Human activities have impacted the global nitrogen cycle; nitrous oxides are released to the atmosphere from fossil fuel combustion, ammonia is released from agricultural production and particulate nitrogen from a range of activities. Evidence of nutrient enrichment has been observed in alpine environments in western
North America (Baron et al., 2000; Saros et al., 2003). Investigations of two lakes in the Colorado Front Range reported increases in nitrogen concentrations beginning
1950, and presently exceed the range of natural variability (Wolfe et al., 2001). The timing of these changes coincides with increases in human population and associated changes to nitrogen inputs; however, research in the Beartooth Mountains, Montana suggests that similar changes are not directly the result of human activities, but climate-driven (Saros et al., 2003). Increased nitrogen in alpine lakes could lead to eutrophication of these dilute systems, which could profoundly impact aquatic ecosystems and food web dynamics. The Uinta Mountains, Utah are situated downwind of the Wasatch Front (Figure 1), a potential source of atmospheric nitrogen to the Uinta Mountains. The overall goal of this limnological and paleolimnological research is to determine whether atmospheric deposition of nitrogen is affecting Uinta Mountain lakes. We will determine which lakes in the Uinta Mountains, Utah have the greatest concentrations of various forms of nitrogen.
If atmospheric deposition is a primary source of nitrogen to Uinta Lakes, then we expect high elevation sites, which receive greater amounts of precipitation, to have greater concentrations of atmospherically derived nitrogen than low elevation sites. Furthermore, nitrate and ammonium concentrations in alpine lakewaters should be similar to precipitation, which is mainly snow. By comparing the timing of changes in the percentage of organics and diatom community composition, a proxy for nutrient concentrations, from lake sediment cores we can infer changes in nutrients, such as nitrogen. If nitrogen loading is related to increased nitrogen to the atmosphere by human activities, than we expect the timing of changes in nutrients to match the historical development of the Wasatch Front. The first settlers to the
Salt Lake City valley arrived in 1847, and large-scale agriculture started shortly after this (Sillitoe, 1996). By 1890 A.D., sheep and cattle grazing were prevalent along the Wasatch Front, and for approximately a century, agriculture was the primary industry. Land disturbances associated with such agriculture can increase dust and atmospheric N and P (Wetzel, 2001). Rapid urbanization has occurred along the Wasatch Front since the 1940s, during which time much farmland has been converted into residential or industrial sects, which contribute nitrous oxides to the atmosphere.
113˚W 112˚ 110˚
Great
Salt
Lake a
R h tc a
W sa g n e
Salt Lake City
W Y O M I N G
Flaming
Gorge
Reservoir
41˚
Uinta Mountains
Utah
Lake
40˚N
U T A H
G re en
R ive r
0 25 50
Kilometers
75 100
State Boundary
Figure 1: Map showing the location of the Uinta Mountains in Utah, approximately 100km downwind of the Wasatch Front and Salt Lake City.
The Uinta Mountains are located in northeastern Utah between 109° to 111° W and 40° to 41° N, near the
Utah-Colorado and Utah-Wyoming borders (Figure 1). For this study, 64 lakes were sampled from across the Uinta Mountain range, and three pairs (six) of these lakes were studied in more detail.
The Uinta Mountains are a Laramide-aged uplift of Precambrian metasedimentary rock, composed
primarily of quartzite and shale, which are flanked by Paleozoic rocks (Hansen, 1965). Uinta Mountain drainages are frequently overlain by glacial deposits (Bryant, 1992). The amount of precipitation varies considerably across the range (Munroe, 2003). Orographic uplift of humid air from the west, generally results in more precipitation on the western half of the range than on the eastern half (MacDonald &
Tingstad, 2007). Climate data recorded from the Uinta Mountains are typically low elevation sites, and indicate that the western Uinta Mountains receive, on average, approximately 254 mm more rainfall annually than the eastern part of the range (Western Regional Climate Center). The timing of maximum precipitation also varies across the range. Western sites generally have maximum precipitation amounts in the cold season, whereas eastern sites generally show peak values in the warm season (MacDonald &
Tingstad, 2007). Both glaciation history and climate influence the hydrology of the region. Along the
U-shaped valleys, chains of glacial lakes, ponds and marshes are found, and these are often separated by glacial moraines. Most lakes are relatively shallow (<15 m deep) and are located above 10,000 feet.
