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Long-term trends of Great Lakes major ion chemistry
Steven C. Chapra
a
, Alice Dove
b,
⁎, Glenn J. Warren
c
a
Civil and Environmental Engineering Department, Tufts University, Medford, MA 02155, USA
b
Water Quality Monitoring and Surveillance, Environment Canada, Burlington, ON, Canada L7R 4A6
c
U.S. EPA Great Lakes National Program Office, 77 W. Jackson Blvd., Chicago, IL 60604, USA
abstractarticle info
Article history:
Received 23 February 2012
Accepted 18 June 2012
Available online 1 August 2012
Communicated by Gerald Matisoff
Index words:
Calcium
Sodium
Chloride
Sulfate
Conductivity
Alkalinity
Data from U.S. and Canadian federal monitoring programs are compiled to assess long-term trends of major
ions in each of the Laurentian Great Lakes. Time series are developed for the primary cations and anions as
well as for specific conductance and alkalinity. When combined with historical estimates, these modern
datasets provide a 150-year overview of each lake's chemical makeup. Because of their long residence
times, lakes Superior, Michigan and Huron exhibit persistent increases in most ions. For lakes Erie and Ontar-
io, several ions (chloride, sodium, calcium and sulfate), as well as specific conductance, reached peak levels
between 1965 and 1975, but then began to decline. The decreases are attributable to different mechanisms:
industrial point discharge reductions (sodium, chloride), atmospheric loading declines (sulfate), and the in-
troduction of exotic dreissenid mussels (calcium). Recent data indicate that these ions are now increasing
again (chloride, sodium) or have leveled off (calcium, sulfate). The results establish how much the chemical
makeup of the Great Lakes has changed due to anthropogenic influences, and underscore the importance of
long-term, systematic, water-quality monitoring.
© 2012 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.
Introduction
Over 40 years ago, Alfred Beeton documented a variety of significant
chemical and biological changes that had occurred in the Laurentian
Great Lakes during the preceding century (Beeton, 1965). Aside from
its scientific merit, this analysis was instrumental in raising public
awareness of the urgent need to improve environmental conditions in
the Great Lakes. The resulting political pressure culminated in a massive
remediation program aimed primarily at reducing pollutant discharges
from municipal and industrial sources (IJC, 1978).
An important feature of Beeton's analysis involved documenting
the progressive increase in total dissolved solids as well as the con-
centrations of several major ions. Although they did not pose a
water-quality threat in their own right, Beeton recognized that salts
could serve as a general indicator of anthropogenic impacts on the
system. This insight was critical because adequate long-term time se-
ries for the parameters of interest (e.g., phosphorus) had not yet been
compiled in the early 1960s.
By the late 1960s and early 1970s, concern over eutrophication, es-
pecially inthe lower Great Lakes (lakes Erieand Ontario) led to concert-
ed efforts to reduce phosphorus discharges, to the development of
phosphorus loading targets and to the signing of the U.S.–Canada
Great Lakes Water Quality Agreement (GLWQA) in 1972. In particular,
the reduction of phosphorus in laundry detergents and upgrades to
wastewater treatmentplants lead to great improvements in water qual-
ity. During the 1970s and 1980s, total phosphorus concentrations and
primary productivity were reduced considerably, particularly in lakes
Michigan (Barbiero et al., 2002) and Ontario (Johengen et al., 1994;
Stevens and Neilson, 1987). As we will subsequently discuss, these
same pollution controls likely resulted in reduced concentrations of
some major ions in the Great Lakes at the same time.
Starting in about 1989, the introduction and subsequent widespread
expansion of the invasive zebra (Dreissena polymorpha) and quagga
(Dreissena bugensis) to each of the Great Lakes, with the exception of
Lake Superior, have again profoundly altered the lakes (Higgins and
Vander Zanden, 2010). Together with other factors, dreissenid mussels
have been implicated in affecting the cycling of nutrients (Hecky et al.,
2004), exacerbating the growth of nearshore nuisance cladophora
(Auer et al., 2010; Ozersky et al., 2009), accelerating oligotrophication
in lakes Michigan (Evans et al., 2010; Fahnenstiel et al., 2010b; Mida
et al., 2010), as well as Huron (Evans et al., 2010) and likely Ontario
(Dove, 2009), altering lower food webs (Fahnenstiel et al., 2010a;
Mills et al., 2003) and lowering prey fish abundances (Environment
Canada, U.S. Environmental Protection Agency, 2012). Dreissenids
have also been implicated in reducing calcium levels and hence the ten-
dency for whiting events in the lower Great Lakes (Barbiero and
Tuchman, 2004; Barbiero et al., 2006), but possible impacts of dreissenids
on other ions has not been documented. Indeed, a systematic review of
majorionstrendshasnotbeenconductedsinceBeeton's (1965) analysis.
Journal of Great Lakes Research 38 (2012) 550–560
⁎Corresponding author. Tel.: +1 905 336 4449.
E-mail address: Alice.Dove@ec.gc.ca (A. Dove).
0380-1330/$ –see front matter © 2012 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jglr.2012.06.010
Contents lists available at SciVerse ScienceDirect
Journal of Great Lakes Research
journal homepage: www.elsevier.com/locate/jglr
Dove (2009) describes long term trends for several major ions for Lake
Ontario, and chloride was recently used as a conservative tracer in the
Great Lakes to describe long-term trends and elucidate loadings
(Chapra et al., 2009). More recently, Winter et al. (2011) examined
30-year trends in Lake Ontario nearshore water quality measured at
water treatment plant intakes. However, the only major ion discussed
was chloride and the results agreed with those of Chapra et al. (2009).
This is the first comprehensive compilation of major ion chemis-
try for the Great Lakes in over 40 years. This paper builds on the pre-
vious work of Chapra et al. (2009),withanexpandedscopeto
include the other ions that contribute significantly to the lakes'
macrochemistry. The current study employs more recent observa-
tions to extend Beeton's (1965) historical trends to the present.
