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Tracking anthropogenic- and climatic-related environmental changes in the remaining habitat lakes of the endangered Atlantic whitefish (Coregonus huntsmani) using palaeolimnological techniques.

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AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
Published online 2 April 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/aqc.934
Tracking anthropogenic- and climatic-related environmental
changes in the remaining habitat lakes of the endangered Atlantic
whitefish (Coregonus huntsmani) using palaeolimnological
techniques
BRIAN K. GINN*, LAURA C. GRACEa, BRIAN F. CUMMING and JOHN P. SMOL
Paleoecological Environmental Assessment and Research Laboratory (PEARL), Department of Biology, Queen’s University,
Kingston, Ont., Canada, K7L 3N6
ABSTRACT
1. Atlantic whitefish (Coregonus huntsmani) is a critically endangered species whose remaining habitat is
restricted to three lakes near Bridgewater, Nova Scotia, Canada. Other lakes in this region have been affected by
a variety of environmental changes over the past 150 years (e.g. acidic deposition, eutrophication, and climatic
changes); however, the extent of the impact on these remaining Atlantic whitefish lakes, and how the current
limnological conditions compare with pre-industrial conditions, is not known.
2. Given the lack of long-term monitoring data, palaeolimnological techniques were used to track
environmental changes in these three lakes to infer historic limnological conditions.
3. Results of this study show that acidic deposition has had no significant impact on these lakes (diatominferred lakewater pH has changed little over time), nor has the nutrient status of these lakes changed as the
sediment profiles have been consistently dominated by oligotrophic diatom taxa.
4. Changes in the dominance of diatom assemblages since 1850, from Aulacoseira distans to Cyclotella
stelligera, are correlated with climatic warming (r ¼ 0:48–0.89, P50:05).
5. Contrary to initial concerns, these only remaining habitats of the Atlantic whitefish have not yet been
affected by acidic precipitation. However, other potential stressors, such as climatic warming and associated
limnological changes, may now be affecting the habitat of this endangered species.
Copyright # 2008 John Wiley & Sons, Ltd.
Received 28 April 2007; Revised 7 September 2007; Accepted 27 October 2007
KEY WORDS:
palaeolimnology; Nova Scotia; Atlantic whitefish; Coregonus hunstmani; climatic change; diatoms; endangered
species
*Correspondence to: Brian K. Ginn, Paleoecological Environmental Assessment and Research Laboratory (PEARL), Department of Biology,
Queen’s University, Kingston, Ont., Canada, K7L 3N6. E-mail: ginnb@biology.queensu.ca
a
Current Address: Canadian Rivers Institute, Department of Biology, University of New Brunswick, Fredericton, NB, Canada, E3B 6E1.
Copyright # 2008 John Wiley & Sons, Ltd.
1218
B.K. GINN ET AL.
INTRODUCTION
Endangered species and, in particular, the impact of
environmental changes on their habitats, are of considerable
scientific and public interest. While most of these
investigations concern large mammals and terrestrial
habitats, critical information is also needed on other species,
and in particular those from aquatic ecosystems (Millennium
Ecosystem Assessment, 2005). The Atlantic whitefish
(Coregonus huntsmani (Scott, 1987)) is a critically endangered
species that is now restricted to just three lakes (Hebb Mill,
Milipsigate, and Minamkeak) in the Petite Riviere basin near
Bridgewater in south-western Nova Scotia (Canada) (Figure
1). It is one of the few fish species that is endemic to Canada
and the only species known to be endemic to Nova Scotia. This
species was formerly anadromous but has become landlocked
after the construction of dams on the Petite Riviere. Dam
construction, over-exploitation, and the effects of acidic
deposition resulted in the extirpation of the only other
known population in the Tusket River basin near Yarmouth
(Edge, 1984; Edge and Gilhen, 2001) (Figure 1). Fish ladders
constructed to allow access to nursery lakes around the dams
were ineffective (Edge, 1984).
