Environmental and not spatial processes (directional and non-directional) shape the phytoplankton composition and functional groups in a large subtropical river basin

Despite wide knowledge of environment-community relationships in inland waters, the relative effects of niche conditions (environment) and dispersal (space) on microorganism communities remain controversial. In lotic waters, non-directional processes (i.e. overland dispersal) and the unidirectional connectivity among sites control species distributions along a gradient. We aimed to detect phytoplankton biogeographical patterns in a large subtropical river (Uruguay River, 1150 km, Brazil) and to isolate the contributions of the environment (limnology, climate and topography) and space in shaping community structure. Phytoplankton (total and functional groups) and abiotic variables were sampled from a hydroplane in motion. To detect biogeographical patterns, we used Mantel's correlation test, and to evaluate the relative roles of environment and space, we used partial redundancy analysis with non-directional symmetrical and directional asymmetrical approaches. Similarity in phytoplankton composition decreased with distance, but only environment significantly explained this pattern. Space was not significant, considering non-directional and directional processes. Phytoplankton communities considering both individual species and their functional groups were more similar between closer sites, because niche conditions play the main role in shaping phytoplankton, not because dispersal is favored among nearby communities. The species-sorting model, based mostly on niche theory, was the main metacommunity mechanism for phytoplankton communities in this river basin.