The major nutrients nitrogen, phosphorus and silica are not included
here; these will instead be the focus of a separate analysis.
Beyond their utility asindicators of human impacts, there are several
other reasons why it is important to understand and quantify the major
ion chemistry of lakes and other freshwater ecosystems. In the broadest
sense, the trends of major anions and cations are relevant as they gen-
erally constitute the lake's background chemical environment or
“macrochemistry”. In this sense, they can be ofsignificance individually
as well as collectively via their effects on ionic strength, pH and acid
neutralizing capacity. For example, pH can affect metal speciation,
which in turn can strongly influence toxicity (e.g., Campbell and
Stokes, 1985; Di Toro et al., 2001), and invasion of exotic species will
occur only if the chemical environment is suitable (e.g., Mellina and
Rasmussen, 1994; Strayer, 1991). Sufficient calcium in lakes, together
with warm temperatures and photosynthetically-induced pH rises,
can lead to calcite precipitation in summer, surface waters (Kelts and
Hsü, 1978). Such “whitings”, which have been observed in the Great
Lakes (Strong and Eadie, 1978), have a profound impact on water qual-
ity as they diminish water clarity (Weidemann et al., 1985). In addition,
calcite precipitation can influence eutrophication via its impact on light
penetration (Homa and Chapra, 2011) as well as nutrient scavenging
(Murphy et al., 1983; Otsuki and Wetzel, 1972).
The observed trends of major ion concentrations will necessarily
reflect to a great degree the underlying hydrology of the system and
the biogeochemistry of each lake's watershed. The physical charac-
teristics of the Great Lakes are provided in Table 1.Asrecommended
by Quinn (1992), the lake volumes correspond to average rather
than base surface elevations. The upper Great Lakes (lakes Superior,
Michigan and Huron) are characterized by larger drainage areas,
greater lake volumes and surface areas, and longer residence times
compared with the lower Great Lakes (lakes Erie and Ontario).
These characteristics have a strong influence on how each lake re-
sponds to inputs of major ions.
Methods
Along with the parameters originally reported by Beeton (TDS,
calcium, chloride, sulfate, sodium/potassium), we also include mag-
nesium. While Beeton reported sodium and potassium collectively,
we have compiled independent measurements of each parameter.
Beyond these individual ionic species, we have also included two ag-
gregate parameters: specific conductance and alkalinity.
The data can be divided into two categories: historical (prior to
the mid-1960s) and modern (after the mid-1960s). Historical data
are taken primarily from Beeton (1965) with additional measure-
ments drawn from a variety of other sources (Bartone and Schelske,
1982; O'Connor and Mueller, 1970; Schelske, 1985a,b; Schindler,
1988; ULRG, 1977a,b; Weiler, 1978, 1981).
Beginning in the late 1960s, much more systematic data collection
efforts were instituted under the auspices of Environment Canada
(EC) and, in 1983, by the U.S. Environmental Protection Agency's
Great Lakes National Program Office (GLNPO). Because they involved
more rigorous quality control, these “modern”data sets exhibit con-
siderably less uncertainty than the historical data. Hence, they pro-
vide a better basis for separating long-term, emerging trends from
interannual natural variability.
Both EC and GLNPO conduct ship-based cruises to collect water
quality samples on the Great Lakes. Methods for EC's Great Lakes Sur-
veillance Program are described in Dove et al. (2009). Sampling and
analytical procedures for GLNPO's Open Lake Water Quality Surveys
are provided in GLNPO (2010). Briefly, EC conducts monitoring in
each of the Great Lakes except Lake Michigan, which is located entire-
ly within the United States. Each lake is generally monitored every
second year, with multiple cruises conducted during that year. All re-
gions (nearshore, offshore and major embayments) are monitored for
the EC program. GLNPO conducts one spring and one summer cruise
on all waters except Georgian Bay, with stations located more along
the central long axis of each lake.
Open-lake, spring, surface water cruise-median values from the
modern programs are presented to extend the historical trends to the
modern period. Open-lake (offshore) waters best indicate long-term
trends because, in contrast to shallower nearshore waters, they are less
influenced by local pollutant discharges. In Lake Superior, open-lake sta-
tions comprise those with depths of at least 150 m; in lakes Michigan,
Huron and Ontario, the cutoff depths are 60, 50 and 100 m, respectively.
Beyond omitting near-shore zones, the major embayments (i.e., Geor-
gian Bay, Green Bay, Saginaw Bay and Lake Huron's North Channel)
are excluded from analysis here.
Because its offshore waters exhibit a persistent west-to-east gradi-
ent, a somewhat different approach is adopted for Lake Erie. As we are
examining dissolved ions that are for the most part conservative, the
eastern basin concentration represents the best indicator of the inte-
grated effect of the entire lake's loadings. Consequently, the analysis is
limited, in both the GLNPO and EC data sets, to deep-water (>30 m)
stations in the lake's eastern basin.
Restricting the modern data to spring, unstratified periods (April
and early May) is done to minimize the impact of biological activity
on the levels of constituents. In particular, biotic effects can reduce
calcium from the epilimnion during the summer stratified period. It
should be noted that although the trend analysis focuses on spring
conditions, the summer data are also used to assess biotic effects on
constituents such as calcium by comparing winter/early spring con-
centrations with summer surface concentrations.
Table 1
Physical characteristics of the Great Lakes. The residence times are calculated as the ratio of volume to outflow.
Basin Elevation
(m)
Drainage area
(km
2
)
Surface area
(km
2
)
Volume
(km
3
)
Mean depth
(m)
Maximum depth
(m)
Outflow
(km
3
/yr)
Residence time
(yr)
Superior 183 127,687 82,103 12,115 147.6 406 67.4 179.8
Michigan 176 115,804 59,600 4947 83.0 282 44.9
a
110.2
Huron 176 131,313 59,750 3567 59.7 229 167.2 21.3
Erie 173 62,263 25,220 499 19.8 64 187.7 2.7
Ontario 74 64,030 18,960 1651 87.1 244 220.3 7.5
a
Includes the outflow via both Straits of Mackinac and Chicago Diversion.
