Responses Of River Phytoplankton Community To Heterogeneous Hydrological Environment Induced By Damming

The Three Gorges Hydropower Station is the largest hydropower project in Chinese history and the largest hydropower station in the world.After the construction of the Three Gorges dam,hydro-environment in its reservoir has been changed tremendously: With the water level of shorline raising,the water surface became wider,the water depth increased,the water flow velocity decreased,the transparency of the water body increased,and the water exchange time became longer.Phytoplankton is important component and the producer of aquatic ecosystem.Any changes of hydro-environment would directly affect the phytoplankton community and,as a result,the structure and function of the aquatic ecosystem.Since the Three Gorges Project has been completed in 2009,how the phytoplankton communities would respond to the new water environment due to dam is a problem which is not answered but worth exploring.In January,September of the year 2010 and 2015,the phytoplankton communities in the main stream of the Three Gorges Reservoir were investigated.The main results are as follows:(1)Characteristics of water environment changesThe establishment of the Three Gorges Reservoir has significant impacts on its water environment.The hydrological characteristics have been changed dramatically,with the reduced flow rate,increased transparency,and increased water temperature.Those changes are particularly significant in the water body near the dam(lacustrine zone).The nutrient level in the reservoir changed dramatically,as shown by the increase of total nitrogen concentration and the decrease of total phosphorus concentration.No significant difference in water flow rate and water temperature between the year of2015 and 2010,while the transparency increased significantly in the year of 2015 comparing with that of 2010.Moreover,the nutrient concentration of reservoir in the year 2015 was lower than the same period of 2010.(2)Characteristics of phytoplankton community distributionIn 2010,a total of 226 species(including varieties)of phytoplankton algae,which belong to 43 genera and 6 phyla,were detected at 13 sampling sites.In 2015,a total of 341 species(including varieties)of 83 genera and 8 phyla were detected.In 2010,the phytoplankton cell density was ranged from 0.18 cell/L× 104 to 2.42 × 104 cell/L,and in 2015,that range was 1.93-5.63 × 104 cell/L.In 2010 and 2015,the phytoplankton of the Three Gorges Reservoir was dominated by Bacillariophyta,Chlorophyta and Cyanophyta.Pyrrophyta,Euglenophyta and Cryptophyta are relatively rare.In 2015,the number of species and the cell density of all kinds of algae are increased,comparing with that of 2010.The proportion of Chlorophyta and Cyanophyta was raised at the largest,and the proportion of diatoms decreased sharply.There are significant differences in algae composition and cell density between the riverine zone and the lacustrine zone.In particular,the composition and cell density of Cyanophyta and Chlorophyta in the simi-lake zone were significantly higher than that of the riverine zone.The phytoplankton diversity of the Three Gorges Reservoir in 2015 was significantly higher than that of 2010.(3)The succession model of phytoplankton communityThe five functional groups of algae,namely Cyclotella,Synedra,Cosmarium,Fragilaria,Navicula,are the main dominant functional groups in different types of water body in all periods of the years 2010 and 2015.Four of the functional groups are mainly dominated by diatoms,and one functional group is dominated by blue and green algae.In 2015,including all the above five functional groups,12 functional groups dominated by green algae,dinoflagellates,cyanobacteria,euglena,and cryptophyta are increased.And the functional groups represented by Asterionella,Scenedesmu,Euglena,Anabaena,Chlorella,Dactyloccopsis,Phormidium,Pandorina,Peridiniopsis appeared at all sampling sites in 2015.The phytoplankton in the Three Gorges Reservoir has a tendency to evolve from the dominant resistant algae(R type)to the mixture of competitive algae(C type)and the stress type algae(S type).The change of hydrological conditions due to the reservoir establishment is the main factor driving the succession of phytoplankton communities.(4)Factors affecting phytoplankton community successionThe cell density of phytoplankton in the main stream of the Three Gorges Reservoir was significantly positively correlated with transparency and water temperature,and negatively correlated with the water flow rate.RDA analysis of the environmental factors related to phytoplankton functional groups in the Three Gorges Reservoir indicated that some functional groups dominated by blue algae and green algae had strong correlations with water temperature and transparency.Some functional groups dominated by diatoms were highly correlated with the flow rate.