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Ecological Processes
Fig. 4 | Ecological Processes

Fig. 4

From: An approach for finding causal relations in environmental systems: with an application to understand drivers of a toxic algal bloom

Fig. 4

Selected results from first step of the approach. a Result of CART: ‘size of tree’, i.e. the number of its end nodes versus ‘X-val Relative Error’, which is equal to unity minus the cross-validated proportion of explained variance of AL (chlorophyll-a concentrations attributed to diatoms). Complexity parameter ‘cp’ is part of CART and is associated with a certain number of end nodes (Breiman et al. 1984). Vertical bars represent one standard deviation of ‘X-val Relative Error’. b Result of CART: optimum sized tree with three end nodes (squares) and the number of hours (n) assigned to each end node. Ovals represent decision nodes with decisions based on the driving variables electrical conductivity (EC) and river flow (Q) with time lags of 95 h and 80 h, respectively. c Result of linear cross-correlations: time lags (h) versus coefficients of correlation for time series of AL and EC. Red line indicates a time lag of 45 h, which was the maximum correlation coefficient. d Linear cross-correlations: time lags (h) versus coefficients of correlation for time series of AL and Q. Red line indicates a time lag of 81 h, which was the maximum correlation coefficient

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