The roles of environmental conditions and geographical distances on the species turnover of the whole phytoplankton and zooplankton communities and their subsets in tropical reservoirs

Understanding the patterns of biological diversity between different regions (beta diversity) and how diversity is generated and maintained is a central goal in ecology. The relative roles of niche differentiation and geographical distances in shaping the phytoplankton and zooplankton structures are not yet fully determined, especially considering the subsets of species that share particular traits. We evaluated whether dissimilarity in phytoplankton and zooplankton assemblages depends on environmental distances and geographical distances on the whole communities and for subsets. We sampled eight hydroelectric reservoirs over a large scale (1500 km) in central Brazil, during three climatological periods. Phytoplankton composition was correlated with niche differentiation; that is, environmental similarity would account for assemblage similarity, due to the high dispersal abilities and low turnover rates of these organisms. For zooplankton, geographical distances were more important, which resulted in a stronger spatial structure with a greater dispersal limitation. For phytoplankton, the Morphologically Based Functional Group formed by diatoms, and for zooplankton, the three major taxonomic groups appeared as spatially structured. Therefore, phytoplankton composition was mostly correlated to niche differentiation and zooplankton to geographical distances; the communities deconstructed into subsets of species with similar traits refined the responses of the community structures.