Impact of flood regime on phytoplankton communities in the large African reservoir, Lake Nasser, Egypt

Lake Nasser is a reservoir of freshwater for drinking and irrigation in Egypt and it constitutes an important share in the fisheries sector. This study aims to acquire a better understanding of the status of phytoplankton distribution in Lake Nasser under the current flood regime and before the construction of the Grand Ethiopian Renaissance Dam (GERD). Samples were collected from fifteen sites representing different lake sectors along the main channel of the lake during 2016−2017. Approximately 103 species were recorded, belonging to six different classes, dominated by Cyanobacteria, Bacillariophyceae, Chlorophyceae, whereas Euglenophyceae, Dinophyceae and Chrysophyceae were rare. Based on the abundance of phytoplankton species, the pre-flood and post-flood seasons were very well discriminated. A major blooming of Cyanobacteria in the Tushka sector was detected during the pre-flood season, whereas this phenomenon diminished in the post-flood period when Bacillariophyceae and Chlorophyceae appeared. The results of this study indicated that the most important factors affecting phytoplankton species distribution during pre-flood and post-flood seasons are temperature, nutrients and total dissolved solids. The study confirms that flood regime substantially affects the phytoplankton abundance and distribution in Lake Nasser.纳赛尔湖是埃及用于饮用水与灌溉的淡水水库，在该国渔业领域占据重要份额。本研究旨在更深入地明晰当前洪水情势下，以及埃塞俄比亚复兴大坝（Grand Ethiopian Renaissance Dam, GERD）建成前，纳赛尔湖浮游植物的分布现状。研究于2016—2017年间，沿湖泊主河道布设15个代表不同湖区的采样点位并采集样本。本次调查共记录浮游植物物种约103种，隶属于6个纲，其中蓝藻纲（Cyanobacteria）、硅藻纲（Bacillariophyceae）与绿藻纲（Chlorophyceae）为优势类群，裸藻纲（Euglenophyceae）、甲藻纲（Dinophyceae）及金藻纲（Chrysophyceae）相对稀少。基于浮游植物物种丰度，可清晰区分洪前与洪后两个季节。洪前季节，图什卡湖区检测到蓝藻大规模水华；而洪后阶段，该现象消退，硅藻纲与绿藻纲成为优势类群。本研究结果显示，影响洪前、洪后季节浮游植物物种分布的核心环境因子为温度、营养盐及总溶解固体。本研究证实，洪水情势对纳赛尔湖浮游植物的丰度与分布具有显著影响。