UC Riverside
UC Riverside Previously Published Works
Title
20th century atmospheric deposition and acidification trends in lakes of the Sierra Nevada,
California, USA.
Permalink
https://escholarship.org/uc/item/8j33v5vf
Journal
Environmental science & technology, 48(17)
ISSN
0013-936X
Authors
Heard, Andrea M
Sickman, James O
Rose, Neil L
et al.
Publication Date
2014-09-01
DOI
10.1021/es500934s
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
20th Century Atmospheric Deposition and Acidification Trends in
Lakes of the Sierra Nevada, California, USA
Andrea M. Heard,*
,†,‡
James O. Sickman,
†
Neil L. Rose,
§
Danuta M. Bennett,
∥
Delores M. Lucero,
†
John M. Melack,
∥,⊥
and Jason H. Curtis
#
†
Department of Environmental Sciences, University of California, Riverside, California 92521, United States
‡
National Park Service, 47050 Generals Highway, Three Rivers, California 93271, United States
§
Environmental Change Research Centre, University College London, Gower Street, London WC1E 6BT, U.K.
∥
Marine Science Institute and
⊥
Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara,
California 93106-6150, United States
#
Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, United States
*
S
Supporting Information
ABSTRACT: We investigated multiple lines of evidence to determine if
observed and paleo-reconstructed changes in acid neutralizing capacity (ANC)
in Sierra Nevada lakes were the result of changes in 20th century atmospheric
deposition. Spheroidal carbonaceous particles (SCPs) (indicator of anthro-
pogenic atmospheric deposition) and biogenic silica and δ
13
C (productivity
proxies) in lake sediments, nitrogen and sulfur emission inventories, climate
variables, and long-term hydrochemistry records were compared to
reconstructed ANC trends in Moat Lake. The initial decline in ANC at
Moat Lake occurred between 1920 and 1930, when hydrogen ion deposition
was approximately 74 eq ha
−1
yr
−1
, and ANC recovered between 1970 and
2005. Reconstructed ANC in Moat Lake was negatively correlated with SCPs
and sulfur dioxide emissions (p = 0.031 and p = 0.009). Reconstructed ANC
patterns were not correlated with climate, productivity, or nitrogen oxide
emissions. Late 20th century recovery of ANC at Moat Lake is supported by increasing ANC and decreasing sulfate in Emerald
Lake between 1983 and 2011 (p < 0.0001). We conclude that ANC depletion at Moat and Emerald lakes was principally caused
by acid deposition, and recovery in ANC after 1970 can be attributed to the United States Clean Air Act.
■
INTRODUCTION
Mountain lakes are sensitive indicators of environmental
change and are especially useful for detecting changes from
regional-scale stressors including air pollution. Sensitivity of
aquatic ecosystems to atmospheric deposition of sulfate and
nitrate is well documented throughout the northern hemi-
sphere.
1−3
Adverse effects from acidification include chronic or
episodic depression of acid neutralizing capacity (ANC) and
changes in the structure of biotic communities.
4,5
Associated
nutrient inputs are contributing to long-term eutrophication
and shifts in nutrient limitation and phytoplankton commun-
ities.
6−8
At the same time, climate change is altering hydrologic
and water temperature regimes, which may further contribute
to variability in productivity and algal community shifts.
9,10
The Clean Air Act and Amendments (CAAA) is the primary
policy for improving air quality and reducing atmospheric
deposition in the United States (US). The legislation targets
decreases in acid deposition and the recovery of surface waters
from acidification and prohibits deterioration of air quality in
national parks and wilderness areas, where a large proportion of
western mountain lakes occur. Chemical recovery of lakes from
acid deposition has been attributed to the CAAA, although
most of the published research and success stories have been in
the northeastern US where negative ecological effects have
been most notable and acid deposition has received the most
attention.
11,12
Recently, western land management agencies
have been adopting the critical load (CL) as a policy approach
to further protect mountain lakes from air pollution.
