Acid deposition released large amounts of aluminium into
streams and lakes during the last century in northern Europe and eastern
North America. Elevated aluminium concentrations caused major environmental
concern due to aluminium's toxicity to terrestrial and aquatic organisms and led to
the extirpation of wild Atlantic salmon populations. Air pollution reduction
legislation that began in the 1990s in North America and Europe successfully
reduced acid deposition, and the aluminium problem was widely considered
solved. However, accumulating evidence indicates that freshwater systems still show
delays in recovery from acidification, with poorly understood implications
for aluminium concentrations. Here, we investigate spatial and temporal
patterns of labile cationic forms of aluminium (Ali) from 2015 to 2018 in
10 catchments in Nova Scotia, Canada; this region was one of the hardest
hit by acid deposition, although it was not considered to have an aluminium problem
due to its high dissolved organic carbon (DOC) concentrations that were expected
to reduce Ali concentrations. Surprisingly, our results show the widespread
and frequent occurrences of Ali concentrations that exceed toxic
thresholds in all sampled rivers despite high DOC concentrations.
Generalized linear mixed model results reveal that DOC, instead of being
inversely related to Ali, is the strongest predictor (positive) of
Ali concentrations, suggesting that the recruitment properties of DOC
in soils outweigh its protective properties in streams. Lastly, we find
that, contrary to the common conceptualization that high Ali levels are
associated with storm flow, high Ali concentrations are found during
base flow. Our results demonstrate that elevated Ali concentrations in
Nova Scotia continue to pose a threat to aquatic organisms, such as the
biologically, economically, and culturally significant Atlantic salmon
(Salmo salar).
Introduction
Increased rates of acid deposition, predominantly deriving from upwind
fossil fuel burning, resulted in the acidification of soils, rivers, and
lakes during the last century (e.g., Kerekes et al., 1986), depleting base
cations and increasing toxic aluminium concentrations in soils and drainage
water. Increased aluminium concentrations caused the extirpation of native
Atlantic salmon (Salmo salar) populations in many rivers (Rosseland et al., 1990), for
example in Scandinavia (Henriksen et al., 1984; Hesthagen and Hansen, 1991),
the eastern USA (Monette and McCormick, 2008; Parrish et al., 1998), and
Nova Scotia, Canada (Watt, 1987). The acidification problem was widely
considered solved following reductions in anthropogenic sulfur emissions in
North America and Europe since the 1990s. Many rivers showed steady
improvements in annual average stream chemistry (Evans et al., 2001;
Monteith et al., 2014; Skjelkvåle et al., 2005; Stoddard et al., 1999;
Warby et al., 2005), including reduced concentrations of aluminium (Al) in
the USA (Baldigo and Lawrence, 2000; Buchanan et al., 2017; Burns et al.,
2006) and Europe (Beneš et al., 2017; Davies et al., 2005; Monteith et
al., 2014). However, recent evidence highlights delayed recovery from
acidification in other regions (Houle et al., 2006; Warby et al., 2009;
Watmough et al., 2016), including Nova Scotia (Clair et al., 2011), raising
questions about the possibility of elevated aluminium concentrations in
freshwater systems.
Al in freshwater can exist as inorganic monomers, inorganic polymers,
in amorphous and microcrystalline inorganic forms, and in fast reactive or
unreactive organic forms (Chew et al., 1988; Driscoll et al., 1980; LaZerte,
1984). While a variety of Al species in circumneutral waters are toxic to
fish (Gensemer and Playle, 1999), including precipitated forms (Gensemer et
al., 2018), the cationic species of Al (Ali), such as Al3+,
Al(OH)21+, and Al(OH)2+, are considered to be the most labile
and toxic to salmonids. Al species bind to the negatively charged fish gills
and cause morbidity and mortality through suffocation (Exley et al., 1991),
reduce nutrient intake at gill sites, and alter blood plasma levels
(Nilsen et al., 2010). The effects of sublethal exposure to freshwater Al
elicit osmoregulatory impairment (Monette and McCormick, 2008; Regish et
al., 2018), which reduces survival in the hypertonic marine environment
(McCormick et al., 2009; Staurnes et al., 1996). Elevated concentrations of
Ali are also toxic to other freshwater and terrestrial organisms
(Boudot et al., 1994; Wauer and Teien, 2010), such as frogs and aquatic
birds (Lacoul et al., 2011).
