Figure 1.
Location map of the study lakes in North America and Greenland.
Numbers correspond to the descriptions in Table 1. Shaded area shows the extent of the Rocky Mountains.
Table 1.
Locations, physical characteristics, pH, conductivity, and β-diversity values from lakes in this study.
Figure 2.
Examples of diatom stratigraphic records.
Stratigraphies of diatom relative abundance are illustrated for one arctic lake (CF-11, A), two alpine lakes (Curator and Emerald, B and C), and one montane-boreal lake (North Barrière, D). Diatom taxa are ordered by descending DCCA axis 1 score from left to right. β-diversity values corresponding to each time interval are shown on the right. Only dominant taxa are shown, however all taxa >1% were included in β-diversity calculations.
Figure 3.
Boxplots of diatom β-diversity from each lake region.
Black dots represent the median value, boxes are the 25th percentile surrounding the mean value, and dashed ticks indicate range. (A) 20th century results; statistically significant differences exist between alpine and montane-boreal (p = 0.003) and arctic and montane-boreal (p = 0.004) lakes. Results for the 19th century (B) and 1550–1800 (C) reveal no statistically significant differences, in contrast to the 20th century.
Figure 4.
Scatterplots of 20th century β-diversity against latitude and altitude.
In (A), only the arctic and montane-boreal lakes are considered (n = 37), whereas (B) includes the alpine and montane-boreal sites (n = 32). Both relationships are highly significant (p<0.01; p = 0.03).
Figure 5.
Latitudinal distribution of 20th century diatom β-diversity in relation to modeled Nr deposition and recent climate change.
In (A), the horizontal line represents the grand mean 20th century β-diversity value (0.82 SD, n = 52), and vertical dashed lines represent the intersection of this value with the a LOESS smooth curve [48] fitted using a span of 0.75. Using the same approach, curves were fitted to the Nr deposition (B) and climate change (C) trends extracted from gridded data [Materials and Methods], revealing their respective latitudinal trends. The 95th percentile confidence interval is shaded for each curve. Lakes illustrated in Fig. 2 are identified on panel (A).
Figure 6.
Temperature anomalies for arctic (A and B) and alpine (C and D) regions.
(A) regional compilation of arctic (>60°N) instrumental temperature data for the period 1870–2005 (data from HadCRUTv2; http://www.cru.uea.ac.uk/). (B) Arctic multi-proxy paleoclimate reconstructions [18]. (C) NCEP/NCAR reanalysis data for alpine regions of the Canadian and American Rocky Mountains (data from http://www.cdc.noaa.gov/). (D) Alpine dendroclimatic reconstruction, showing temperature anomalies of the last 400 years [19]. The reanalysis data are expressed in relation to the 1961–1990 mean, whereas proxy reconstructions are calculated relative to 1901–1960. The shaded area in (B) indicates standard errors of prediction.
Figure 7.
PCA of site-specific temperature and Nr deposition trends, lake pH, and lake∶catchment area ratio.
(A) Biplot of the two first PCA axes, with 20th century diatom β-diversity included in the PCA as a passive variable, contoured at 0.05 SD intervals using thinplate splines over the ordination space. (B) Lake scores ordered by PCA axis 1 score, with lake numbers corresponding to Table 1, and shading by region.