Phytoplankton community in the last undammed stretch of the Paraná River: considerations on the distance from the dam

Abstract Aim: To evaluate the influence of upstream reservoirs on the temporal and spatial variation of the phytoplankton community in the Paraná River channel and the importance of its main tributaries in reducing the oligotrophication process along this stretch of the river. Materials and Methods Sampling of phytoplankton and physical and chemical variables was performed quarterly between August 2013 and May 2015, in the Paraná River main channel and in the mouth of the main tributaries, at the stretch located between downstream Porto Primavera reservoir and the backwaters of Itaipu reservoir. To summarize the variation of phytoplankton density in relation to the main physical and chemical variables, we performed a Redundancy Analysis (RDA). Results A total of 214 taxa were identified throughout the study. We registered low values of phytoplankton species richness and abundance, which showed no patterns of temporal and spatial distribution, both for the Paraná River and the tributaries. However, RDA results showed spatial segregation among samples from the Paraná River main channel, such that the stretch located closer to the dam was associated with higher values of water transparency and Cyanobacteria density, exported by upstream reservoirs, whereas the middle and lower stretches were associated with higher phytoplankton complexity and higher nutrient concentration and turbidity. Conclusion Through the analysis of the phytoplankton community we could verify a reduction in the effects of oligotrophication along the longitudinal axis of the Paraná River and the important role of the tributaries in diminishing this effect.

**摘要 研究目的**：评估上游水库对巴拉那河主河道浮游植物（Phytoplankton）群落时空分布特征的影响，以及其主要支流在缓解该河段寡营养化过程中的重要作用。
**材料与方法**：2013年8月至2015年5月期间，每季度对位于波尔图普里马韦拉水库下游与伊泰普水库回水区之间的巴拉那河主河道及主要支流河口开展采样，测定浮游植物与理化指标。为探究浮游植物密度随主要理化变量的变化规律，本研究采用冗余分析（Redundancy Analysis，RDA）进行统计分析。
**研究结果**：本研究共鉴定出214个浮游植物分类单元。巴拉那河主河道及支流的浮游植物物种丰富度与丰度均处于较低水平，且未呈现显著的时空分布规律。但冗余分析结果显示，巴拉那河主河道的采样样本存在空间分异：靠近大坝的河段与更高的水体透明度及上游水库输入的蓝藻（Cyanobacteria）密度呈显著关联；而中、下游河段则与更高的浮游植物群落复杂性、营养盐浓度及浊度相关。
**研究结论**：通过对浮游植物群落的分析，本研究证实巴拉那河纵向上的寡营养化影响呈逐步减弱趋势，且主要支流在缓解该效应中发挥了重要作用。