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 Parana, Ivinhema and Baia Rivers on the floodplain of the Upper Parana River. For this purpose, we evaluated zooplankton alpha, beta and gamma diversity and applied dispersal analysis to identify the dispersal of species among lakes. In addition, we investigated whether the Baia and Ivinhema Rivers could disperse species to the Parana 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 Parana River. Dispersal of species among all lakes was identified. The highest alpha-diversity values were found in the environments connected to the tributaries, the Ivinhema and Baia Rivers, which also contributed with more than 50 % of the composition of the lakes associated with the Parana River. There was also greater beta-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 gamma-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.