13
ACLisa
quantitative estimate of an input or exposure to a pollutant at
which unacceptable impacts occur to sensitive ecosystem
components.
14
Paleolimnological research suggests that Sierra Nevada lakes
may have been affected by acid deposition as early as 1920.
Sickman et al.
15
reconstructed the last 1600 years of ANC using
diatom inference models in Moat Lake, a subalpine lake located
in the Sierra Nevada, and observed a decrease in ANC from
1920 to 1970. ANC increased after 1970 and returned to pre-
1920 levels by 2005. The causes underlying Moat Lake ANC
patterns are uncertain, but it is possible that the CL for acid
Received: February 23, 2014
Revised: July 28, 2014
Accepted: July 31, 2014
Published: July 31, 2014
Article
pubs.acs.org/est
© 2014 American Chemical Society 10054 dx.doi.org/10.1021/es500934s | Environ. Sci. Technol. 2014, 48, 10054−10061
deposition was exceeded well before regular monitoring in the
1980s. Sierra Nevada lakes have been affected by other stressors
during the 20th century such as nutrient inputs, climate change,
and non-native fish species so additional information is needed
to understand the primary drivers of ANC change.
Multiproxy approaches can aid interpretation of the Moat
Lake ANC reconstruction.
16
Spheroidal carbonaceous particles
(SCPs) found in lake sediments have been used to investigate
historic atmospheric deposition.
17
SCPs are porous spheroids
composed primarily of elemental carbon that are chemically
resistant and well-preserved.
18
They are unambiguous
indicators of anthropogenic atmospheric deposition because
they are produced only by industrial combustion of fossil fuels;
there are no natural sources. Biogenic silica (BSi) and δ
13
C are
proxies for algal productivity and are used to assess effects of
nutrient inputs and climate change on aquatic ecosystems.
19,20
Paleolimnological studies are further strengthened by compar-
ison with long-term climate and lake chemistry data.
The primary goal of our research was to determine if the
ANC changes observed in Moat Lake
15
are a result of acid
deposition. In our study we synthesize multiple lines of
evidence, including measurements of SCPs, BSi, and δ
13
Cin
lake sediments, emission inventories for oxides of sulfur (S) and
nitrogen (N), climate, and long-term hydrochemistry records to
test the hypothesis that high elevation lakes in the Sierra
Nevada were aff ected by atmospheric deposition early in the
20th century. Using these analyses we evaluate the effectiveness
of the CAAA in protecting Sierra Nevada lakes and contribute
to the development of air pollution standards, including CLs.
■
METHODS
We analyzed SCPs, BSi, and δ
13
C in Moat Lake sediments from
the same core where Sickman et al.
15
reconstructed ANC. Moat
Lake is located on the eastern slope of the Sierra Nevada,
California, United States of America on Humboldt-Toiyabe
National Forest at 3224 m (Figure S1). It has a maximum
depth of 7 m, lake surface area of 2.8 ha, and watershed area of
59 ha. The bedrock geology of the watershed is dominated by
metasedimentary rocks including quartzite and argillite. To
increase our spatial understanding of atmospheric deposition
and ANC chemistry in the Sierra Nevada, we also investigated
SCP patterns at Pear and Emerald lakes and hydrochemistry
trends at Emerald Lake. Pear and Emerald lakes are located in
adjacent watersheds on the western side of the Sierra Nevada in
Sequoia National Park at 2904 and 2800 m, respectively. Pear
Lake has a maximum depth of 24 m, lake surface area of 7.3 ha,
and watershed area of 136 ha. Emerald Lake has a maximum
depth of 10 m, lake surface area of 2.7 ha, and watershed area of
120 ha. The bedrock geology of both watersheds is dominated
by granite and granodiorite. Less than 10% of the three
watersheds are vegetated.
At Moat Lake we used a rod-corer to collect a 210 cm core in
September 2008 and field sectioned the core at 1 cm
intervals.