Al speciation varies with pH (Helliweli et al., 1983; Lydersen, 1990):
positive Al species dominate over neutral and negative species below pH 6.3
at 2 ∘C and below pH 5.7 at 25 ∘C (Lydersen, 1990), and the most
toxic Al species, Al(OH)2+1 (Helliweli et al., 1983), dominates Al
speciation between pH 5.0 and 6.0 at 25 ∘C, and between 5.5 and 6.5 at 2 ∘C
(Lydersen, 1990). Thus, the toxicity of Al increases with increasing pH up to
approximately 6.0 at 25 ∘C or 6.5 at 5 ∘C, when aqueous aluminium
precipitates, forming gibbsite (Lydersen, 1990; Schofield and Trojnar,
1980); colder water will have a higher proportion of toxic species at
higher pH values than warmer water (Driscoll and Schecher, 1990).
The bioavailability of Al is reduced by the presence of calcium (Ca; Brown,
1983), which can occupy the negatively charged gill sites. Dissolved organic
carbon (DOC) also reduces the bioavailability of aluminium via the formation of
organo-Al complexes (Alo; Erlandsson et al., 2010; Neville, 1985).
High levels of DOC in rivers have been believed to be sufficient to protect
fish gills from adverse Al effects (Lacroix and Kan, 1986; Vogt and Muniz,
1997; Witters et al., 1990).
Despite being the most common metal on Earth's crust, Al is usually
immobilized in clays or hydroxide minerals in soils. Rates of Al release
into soil water from soil minerals increase with three drivers: (1) low soil
pH, (2) low soil base saturation, and (3) high soil DOC concentrations.
Lowered pH increases the solubility of secondary minerals containing Al;
Ali concentrations in stream water are generally negatively correlated
with pH (Campbell et al., 1992; Kopáček et al., 2006; Seip et al.,
1989). Low levels of base saturation can cause charge imbalances resulting
in the release of Al into soil water and later into drainage water
(Fernandez et al., 2003); thus, chronic acidification shifts available
exchangeable cations in the soil water from Ca and magnesium (Mg) towards
Al (Schlesinger and Bernhardt, 2013; Walker et al., 1990). Higher
concentrations of DOC in soil water increase the release of Al via two
mechanisms: (1) as an organic acid, DOC decreases soil pH, thereby increasing Al
release (Lawrence et al., 2013); and (2) by forming organic complexes with
Ali, DOC maintains a negative Al concentration gradient from the cation
exchange sites to the soil water, increasing the rates of Al release (Edzwald
and Van Benschoten, 1990; Jansen et al., 2003). Field studies confirm Al
concentrations to be positively correlated with DOC (Campbell et al., 1992;
Kopáček et al., 2006).
The general consensus in the literature is that elevated Ali
concentrations occur during episodic storm events due to three possible
mechanisms: (1) dilution of base cations during storm events, where flow
paths move through shallower, more organic-rich soil layers; (2) added anions
in snowmelt or rainfall (e.g., Cl-, SO42-, and F-) providing
mobile anions that increase Ali export to streams; and (3) low pH
associated with storm events that redissolves Al accumulated in the soil
(Hooper and Shoemaker, 1985) and/or from the streambed (Norton et al.,
1990). For example, from 1983 to 1984, Al concentrations for the River
Severn in Wales increased tenfold during the storm flow peak compared with the
base flow (Neal et al., 1986). The general consensus in the literature is
also that Ali is seasonally elevated during spring snowmelt and autumn
rainfall events, and it is seasonally depressed during summer months due to higher
levels of DOC, such as in Quebec (Campbell et al., 1992), Russia (Rodushkin
et al., 1995), and along the Czech–German border (Kopáček et al., 2000;
Kopáček et al., 2006). Timing of the Ali peaks is important. If
peak Ali concentrations coincide with vulnerable life stages of
Atlantic salmon, such as during the spring when salmon transition from parr
to smolt (i.e., smoltification) in preparation for life in the ocean
(Kroglund et al., 2007; Monette and McCormick, 2008; Nilsen et al., 2013) or
during the emergence of salmon fry from eggs (e.g., Farmer, 2000), the
potential for large biological impacts from elevated Ali is
particularly high.