Little is known about Atlantic whitefish biology including
the spawning period, habitat requirement, and marine habitat
locations. There are a few sporadic records of this species in
the Gulf of Maine and the Bay of Fundy (Edge and Gilhen,
2001). Compounding conservation efforts is the lack of an easy
method to distinguish C. huntsmani from the lake whitefish
(C. clupeaformis (Mitchill 1818)), which is smaller, exclusively
freshwater, and is a popular recreational sport fishery in Nova
Scotia. Lake whitefish is an introduced species to southern
Nova Scotia, as are brown trout (Salmo trutta L. 1758) and
chain pickerel (Esox niger Lesueur 1818), which are known to
have had impacts on other indigenous fish populations (Edge,
1984). In addition to the threats mentioned above,
Figure 1. Map of study area near Bridgewater (Nova Scotia) with locations of Hebb Mill, Milipsigate and Minamkeak lakes. Inset A shows the
location of Nova Scotia within context of the rest of North America.
Copyright # 2008 John Wiley & Sons, Ltd.
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
TRACKING ANTHROPOGENIC- AND CLIMATIC-RELATED ENVIRONMENTAL CHANGES IN LAKES
environmental stressors (e.g. nutrient loading, landscape
development, acidic deposition, and climatic change) are
affecting lakes in south-western Nova Scotia (Ginn, 2006).
Owing to the lack of long-term monitoring, however, it is not
known how the limnological conditions of these lakes compare
with the pre-industrial background conditions, or, how the
habitat of this endangered species has been affected by human
activities. Aside from providing important insights to the
protection of a critically threatened species, long-term data are
of interest to lake managers as the three study lakes also supply
drinking water for the town of Bridgewater.
Palaeolimnology is an excellent method for determining predisturbance limnological conditions, defining long-term trends,
and assessing impacts of environmental stressors (Smol, 2008).
This approach has been applied successfully in the study
of many lake management issues, such as acidification
(Ginn et al., 2007a, b), eutrophication (Hall et al., 1999;
Finsinger et al., 2006; Weckström, 2006), impacts of mining
(Salonen et al., 2006), urban development (Meriläinen et al.,
2003), land use (Augustinus et al., 2006; Davis et al., 2006),
contaminant transport (Donahue et al., 2006), and climatic
change (Laird et al., 2003; Moos et al., 2005; Harris et al.,
2006; Westover et al., 2006). By using biological (or
geochemical) indicators archived in lake sediments, past
environmental conditions can be inferred (Smol, 2008).
Indicator organisms, such as diatoms (Bacillariophyceae),
from the top layer (or surface) sediments, usually
representing the last 1–2 years of sediment accumulation, can
be identified and ‘calibrated’ to current water chemistry data
and other environmental variables (Smol, 2008). When
repeated for a large number of calibration lakes that span
the environmental gradients of interest (in this case the 494lake calibration set from north-eastern North America
(NENA dataset)) (Ginn et al., 2007c) the environmental
optima of the indicators can be estimated and limnological
conditions can be inferred using the fossil diatom assemblages
recorded in sediment cores. Once the pre-disturbance
conditions have been defined, and natural environmental
variations taken into account, the impact of environmental
1219
stressors can be determined and realistic mitigation goals can
be set (Smol, 2008).
This paper analyses diatom assemblage changes from dated
sediment cores retrieved from three lakes in the Petite Riviere basin
of south-western Nova Scotia (Figure 1) } Hebb Mill (44820.50 N,
64843.00 W), Milipsigate (44820.00 N, 64836.00 W), and Minamkeak
(44817.50 N, 64836.50 W) lakes, which are the last known habitat for
Atlantic whitefish (C. huntsmani). Diatom-based inference models
for lake water pH, Gran-alkalinity, and total phosphorus
developed from a 494-lake calibration set (NENA) from northeastern North America (Ginn et al., 2007c) were applied to the
diatom profiles from each lake. The diatom-inferred changes were
used to assess the impact of acidic deposition, eutrophication, and
climatic change on these study lakes.