15
The Pear and Emerald cores were collected in the
summer of 2003 as part of the Western Airborne Contaminants
Assessment Project (WACAP) using a gravity corer fitted with
a Plexiglas tube and field sectioned at 0.5 cm intervals.
21
The
chronology of the Moat Lake core was established with
210
Pb
and
14
C dating
15
and the Pear and Emerald lake cores using
210
Pb dating.
21
SCPs (gDM
−1
) were analyzed in the Moat, Pear, and
Emerald cores per methods in Rose,
22
and diatom BSi (mg g
−1
)
and δ
13
C(‰) of bulk organic matter were analyzed in the
Moat core per methods in Conley and Schelske
23
and
Sickman.
15
Methods are described in the SI.
Sickman et al.
15
described the methods and results for the
Moat Lake ANC reconstruction based on a diatom inference
model. In this paper we compare the Moat Lake ANC
reconstruction to SCP sediment profiles (Moat, Emerald, and
Pear lakes) and 20th century sulfur dioxide (SO
2
) and nitrogen
oxide (NO
x
) emissions estimated by the EPA
24
and Smith et
al.
25
National emissions (1900−2012) were used as regional
data were only available back to 1990, and we have provided a
Kendall correlation analysis of national and regional emissions
in the SI to demonstrate that national emissions provide a good
proxy for understanding multidecadal trends. ANC variations
are strongly influenced by precipitation and snow water
equivalent (SWE) and to a lesser extent by temperature
which affects snowpack dynamics and watershed weathering
rates.
26
Using available long-term records, we compared ANC
changes to mean annual air temperature and precipitation, and
April 1st SWE. Temperature and precipitation data were
obtained for the Sierra climate region, as defined by Abatzoglou
et al.,
27
from the Western Regional Climate Center’s California
Climate Tracker (http://www.wrcc.dri.edu/monitor/cal-mon/
).
27
SWE data were examined from two snow courses: (i)
Donner Summit located near Lake Tahoe (39.3100 N,
120.3380 W) (1910−2011) and (ii) Virginia Lakes located
near Moat Lake (38.0570 N, 119.2470 W) (1947−2011)
(http://cdec.water.ca.gov). Climate data were analyzed using
the Mann-Kendall (MK) test for trend
28
with Theil-Sen slope
estimator.
29,30
Correlations among variables were tested using
Pearson and principal component analysis (PCA).
We investigated trends for 30 years (1982−2012) of sulfate
chemistry and 28 years (1983−2011) of fall ANC and base
cation (BC) chemistry from Emerald Lake. Sampling and
laboratory methods are described in Melack et al.
31
Sulfate
trends were analyzed using the Seasonal Kendall test (SKT)
32
with Theil-Sen slope estimator
29,30
and compared to sulfate
deposition trends from the National Atmospheric Deposition
Program site CA75 in Sequoia National Park (http://nadp.sws.
uiuc.edu/). Deposition trends were analyzed using the MK
28
test with Theil-Sen slope estimator,
29,30
and correlations
between lake chemistry and deposition were analyzed using a
Kendall tau. We used two approaches to test for ANC and BC
trends that control for the effect of SWE as the correlation
between these variables and SWE coupled with the high annual
variability of SWE makes it challenging to detect temporal
trends. Method (i) is computing the residuals for the SWE-
ANC linear regression, plotting them in chronological order,
and computing a linear regression between residuals and year
and (ii) is using multiple linear regression (MLR) to predict
ANC using year and SWE as explanatory variables.
33
We used
the residuals approach (i) to test for BC trends.
We calculated a CL for acid deposition by hindcasting
deposition rates back to the time period when changes in
diatom communities and reconstructed ANC were initially
observed by Sickman et al.
15
We assumed acid deposition to be
the sum of nitrate and sulfate, estimated deposition rates for
these constituents separately, and then summed them.
Hindcasting methods are based on Baron
34
and described in
the SI section.