551S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
For each ion, yearly medians are determined separately for both
EC and GLNPO data. The resulting values are then plotted versus
time and linear regression is used to generate a best fit line for each
ion for each lake,
ct
ðÞ¼c1970
ðÞ
þrate t–1970
ðÞ ð1Þ
where c(t) = concentration (mg L
−1
) at time t(yr), and rate = linear
rate of change (mg L
−1
yr
−1
). Although some modern data were col-
lected earlier, 1970 is chosen as the base year (i.e., the intercept) of
Eq. (1) as the best reference point prior to the implementation of
major water-quality controls and the approximate onset of systemat-
ic water quality monitoring by the Canadian federal government.
In addition to the trends of the individual ions, charge balances
were computed for the beginning and the end of the modern data re-
cord. Because not all lakes were monitored in all years, the best fit
lines were extended to 1970 and 2009, where necessary.
In order to place the recent data in a longer context, the historical and
modern data sets for some of the major ions (chloride, sodium, calcium
and sulfate) are also plotted together. Laboratory methods have im-
proved over time; in general, colorimetric and nephelometric methods
were replaced by atomic absorption methods, and these have in turn
been replaced by ion chromatography. Method comparisons have been
conducted as laboratory methods changed, interagency comparisons
have been conducted (e.g., Esterby and Bertram, 1993), and there is
greater confidence in the accuracy of more recent measurements.
Wherever possible, best-fit lines are superimposed to aid visuali-
zation of the underlying long-term trends. Because of their high un-
certainty, the fits of the historical data rely heavily on the original
trend lines drawn by Beeton (1965). These historic lines are sampled
at ten-year intervals and then combined with the more recent EC and
GLNPO data to create long-term trend lines with linear fits or smooth-
ing splines (de Boor, 2001, 2008).
To more definitively establish the Lake Erie calcium trend, we
also compiled data collected by EC as part of the Niagara River
Upstream-Downstream Program (Niagara River Secretariat, 2007).
At Niagara-on-the-Lake, sampling has been conducted approxi-
mately biweekly since 1975, providing a consistent, long-term,
year-round and high-frequency data set that is well correlated
with the eastern Lake Erie measurements.
Historical total dissolved solids (TDS) data have been compiled
(Beeton, 1965; Weiler, 1978), but measurements for specific conduc-
tance and alkalinity are not available for the same period. Further, di-
rect TDS measurements based on gravimetric methods are no longer
routinely made by either EC or GLNPO. In the absence of direct mea-
surements, calculations based on the individual constituents provide
a means to estimate long-term trends of such aggregate variables.
We do not sum the concentrations of the individual dissolved con-
stituents to estimate dissolved solids concentrations because a portion
of the bicarbonate ion is volatilized as carbon dioxide during drying
(Hem, 1985). Instead, we relate specific conductance to dissolved solids
for the period when both measures were available, and use specificcon-
ductance to estimate dissolved solids when direct measurements are
not available. Beeton's (1965) historical TDS trends are therefore ex-
tended linearly to 1965 when the modern data set provides measure-
ments of specific conductance. The average ratio of (historical) TDS to
(modern) specific conductance is calculated for each lake (Table 2).
Results
Modern data
The statistics for the linear fits are summarized in Table 3 and classi-
fied based on P-values for the slope (i.e., rate in Eq. (1)) as strong
(b1×10
−4
), moderate (1× 10
−4
to 0.05), or weak(>0.05). This classi-
fication is also depicted in the accompanying graphs using heavy solid
(strong fits), dashed (moderate), and dotted (weak) lines. Note that be-
cause they are clearly curvilinear, the chloride and sodium trends for
the lower lakes are represented graphically with smoothing splines
(de Boor, 2001, 2008).
Chloride and sodium
The modern chloride (Fig. 1a) and sodium (Fig. 1b) measure-
ments for lakes Michigan and Huron indicate strong upward trends
whereas Lake Superior manifests a moderately significant upward
trend. Starting in the early 1970s, first Lake Erie and then Lake On-
tario showed dramatic decreases in chloride and sodium concentra-
tions. However, they are now increasing again, albeit at a somewhat
slower rate. The 6-to-10-year lag in Lake Ontario's response relative
to Lake Erie is consistent with Lake Ontario's 7.5 year residence
time and suggests that a significant component of the Lake Ontario
trendisinducedbytheLakeEriesignal.Aswouldbeexpectedfor
non-reactive ions, concentrations increase with distance down-
stream in the Great Lakes basin.
Calcium and sulfate
The trends of calcium are depicted in Fig. 2a. There is a strong but
gradual positive trend for Lake Superior. Otherwise the calcium con-
centrations in the upper lakes are relatively constant, with Lake
Huron values intermediate between lakes Superior and Michigan. In
contrast, the levels in both Lake Erie and Lake Ontario have dropped
significantly over the past 40 years, so that the calcium concentration
in Lake Michigan now exceeds that in lakes Erie and Ontario. Al-
though the Lake Erie data exhibit considerable variability, there is a
suggestion that the calcium decline accelerated in the early 1990s,
with minimum values reached in the mid-1990s. The acceleration
may be connected with the dreissenid mussel invasion in the early
1990s as discussed later in this paper. For Lake Ontario, after peaking
in the early 1970s, the rate of decrease has been more constant, with a
minimum reached in about 2000 and little change since that time.
As depicted in Fig. 2b, the trends of sulfate are qualitatively similar
to those for calcium; that is, increases in the upper lakes and de-
creases in the lower lakes. Although a statistically significant trend
is not evident for Lake Huron, linearly increasing trends are evident
for lakes Superior (strong) and Michigan (moderate). Unfortunately,
there are no sulfate data for Lake Michigan since 1992, and so it is
not known if the linear trend observed between 1976 and 1992 con-
tinues to the present. In contrast, the levels in both Lake Erie (moder-
ate) and Lake Ontario (strong) have declined over the past 40 years.