STUDY AREA
Nova Scotia is a province in eastern Canada that is
characterized
by
many
freshwater
habitats
with
approximately 9400 lakes larger than 1 ha (Figure 1). This
region is dependent on its natural fish resources, relying on
fishing for recreational and economic purposes. In 2000, for
instance, the inland sport fishery grossed $86 million Canadian
dollars (J. MacMillan, pers. comm.). The province has a
maritime climate (no lake is more than 50 km from salt water;
Clair et al., 2001), which is moderated by the Atlantic Ocean,
resulting in warmer winters and cooler summers in comparison
with more inland regions (Davis and Browne, 1996). Current
environmental issues that are of most concern to Nova Scotia
lakes include acidic deposition, increased nutrient loading,
catchment development, and climatic change.
The study lakes are situated on poorly weathered
metamorphic bedrock belonging to the Halifax Formation
of the Meguma Group, with shallow soils derived from glacial
till. A summary of some available physical and chemical
data is given in Table 1. Catchments are mixed deciduous–
coniferous forest (Acadian forest dominated by spruce–
pine–maple–birch) with some areas cleared for agriculture or
Table 1. Mean physical and chemical data recorded for the three Atlantic whitefish study
lakes (2004/2005), Nova Scotia, Canada
pH
Total phosphorus (TP) (mg L1)
Surface area (ha)
Maximum depth (m)
Total organic carbon (TOC) (mg L1)
Dissolved O2 (surface) (mg L1)
Dissolved O2 (bottom) (mg L1)
Copyright # 2008 John Wiley & Sons, Ltd.
Hebb Mill
Milipsigate
Minamkeak
6.1
7.5
431.0
15.0
3.8
7.9
5.2
6.3
10.2
335.8
16.0
3.6
8.6
4.4
7.4
11.2
788.6
16.0
3.0
8.6
1.0
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
1220
B.K. GINN ET AL.
residential use. The area has a low topography (100 m above
sea level) and a climate moderated by the nearby Atlantic
Ocean. The lakes are also important to nearby communities as
they provide the industrial and domestic water supply to the
town of Bridgewater (Edge, 1984). With the exception of a few
private lakeshore landholdings, direct human use of these
lakes, such as new developments, is prohibited by law so as to
protect the water supply (Edge, 1984). Recreational lake use,
including boating and swimming, is also prohibited. Although
these regulations provide some protection against human
influence, they do not guard the lakes against the effects from
nearby forestry and agricultural practices, as well as threats
such as airborne pollution (i.e. acidic deposition) or climatic
change. Any disturbances that might affect these lakes are
clearly important, because these lakes are the last known
habitats of the Atlantic whitefish.
METHODS
Sampling protocols
Sediment cores were collected from Minamkeak Lake (July
2003) and Hebb Mill and Milipsigate lakes (July 2004) using
a Glew gravity corer (Glew, 1989; Glew et al., 2001; J.R. Glew,
Queen’s University, Kingston, Ontario, Canada) equipped
with a 7.6-cm internal-diameter Lexan1 core tube (General
Electric Polymer Shapes, Ottawa, Ontario, Canada). Cores
were sectioned into 0.5-cm intervals using a Glew (1988)
extruder, with separate intervals placed in Whirlpack1 bags
(Fisher Scientific, Ottawa, Ontario, Canada) stored on ice and
kept below 48C for transport back to PEARL (Queen’s
University at Kingston) and subsequent storage in a cold-room.
Radiometric dating
Sediment samples were freeze-dried with 0.5 g from each
interval being placed in individual plastic tubes for 210Pb
gamma dating analysis following the procedures of Schelske
et al. (1994) and Appleby (2001). Radioactive decay was
measured for 80 000 s using an Ortec1 germanium (Gr) crystal
well detector (Ortec, Oak Ridge TN, USA). 210Pb, 226Ra, 214Bi,
and 137Cs activities were calculated based on procedures
outlined by Schelske et al. (1994) and dates calculated using
the Constant Rate of Supply (CRS) program developed by
Binford (1990).
