OTU distribution and protist biogeographic regions in Europe. A) Richness and contribution to beta diversity of individual lakes. Richness is illustrated by grey scale of the symbols: dark shades indicate a high richness whereas light shades indicate a low richness. The size of the symbols indicates the relative contribution to beta diversity, with large symbols indicating a high contribution. Richness of individual lakes varied between 15 and 4092. Lakes in high mountain regions tended to contribute at an above-average level to beta diversity. B) Protist biogeographic regions as calculated by geo-constrained hierarchical clustering. High mountain ranges appear as biogeographic islands; the geographic regions largely comprising high mountain protist communities are depicted in yellow. Furthermore, the lowland areas are biogeographically structured which is mainly due to an east- west-gradient. Even though mountain ranges affect protist community similarity between sites, they do not necessarily separate biogeographic regions.