We will explore these trends in more detail when we integrate the
modern sulfate data with historical data later in this paper.
Magnesium and potassium
The modern data for magnesium and potassium are displayed in
Figs. 3a and b, respectively. Lake Superior exhibits the lowest concentra-
tions. The highest magnesium levels occur in Lake Michigan, whereas
potassium concentrations rise as the waters move through lakes Erie
Table 2
TDS and specific conductances for the Great Lakes. The ratio is used to convert modern
specific conductance data to TDS so that both measures can be displayed on the same
plot (Fig. 6).
The TDS for 1965 comes from Beeton (1965).
Lake TDS
(mg L
−1
)
Estimated spec
cond circa
1965
(μScm
−1
)
Ratio
(mg L
−1
)
(μScm
−1
)
−1
Measured
spec cond
circa 2009
(μScm
−1
)
Estimated
TDS circa
2009
(mg L
−1
)
Superior 56 97 0.58 102.3 59
Michigan 154 269 0.57 297.0 169
Huron 114 203 0.56 216.2 121
Erie 195 315 0.62 272.6 169
Ontario 199 342 0.58 305.3 177
552 S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
and Ontario. Although the magnesium trend is not statistically evident
for Lake Michigan and the potassium trend is weak for Lake Superior,
both ions appear to be rising in all lakes, with the strongest linear in-
creases observed for magnesium in the lower lakes. The potassium
data exhibit greater within-lake variability than the other ions exam-
ined. Similar to sulfate, there are no potassium data since 1992 for
Lake Michigan, so it is not known if the high rate of increase suggested
by the earlier data continues to the present.
Specific conductance and alkalinity
Reflecting the integrated effect of the dynamics of the major ions,
the plot of specific conductance (Fig. 4a) indicates no discernable
trend for Lake Superior, a moderate upward trend for Lake Huron,
and a strong upward trend for Lake Michigan. In contrast, lakes Erie
and Ontario exhibit strong decreasing trends. In addition, it appears
that Lake Erie's conductance leveled off in the mid-1990s and may
now be increasing again.
The alkalinity of the upper lakes has not changed measurably dur-
ing the modern period (Fig. 4b), while the long-term trends in the
lower lakes appear to be decreasing. Alkalinity is lowest in Lake Supe-
rior, and highest in Lake Michigan, and roughly equivalent in lakes
Erie and Ontario. The data suggest that Lake Erie alkalinity increased
until 1990, dropped sharply at that time and rebounded again to
the present, although current values remain lower than the maxi-
mum observed during the 1980s. The Lake Ontario data appear to
mimic Lake Erie data, with some dampening of the variations as
expected because of its longer residence time.
Charge balance
The charge balances for 1970 and 2009 are summarized in Table 4.
All balances are well within the criterion of ±0.2 meq/L for anion
sums in the range 0–3 meq/L established by APHA (2012). Aside
from providing a quality assurance check for the chemical analyses,
the charge balances also document the changes that have occurred
over the past 40 years. In particular, the results generally indicate
how salts have increased in the upper Great Lakes whereas the salt
content of the lower lakes have generally decreased. For the latter,
the ion balance indicates that the drop is attributable to the combined
impact of input reductions (for sodium, chloride and sulfate) and the
introduction of dreissenid mussel filtration and primary production
reductions (for calcium).
Table 3
Summary statistics for linear fits of Great Lakes chemical data from 1965 to 2009. Strong fits are indicated by bold-face and shading (P-valueb1×10
−4
), and moderate fits by
bold-face (b0.05). Note that the statistic s
y/x
is the standard error of the estimate (i.e., the root-mean-squared error of the residuals, adjusted for the two estimated coefficients).
Lake Statistic
Ca
(mg L–1)
Mg
(mg L–1)
Na
(mg L–1)
K
(mg L–1)
Cl
(mg L–1)
SO4
(mgSO4L–1)
Spec. conduct.
(µS cm–1)
Alkalinity
(mgCaCO3L–1)
Superior n22 21 21 19 34 20 33 33
c(1970) 12.98 2.62 1.25 0.484 1.17 2.61 97.3 41.1
c(2009 ) 13.62 2.83 1.44 0.505 1.42 3.85 102.3 41.9
rate 0.0162 0.0053 0.0048 0.00053 0.0064 0.0319 0.128 0.020
r20.680 0.445 0.736 0.064 0.353 0.671 0.104 0.051
sy/x0.149 0.081 0.040 0.02 70 0.106 0.290 4.66 1.00
P-value 2.3×10−69.6×10−46.5×10−70.294 2.1×10−41.0×10−50.068 0.207
Michigan n16 15 16 13 25 13 25 25
c(1970) 35.13 11.09 4.45 1.096 6.53 20.46 2 72.18 110.4
c(2009) 35.95 11.28 6.26 1.406 12.05 24.01 296.99 107.9
rate 0.0209 0.0048 0.0464 0.00795 0.1414 0.0910 0.6363
r20.017 0.015 0.703 0.389 0.963 0.680 0.816 0.093
0.062
sy/x1.344 0.319 0.255 0.0525 0.207 0.306 2.271 1.46
P -value 0.630 0.658 4.9×10−50.023 5.2×10−18 5.2×10−46.3×10−10 0.138
Huron n32 31 31 29 43 29 42 44
c(1970) 26.45 6.84 2.99 0.809 4.99 15.60 204.8 77.0
c(2009 ) 26.40 7.46 3.86 0.939 6.58 15.83 215.9 78.5
rate 0.0158 0.0222 0.00334 0.0408 0.0062 0.284 0.039
r20.000
0.0014
0.311 0.632 0.293 0.812 0.024 0.136 0.048
sy/x0.955 0.237 0.170 0.0500 0.207 0.379 7.73 1.84
P-value 0.937 1.1×10−39.1×10−82.4×10−31.8×10−16 0.423 0.016 0.154
East Erie n26 26 26 24 36 24 40 35
c(1970) 37.52 7.75 9.56 1.235 20.64 24.80 309.22 94.7
c(2009 ) 32.11 8.89 8.58 1.431 14.58 22.81 274.06 88.9
rate 0.0292 0.00503
r20.560 0.525 0.081
0.0250
0.296 0.340
0.1553
0.322
0.0511
0.467
0.9018
0.259
0.149
sy/x1.465
0.1389
0.331 1.006 0.0909 2.494 0.878 13.329 3.01
P -value 1.1×10−52.9×10−50.160 6.0×10−31.9×10−43.8×10−31.2×10−61.8×10−3
Ontario n36 36 36 34 48 35 52 47
c(1970) 42.03 7.91 12.74 1.407 28.30 28.58 337.82 95.96
c(2009 ) 33.55 8.61 11.56 1.501 19.56 25.54 305.32 90.08
rate 0.0179 0.00241
r20.893
0.2173
0.568 0.302
0.0305
0.151 0.763
0.2243
0.434
0.0781
0.430
0.8334
0.301
0.1507
sy/x0.741 0.153 0.456 0.0535 1.314 0.826 11.450 2.371
6.5×10−5
P -value 4.6×10−18 1.1×10−75.1×10−40.023 5.3×10−16 1.7×10−51.3×10−7
553S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
Historical closure
The historical data can be plotted together with the modern data
for four of the major ions (Fig. 5). There is no major discontinuity
for any ion at the late 1960s junction, indicating consistency between
the historic and the modern data sets.
