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Permission to access the Turkey Lakes Watershed for lake sediment core sampling was granted to SAF in May 2011 from the Canadian Forest Service (Natural Resources Canada). Lake cores were collected June 2011 at a water depth of 4.5 m, approximately the deepest point in the lake; an anchored coring platform was used to maintain position during core collection. A 7-m lake sediment core sequence (Core WS03) was collected in 1-m sections using a Livingstone piston corer [44]. Woody debris at the base of the core prevented the recovery of sediments below 7 m depth. Because this coring system is not able to collect the uppermost sediments in an undisturbed fashion, a gravity corer [45] was used to collect replicate 26-cm long surface cores (Cores WS02 and WS04) that included the undisturbed sediment-water interface. The two surface cores were taken within <1 m of each other, both contained an undisturbed sediment-water interface, and thus were considered coeval. The Livingstone piston core sequence was collected from the same core hole as the WS02 surface core; the first drive of the Livingstone core sequence was set to commence at the base of the WS02 surface core section. Livingstone core sections were extruded in the field into PVC tubing, and wrapped in plastic wrap and then aluminum foil. The surface cores (WS02 and WS04) were immediately extruded in 1-cm increments.
An age-depth model was produced from the combination of 210Pb activity, the rise in Ambrosia pollen, and radiocarbon dating. 210Pb dating was performed by drying sediments at 60°C and grinding samples at 1-cm increments. Samples from the surface of the core to depth of 70 cm (WS04/WS03 sequences) were submitted for measurement of 210Pb activity by alpha spectroscopy at Flett Research Laboratory (Winnipeg, Manitoba) (Table 2). Five 14C dates were selected between the depths of 150–700 cm in the WS03 core sequence; the selected sediment samples were washed with distilled water through a 90-μm mesh. Plant material was selected using a stereomicroscope, and submitted to Beta Analytic Inc. (Miami, USA) for Accelerator Mass Spectrometry (AMS) dating (Table 3). Dates were calibrated using the IntCal09 calibration curve [46] and the program CALIB [47].
Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0159937.t002 Po-210 activities used to model ages in Wishart Lake core WS04/WS03.Po-210 is assumed to be in secular equilibrium with Pb-210. Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0159937.t003 Wishart Lake Sediment Core (WS03) AMS radiocarbon dates.Dates were calibrated using the INTCAL09 calibration curve and the program Calib v6 [47]. The Ambrosia pollen rise was used to confirm the recent chronology and the 210Pb dates. The Ambrosia pollen rise in Ontario is known to have occurred between 1830 and 1880 AD with precise ages for the Ambrosia rise assigned based on local history of forest clearance and disturbance associated with Euro-Canadian settlement [48]. The Ambrosia rise in the TLW region is placed at ~1880 AD as this was the time of initial railway development, early mining and settlement in the local area [49, 50]. The Ambrosia peak was determined in the Wishart Lake record by processing 32 samples for pollen analysis from the top 65 cm of the record using standard procedures involving acid digestion and sieving [51]. Frequencies of Ambrosia and non-Ambrosia pollen were identified on a light microscope at 400x magnification for a total of 200 pollen grains. Exotic Lycopodium spores were used as markers to determine concentrations of pollen per ml sediment. Two cores collected from Wishart Lake in 1980 AD [52] have data available on the rise in Ambrosia pollen and these were used to validate the chronology proposed here. An age-depth model was developed using the date of core collection as an upper constraint (2011 AD), the five radiocarbon dates, the lower-most 210Pb date (50 cm depth), and the Ambrosia rise (40 cm depth), using the clam package for R [53]. The model was based on linear interpolation between dating points, with calculations at 95% confidence ranges and 1000 iterations. Ages were calculated every 1 cm from 0 to 697 cm (Fig 2). Ages for the pre-industrial Holocene are discussed as calibrated calendar years before 1950 AD (cal yr BP); more recent ages are discussed using calendar date (ie. 1950 AD).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0159937.g002 Age-depth model for the Wishart Lake sediment record.The model was developed using a linear regression model fit to calibrated radiocarbon dates, recent chronological data points derived from the Ambrosia pollen rise (depth 40 cm) and activity of 210Pb (lower boundary used, depth 50 cm). Radiocarbon dates (the five lowermost points) are shown with histograms corresponding to the probability distribution of calibrated dates [53]. Figure generated using clam for R [53].
Point samples from the Wishart Lake core were taken at 10-cm intervals for diatom and chrysophyte cyst and scale analysis, providing an inter-sample resolution of ~100 years. Higher resolution sampling at 1-cm intervals was performed for the upper-most 100 years of the core for an inter-sample resolution of ~4–5 years. Diatoms, chrysophyte cysts and scales, were concentrated using 10% HCl, followed by 30% hydrogen peroxide [54]. Residues of known concentration were mounted with Naphrax®. Four hundred and fifty diatom valves were counted per slide using an oil immersion DIC objective on a light microscope at 1000X magnification. Taxa were identified using Antoniades et al. [55], Fallu et al. [56], Krammer and Lange-Bertalot [57], Lavoie et al. [58], and Patrick and Reimer [59]. Diatom taxonomy follows current nomenclature used in Algaebase (http://www.algaebase.org/). Nomenclature for Cyclotella sensu lato follows [60]. Loss-on-Ignition (LOI) estimates of inorganic and organic matter in lake sediments were used as a general proxy for sediment provenance, including erosion in the watershed and biological productivity. Standard methods for LOI were applied, with combustion at 550°C and 950°C [61]. LOI550 and LOI950 were measured every 5 cm, except for the upper 26 cm, where it was measured at 1-cm increments. ANOVA was used to compare the LOI550 and LOI950 among the diatom-determined zones. Cluster analysis was used to determine boundaries between diatom assemblage zones in the stratigraphy, with the number of significant zones evaluated by the broken stick model, using the rioja package in R [62, 63]. Diatom and pollen assemblages were plotted as stratigraphies using C2 [64]; C2 was also used to ordinate the samples using Detrended Correspondence Analysis (DCA). To compare our analysis of diatom and chrysophyte indicators with regional paleovegetation and paleoclimate, we extracted pollen and geochronology data for Upper Mallot Lake [20] from the Neotoma database (http://www.neotomadb.org/). Ages were re-modelled for the Upper Mallot Lake pollen record using the top of the core (1993 AD), the three radiocarbon dates included in the database record, and the Ambrosia rise (15 cm depth) with the clam package for R [53]; pollen assemblages were ordinated using DCA and axis scores were plotted. All diatom assemblage data, geochronology results and other core data have been deposited in the publicly accessible Neotoma Database (http://apps.neotomadb.org/Explorer/?datasetid=19788). All slides have been permanently archived by the lead author.
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