Despite much progress in acidification research, the processes affecting
Ali dynamics are not well understood (e.g., Mulder et al., 1990). Our
understanding of Ali is limited by the relative paucity of samples;
Ali is not measured as part of standard analyses. Thus, comprehension of Ali is
also limited by the difficulty involved in comparing the wide variety of methods for
estimating Ali; different definitions, often operational, of toxic Al
include inorganic Al, inorganic monomeric Al, labile Al, Al3+, and
cationic Al (Supplement Table S1.1).
Acid-sensitive areas of Nova Scotia, Canada, here abbreviated as NSA
(see Clair et al., 2007), with once-famous wild Atlantic salmon populations,
were heavily impacted by sulfur deposition at the end of the last century,
which originated from coal burning in Central Canada and the Northeastern USA
(Hindar, 2001; Summers and Whelpdale, 1976). NSA catchments are
particularly sensitive to acid deposition due to base-cation-poor and slowly
weathering bedrock that generates thin soils with a low acid neutralizing
capacity (ANC), extensive wetlands, and episodic sea salt inputs (Clair et
al., 2011; Freedman and Clair, 1987; Watt et al., 2000; Whitfield et al.,
2006). Al was not considered to be a threat to Atlantic salmon in Nova
Scotia because of the high natural levels of DOC in NSA rivers
following preliminary research by Lacroix and Townsend (1987) and Lacroix (1989). A 2006 fall survey, however, (Dennis and Clair, 2012) found that Ali concentrations in NS exceeded the 15 µg L-1 toxic threshold for aquatic health, as determined from an
extensive review of toxicological and geochemical literature by the European
Inland Fisheries Advisory Council (EIFAC), in
7 of 42 rivers surveyed (Howells et al., 1990). No
assessment of Ali has been carried out in NSA since that time, and little
is known about the current extent and patterns of Ali in the region.
Here, we conduct a 4-year survey of Ali concentrations in 10
streams in NSA in order to test the hypothesis that elevated DOC concentrations
are sufficient to protect life from Ali and to identify the hydrologic
conditions associated with elevated Ali concentrations.
Materials and methodsStudy area
We surveyed Ali concentrations at 10 study catchments in NSA,
ranging from headwater to higher-order systems: Mersey River (MR), Moose Pit
Brook (MPB), Pine Marten Brook (PMB), Maria Brook (MB), Brandon Lake Brook
(BLB), above the West River lime doser (ALD), Upper Killag River (UKR),
Little River (LR), Keef Brook (KB), and Colwell Creek (CC) (Table 1, Fig. 1). Our study catchments are predominantly forested with a mix of coniferous
and deciduous species, and they drain slow-weathering, base-cation-poor bedrock,
producing soils with a low ANC (Langan and Wilson, 1992; Tipping, 1989). The
catchments also have relatively high DOC concentrations (Ginn et al., 2007)
associated with the abundant wetlands in the region (Clair et al., 2008;
Gorham et al., 1986; Kerekes et al., 1986).
Study site locations showing (a) mean Ali concentrations and
(b) the proportion of samples in which Ali concentrations exceeded the 15 µg L-1 toxic threshold between spring 2015 and fall 2018. The shaded region
corresponds to the catchments of monitoring sites. For additional site
details, refer to Table 1.
Study site characteristics. “n” refers to the number of sampling
events. Numbers in parentheses after the mean concentration represent the standard
deviation. One Ali outlier was removed for MR (a value of 2 µg L-1 from 30 April 2015). pH is calibrated using the
method outlined in Sect. S4.4.