Put into context with the historical data, the rises of most ions ap-
pear more pronounced than the modern data indicate alone. Histori-
cal values were highest in the lower lakes, although calcium was
perhaps historically highest in Lake Michigan. The 1910 values of
chloride and sodium in Lake Michigan are suggestive of some possible
anthropogenic impact at that time.
The patterns for chloride (Fig. 5a) and sodium (Fig. 5b) show that
concentrations in Lake Superior have been relatively constant over
the longer term whereas lakes Michigan and Huron show persistent
and significant upward trends. The lower lakes exhibit exponential
increases up to about 1970 and subsequent declines to minimum
concentrations in the 1990s. Since the turn of the current century,
concentrations are rising again.
There is strong evidence of slow but significant increases in calcium
(Fig. 5c) and sulfate (Fig. 5d) in the upper Great Lakes, with Lake Huron
values again intermediate between lakes Superior and Michigan. In the
lower lakes, the trends are curvilinear. While not as definitive as for
chloride and sodium, the data suggest increases in both lower lakes to
peak values in the 1960s and 1970s, followed by gradual declines. The
most recent information suggests that concentrations of both ions
may be increasing again.
Historic values of specific conductance, estimated from TDS mea-
surements as described previously, are shown along with the modern
data in Fig. 6. The Lake Superior data suggest no change over time,
with a weak upward trend implied in the modern data. In contrast,
conductance in lakes Michigan and Huron has risen measurably
since the late nineteenth century. Lake Michigan exhibits a steady in-
crease of about 0.64 μScm
−1
yr
−1
whereas Lake Huron is also rising,
(a) (b)
Fig. 1. Modern trends of (a) chloride and (b) sodium concentration for the Great Lakes.
(a) (b)
Fig. 2. Modern trends of (a) calcium and (b) sulfate concentration for the Great Lakes.
554 S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
but at a slower rate of approximately 0.28 μScm
−1
yr
−1
. Both lower
lakes manifest the N-shaped pattern of increase, decrease and
rebound.
Discussion
The number of people inhabiting the Great Lakes watersheds has
greatly increased over time, from approximately 10.5 million people
in the early 1900s to approximately 54 million in 2000. Growing con-
cern and mounting evidence of environmental damage to the lakes
gave rise in the 1960s and 1970s to major pollution control initiatives,
embodied by the signing of the bi-national (U.S.–Canada) Great Lakes
Water Quality Agreement in 1972 and its amendment in 1978.
Large-scale pollution controls were implemented in both countries,
targeted mainly toward industrial processes and sewage treatment.
The resulting decrease of pollutant loading to the Great Lakes was
perhaps most famously responsible for the dramatic decline in total
phosphorus observed during the 1970s and 1980s (Chapra, 1980;
Lesht et al., 1991; Stevens and Neilson, 1987).
The major ion loadings to the Great Lakes were doubtlessly dimin-
ished by these pollution controls. In this way, the ion loadings likely de-
clined during the 1970s and 1980s. Recent population growth and
human development in the lower lakes' watersheds, and continuing ad-
justment of the lower Great Lakes to historic loads to the upper Great
Lakes, account in large part for the rebounds that have occurred begin-
ning in the early 2000s. In addition, as described next, these trends have
also been influenced by other factors such as air-pollution control and
the introduction of invasive species.
Chloride and sodium
The rising trends of chloride in the upper Great Lakes are primarily
the result of increases of industrial discharges and road salt runoff
that started in the early twentieth century (O'Connor and Mueller,
1970). Although major industrial load reductions occurred between
1965 and 1980, their impact on the chloride concentrations in the
upper Great Lakes is difficult to perceive because of their slow re-
sponse times for conservative constituents (Chapra et al., 2009). The
(a) (b)
Fig. 3. Modern trends of (a) magnesium and (b) potassium concentration for the Great Lakes.
(a) Specific conductance (b) Alkalinity
Fig. 4. Modern trends of (a) specific conductance and (b) alkalinity for the Great Lakes.
555S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
industrial reductions have in fact slowed the rate of increase in each
of these lakes. However, such improvements are confounded by
their continuing adjustment to pre-1970 increases in non-industrial
chloride inputs such as road salt. The net result is that despite signifi-
cant industrial load reductions, the concentrations in these lakes should
continue to rise for decades into the future (Chapra et al., 2009).