Diatom preparation and analysis
Diatoms were isolated from sediment using a 1:1 molar ratio of
HNO3 and H2SO4 and procedures described by Battarbee et al.
(2001). Diatom preparations were mounted on slides using
Copyright # 2008 John Wiley & Sons, Ltd.
Naphrax1 as a mountant (Brunel Microscopes, Wiltshire, UK).
A minimum of 300 diatom valves were enumerated per sample
using a 100 oil immersion objective ðNA ¼ 1:3Þ on a Leica1
DMRB microscope equipped with differential interference
contrast (DIC). Diatoms were identified using Patrick and
Reimer (1966, 1975), Krammer and Lange-Bertalot (1991–
2000), Round et al. (1990), Camburn and Charles (2000), Fallu
et al. (2000), Wehr and Sheath (2002), and other references fully
documented in Ginn et al. (2007c).
Statistical analysis and inference of limnological variables
Diatom–environment inference models were developed using
the 494-lake NENA dataset from north-eastern North
America (Ginn et al., 2007c). Inferences of pH, total
phosphorus (TP), and Gran-alkalinity were made using a
model based on Gaussian logit regression on the individual
taxa and maximum likelihood calibration, since it performed
best, although other methods (weighted averaging and modern
analogs technique) yielded similar results (Ginn, 2006; Ginn
et al., 2007c). Inference models performed well as established
by the strength of relationship between the observed and
predicted values (i.e. bootstrapped coefficient of determination
ðr2boot Þ and root mean squared error of prediction (RMSEP)).
The pH inference model was strongest and highly significant
(RMSEP ¼ 0:45; r2boot ¼ 0:88), but other limnological
inference models were less rigorous (log Gran-alkalinity:
RMSEP ¼ 0:38; r2boot ¼ 0:75; log TP: RMSEP ¼ 0:46;
r2boot ¼ 0:24) (Ginn, 2006), although similar to those obtained
in other studies (Hall et al., 1999). Changes in relative
abundances of Cyclotella stelligera, Aulacoseira distans,
and principal components analysis (PCA) axis-1 site
scores in the dated sediment cores were correlated with 10and 15-year mean temperatures recorded at Halifax using
a Pearson correlation with Bonferoni adjustment.
Relative frequency diagrams of diatom assemblages were
constructed using C2 version 1.4 (S. Juggins, University of
Newcastle) with diatom taxa arranged using (PCA) axis-1
species scores.
RESULTS
All three study lakes showed the same trend with a shift in
diatom assemblages from a pre-1850 dominance by
tychoplanktonic A. distans to current dominance by
planktonic C. stelligera (Figure 2(a)–(c)). The trends in
diatom-inferred limnological values were also similar among
the three study lakes. Diatom-inferred TP (B.K. Ginn
unpublished, data not shown) remained relatively stable
throughout the cores and there were no increased
abundances of diatoms typically found in eutrophic systems
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
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TRACKING ANTHROPOGENIC- AND CLIMATIC-RELATED ENVIRONMENTAL CHANGES IN LAKES
Table 2. Pearson correlation coefficients between PCA axis-1 site
scores, and abundances of Cyclotella stelligera and Aulacoseira distans
with 10-year mean temperature at Halifax (1870–2001, n ¼ 128) for
Hebb Mill, Milipsigate, and Minamkeak lakes, Nova Scotia. All
correlation coefficients are significant and a P-level 50.01
PCA
Cyclotella stelligera
Aulacoseira distans
Hebb Mill
Milipsigate
Minamkeak
0.29
0.41
0.78
0.84
0.88
0.87
0.85
0.81
0.98
in species assemblage and diatom-inferred pH (Figure 2(a)–
(c)). The timing of changes in species abundances and diatominferred limnological variables was similar in the three cores
and could be dated to the late 19th century, although low
unsupported 210Pb activities in Hebb Mill Lake hindered
accurate dating below 12.0 cm (1906).