We measured Ali concentrations at 3 of the 10 catchments from
April 2015 to September 2017 (MR, MPB, and PMB) at a weekly to monthly
frequency during the snow-free season (approximately April to November,
Table S1.2). In the period from 2016 to 2018, seven sites were added and were sampled biweekly
to monthly during the snow-free season.
Ali sampling events comprise grab samples for laboratory analysis and in situ
measurements of pH and water temperature (Tw). We calculate Ali as
the difference between dissolved Al (Ald) and Alo following Dennis
and Clair (2012) and Poléo (1995), as in Eq. 1, separating the species in the
field to reduce errors caused by Al species change due to variations in
temperature and pH during transport from the field to laboratory.
Ali=Ald-AloAld is measured as the Al concentration of a filtered (0.45 µm) sample and Alo is measured as the eluate from passing filtered water
through a 3 cm negatively charged cation exchange column (Bond Elut Jr.
strong cation exchange column). Samples were passed through the cation
exchange column at a rate of approximately 30 to 60 drops per minute. Using
this method, Alo is operationally defined as the non-labile,
organically complexed metals and colloids; and Ali is defined as the
positive ionic species of Al (e.g., Al3+, Al(OH)2+, and
Al(OH)2+).
Thus, the cation exchange method determines the concentrations of weak monomeric
organic Al complexes (passed through the column), monomeric inorganic Al
(retained in the column), and colloidal, polymeric, and strong organic
complexes that are measured after acid digestion of the sample (Gensemer and
Playle, 1999). An assumption here is that the Al species retained on the
exchange column would also be retained on the negatively charged fish gills
and, therefore, have a potentially toxic effect (see Gensemer and Playle,
1999). The eluate is generally considered to be nontoxic; however, there is
some evidence that precipitated polymeric Al and colloidal Al can be toxic
to aquatic life (Parent and Campbell, 1994; Gensemer and Playle, 1999;
Gensemer et al., 2018), although the nuances of this toxicity are unclear. To
this end, the calculated Ali reported in this study represents a
minimum concentration. Ultrafiltration (following Simpson et al., 2014) may
improve the accuracy of the estimations of recently precipitated colloidal Al
concentrations.
Stream chemistry samples (50 mL) were collected using sterilized
polyethylene syringes into sterilized polyethylene bottles. Samples for
sulfate (SO42-) analysis were not filtered. Trace metal
samples were filtered (0.45 µm) and preserved with nitric acid
(HNO3). Samples for DOC analysis were filtered (0.45 µm) and
transported in amber glass bottles containing sulfuric acid preservative
(H2SO4) to prevent denaturation. All samples were cooled to 7 ∘C during transport to the laboratories. Samples were delivered
to the laboratories within 48 h of collection, where they were further
cooled to ≤4∘C prior to analysis (Sect. S4).
We examined correlations between Ali and the following water chemistry parameters:
Ald, Ca, DOC, pH, SO42-, Tw, fluoride (F-), nitrate
(NO3-), and runoff (where data were available). Correlations were
analysed within and across sites. For the purposes of this study, we use the
toxic threshold of Ali of 15 µg L-1, as the majority of
our pH observations were greater than or equal to 5.0 (Table S1.2, Sect. S4.3).
We developed a generalized linear mixed model (GLMM) to identify the main
drivers of the Ali concentration at the study sites. We tested Ald,
DOC, Ca, SO42-, F-, NO3-, season, and Tw as
potential drivers. The GLMM analysis was implemented with R version 3.6.2.
(R Core Team, 2019) using the lme4 package (Bates et al., 2015). Due to the
nonnormality of the Ali concentration data, the glmer function was
employed for model fitting which uses the method of maximum likelihood for
parameter estimation via Laplace approximation (Raudenbush et al.,
2000). The study sites in the analysis were included as the random effect
with the fixed effects provided in Table S1.3; Ald was not included in the
GLMM, as it results in an overfit model (singularity). The Wald t test
statistic and the Akaike information criterion (AIC) were used as measures
of goodness of fit (Akaike, 1974; Bolker et al., 2009). Numerous iterations
of fixed effects and interactions were considered in the GLMM development
(Table S1.3). Multiple fixed effects were initially considered; however,
several of the effects were not significant, although the overall model fit
provided a low AIC. GLMMs were applied to assess seasonal drivers of Ali
concentrations; however, owing to the limited amount of seasonal data
collected, an analysis of both the site and seasonal random effects could
not be carried out due to model singularities.