We have sampled 64 lakes in the Uinta Mountains. Sampling occurred between July and August from 2000 to 2004 (Figure 2). These lakes span a longitude gradient
of ~1˚ and an elevation gradient of ~3000 feet. At each lake, surface sediment (the top 0-1 cm of sediment from the center of the lake, which represents a spatially and temporally integrated sample) and water samples from the upper 0.5 m were collected (Figure 3). At each lake maximum depth, conductivity, salinity, pH, Secchi depth, temperature, alkalinity, and concentrations of major anions, cations and trace metals were measured. Water analysis was conducted by Rick Knurr at the
University of Minnesota.
Figure 3: Water samples and surface sediment samples were collected from each lake, and limnological variables were measured. Figure 3a shows sediment core being retrieved using a mini-Glew corer (Glew et al., 2001). Figure 3b shows specific conductivity being measured.
Figure 3c shows sediment core from Lake 7.
3
2
Principal Components Analysis (PCA) was performed on data from 64 lakes to determine relationships between limnological variables and to examine spatial patterns of the values of these variables (Figure 4). High elevation sites are characterized by greater concentrations of NO
3
NO
2
compared to lower elevation sites.
These greater concentrations are because of greater snow cover and reduced plant cover. Nitrogen from atmospheric deposition is stored in snow and released during spring runoff (Sievering et al., 1992). As it percolates through soils, plants can take up the nitrogen, however, the poor soils and reduced vegetation coverage of alpine areas, result in less uptake and nitrogen loading to lakes and streams. Although the PCA indicates that atmospheric deposition is an important source of NO
3
NO
2
to alpine sites, there may be other characteristics of alpine sites contributing to elevated NO
3
NO
2
values.
The limnological and diatom data from the Group 3 sites revealed that in some cases lakes side-by-side were different in terms of the concentrations of NO
3
NO
2 and the abundance of
, a diatom shown to be indicative of greater nitrogen concentrations in alpine areas (Saros et al., 2005). Three pairs of these lakes were selected for nutrient and paleolimnological analyses to further our understanding of the factors causing greater concentrations of NO
3
NO
2
. These sites were similar in terms of underlying geology (quartzites), pH (7.5-7.9), specific conductivity (10.2-15.6 uS cm -1 ) and temperature (12.9-13.5 ˚C). PCA shows that one of each of the paired lakes (8, 55 and 57) had nutrient concentrations similar to snow in the Uinta Mountains (Figure 5) suggesting, as others have, that a major source of nitrogen to oligotrophic lakes of mountainous regions is atmospheric inputs (Likens et al., 1977; Jassby et al., 1994). One lake of each pair, however, was distinct from snow in terms of nutrient concentrations, indicating that other factors affect nutrient loading to these lakes. One lake (7) had greater concentrations of TN, TP and DOC compared to snow (Figure 5, 6), which could be due to greater vegetation coverage in the catchment (Table 1). Surface waters running through forest and forest tundra sites have been reported to have increased concentrations of DOC, TP and TN compared to tundra sites (Pienitz et al.,
1997). Two lakes (56 and 58) have greater concentrations of NO slopes. Although all of the lakes except Lake 7 have significant talus coverage in their catchments (Table 1), only lakes 56 and 58 have very steep, barren talus slopes with significant streams running from the talus into the lakes. Recent research indicates that microbial activity in talus slopes may contribute significant amounts of NO
3
to surface waters (Campbell et al., 2000). Chla values at all lakes, except 56, are similar (Figure 6). The greater concentrations of Chla at Lake 56 may be related to greater concentrations of both NO
3
and NH
4
compared to snow. Both of these lakes have streams entering from beneath barren talus
3
and TP, which only occur at this site.
+1.0
-1.0
-1.0
Figure 4
Group 1 (8700’ to 9799’, n=17) low elevation sites
Group 2 (9800’ to 10,500’, n=20) mid-elevation lakes
Group 3 (10501’ to 11,679’ n=27) high elevation lakes
ELEVATION
LONGITUDE
NO
3
Depth
Secchi
Fe
PO
4
Mn
Temp
DOC
LATITUDE pH
Na
Spcond
Alkalinity
+1.0
Figure 4: Principal Components Analysis (PCA) of water chemistry analyses of 64 lakes.
