The DDG patterns of macrophytes
We showed, that submerged macrophytes in deep lakes have in general a hump-shaped Depth Diversity Gradients (DDG) for alpha richness, a prevailing decreasing pattern for beta richness and a dominantly hump-shaped pattern for gamma richness (Fig. 2a-c) (question 1.1). As we had only significant differences between mid and greater depths for all richness components an even coarser species mapping resolution might be helpful. Our results are congruent to the few existing studies, that show also hump-shaped pattern (Ye et al. 2018) for alpha richness of submerged macrophytes. A simple explanation for the predominantly hump-shaped pattern of alpha and gamma richness might be the mid-domain effect (Colwell et al. 2004): Niches along environmental gradients overlap and build a peak of richness following geometric constrains. Furthermore, the generally decreasing beta DDG might be explained by a change in local species between transects in shallower depths. In shallow water, disturbances resulting from the surface might be more diverse, which may increase coexistence with spatial partitioning of occurrences. As disturbances are coming from the surface, we make the hypotheses that shallow water has a higher environmental heterogeneity which might be the reason for an increased beta diversity.
We see a high variety of DDG shapes between lakes as all these DDGs vary in their maximum richness (Rmax ) and the corresponding depth (Dmax ), but a robust hump-shaped pattern can be seen for alpha richness (Fig. 2d-f) (question 1.2). The high variety of Rmax andDmax is not surprising, as we studied lakes showing a range of physical-chemical properties (Table 1) and gamma richness between 2-35 species per lake. This wide range of species richness and environmental conditions broadens our understanding of the DDG, before limited to one single lake in China (Fu et al. 2014a, b). Although for alpha and gamma richness, hump-shaped curves along depth are predominant (Fig. 2g-i), we also see increasing and decreasing patterns at single lakes. Increasing curves must be hump-shaped as we can safely assume that plant richness should decrease to zero further down in deep lakes. We detected more decreasing DDGs for gamma than for alpha richness, which reflects predominantly decreasing beta richness curves. Nevertheless, besides geometry and disturbances, there must be further variables affecting the DDG, as DDG shape varies between lakes, which themselves have different properties.