Results and discussionPrevalence of Ali
Ali concentrations exceed toxic levels (15 µg L-1) at all sites
during the study period (Table S1.2) despite relatively high DOC
concentrations (mean values ranging from 7.2 to 23.1 mg L-1 among the
catchments; Table 1). Mean Ali concentrations across all sites range
from 13 to 60 µg L-1 (Table 1), with the highest mean concentrations also
occurring in the eastern part of the study area (Fig. 1a) where one site
had 100 % of samples in exceedance (Fig. 1b). Ali concentrations
exceed 100 µg L-1 (approximately 7 times the threshold) at three
sites (Table S1.2). Our Ali concentrations are consistent with the
6.9–230 µg L-1 range of Ali concentrations measured across
NSA by Dennis and Clair (2012) and are higher than concentrations
measured in Norway from 1987 to 2010 (5–30 µg L-1; Hesthagen et al.,
2016).
The percentage of Al not complexed by DOC (% Ali/Ald) ranges from
a minimum of 0.6 % to a maximum of 50 %, with a median value of
10.7 %, across all sites. These findings are similar to those reported in
NSA by Dennis and Clair (2012) with respect to the proportion of Ali in total
aluminium (Alt; min. = 4 %, max. =70.1 %, median =12.4 %),
and less than those found by Lacroix (1989; over 90 %
Alo/Ald). These speciation results are also quite similar to other
diverse environments, such as acid sulfate soil environments in
Australia (Simpson et al., 2014). However, even when the percentage of
Ali/Ald is low, Ali concentrations remain well above the
thresholds for toxicity (Supplement Figs. S2.2–S2.11). Similarly to our findings, previous
studies show that Ali/Ald is low during base flow (Bailey et al., 1995; Murdoch and Stoddard, 1992; Schofield et al., 1985; Figs. S2.2–S2.11 in this paper).
The highest concentrations of Ali (> 100 µg L-1)
occurred in early summer (late June or early July in 2016–2018) when
Ald, Ca, and DOC concentrations had not yet reached their annual peak
(Table S1.2). The spring/summer extreme events occurred on the first
exceptionally warm days (> 21 ∘C) of the year, in dry
conditions, and when the proportion of Alo/Ald was low (decreasing
to approximately 60 %–70 % from higher levels of around 80 %–90 %; Figs. S2.2–S2.11). pH was not abnormally low during these events (ranging from 4.8 to
6.13), Ca concentrations were low (less than or equal to 800 µg L-1), and DOC concentrations ranged from 15 to 21 mg L-1.
At the sites with the longest and most frequent data collection (MR and
MPB), Ali concentrations exceed the toxic threshold in consecutive
samples for months at a time, particularly in the late summer (Fig. S2.1).
Potential Ali drivers
GLMM results reveal, via multiple fixed effect model combinations, that
DOC and Tw are the most significant predictors of Ali concentrations (Table S1.3). When both DOC and Tw were included in the
GLMM model, the resulting AIC was markedly lower, indicating that the
inclusion of both parameters provided better predictive potential than DOC
or Tw alone. Furthermore, the interaction between DOC and Tw was
not significant in the model, provided a nominally lower AIC, and the
correlation between the effects was low (-0.378), indicating that distinct
processes are responsible for Ali concentrations. The strength of the
Tw relationship is likely due to the role that increased temperature
plays in activating biological drivers that mobilize Al (Hendershot et al.,
1986).
Thus, the GLMM results show that DOC is positively correlated with Ali
in the study area, in contrast to the standard conceptualization that DOC is
inversely correlated with Ali; this suggests that the increased
recruitment of Al in soils by DOC may outweigh the protective effects of DOC in
freshwater, which is consistent with observations from other studies (e.g., Campbell et
al., 1992; Kopáček et al., 2006).