Each point represents a site, and sites are plotted such that the more similar two sites are in terms of limnology, the closer together they plot. Each arrow represents a limnological variable, and they are plotted such that the smaller the angle between two arrows the more correlated the variables. Sites plotting closer to the head of an arrow have greater values of that variable. The variance explained by the first axes is 30%, and by the second axis is 12%. The first axis is most influenced by variables related to pH and salinity; whereas the second axis is most influenced by NO
3
NO
2
and Secchi and lake depth. Sites have been classified into three groups based on elevation, and corresponding vegetation. Group 1 are low elevation sites, and owing to warmer temperatures and greater evaporation, they tend to have greater ionic concentrations. Although there is considerable overlap between Group 2 and 3 lakes, Group 3 sites, generally located at or above treeline, are characterized by greater concentrations of NO
3
NO
2 these sites are characterized by higher levels of metals, which may be related to higher amounts of dissolved organic carbon (DOC), which many metals adsorb onto. DOC also reduces Secchi depth, which is low at these sites.
and greater Secchi depths. Group 2 are mid-elevation sites and are surrounded by fir and spruce forest;
+1.0
Figure 5: PCA of nutrient analyses of paired lakes and snow. A seventh lake, Dead Lake
(72), is added because it is grazed today. The first axis explains 34% of the data variance and is most influenced by NO
3
and NH
4
, whereas the second axis explains 22% of the data variance and is most influenced by TP, TN and DOC. Sites 8, 55 and 58 are most similar to the snow sample. Site 72 is distinct from the other lakes in having higher values of TP and low values of NO
3
and NH
4
. -1.0
-1.0
Figure 5
Dead (72)
Denise (7)
TP
DOC
Total Chla
TN
PO
4
East Carroll (56)
57
Snow
Upper Carroll (55)
Taylor (8)
NH
4
58
NO
2
NO
3
+1.0
210 Pb dating was used to establish a core chronology (Figure 7). Lake 8 has a slower sediment accumulation rate than Lake 7, which could be explained by Lake 8’s smaller catchment to lake ratio
(Table 1). The difference in sedimentation accumulation rates is less likely related to differences in in-lake productivity, because Chla concentrations are nearly the same (Figure 6). Organics deposited in lakes are derived from the lake catchment and from the lake itself. Lake 8 presently has lower percentages of organics than Lake 7 (Figure 8). The percentage of organics in sediments at both Lake
7 and 8 increases beginning in the 1930’s (Figure 8). This could be related to increased in-lake productivity as a result of increased nutrient loads or a reduction of inorganics from the terrestrial system. Based on historical information we would expect nitrogen deposition to the Uinta Mountains from the Wasatch Front to have begun to increase in the late 1800’s, although more dramatic increases would have occurred after ~1940 A.D. Instrumental temperature data and chironomidinferred temperatures, indicate that temperatures in the Uinta Mountains have increased beginning
~1950 A.D. (Figure 9). It is also possible that a reduction in inorganics could be due to increased stability with reduced grazing in this catchment. However, results from other sites, not presented here, indicate that in sites still grazed, organics are also increasing.
10
12
2
4
6
8
0
1840 1860 1880 1900
Age (Years A.D.)
1920 1940 1960 1980 2000 2020
14
Figure 7: 210 Pb determined ages for given depths for Lake 7 and Lake 8.
Taylor Lake (8)
Denise Lake (7)
Sediment samples from the six lakes have now been processed for diatom analyses. Initial scans of the topmost samples (representing today) and bottommost
(pre-1850) samples have been made and the results suggest that nutrients at some lakes have increased, but not at all lakes. In Lake 8 there is a dramatic change from a lake dominated by
complex to one dominated by
(Figure 8). Increases in
have been noted in other alpine regions of the southwestern USA (Saros et al., 2003; Wolfe et al., 2001). Experiments have shown that in oligotrophic, alpine lakes,
increase rapidly in response to increased nitrogen (Saros et al., 2005). In Lake 7, beside Lake 8, there appears to be an increase in diatom diversity. Lake 7 is shallower than Lake 8 (Table
1), so that Lake 8 is dominated by planktonic diatoms, whereas Lake 7 is dominated by benthic diatoms. The change in diatoms in Lake 7 represents an increase in the number of species of diatoms living in the littoral zone. We speculate that this may be as a result of a decrease in ice cover related to warming temperatures and/or increase in aquatic plant diversity which could be related to increased temperatures or nutrients. However, some lakes show no change at all. In Lake 55, for example, the top and bottom sample are nearly identical and are both dominated by Asterionella formosa (~35%). These results indicate that some sites are more sensitive than others to changes in nutrients or that the changes in diatom community composition are driven by local variations, such as changes in grazing activity, rather than regional changes, such as climate or atmospheric deposition. Future research, including further diatom analyses and nitrogen isotope analyses of lake waters and sediments, will hopefully provide further insights into why some lakes are recording dramatic changes, whereas others record no change.
