The Role of Habitat, Climate, and Space in the Species and Traits Variation of Phytoplankton in Large Cascade Reservoirs.

The construction of cascade dams on large rivers reduces river connectivity, increases environmental homogenization by altering the longitudinal processes responsible for high biodiversity, and may restrict the establishment and survival of species without functional traits in these locations. Our study evaluated phytoplankton beta diversity regarding taxonomic and functional facets in the seven cascade reservoirs of the River Tocantins (Brazil), along 1500 km in length, between 2006 and 2014. We tested whether (1) younger reservoir age leads to higher values of taxonomic and functional phytoplankton beta diversity than the others; (2) whether there is a positive correlation between functional and taxonomic phytoplankton beta diversity in cascade reservoirs; (3) and the taxonomic and functional beta diversity of phytoplankton is more strongly determined by spatial mechanisms than by local hydroclimatic and environmental conditions, with a stronger explanation expected for functional beta diversity. We demonstrated that younger and more distant reservoirs present higher values of taxonomic and functional phytoplankton beta diversity, and that reservoirs in the intermediate regions of the cascade, close to each other and with greater environmental similarity, presented taxonomic and functional homogenization of phytoplankton. Finally, we found that spatial variables were more explanatory of the variation in species and functional characteristics of phytoplankton communities compared to local environmental and hydroclimatic variables.