■
RESULTS
SCPs were first consistently detected in 1869 ± 96 in Moat
Lake, 1842 ± 33 in Pear Lake, and 1932 ± 10 in Emerald Lake
Environmental Science & Technology Article
dx.doi.org/10.1021/es500934s | Environ. Sci. Technol. 2014, 48, 10054−1006110055
(Figure 1). In Moat Lake a few SCPs were detectable prior to
industrial fossil fuel combustion. These SCPs may be
contamination-derived, although the core chronology suggests
limited contamination overtime. Following initial detection,
SCPs show an increasing trend at all three sites until maximum
SCP concentrations were reached in 1964 ± 6 at Emerald Lake
(1,500 gDM
−1
, 90% CI [910, 2100]), 1972 ± 9 at Pear Lake
(1,800 gDM
−1
, 90% CI[1200, 2400]), and 1988 ± 4 at Moat
Lake (2,400 gDM
−1
, 90% CI [1700, 3100]). A decreasing trend
in SCP concentrations was then observed through ca. 2000,
with the exception of Emerald Lake where SCPs decrease but
then increase again after 1999 ± 2. SCP concentrations
measured in surface sediments are 560 gDM
−1
, 90% CI [240,
870] in Pear, 590 gDM
−1
, 90% CI [260, 900] in Moat, and 860
gDM
−1
, 90% CI [480,1200] in Emerald.
Moat Lake BSi ranged from 175.1 to 227.9 mg g
−1
from 1834
± 277 to 2004 ± 3 (Figure 2). We observed an initial increase
between 1920 and 1930 and a second increase between 1980
and 2000. Sediment δ
13
C ranged from −22.3 to −21.4‰ from
ca. 1830 to ca. 1900 with no notable trend (Figure 2). We
observed an increasing trend in δ
13
C between 1900 and 2000
where δ
13
C ranged from −21.9 to −19.2‰. By 1990, δ
13
C had
increased from the most depleted value of −22.2‰ in 1903 ±
36 to −20.20‰. In the analysis we focus on δ
13
C trends from
before 1990 as sediment diagenesis complicates interpretation
of the most recent 10 years of sediment accumulation.
35
The PCA identified three principal components, which
together account for about 80% of the variance (Figure 3 and
Table S1). The first axis was positively correlated with air
quality indices (NO
x
and SO
2
emissions) and temperature and
negatively correlated with ANC and precipitation indices. The
second axis positively correlated with phytoplankton produc-
tivity indices and negatively correlated with SCP and SO
2
emissions. The third axis was posit ively correlated with
precipitation indices.
The PCA and Pearson correlation demonstrated negative
correlations between ANC and SCPs (p = 0.031) and ANC and
SO
2
emissions (p = 0.009) (Figure 3 and Table S2). Relatively
low levels of SCPs were observed in Moat Lake prior to any
notable change in ANC (Figure 1). A decreasing trend in ANC
began after 1920 and continued until the late 1970s. The ANC
decrease coincides with increasing concentrations of SCPs
between 1920 and 1980 and rising SO
2
emissions between
1920 and 1970. ANC in Moat switches to an increasing trend
within the same decade that SCPs and SO
2
emissions switch to
a decreasing trend. Pear and Emerald SCPs showed similar
declines between 1970 and 2000. Moat ANC patterns correlate
less with NO
x
emissions (p = 0.185), although the time series
suggests a stronger correlation after 1950 (Figure 4).
Figure 1. a) Moat Lake SCP concentrations and reconstructed ANC, b) Pear Lake SCP concentrations, and c) Emerald Lake SCP concentrations.
Figure 2. BSi and δ
13
C profiles for Moat Lake.