The characteristic “N-shaped”pattern of increase, decrease, and
rebound for chloride that is seen for the lower Great Lakes can be as-
cribed to the pattern of century-long deterioration of water-quality
followed by improvements due to the post-1970 load reductions.
The current increases are attributable to the system's continued ad-
justment to the lagged response of the upper lakes, uncontrolled dif-
fuse sources, and possibly to new point sources (Chapra et al., 2009).
The modern trends for sodium are strikingly similar to those for
chloride. The correspondence between these ions suggests that much
of their dynamics are driven by loadings of their salt, sodium chloride.
This hypothesis is reinforced by the fact that some of the major anthro-
pogenic salt inputs to the system (e.g., road salt) are predominantly in
the form of sodium chloride (Environment Canada, Health Canada,
2001). Due to the high correspondence between road salt usage and
human population (Morin and Perchanok, 2003) and the projected
population increase in the Great Lakes basin (Manson, 2005), road salt
usage will likely continue to increase, and the trends of sodium and
chloride are likely to continue to reflect each other well into the future.
Further insight can be developed by examining the molar ratio of
chloride to sodium, which should be unity if sodium chloride is the
predominant source of both ions. As indicated by Fig. 7, the actual sit-
uation is a bit more complex. Lake Superior differs from the other
lakes in having a Cl/Na ratio that is much lower than 1. This is not un-
expected as Lake Superior has been the least impacted by human de-
velopment. Hence, its levels should be primarily dictated by diffuse
sources such as direct precipitation and natural runoff.
Based on the data collected at 23 locations around the Great Lakes
(USGS, 2008), the Cl/Na ratio for wet deposition is about 0.76, which is
consistent with global average values for continental (i.e., non-coastal)
precipitation in North America (Berner and Berner, 1987). Hence, pre-
cipitation would tend to lower the Cl/Na ratio below unity. However, be-
cause the actual concentrations of the ions in rainwater are muchlower
than the in-lake concentrations, the influence of precipitation to the
lakes is negligible.
The more likely explanation for Lake Superior's low Cl/Na ratio is
that sodium is a major component of the silicate rocks that comprise
most of its watershed. In contrast, the chloride content of such rocks
is minimal. Hence, the long-term weathering and dissolution of these
silicates should result in greater runoff of sodium relative to chloride.
In contrast to Lake Superior, the other lakes all have Cl/Na ratios
greater than unity, suggesting that they may have significant sources
of chloride in addition to sodium chloride. For example, chloride can
be introduced with other cations as ferric chloride (steel pickling pro-
cess, wastewater treatment), potassium chloride (potash fertilizer),
and calcium chloride (snow and ice control).
Calcium
The reason for the halting of the calcium declines in the lower
Great Lakes, is less straightforward than for sodium and chloride.
Closer inspection of the calcium trends in Lake Erie suggests that
there was a major drop between 1990 and 1995 with levels stabiliz-
ing thereafter. In addition, since 2000 the Lake Ontario calcium levels
also appear to be constant. Although the Lake Erie data exhibit con-
siderable variability, there is a suggestion that the decline accelerated
in the early 1990s following the dreissenid mussel invasion as noted
previously by Barbiero et al. (2006). The post-1995 data appear to
be rising again.
For Lake Ontario, after peaking in the early 1970s, the decrease has
been smoother with a minimum reached in about 2000 with no dis-
cernable upward trend thereafter. The downward trend in Lake On-
tario calcium concentrations has been noted elsewhere (Barbiero et
al., 2006) and has been attributed to uptake by dreissenid mussels.
While the mussels may have had a significant impact, Fig. 2a suggests
that calcium in the lower lakes began decreasing prior to the intro-
duction of the mussels in the late 1980s. In particular, theLake Ontario
data indicate a persistent downward trend starting in the mid-1970s.
This implies that factors other than dreissenids may have also contrib-
uted to the reductions. A reduction in primary production in Lake On-
tario due to the continued reduction in nutrient concentrations and
other factors (as summarized by Mills et al., 2003) may also be reducing
calcite levels in the lake.
Based on the patterns of chloride and sodium, a rise in Ontario's
calcium concentration is anticipated. Using chloride as a conservative
tracer, a lag of 6 to 10 years between lakes Erie and Ontario is expected.
Therefore, we hypothesize that we may later confirm that calcium levels
increased in Lake Ontario starting in the early 2000s.
In order to gain more insight into the Lake Erie calcium trends, we
have supplemented the EC and GLNPO open-lake data with calcium con-
centrations measured by EC in the Niagara River at Niagara-on-the-Lake
(NOTL). As shown in Fig. 8, the Niagara River data are generally consis-
tent with the open-lake measurements. However, because they provide
a more continuous and high-frequency time series, the river concentra-
tions allow us to more definitively identify the onset and duration of the
decline. Thus, the major reduction began between the summer of 1989
and the winter of 1989/90 and persisted until about 1994. Subsequent
data suggest that concentrations may now be increasing slowly. This
Table 4
Charge balances for the Great Lakes for 1970 and 2009 with values in meq/L. Both the difference (Σcations −Σanions) and the error [(Σcations −Σanions) / (Σcations+Σan-
ions)× 100%] are indicated.
