Fig. 3. Threshold analysis for microbial beta diversity (Bray–Curtis dissimilarities) along the chlorophyll a gradient.

The color scale represents Bray–Curtis dissimilarity values computed between sites. The axes stand for chlorophyll a concentration (mg L⁻¹) at each site. In (a), the observed beta diversity patterns are presented along the chlorophyll a gradient, as modeled using XGboost. In (b), a hypothetical relationship is represented where dissimilarity between communities increases linearly as a function of the difference in chlorophyll a between sites. The mean value of the response surface in (a) can be treated as the baseline beta-diversity across all sites. Data points with values below the mean represent higher similarity between sites; likewise, higher values represent lower similarity. To interpret the response surfaces of observed values, one may begin by looking at a point bordering the diagonal and then follow a line of points further up on the chlorophyll a axis. Here, a ridge indicates a chlorophyll a concentration to be a likely threshold from which the shift in bacterial community composition is greater than average. In the same manner, a valley indicates a chlorophyll a concentration likely located on an interval of the chlorophyll-a gradient along which bacterial communities do not shift substantially. More details on the XGboost approach and its interpretation are given in Fontaine et al.⁵⁷.