. A possible exception might be HCO~. This indicates that soil water was diluted by snowmelt inputs, and that concentrations of most ions were slow to recover until the following autumn or early winter. This was further evidence of the rather slow weathering rates in Fraser soils. (3) A more gradual initial melt period followed by a sustained peak discharge, as might occur with a large but relatively slow-melting snowpack, increases the likelihood of significant piston action being exerted by meltwaters on soil water which then increases the relative contribution of soil water to streamflow. These conditions result in significantly larger annual ion outputs especially for those species with high concentrations in soil water. (4) Large diurnal variation in stream discharge, which is especially prevalent in high-elevation, smaller catchments, increases the likelihood of rapid change in the relative proportions of soil and meltwater, and decreases

206

R. STOTTLEMYER AND C.A. TROENDLE

the l i k e l i h o o d o f significant r e l a t i o n s h i p s b e t w e e n s t r e a m w a t e r d i s c h a r g e a n d ion c o n c e n t r a t i o n . ACKNOWLEDGEMENTS W e t h a n k M a n u e l M a r t i n e z o f the R o c k y M o u n t a i n F o r e s t a n d R a n g e E x p e r i m e n t Station, w h o c o n d u c t e d the field s a m p l i n g a n d processing, Patricia T o c z y d l o w s k i o f the G r e a t L a k e s R e s o u r c e Studies Unit, M i c h i g a n T e c h n o l o g i c a l U n i v e r s i t y , w h o c o n d u c t e d m o s t o f the l a b o r a t o r y chemical analyses, a n d D a v i d T o c z y d l o w s k i for d a t a analysis a n d graphics. REFERENCES Alexander, R.R., Troendle, C.A., Kaufmann, M.R., Shepperd, W.D., Crouch, G.L. and Watkins, R.K., 1985. The Fraser Experimental Forest, Colorado: Research program and published research 1937-1985. U.S. For. Serv. Rocky Mount. For. Range Exp. St. Gen. Tech. Rep., RM-118, 46 pp. American Public Health Association, 1989. Standard Methods for the Examination of Water and Wastewater. Washington, D.C., pp. 2-36, 38. Bales, R.C., Sommerfield, R.A. and D.G. Kebler, 1990. Ionic tracer movement through a Wyoming snowpack. Atmos. Environ., 24A(11): 2749-2758. Baron, J. and Bricker, O.P., 1987. Hydrologic and chemical flux in Loch Vale watershed, Rocky Mountain National Park, In: R.C. Averett and D.M. McKnight (Editors), Chemical Quality of Water and the Hydrologic Cycle. Lewis, Chelsea, MI, pp. 141-155. Baron, J., Norton, S.A., Beeson, D.R. and Herrmann, R., 1986. Sediment diatom and metal stratigraphy from Rocky Mountain lakes with special reference to atmospheric deposition. Can. J. Fish, Aquat. Sci., 43: 1350-1362. Binkley, D., Driscoll, C.T., Allen, H.L., Schoeneberger, P. and McAvoy, D., 1989. Acidic deposition and forest soils: context and case studies of the Southeastern United States. In: W.D. Billings, F. Golley, O.L. Lange, J.S. Olson and H. Remmert (Editors), Ecological Studies, Vol. 72. Springer-Verlag, New York, 146 pp. Bond, H.W., 1979. Nutrient concentration patterns in a stream draining a montane ecosystem in Utah. Ecology, 60(6): 1184-1196. Cadle, S.H., Dasch, J.M. and Grossnickle, N.E., 1984. Retention and release of chemical species by a northern Michigan snowpack. Water Air Soil Pollut., 22: 303-319. Driscoll, C.T. and Newton, R.M., 1985. Chemical characteristics of Adirondack Lakes. Environ. Sci. Technol., 19: 1018-1024. Driscoll, C.T., Cosentini, C.C. and Newton, R.M., 1987. Processes regulating episodic acidification of an Adirondack, New York, stream. In: B. Moldan and T. Paces (Editors), Proc. Int. Workshop on Geochem. and Monit. in Representative Basins. Czechoslovakia Geol. Survey, Prague, pp. 244-246. Driscoll, C.T., Johnson, N.M., Likens, G.E. and Feller, M.C., 1988. Effects of acidic deposition on the chemistry of headwater streams: a comparison between Hubbard Brook, New Hampshire, and Jamieson Creek, British Columbia. Water Resour. Res., 24(2): 195-200. Goodison, B.E., Louie, P.Y.T. and Metcalf, J.R., 1986. Snowmelt acidic shock study in south central Ontario. Water Air Soil Pollut., 31: 131-138. Hirsch, R.M., Slack, J.R. and Smith, R.A., 1982. Techniques of trend analysis for monthly water quality data. Water Resour. Res., 18(1): 107-121.

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