Fig. 1

Conceptual diagram illustrating different perspectives on the relative importance of ecological processes in bacterioplankton metacommunity assembly along hydrological network. The colored circles represent local aquatic conditions. The arrows represent dispersal (width reflects relative importance and, color represents ecosystem connectivity: aquatic in white and soil-aquatic in black). The balance between ecological processes (selection and dispersal) reflects species sorting or mass effect change along the hydrological network. Font size corresponds to relative importance of each process along the network, and color is related to the potential source (water in blue and soil in red). The first perspective (A) predicts that (i) headwater communities are regulated by selection because they are isolated, thus community assembly is described by species sorting, whereas (ii) downstream communities are described by mass effects due to a surplus of dispersal. Therefore, β diversity is expected to decrease with Strahler order. The second (B) and third (C) perspectives add the terrestrial connections of aquatic network with the surrounding soil (dotted rectangle). However, B is a special case of C. B proposes that i) community assemblages in headwaters are predominantly shaped by mass effects, driven by the high dispersal of soil taxa into the water channel (this perspective overlooks terrestrial heterogeneity, assuming that dispersed soil bacteria are uniform across different headwaters), and ii) downstream communities are increasingly influenced by selection exerted by similar local aquatic conditions due to higher water retention time. Hence, β diversity is expected to be more similar in communities from low- and high-order Strahler sites. C proposes an increase in headwater selection due to local environmental changes resulting from landscape heterogeneity (colored patches in background). Therefore, β diversity patterns along the network are expected to be modulated by the degree of landscape heterogeneity (boxplot boxes size).