A significant (r ¼ 0:45; P50:001; n ¼ 128) warming trend of
1.58C in temperatures recorded at Halifax (1870–2001) (raw
data from Environment Canada, 2002) correlated with the
increases in the relative abundances of C. stelligera (r ¼ 0:41 to
0.88) and a decrease in A. distans (r ¼ 0:78 to 0.98)
(Table 2). While the temperature increase was somewhat
noticeable in the seasonal temperature trends (Figure 3(a)–
(d)), it was most obvious in the mean annual temperature
(Figure 3(e)) with a five-year running mean smoothing line
ðr2 ¼ 0:21Þ:
DISCUSSION
Figure 2. Relative frequency diagrams showing dominant (>5%)
diatom taxa (arranged by PCA axis-1 score) with diatom-inferred pH
and Gran-alkalinity with PCA axis-1 scores, showing the main
direction of variation for: (a) Hebb Mill Lake; (b) Milipsigate Lake;
and (c) Minamkeak Lake.
(Figure 2(a)–(c)). Diatom-inferred pH increased slightly in
conjunction with the change in diatom assemblage, and
diatom-inferred Gran-alkalinity increased with the changes
Copyright # 2008 John Wiley & Sons, Ltd.
Results of the diatom species assemblage analyses revealed
similar overall trends in Hebb Mill, Milipsigate and
Minamkeak lakes (Figure 2(a)–(c)). Most apparent is the
decrease in the more heavily silicified, tychoplanktonic A.
distans species with a parallel increase in the smaller,
planktonic C. stelligera.
The shift in dominant diatom taxa since 1850 compares
with species changes recorded by other studies (Catalan et al.,
2002; Sorvari et al., 2002; Rühland et al., 2003; Rühland and
Smol, 2005; Smol et al., 2005; Harris et al., 2006), which have
been interpreted as indicative of climatic warming. The timing
of the changes in diatom assemblages (1850) was correlated
with a warming trend in the instrumental record. Instrumental
temperature data (Environment Canada, 2002) from Halifax
(Figure 3(e)) record an increase of 1.58C since 1870 and
a significant increasing linear trend (r ¼ 0:45; r2 ¼ 0:21;
P50:001). This trend was recognizable in the winter season
but was most apparent when analysed on a mean annual basis
(Figure 3(e)) with a 5-year running mean smoothing line.
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
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B.K. GINN ET AL.
Figure 3. Air temperature data recorded at Halifax (Nova Scotia) during the period 1870–2002 (data from Environment Canada, 2002) showing
trends for: (a) spring; (b) summer; (c) autumn; (d) winter; and (e) mean annual temperature. A five point (5 year) smoother trend line is shown as a
solid black line on the mean annual temperature graph.
Similarly, instrumental data from the neighbouring province
of New Brunswick has also shown an increase in temperature,
with a related change in diatom assemblages, although most
pronounced during summer and starting 1900 (Harris et al.,
2006).
Analysis of the diatom stratigraphies for the three study
lakes showed that the diatom assemblages began to change
markedly 1850 for Hebb Mill and Milipsigate lakes (Figure
2(a)–(b)), and in the late 19th century for Minamkeak Lake
(Figure 2(c)). These observations are consistent with other
studies (Karst-Riddoch et al., 2005; Harris et al., 2006) which
also recorded major changes in diatom assemblages beginning
around the turn of the 20th century and related to climatic
warming. As with all palaeolimnological studies, it is
acknowledged that 210Pb dating provides only an estimate of
the time horizons. While unsupported 210Pb dating was
relatively high and estimated the expected exponential decay
with core depth for Milipsigate and Minamkeak lakes, results
from Hebb Mill Lake were lower and had larger error
estimates below 12.0 cm (1906). The resultant errors in
estimated 210Pb dates prevents the direct comparison of Hebb
Copyright # 2008 John Wiley & Sons, Ltd.
