Summer Phytoplankton Community Structure in the Cascaded Reservoirs of the Hongshui River Basin

Hongshui River stretches across Yunnan,Guizhou and Guangxi Provinces,with a length of 1 553 km and basin area of 130 870 km2.The cascaded hydropower development on the Hongshui River has provided huge benefits in terms of electricity generation,irrigation,water supply,tourism as well as the conservation of water and soil.However,changes in the water quality,hydrologic condition and river morphology of the Hongshui River caused by water resource development has resulted in eutrophication,loss of biodiversity and habitat degradation.In this study phytoplankton species composition,density,biomass,spatial distribution and biodiversity were investigated in the six cascaded reservoirs(Longtan,Yantan,Dahua,Bailongtan,Letan and Qiaogong) and the main tributaries of the Hongshui River basin.Sampling was conducted at 28 sites in June and July,2009.Phytoplankton samples for qualitative analysis were collected with a No.25 plankton net and samples for quantitative analysis were collected using 2 L of raw water concentrated to approximately 30 m L.Subsequent counting and classification under a microscope gave 84 phytoplankton species,belonging to 75 genera and 6 phyla,dominated by Bacillariophyta(51species),Chlorophyta(17 species) and Cyanophyta(10 species) accounting,respectively,for 60.71%,20.24% and 11.90% of the total phytoplankton species and Pyrrophyta,Chrysophyta and Rhodophyta accounted for 2 species(2.38%) each.The dominance index indicated that Bacillariophyta and Chlorophyta were the dominant populations in Longtan and Bailongtan Reservoirs;Bacillariophyta,Chlorophyta and Cyanophyta dominated in Yantan,Dahua and Letan Reservoirs;Bacillariophyta,Chlorophyta,and Chrysophyta dominated in Qiaogong Reservoir.Phytoplankton density in the cascaded reservoirs ranged from 2.12 × 105 ind./ L to 6.03 × 105 ind./ L,with the highest value 6.03 × 105 ind./ L in Bailongtan Reservoir and the lowest value 2.12 × 105 ind./ L in Longtan Reservoir.Phytoplankton biomass in the cascaded reservoirs ranged from 0.60 to 2.45 mg / L,with the maximum value in Longtan Reservoir and the minimum value in Dahua Reservoir.The Shannon-Wiener diversity index,Pielou evenness index and the similarity index of phytoplankton composition ranges were 2.14-3.22,0.56-0.85 and 0.35-0.70,respectively.The highest degree of similarity was between Longtan Reservoir and Qiaogong Reservoir,while the lowest degree of similarity was between Yantan Reservoir and Bailongtan Reservoir.As a result,the water quality of Longtan Reservoir was mesotrophic-oligotrophic and the water quality of the other five reservoirs was oligotrophic based on the comprehensive assessment of the phytoplankton community indexes in these areas.