Zooplankton diversity in a dammed river basin is maintained by preserved tributaries in a tropical floodplain

In this study, we tested the hypothesis that in floodplain lakes connected to a primary river with regulated flow, the zooplankton composition is maintained by the species found in lakes connected to preserved tributaries. Zooplankton were sampled from 23 lakes connected to the Paraná, Ivinhema and Baía Rivers on the floodplain of the Upper Paraná River. For this purpose, we evaluated zooplankton α, β and γ diversity and applied dispersal analysis to identify the dispersal of species among lakes. In addition, we investigated whether the Baía and Ivinhema Rivers could disperse species to the Paraná River Basin through an analysis of source environments. A total of 150 species were observed, and the greatest number of species (115 species) occurred in environments associated with the flow-regulated Paraná River. Dispersal of species among all lakes was identified. The highest α-diversity values were found in the environments connected to the tributaries, the Ivinhema and Baía Rivers, which also contributed with more than 50 % of the composition of the lakes associated with the Paraná River. There was also greater β-diversity in these environments, where the composition was significantly correlated with turbidity, chlorophyll-a and conductivity. The results supported our hypothesis and showed that when the main river is regulated by a dam, the tributaries take on the role of maintaining γ-diversity in the floodplain, favouring the maintenance of the natural characteristics of the system. We propose that preserving the integrity of natural floodplain environments assists in maintaining the regional diversity of the ecosystem as a whole.