Linear regressions show that Ald is significantly (α=0.05)
and positively correlated with Ali at 7 of the 10 study sites
(ALD, KB, LR, MB, MPB, MR, and PMB; Fig. 2, Table S1.5). Ca is significantly and
positively correlated with Ali at two sites (MPB and MR; Fig. 2, Table S1.5). The positive relationship between Ca and Ali is the opposite of
what was expected (following Rotteveel and Sterling, 2019). We hypothesize that
this is due to the two study sites having very low Ca concentrations (mean
concentrations below 1 mg L-1), below which soil water Ca
concentrations are too low to retard Al release.
Correlation among water chemistry parameters and Ali
concentration, where red polygons and lines indicate a positive correlation
with Ali, and blue polygons and lines indicate a negative correlation
with Ali. One Ali outlier was removed for MR (a value of 2 µg L-1 from 30 April 2015). Correlation data are listed in Table S1.5.
Discharge is significantly and negatively correlated with Ali at one
site MPB (Fig. 2, Table S1.5), which is in contrast with previous observations that
Ali concentrations are positively correlated with discharge (Hooper and
Shoemaker, 1985; Neal et al., 1986; Seip et al., 1989; Sullivan et al.,
1986). Runoff data are available for only two of the study sites (MR and MPB);
more runoff data are needed to improve our understanding of the relation
between runoff and Ali in NSA.
We did not observe the negative association between pH and Ali that has been observed
in previous studies (Campbell et al., 1992; Kopáček et al., 2006) –
pH is negatively correlated with Ali at 4 of the 10 sites, but none
of these relationships are statistically significant (Fig. 2, Table S1.5). The
lack of a significant correlation may be due in part to other mechanisms that
could potentially cloud the strength of the inverse relationship between pH and Al, such
as increased DOC solubility at higher pH levels that leads to increased Al solubility
in soils (Lydersen, 1990), pH buffering by Al in the lower pH range
(Tomlinson, 1990), and the limited pH range in the dataset. We did
observe a statistically significant positive relationship between pH and
Ali/Ald (Table S1.4); thus, it seems that pH may play a more
important role in determining the proportion of different Al species rather
than the absolute value of Ali present in stream water in chronically
acidified conditions such as those found Nova Scotia. F- has also been
found to be a complexing agent that affects the speciation of Al at low pH
levels and relatively high concentrations of F- (> 1 mg L-1; Berger et al., 2015). The concentrations of F- at the study
sites are mostly below this threshold (mean across all sites of 0.045 mg L-1); however, there is still a significant positive effect of F-
on Ali concentrations at two sites (KB and MPB; Fig. 2, Table S1.5).
NO3- and SO42- are also potential complexing ligands of
Al; however, we did not observe any correlation between Ali and either
of these parameters, except for a significant negative correlation between
SO42- and Ali at MB.
Conceptual model of a new mechanism that can produce high
concentrations of Ali in freshwater systems. Warm days increase biological
activity that helps to mobilize Al from secondary minerals and enhances
production of DOC; this, in turn, reduces pH and decreases Al saturation in soil
solution by forming Al-organic complexes, thereby maintaining the Al concentration
gradient away from secondary minerals. In rivers, the amount of DOC is
insufficient to protect the fish from the amount of Ali mobilized in
soils.
Scatterplot relationships among water chemistry parameters for
seasons 1, 2, and 3 at MR and MPB. R2 values are listed in Table S1.7.
One runoff outlier was removed for MR (a value of 17 m3 s-1 from 22 April 2015). One runoff outlier was removed for MPB (a value of 35 m3 s-1 from
22 April 2015).
Possible seasonal groupings of Ali in NSA
At the two sites with the most samples, MPB and MR, groupings of data are
visible that are temporally contiguous, suggesting seasonally dependent
Ali behaviour (Fig. 4). This is supported by stronger linear
correlations (r2) among variables when grouped by “season” (Table 2);
for example, for the correlation between pH and Ali at MR, the r2 value
improves from 0.02 for year-round data (Fig. S2.12) to up to 0.78 in season 1
(Fig. 4). The transition dates between the seasons are similar for the two
catchments, but they are not the same (Table S1.2) and vary by year. Here, we propose
an initial characterization of potential “seasons”; more research is
needed to test these hypotheses on seasonal divisions and their drivers
using larger datasets to test for statistical significance among the
potential seasonal groupings.