Environmental Science & Technology Article
dx.doi.org/10.1021/es500934s | Environ. Sci. Technol. 2014, 48, 10054−1006110056
The PCA and Pearson correlation indicate no direct
correlation between Moat Lake ANC and the climate variables
considered (Figure 3 and Table S2). Sierra Nevada air
temperature records demonstrate a 20th century warming
trend of 0.1 °C decade
−1
(p < 0.0001) (Figure 4). The ca. 1920
decrease in ANC coincided with a slight warming trend
observed during the same period. However, temperature
continued to rise through 2010, whereas the ANC trend in
Moat Lake reversed after 1980. There were no statistically
significant trends in precipitation (p = 0.4129) or SWE
(Donner Summit: p = 0.257;Virginia Lakes: p = 0.4129), nor
were these variables correlated with ANC. Temperature was
more closely correlated with productivity (δ
13
C p = 0.007) and
NO
x
emissions (p = 0.021). Temperature, δ
13
C, and BSi
increased throughout the 20th century. Temperature and NO
x
emissions both increased throughout the 20th century but
diverge after 2000 when NO
x
begins to decline as temperature
continues to increase.
Emerald Lake sulfate significantly declined over the 30 year
monitoring period (Sen slope 95% CI: −0.080 to −0.053; p <
0.0001) as did annual sulfate deposition concentration (Sen
slope 95% CI: −0.273 to −0.088; p = 0001) and loading (Sen
slope 95% CI: −0.077 to −0.003; p = 0.0338) (Figure S2).
Emerald Lake sulfate concentrations were positively correlated
with deposition concentrations (tau = 0 .519; p < 0.0001) and
loading (tau = 0.262; p-value = 0.0470). The residuals from the
Emerald Lake SWE-ANC linear regression demonstrate a
concurrent increasing trend in ANC between 1983 and 2011
(slope = 0.139 μeq L
−1
yr
−1
; p = 0.006) (Figure 5). The 95%
confidence interval for the slope fell between 0.042 and 0.236
further supporting a nonzero change in the residuals through
time. Over the 28 year record, ANC increased approximately 4
μeq L
−1
. MLR results confirm that Year (p = 0.005) and SWE
(p < 0.001) are both significant predictors of ANC (R
2
= 0.89)
and the coefficient for Year is positive:
=− + −
A
NC 245 [(0.142)(Year)] [(0.00814)(SWE)]
(1)
The variable inflation factor for the MLR was 1.002 indicating
no significant covariance between SWE and YEAR. We expect
SWE to be a significant predictor as it controls ANC
concentrations through dilution when the snowpack melts.
The significance of YEAR as a copredictor suggests an ANC
trend overtime. The residuals from the SWE-BC linear
regression indicate no significant trend in base cations between
1983 and 2011 (p-value = 0.16 and 95% CI slope: −005 to
0.29) (Figure S3).
Mean nitrate and sulfate deposition from 1985−1999 at
Emerald Lake was 99 ± 17 eq ha
−1
yr
−1
and 52 ± 15 eq ha
−1
yr
−1
, respectively, and the sum of acid anions was 150 ± 30 eq
ha
−1
yr
−1
(Table S3). The resulting hindcast model equation
used to compute historical nitrate deposition was
=×
−
Nitrate Deposition (8 10 )(e )
19 0.0232Year
(2)
Hindcasted sulfate deposition was modeled to match US
emissions records. We estimate that annual deposition varied
from 13 to 76 eq ha
−1
yr
−1
over the 20th century (Figure S4).
The CL was defined as the rate of nitrate and sulfate
deposition during 1920−1930 as estimated from hindcasting
Figure 3. PCA plot for Moat Lake sediment, emissions, and climate
variables.
Figure 4. Time series graphs for a) Sierra Nevada mean annual air
temperature, b) Sierra Nevada mean annual precipitation, c) April 1st
SWE at Donner Summit (DNS) and Virginia Lakes (VGL), d)
national NO
x
and SO
2
emissions, and e) diatom reconstructed ANC at
Moat Lake. The slopes are reported as the 95% CI range.
Environmental Science & Technology Article
dx.doi.org/10.1021/es500934s | Environ. Sci. Technol. 2014, 48, 10054−1006110057