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Expanded Description of Materials and Methods
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Site Description
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This study focuses on assessing changes to lake ecosystems in the uplands east of the
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Mackenzie Delta, near the northern town of Inuvik, Northwest Territories, Canada (Fig. 1).
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Climate in the region is seasonally extreme. Winters are cold due to the shielding effect of the
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cordillera, which blocks warm, maritime Pacific air, ensuring cold Arctic air masses dominate
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the region (Dyke, 2000). In contrast, during the summer months, southern air masses move
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northward, giving the region the warmest summer temperatures in Canada for its latitude (Dyke,
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2000). The study region was covered by the Wisconsinan ice sheet which reached a maximum
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extent ~38.8 thousand years ago (Duk-Rodkin & Lemmen, 2000). The region is underlain by
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thick, continuous permafrost, with taliks that penetrate through the permafrost present under
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lakes that do not freeze to the bottom due to the insulating properties of water (Burn, 2002).
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Near-surface permafrost is enriched with solutes (particularly calcium and sulphate, which are
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glacially derived from carbonate and shale bedrocks) in comparison to the overlying active layer
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(Kokelj & Burn, 2003, 2005). Vegetation communities common in the lowland shrub tundra just
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north of treeline include primarily members of the Cyperaceae, especially Eriophorum and
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Carex (Ritchie, 1984), although shrub birch (Betula nana subsp. exilis), green alder (Alnus
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viridis subsp. fruticosa), and willow (Salix spp.) are common (MacDonald, 2000). Patches of
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black (Picea mariana) and white (Picea glauca) spruce in a tundra matrix are common where
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climatically favourable sites exist (Ritchie, 1984).
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Water chemistry analyses, sediment core collection and diatom analyses
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Samples for water chemistry analyses were collected from the central portion of the lake
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approximately 1 m below the surface. Samples for determination of major ion concentrations
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were analyzed by ion chromatography at the Taiga Environmental Laboratory (Yellowknife,
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NT). Samples for dissolved and unfiltered total phosphorus (TP) concentration determination
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were analyzed using the colorimetric method for low yield results at the National Laboratory for
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Environmental Testing, Canadian Centre for Inland Waters (Burlington, ON). For lakes from
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which sediment was sampled through the winter ice, nutrient samples were collected the
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following open-water season. Lakes 2a, 2b and 5b were not re-sampled, and thus unfiltered TP
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concentrations were taken from Thompson (2009). Further water chemistry analyses for the 12
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study lakes are available in Kokelj et al. (2009b) and Thompson (2009), including other nutrients
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such as total dissolved nitrogen, which was not sampled at time of sediment sampling.
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Sediment cores from the 12 lakes studied were collected from the centre (deepest
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location) of each lake over a two year period, in April of 2007 (coring through the late winter
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ice) and in July of 2008 (coring from an inflatable raft). In all cases lakes were accessed by
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helicopter. Sediment cores were collected using a Glew-type (1989) gravity corer (internal
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diameter 7.62 cm) and sectioned using a Glew (1988) vertical extruder. To obtain high temporal
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resolution paleolimnological profiles, cores were sectioned at 0.25 cm (for the top 15 cm) and
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0.5 cm (for the remainder of the core) intervals, and stored in individual WhirlPak® bags at 4 °C
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until analysis. Sediment age estimation was established on 10-16 sedimentary intervals from
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each core using 210Pb and 137Cs radioisotopes following standard methods for gamma dating
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(Schelske et al., 1994; Appleby, 2001). For all sediment cores, the constant rate of supply (CRS)
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model (Appleby & Oldfield, 1978) was used to calculate dates, and whenever possible, the peak
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in 137Cs (corresponding to 1963) was used to corroborate the 210Pb dates (Appleby, 2001). For
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each core the cumulative dry weight of the sediment core was compared to the log-transformed
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unsupported 210Pb activity, in order to detect changes in sediment accumulation over time
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(deviation from a linear fit used to detect changes). Linear regression for all cores were found to
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be strong (r2 >0.60), with the slump-affected lakes showing no difference from reference sites,
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with the exception of Lake 9b, which showed a somewhat less robust relationship (r2 = 0.45).
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Laboratory methods for siliceous indicator identification followed Battarbee et al. (2001).
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In summary, sediment was digested in a mixture of sulphuric and nitric acids, and heated in a
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water bath to facilitate the digestion of the organic matrix. After being allowed to settle, samples
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were rinsed daily until a near neutral pH was established. Aliquots of diatom slurry were plated
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onto coverslips, and mounted on microscope slides using the high-refractive index mounting
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medium Naphrax© (Brunel Microscopes, Wiltshire, UK). For each sedimentary interval, at least
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300 diatom valves were identified and enumerated along transects at 1000x magnification.
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Diatoms were identified using a variety of sources (primarily Krammer & Lange-Bertalot, 1991,
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1997, 1999, 2000; Cumming et al., 1995; Fallu et al., 1999).
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Statistical Analyses
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Climate data from Inuvik were obtained from the Adjusted Historical Canadian Climate
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Database (http://www.cccma.ec.gc.ca/hccd/). The SiZer (Sonderegger, 2010) package for the R
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statistical environment (R Development Core Team, 2010) was used to conduct a piecewise
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linear regression (Toms & Lesperance, 2003) on these climate data, with 95% confidence
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intervals determined by bootstrapping (n=1000). Longer term climate reconstructions (~A.D.
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1750 to 1988) based on dendrochronological analyses of samples (Szeicz & MacDonald, 1995;
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Wilson et al., 2007) were obtained online from the National Oceanic and Atmospheric
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Administration (NOAA) National Climate Data Center.
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Relative frequency diagrams of the most common diatom taxa were prepared using the
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computer program Tilia v1.7.16 (Grimm, 2011), with a constrained incremental sum of squares
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(CONISS) cluster analysis included (Grimm, 1987). Detrended canonical correspondence
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analyses (DCCA) were conducted using CANOCO v4.5 (ter Braak & Šmilauer, 2002). The
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approach used here follows Smol et al. (2005) where compositional change in diatoms,
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chrysophytes, chironomids and cladocerans over the past 150 years was quantified and compared
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among a suite of Arctic lakes. For all sedimentary diatom profiles, DCCAs were conducted using
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square-root transformations, no down-weighting of rare taxa and detrending by segments.
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Trends in the relative abundance of benthic fragilarioid taxa in each lake over time were
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standardized using z-scores (i.e. standard scores) and combined into two records (reference and
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slump-affected sites) by calculating the mean z-score for each 210Pb year estimate where multiple
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samples existed. These standardized trends were compared to the yearly air temperature records
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(mean annual and mean winter) from the Inuvik climate station using Pearson correlation
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analyses (temperature data were not smoothed prior to analyses). Significance levels were
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adjusted using Bonferroni probabilities. Hill’s N2 diversity (Hill, 1973) determinations were
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conducted using the vegan package for R (Oksanen et al., 2010).
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