Ali relations with other stream chemistry parameters separated
by possible seasons. Dark shading represents an r2 value greater than 0.6.
Medium shading represents an r2 value between 0.2 and 0.6. Light shading represents an r2 value between
0.0 and 0.2. Green indicates a negative relation. Orange indicates a positive
relation.
Season 1 (approximately April/May) is coincident with snowmelt runoff and
is characterized by relatively low concentrations of Ali (2–46 µg L-1), a low pH (4.5–5.3), and lower concentrations of most constituents,
including DOC, as well as cold temperatures (< 4 ∘C). During this
season, Ali is strongly coupled with pH, DOC, Ald, and Ca in MR,
but less so in MPB. A possible explanation for this is that season 1 is dominated by
snowmelt hydrology in which cation exchange between soil and discharge
occurs less efficiently, which has been attributed to ice and frozen soil
potentially limiting water contact time with soil (Christophersen et al.,
1990). It is important to note that we likely did not capture the first
flush effect of increased Ali as has been noted in other studies (e.g.,
Hendershot et al., 1996). The onset of season 2 (approximately late June) is
characterized by increasing Ali concentrations, temperature, and DOC.
Ali and pH values are higher in this season, and Ali becomes
strongly negatively correlated with pH as pH increases to the lower
threshold for gibbsite. In MR in season 2, Ali has a strong positive
relationship with DOC. The highest observed Ali concentrations of the
year occur in season 2 (Fig. 3). Ali relations are weak in MR in season
3 (approximately September through March), which is likely due to the lower frequency
of measurements during the winter. Season 3 in MR has the highest
concentrations of dissolved constituents (Ald, Ca, and DOC), whereas only Ca has the highest concentrations in
MPB.
With the inclusion of season as the random effect in the GLMM analysis
(Table S1.6), limited data remains to undertake a robust comparative
analysis, but it is included nonetheless to highlight the seasonal impacts
that can be garnered from the limited dataset. pH and Tw were omitted
due to singularities. These results reinforce that DOC concentrations are
associated with Ali concentrations on a seasonal basis; however, more
data are required to ascertain the effects of Tw on seasonal Ali
concentrations.
In contrast with the conceptualization that peak Ali concentrations
occur during storm flow (e.g., Campbell et al., 1992; Kopáček et al., 2000;
Neal et al., 1986; Rodushkin et al., 1995), our data show elevated Ali
concentrations during base flow conditions. These results suggest a new
pathway for the generation of elevated Ali concentrations that is
associated with base flow, warmer summer temperatures, and high DOC
concentrations and is, thus, likely more chronic in nature. Nilsson (1985)
suggested that this flow path has important consequences for Al
concentrations in Swedish catchments. We hypothesize that this pathway is
caused by increased temperatures resulting in higher levels of biological
activity that mobilize Al in soils (Fig. 3; following Nilsson and
Bergkvist, 1983). Biological activity further generates DOC that mobilizes
Al to drainage water during summer base flow (Fig. 3). Other cases of
increased Ali concentrations occurring during low flow and warming
temperatures can be found in the literature in locations such as Ontario and
Quebec (Hendershot et al., 1986, 1996) and in Virginia,
USA (Cozzarelli et al., 1987).
Ecological implications
While the summer peak in Ali that we observed in NSA does not
coincide with the smoltification period, continued exposure throughout the
year may still negatively affect salmon populations, as the accumulation of
Ali on gills reduces marine and freshwater salmon survival (Kroglund et
al., 2007; Kroglund and Staurnes, 1999; Staurnes et al., 1996; Gibson et
al., 2011).