Superior Michigan Huron East Erie Ontario
1970 2009 1970 2009 1970 2009 1970 2009 1970 2009
Calcium 0.648 0.680 1.753 1.794 1.320 1.317 1.872 1.602 2.097 1.674
Magnesium 0.216 0.233 0.913 0.928 0.563 0.614 0.638 0.732 0.651 0.708
Sodium 0.054 0.063 0.194 0.272 0.130 0.168 0.416 0.373 0.554 0.503
Potassium 0.012 0.013 0.028 0.036 0.021 0.024 0.032 0.037 0.036 0.038
Σanions 0.930 0.988 2.887 3.030 2.034 2.123 2.958 2.744 3.338 2.924
Chloride 0.033 0.040 0.184 0.340 0.141 0.186 0.582 0.411 0.798 0.552
Sulfate 0.054 0.080 0.426 0.500 0.325 0.330 0.516 0.475 0.595 0.532
Nitrate 0.020 0.027 0.018 0.024 0.018 0.026 0.011 0.021 0.017 0.032
Bicarbonate 0.818 0.834 2.186 2.140 1.527 1.563 1.883 1.758 1.898 1.789
Carbonate 0.003 0.004 0.021 0.018 0.013 0.009 0.011 0.019 0.020 0.012
Σcations 0.929 0.985 2.836 3.022 2.024 2.113 3.003 2.685 3.329 2.917
Difference 0.001 0.003 0.051 0.009 0.010 0.010 −0.046 0.058 0.009 0.007
Error 0.06% 0.15% 0.90% 0.14% 0.25% 0.23% −0.76% 1.08% 0.14% 0.12%
556 S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
represents strong circumstantial evidence of the intense impact of the
mussel invasion on Lake Erie's calcium levels in the early 1990s.
Beyond the concentration trends, the difference between each
year's winter and summer values provides an estimate of calcite pre-
cipitation in the lower lakes. We calculate that the difference for Lake
Erie averaged 1.68 mg L
−1
prior to 1989 and 1.35 mg L
−1
after 1994.
These values can be converted to equivalent calcite concentrations of
4.2 and 3.4 mg CaCO
3
L
−1
, respectively.
Even more dramatic trends are evident for Lake Ontario. Fig. 9,
which shows spring (whole water column) and summer (surface)
values, indicates that the summer concentration (generally measured
in August) is less than the spring value (generally measured in late
March or early April) but that the difference appears to be decreasing
with time. Based on the linear fits in Fig. 9, the spring-summer differ-
ence dropped from about 3.3 mg L
−1
in 1970 to 1.0 mg L
−1
in 2005.
If the difference is attributed to calcite precipitation, these observa-
tions correspond to a reduction of about 5.75 mgCaCO
3
L
−1
. Although
some of the generated calcite would settle out of the epilimnion, the
impact on water clarity would nevertheless be considerable owing to
calcite's propensity to scatter light (Homa and Chapra, 2011; Peng
and Effler, 2005; Weidemann et al., 1985). As suggested previously
by Barbiero et al. (2006) and Dove (2009), this is undoubtedly one
of the reasons for the remarkable increase in water clarity that has
been observed in Lake Ontario over this period.
Sulfate
Sulfate trends reflect the increase in fossil-fuel usage and subse-
quent air-pollution controls (Likens and Bormann, 1974; Schindler,
1988). The upper lakes, with their long residence times, are still equil-
ibrating to the increased loading from many decades ago. The lower
lakes, which are more temporally responsive, are currently showing
improvement due to air-pollution controls (Holland et al., 1999;
Husain et al., 1998; Lehmann et al., 2007; Malm et al., 2002). Some ef-
fect of acid-rain leaching from watersheds may play a role here; as
acid rain worsened and subsequently improved, sulfate in runoff
may have increased and then reduced correspondingly.
The early sulfate record for Lake Michigan is probably suspect; we
believe that Lake Michigan values were likely constant until about
1900, at which time they increased in a fashion similar to that seen
for Lake Ontario. It is not plausible that the steep increase implied by
the historical data for Lake Michigan could have occurred prior to
1900, when fossil-fuel combustion and the population in the Lake
Michigan watershed were still relatively low (Government of Canada,
United States Environmental Protection Agency, 1995).
Although the sulfate trends (Fig. 5d) appear qualitatively similar
to the other major ions, there are some significant differences that
(c) Calcium (mgCa L−1)
−1)(a) Chloride (mgCl L
−1)(b) Sodium (mgNa L
−1)
(d) Sulfate (mgSo4 L
Fig. 5. Long-term trends of major ions for the Great Lakes.
Fig. 6. Long-term trends of TDS/specific conductance for the Great Lakes. The lines
were originally developed by Beeton (1965) to depict the underlying trends of histor-
ical TDS data. Fig. 7. Molar ratios of chloride to sodium for the Great Lakes.
557S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
bear mention. Whereas Lake Superior's sulfate levels have been rising
mildly since the early 1970s (recall Fig. 2b), the levels have risen signif-
icantly in Lake Michigan. Unfortunately, sulfate levels are unavailable
for Lake Michigan after 1992. Hence, it is unclear whether concentra-
tions have continued to rise since that time.
Although not quite as dramatic, Lake Huron also exhibited a con-
siderable increase during the first half of the twentieth century. Be-
cause sulfate measurements in Lake Huron have extended to the
present, Fig. 5d suggests that the trend has leveled off.
The sulfate trends in the lower lakes again suggest a build-up dur-
ing the early twentieth century with peaks in the early 1970s and
subsequent declines into the 1990s. As with calcium, future measure-
ments will bear watching to establish if sulfate levels are now stable
or are beginning to rise.
Magnesium
A large portion of the system's magnesium comes from the erosion
of limestone in the Lake Michigan basin. Hence, the highest levels
occur in that lake. In contrast, Lake Superior exhibits the lowest concen-
trations and, as suggested by historical data (Weiler, 1978), levels have
been relatively constant over the past century. Lake Huron, which re-
ceives comparable levels of inflow from lakes Superior and Michigan,
has an intermediate concentration (~7 mg L
−1
). Due primarily to the
high dolomite content of the bluffs along the north shore of Lake Erie
(Kemp and Dell, 1976; Kemp et al., 1976), lakes Erie and Ontario have
slightly higher levels, on the order of 8 mg L
−1
.
Potassium
Similar to sodium, potassium is a dominant ion in the Earth's crust,
but it is present at lower concentrations in surface waters because it is
readily incorporated into clay minerals. Of all the ions presented here,
the modern potassium record demonstrates the most straightforward
long-term trends, with gradual linear increases in all the lakes, and
higher concentrations downstream in the basin. This implies that
loadings of potassium have not fluctuated as dramatically as other
ions, and that diffuse sources (such as the contribution of KCl as a
constituent of fertilizer and road salt) may comprise the dominant
anthropogenic potassium loads to the system.