Mill Lake diatom assemblages from 1850–1870 with the
instrumental record and consequently diatom assemblages
from 1816 (20.0 cm interval of the sediment core) were used
as a proxy of background conditions. We are confident in
using this (20.0 cm) interval as a direct comparison to 1870
instrumental data, as the other two study lakes, and 164 other
Nova Scotia study lakes (B.K. Ginn unpublished), do not
show any significant change in diatom assemblages before
1850.
The actual shift in diatom species dominance from A. distans
to C. stelligera is correlated with a warming temperature trend
(Table 2) and has been found to be related to climate-induced
thermal stratification in other lakes (Gibson et al., 2003;
Forsström et al., 2005). A climatic warming trend would
probably lead to declines in the duration of ice cover and
increased periods of open-water conditions (Smol, 1988). As a
result there are longer periods of lake stratification leading to
an increase in planktonic habitats. This could explain the
increase in the planktonic C. stelligera abundances as the lakes
became more habitable for this species owing to increased
planktonic habitats. C. stelligera frustules are relatively small
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
TRACKING ANTHROPOGENIC- AND CLIMATIC-RELATED ENVIRONMENTAL CHANGES IN LAKES
and not heavily silicified, and are likely to be favoured by
thermal stratification because they are able to remain in the
photic zone for longer periods (Catalan et al., 2002). Previous
studies have also linked the abundance of Cyclotella taxa with
stratification and longer growing seasons (Sorvari et al., 2002;
Rühland et al., 2003; Forsström et al., 2005). In contrast, A.
distans is a more heavily silicified diatom species, which
requires turbulent waters to maintain its position in the photic
zone (Agbeti et al., 1997; McCausland et al., 2001; Gibson
et al., 2003).
The observed trend in changing diatom species assemblages
(relative decrease in A. distans and an increase in C. stelligera)
is very apparent for Hebb Mill and Milipsigate lakes (Figure
2(a), (b)); the trend is less apparent and occurs 20 years later
for Minamkeak Lake (Figure 2(c)). This observation may be
due to the fact that Minamkeak is larger, and as such will
stratify less and later than the other two study lakes, and thus
respond more slowly to thermally induced changes (Wetzel,
2001).
In addition to these diatom records from Nova Scotia, and
the study undertaken in nearby New Brunswick (Harris et al.,
2006), the trend of increases in Cyclotella coinciding with a
decrease in Aulacoseira has also been observed in other parts
of the world and related to climatic warming. For instance,
Rühland and Smol (2005) found similar trends of increasing C.
stelligera abundances with decreasing Aulacoseira species
abundances (also 1850) in a tundra lake of the Northwest
Territories, Canada. Similarly, Rühland et al. (2003) noted
increased relative abundances of C. stelligera since preindustrial times in 50 lakes studied near the Canadian Arctic
treeline. They, too, concluded that recent climatic warming
was the most likely driver for diatom shifts. However, current
research is revealing that this trend is also occurring in more
temperate regions (Forrest et al., 2002; Harris et al., 2006).
Climate is known to influence a host of limnological
variables and could be responsible for other small changes in
diatom-inferred limnological variables recorded in these
sediment cores. For instance, as temperatures increase, inlake alkalinity generation may increase (Webster et al., 1990;
Psenner and Schmidt, 1992; Schindler et al., 1996). Higher
diatom-inferred alkalinity levels were recorded in all three
study lakes and coincides with changes in diatom assemblages
(Figure 2(a)–(c)). Thus, warmer mean temperatures may also
be driving increased in-lake alkalinity generation (and
subsequent pH increases) in these three lakes. In addition,
with the longer ice-free periods that result from warmer
temperatures, lakes may experience increased biological
activity (Schindler et al., 1996).
While other lakes in south-western Nova Scotia have been
affected by acidic deposition (Ginn et al., 2007(a), (b), (d)), the
three study lakes are similar to most of the Ginn et al. (2007(a),
(b), (d)) study lakes in the Bridgewater area which, despite
Copyright # 2008 John Wiley & Sons, Ltd.