In addition, elevated Ali concentrations appearing during low flow in
the summer months suggest a more chronic delivery of elevated Ali to
rivers, for which increases in the length and severity of droughts and
heat waves due to climate change may further exacerbate effects on aquatic
life. Because many peak Ali concentrations occur on the first
exceptionally warm day in late spring, springtime warming associated with
climate change may cause Ali peaks to occur earlier, thereby increasing the
chance of the peak Ali concentrations overlapping with the
smoltification
season and emergence of salmon fry, which are the most vulnerable life stages of
Atlantic salmon (e.g., Farmer, 2000), although the phenology of the smolt
run is expected to similarly advance earlier in the year.
Conclusions
Our 4-year study of stream chemistry in NSA has two important
findings. First, high DOC concentrations in rivers may not protect aquatic
life against Ali as previously thought; our GLMM analysis suggests
rather the opposite: higher DOC concentrations drive higher levels of
Ali, even possibly on a seasonal basis. Thus, our study reveals that
despite high DOC levels, widespread and persistent toxic concentrations of
Ali in NSA freshwater systems pose a risk to aquatic, and potentially
terrestrial, life. Second, our study highlights an overlooked hydrological
pathway that is associated with high Ali concentrations – base flow –
suggesting a chronically acidified/aluminium dynamic, in addition to episodic
Ali peaks associated with storm flow. This base flow pathway demonstrates
that Ali concentrations are chronically elevated during warmer summer
months and that this Al pathway may, thus, be exacerbated by atmospheric
warming. Our results suggest that the recent 88 % to 99 % population decline
of the Southern Uplands Atlantic salmon population in NSA (Gibson et
al., 2011) may be partially attributable to Ali, which is in contrast to earlier
studies that downplayed the role of Ali in Atlantic salmon mortality
(Lacroix and Townsend, 1987). These high Ali concentrations in NSA
highlight the need to increase our understanding of the influence of
Ali on both terrestrial and aquatic ecosystems as well as its implications
for biodiversity. The catchments with the highest Ali levels had
particularly low Ca levels; this is cause for concern, as Ca protects against
Ali toxicity, and highlights the coincident threats of Ca depletion and
elevated Al. Recent work has identified globally widespread low levels and
declines in Ca (Weyhenmeyer et al., 2019), raising the question of what
other regions may also have Ali levels exceeding toxic thresholds. The
serious potential consequences of high Ali highlight the importance of
actions to further reduce acid emissions and deposition, as critical loads
are still exceeded across the province (Keys, 2015), and to adapt forest
management practices to avoid base cation removal and depletion. The addition of
base cations through liming and enhanced weathering of soils and freshwater
may accelerate recovery from acidification.
Data availability
All of the data presented and discussed in this paper are provided in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/hess-24-4763-2020-supplement.
Author contributions
SMS conceived the idea and led the writing of the paper. SM led the field data
collection. SM and TAC designed the protocol for Ali sampling, assisted
with data analysis, and helped with writing the article. LR performed spatial and
statistical analysis, produced figures, and assisted with sample collection
and draft writing. KH assisted with data analysis, figure production and
editing, and contributed to drafting the paper. NLO led the GLMM analyses and
contributed to the paper. TAC provided information on analytical and
field sampling methods and helped with the selection of sampling sites. EAH contributed
field samples, assisted with data analysis, and contributed to the
article.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The two anonymous referees contributed suggestions that improved the
paper. The Atlantic Salmon Conservation Foundation, the Atlantic Canada
Opportunities Agency, the Nova Scotia Salmon Association, and Fisheries and
Oceans Canada provided financial support for the field data collection and
the laboratory analyses. Marley Geddes, Siobhan Takla, Franz Heubach, Lorena Heubach, Emily Bibeau, and Ryan Currie provided field assistance.
Financial support
Financial support was received from the National Science and Engineering Council of Canada (grant no. RGPIN06958-19), the Nova Scotia Salmon Association, Fisheries and Oceans Canada, the Atlantic Canada Opportunities Agency, and
the Atlantic Salmon Conservation Foundation (research grant).
Review statement
This paper was edited by Matthew Hipsey and reviewed by two anonymous referees.
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