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Published online 2012 March 16. doi: 10.1186/2046-9063-8-6

Figure 2

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Results of circular statistics to quantify direction and magnitude of temporal variation (Schmidt et al., 2007) of δ13C and δ15N values in the 20 study lakes for selected bioata. Periods of temporal change were either seasonal: (A) June to August 2007, (B) June to August 2008, or annual: (C) June 2007 to June 2008 (D) August 2007 to August 2008. The arrow diagrams represent the directional changes in δ13C and δ15N values between time intervals for cladocerans (blue square), copepods (green triangle), and littoral macroinvertebrates (black circle) in each individual lake. Blue arrows represent the angle of change (θ) and the length of the arrow represents the magnitude of change (vector). The mean vector of change (across all lakes and taxonomic groups, μ) is represented by the black line from the origin to the outside of the diagram. The curved line at the end of the mean vector line on the outside of the graph is the 95% confidence interval of the mean vector of change, and p-values indicate if the direction of change was significant for a particular time period. For time period (A) a significant shift was observed, which was associated with increases in both δ13C and δ15N. For time periods (B) and (C), significant shift were associated with either δ13C or δ15N, respectively, and for time period (D) no significant changes were recorded.

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