0
0 10
20 30 40
5
1931
10
15
1936
20
25
30
35
Figure 8: Changes in percentage organics (determined using loss on ignition) for
Lake 7 and 8. Photos illustrating the most dramatic diatom community composition changes. The top sample is dominated by Asterionella formosa, whereas the bottom is dominated by Cyclotella stelligera complex.
Figure 2: Location map of 64 lakes sampled. Topographic maps and photos illustrate the six lakes selected for more detailed analyses.
Three pairs of lakes were selected for more detailed analyses based on the results from the 64 lakes. Detailed nutrient and paleolimnological (including sedimentological, nitrogen isotope, diatom and chironomid) of these six lakes are presently underway. Concentrations of ammonium (NH phate (PO
4
), silicate (SiO2), nitrite (NO
2
4
), nitrite and nitrate (NO
3
NO
2
), phos-
), total dissolved phosphorus (TP) and total dissolved nitrogen (TN), chlorophyll, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in lakewater were determined by Carl Zimmermann at the University of Maryland. Sediment cores were retrieved using a Kajak-Brinkmann gravity corer (Glew et al., 2001) (Figure 3). These cores were sub-sectioned in the field at 0.5 cm intervals. Chronology for the cores is based on 210 Pb dating of 15 sediment samples in each core completed by MyCore (Chalk River, Ontario, Canada). Percentage organics were determined using loss on ignition (Dean, 1974). Samples were processed for diatom analyses using standard techniques (Battarbee et al., 2001) and identification and enumeration was done at 1000X magnification using a
Leica DM2500 M microscope equipped with differential interference contrast (DIC) optics. Although cores from all six lakes are being analyzed, only results from Lake
7 and 8 are shown here.
Lake Number
7
8
55
56
57
58
Table 1: Lake Characteristics
Lake Depth (m) Lake Area
(Ao) (ha)
2.4
9.7
13.8
5.3
5.2
7.7
1.09
9
11.62
3.4
2.64
3.26
Catchement Area
(Ad) (ha)
61
268
254
23
47
159
(Ad+Ao)/Ao
57
23
50
31
8
19
Secchi Depth
(m)
>2.4
5.4
3.9
3.5
>5.2.
2.0
Catchment Talus
(%)
20
80
75
85
70
70
Catchment
Vegetation (%)
80 (trees)
20 (trees)
25
25 (trees)
30 (trees)
30 (trees)
Immediate Lake
Vegetation
1
0.5
1
0.5
0
0.5
0.020
0.015
0.010
0.005
0
0.035
0.030
0.025
7 8 55 56
Snow = 0.011mg N/l
58 57
0.30
0.25
0.20
0.15
0.10
0.05
Snow = 0.18mg N/l
0
7 8 55 56 58 57
Figure 6: Histograms comparing nutrient and Chla concentrations in paired lakes.
0.025
0.020
0.015
0.010
0.005
0
0.006
0.005
0.004
0.003
0.002
0.001
0
7
7 8
8 55 56
55 56
Snow = 0.012mg N/l
58
Snow = 0.0026mg N/l
58
57
57
1.5
1
0.5
0
3.5
3
2.5
2
7 8
3.5
3
2.5
2
1.5
1
0.5
0
5
4.5
4
7 8
55 56
55 56
58 57
58 57
1 3
2
0.5
0
1
0
-1
-0.5
-2
-1
-3
1900 1920 1940 1960 1980 2000 1900
Year
Figure 9: Plots showing instrumental temperature from Vernal, Utah and chironomid-inferred temperature from an alpine lake, Wild Lake, just east of Lake 7 and 8.
1920 1940
Year
1960 1980 2000
* High elevation lakes in the Uinta Mountains have greater concentrations of NO
3
NO
2
than lower elevation lakes probably due to greater snow cover and decreased
vegetation cover.
* Three of the six paired lakes have nutrient concentrations similar to snow indicating that atmospheric inputs of nutrients are important at these ultraoligotrophic
sites. Landscape factors, such as the amount and steepness of talus slopes and the percentage coverage and type of vegetation, may result in enrichment of some
nutrients.
* Preliminary results indicate that the percentage of organics at one set of paired lakes (7, 8) have increased beginning in the 1930’s. Changes in diatom community
composition from the top-most (today) and bottom-most (pre-1850) samples would suggest that in some lakes nutrients have increased, possibly due to increased
atmospheric deposition or possibly due to warming temperatures. However, other sites show no change in diatom community composition, suggesting variations
in lake sensitivity or more local drivers of change.
Funding for this research has been provided by the National Science Foundation (NSF), the US Forest Service and the National Sciences and Engineering Research
Council of Canada (NSERC). Special thanks go to the Ashley National Forest for continued support of this project.