Potential and observed submerged species richness in different lake types (Q1.1)

The comparison of the species richness of submerged macrophytes across lake types shows that in both – observed and potential species richness – the highest species richness appears in intermediate lakes. Intermediate peaks can be explained by geometric constraints of boundary effects, called “mid-domain effect” (Colwell & Lees, 2000). This could play a role especially for the potential species richness as the selected parameter spaces correspond to the given conditions and might create overlap effects. Furthermore, the resources that macrophytes need for growth are differently distributed between lake types. Most nutrients are available in turbid lakes, while most light is available in clear lakes. Intermediate lakes are situated in the middle of that gradients, providing an intermediate level of both resources at the same time (Figure 5). Therefore, the highest share of species can find their niches at the crossing point of multi-dimensional resource availability. Species richness along multi-dimensional, interdependent, and opposing gradients of resources can create non-linear responses (Pausas & Austin, 2001). Single scale and single dimension are not appropriate to depict the patterns of species richness (Mirochnitchenko et al., 2021). In future studies, the community structures could be explored more in depth with this framework. In line with other studies (Lewerentz et al., 2021; Mellin et al., 2010; Ruiz-Benito et al., 2012; Toledo et al., 2012), we see that the multidimensionality of environmental gradients are worth disentangling to understand species richness patterns.