1223
being on poorly weathered bedrock and having precipitation
of a low pH (Wiltshire and Machell, 1981), have not recorded
an acidification trend (Ginn et al., 2007a). In fact, as noted
above, the diatom-inferred pH and alkalinity in each of the
three study lakes increased slightly (Figure 3(a)–(c)) in the
second half of the 19th century.
Eutrophication due to increased loading of limiting
nutrients (nitrogen and phosphorus) is not recorded in the
palaeolimnological profiles. While the diatom inference model
for total phosphorus is relatively weak (r2boot ¼ 0:24;
RMSEP ¼ 0:46), no diatom taxa which are typical of
nutrient-enriched conditions were recorded in high
abundance. In addition, C. stelligera is known to indicate
generally nutrient-poor or oligotrophic conditions (Wunsam
et al., 1995; Ginn et al., 2007c), and the current lake water total
phosphorus concentrations of these lakes are also considered
oligotrophic (TP ¼ 5–10 mg L1). It is unlikely that changes in
diatom assemblages are related to other human activities (e.g.
land clearance, construction of dams) as the area has been
continuously settled since 1632 with the first impoundments on
Petite Riviére 1790 for tanning and wool carding. The dams
on this river were constructed between 1901 and 1903
(Fisheries and Oceans Canada, 2006), which does not
correspond with the timing of the observed diatom changes
(Figure 2(a)–(c)).
Based on diatom reconstructions, this study suggests that
climatic change has probably had the most significant impact
on the Atlantic whitefish lakes in comparison with stressors
such as acidic deposition, eutrophication, and landscape
alteration. Thus, it is important to determine what effect
warmer temperature regimes may have on these endangered
species. While little is known about the ecological requirements
of this taxon, it has been suggested that the Atlantic whitefish
has preferences for deep water habitats (Edge and Gilhen,
2001). If Atlantic whitefish reside in the lower (colder)
hypolimnion, a warmer climate may result in less available
habitat owing to the shrinkage of the hypolimnetic, cold-water
layer of lakes (Schindler, 1998). Temperature profiles taken of
Hebb Mill Lake in 1983 showed a lack of cool hypolimnetic
waters, which may indicate a threat to Atlantic whitefish
habitat (Edge and Gilhen, 2001), although current monitoring
of limnological variables on all three lakes is being used to
assess environmental changes. With the results of this study,
we can now assess how current environmental conditions
compare with those from periods pre-dating instrumental
records.
In conclusion, changes in diatom assemblages in Hebb Mill,
Milipsigate, and Minamkeak lakes appear to be the result of
a climatic warming trend that may eventually alter habitat for
Atlantic whitefish. While these lakes at present are not greatly
affected by acidic deposition or nutrient loading, as are some
other Nova Scotia lakes, climatic changes seem to be the main
Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 1217–1226 (2008)
DOI: 10.1002/aqc
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B.K. GINN ET AL.
driver of ecological changes. So far, these limnological changes
have been relatively minor but should be taken into
consideration for managing these lakes as both a freshwater
resource and habitat for this critically endangered species.
In addition, further studies should be carried out on the
physiology and ecology of Atlantic whitefish to determine
their habitat requirements (e.g. temperature and oxygen)
so that the future impacts of climatic warming can be
more thoroughly assessed. In a broader sense, this study
has shown how palaeolimnological data can provide useful
information to conservation biologists, lake managers, and fish
habitat specialists regarding the management of ecosystems.
ACKNOWLEDGEMENTS
This study would not have been possible without the assistance of
J. LeBlanc, J. McMillan, T. Crandlemere (NS Inland Fisheries);
D. Taylor (NS Environment); L. Lawlor, L. Stewart, A. Tropea,
E. Johnson, and S. Pla for field assistance; as well as K. Rühland
for helpful comments. We thank R. Flower and another anonymous reviewer for their constructive comments, which improved
the quality of this manuscript. This project was funded by an
NSERC Strategic Grant to JPS, BFC and P.J. Dillon.
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