TDS and specific conductance
Although Beeton (1965) sketched a slight decrease in TDS for Lake
Superior, he noted that the trend was not significant due to the high un-
certainty of the early data. Based on the entire data record, we concur
with Beeton's (1965) conclusion that the TDS of Lake Superior has not
changed greatly over the past century. However, as noted elsewhere
(Chapra et al., 2009), because of its extremely long residence time
(~180 yr), the apparent lack of a trend should not necessarily be con-
strued as evidence that Lake Superior's dissolved inorganic solids levels
have not been elevated somewhat due to human activities. As for the
other lakes, conductance in lakes Michigan and Huron has risen mea-
surably since the late nineteenth century and lakes Erie and Ontario
manifest the N-shaped pattern of increase, decrease and rebound.
Conclusions
To summarize, Lake Superior shows the lowest concentrations of
many ions. This is to be expected, as it is the Great Lake least impact-
ed by human influences (Government of Canada, United States
Environmental Protection Agency, 1995). However, the long-term
record presented here demonstrates strong upward trends of calci-
um, sodium and sulfate. These findings indicate that anthropogenic
water-quality impacts have indeed occurred. It should be noted
that such increasing trends for Lake Superior have also been ob-
served for non-salts such as nitrogen (Bennett, 1986; McDonald et
al., 2010; Sterner et al., 2007).
Lake Michigan shows the highest concentrations and the highest
rates of increase for many ions. Of the six ions described in Table 3,
three of them (sodium, potassium and sulfate) show the highest
rate of increase occurring in Lake Michigan. Unlike the lower Great
Lakes, where some constituents have shown periods of improvement,
the trends in Lake Michigan tend to reflect unceasing linear increases
over the period of record.
Concentrations of most ions in Lake Huron are intermediate be-
tween lakes Superior and Michigan, representing the nearly equiva-
lent influence of these two lakes on its water quality (Chapra and
Sonzogni, 1979; Schelske, 1985b). Because the water quality in Lake
Superior is much more stable, the direction of trends observed for
Lake Huron tends to follow that of Lake Michigan.
Water quality is the most variable in Lake Erie due to its short water
residence time and high watershed loads (Chapra et al., 2009; Dolan
and McGunagle, 2005). Recall that only deep stations from the eastern
basin are considered here; this helps greatly to reduce the scatter in
the ion plots and elucidate longer-term trends. Because Lake Erie re-
ceives waters from the upper Great Lakes as well as its own loads, ion
concentrations tend to be greater than those observed in the upstream
waters.
In general, the highest concentrations of many ions are observed in
Lake Ontario, a natural consequence of its most downstream position in
the basin, but also the result of high anthropogenic ion loads from its wa-
tershed (Doerr et al., 1994; Effler, 1987; Effler and Matthews, 2003).
The very long residence times of the upper Great Lakes mean that
water quality impacts in these lakes can continue to occur and im-
pact the downstream lakes, long after the causes of the impacts
have been abated. In this way, the concentrations of chloride, sodi-
um, sulfate and calcium are increasing in lakes Superior, Michigan
and Huron, even while the concentrations are varying or declining
(in the case of calcium) in the lower lakes. Using the example of
chloride, Chapra et al. (2009) previously demonstrated that the con-
centrations can be expected to increase in Lake Ontario to unprece-
dented levels over the next few decades due to the influence of
previous loads to the upper lakes, even though those loads may
have subsequently declined.
Fig. 8. Winter and summer calcium concentrations for the Niagara River at NOTL along
with spring, open-water measurements for eastern Lake Erie.
Fig. 9. Trends of spring, water column (filled squares; solid line) and summer, surface
(open squares; dashed line) calcium concentrations for Lake Ontario.
558 S.C. Chapra et al. / Journal of Great Lakes Research 38 (2012) 550–560
In the lower Great Lakes, the shorter residence times result in trends
that are more dynamic and responsive to watershed and upstream loads.
For several major ions (e.g., chloride, sodium and possibly sulfate), an
oblique N-shaped trend is observed as lake concentrations increased, de-
creased and rebounded. We believe that the N-shaped trends observed
in the lower Great Lakes reflect the broad patterns of urban development
and industrialization (as concentrations increased), the implementation
of significant source controls (when concentrations decreased), followed
by the system's adjustment to (still-increasing) upstream loads and pos-
sibly new sources that are fuelled by the population growth and acceler-
ated development in the lower lakes' watersheds.
The trends documented in this paper could not have been established
without the systematic regular monitoring programs conducted by both
Canada and the United States governments. It will be essential, moving
forward, that these important long-term monitoring programs continue
in the face of increasing resource pressures and despite any perceived
lack of immediate utility. Well-run, accessible environmental monitoring
programs provide contextual information for shorter-term research
initiatives as well as contributing important scientific advances and cru-
cial information to guide environmental policy (Lovett et al., 2007;
Magnuson, 1990; Stow et al., 1998). The long-term data sets utilized
here comprise some of the most comprehensive, systematic and detailed
information that is available for such a large freshwater system. The use-
fulness of these data sets for retrospective assessment of ecosystem re-
sponse to anthropogenic stresses, to important environmental policy, or
to global climate change, cannot be underestimated.
Acknowledgments
EC's Surveillance Program is indebted to the Canadian Coast Guard
captains and crews of the Limnos, the Research Support Branch for its
tireless and high quality field work, the National Laboratory for Envi-
ronmental Testing for water quality analyses, and the previous Sur-
veillance Officers who have overseen the program. GLNPO thanks
the captains and crews of its research vessels, past program scientists
and the chemists whose analyses are used here. In particular, we
would like to thank Marvin Palmer and Michael Yusim for their con-
tinuing analytical expertise. The authors would also like to thank Brad
Hill of Environment Canada for contributing Niagara River data, and
Robert J. Wilcock for discussions related to ion chemistry.
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