Distribution Pattern Of Phytoplankton In The Lower Reaches Of The Yangtze River And Its Correlation With Environmental Factors

The Yangtze River is the largest river in China,and the lower reaches of the Yangtze River is an important support for the“Yangtze River Economic Zone”,the“Belt and Road”,the“Golden Waterway”and the“Urban Agglomeration in the Yangtze River Basin”.Phytoplankton is the most important primary producer and sensitive indicators in the aquatic ecosystem.Studying the distribution pattern of phytoplankton in the lower reaches of the Yangtze River and its correlation with the environment is of great significance for resolving the challenges of resource utilization and ecological protection in the Yangtze River Basin.Based on four surveys of phytoplankton in the lower reaches of the Yangtze River from Poyang Lake Estuary to the Yangtze Estuary with 109 sampling sites from October 2016 to August2017,the community structure of phytoplankton in the main stream(river bend,tributaries,peripheries)was analyzed,the correlation between phytoplankton community structure and environmental factors was discussed,and the water environment status of the main stream of the lower reaches of the Yangtze River was evaluated,which provided scientific basis for water quality assessment and ecological protection of the Yangtze River.The main results are as follows:1.Species composition of phytoplankton in the lower reaches of the Yangtze River:467 species of phytoplankton belonging to 178 genera and 8 phyla in the lower reaches of the Yangtze River were identified,of which,the most abundant was Bacillariophyta(231 species of 82 genera),followed by Chlorophyta,Euglenophyta and Cyanophyta,and fewer species belonged to Chrysophyta,Dinophyta and Cryptophyta,while only 1 genus and 1 species belonged to Xanthophyta.Species number in spring was the most abundant,followed by that in autumn and summer,and the least in winter.From the Hukou to the Yangtze River estuary,the species number decreased gradually.Among them,Anqing-Chizhou section had the largest number of types,followed by Hukou,Zhenjiang section.And Nantong section was the least.Quantitative analysis of river bends,tributaries and periphery showed that there were 176 species of phytoplankton belonging to 85 genera in 7 phyla,196 species belonging to 98 genera in 8 phyla and 196 species belonging to 95 genera in 8 phyla,respectively.The order of species number in the river bends was Chlorophyta,Dinophyta,Euglenophyta and Cyanophyta.And that was Chlorophyta,Dinophyta,Cyanophyta and Euglenophyta in tributaries and periphery.In terms of seasonal variation,species number of the river bends and tributaries were higher in autumn than in winter and the least in summer;species number of the periphery was also high in autumn,followed by spring and the least in summer.2.Phytoplankton density and biomass in the lower reaches of the Yangtze River:The average density of phytoplankton in the lower reaches of the Yangtze River was 8.92×10~4 Cell/L.Bacillariophyta accounted for 51.14%,followed by Cryptophyta,Chlorophyta,Cyanophyta,Chrysophyta,Euglenophyta and Dinophyta.Xanthophyta could only be detected in summer of the third section.The average biomass of phytoplankton was 0.033 mg/L and Bacillariophyta accounted for 54.24%,followed by Cryptophyta,Euglenophyta,Chlorophyta,Dinophyta,Cyanophyta and Chrysophyta.The Xanthophyta was very low.The density and biomass were the highest in winter,lower in summer and autumn,and the lowest in spring.From the Hukou to the Yangtze River estuary,the density and biomass of phytoplankton showed a decreasing trend with diatom dominated.The average density of phytoplankton in the river bend was 371.38×10~4 Cell/L,and the biomass was 1.05 mg/L.Density showed the advantage of Cyanophyta,Chlorophyta,Bacillariophyta and biomass showed the advantage of Bacillariophyta,Euglenophyta,Cryptophyta.The average density and biomass of the tributary were105.52×10~4 Cell/L and 0.46 mg/L respectively.The density showed the advantage of Chlorophyta,Cryptophyta,Bacillariophyta and the biomass showed the advantage of Euglenophyta,Bacillariophyta,Cryptophyta.The average density of peripheral phytoplankton was 443.59×10~4 Cell/L,and the biomass was 1.63 mg/L.Density showed the advantage of Chlorophyta,Bacillariophyta,Cryptophyta and biomass showed the advantage of Bacillariophyta,Cryptophyta,Chlorophyta.3.Distribution and variation of dominant species of phytoplankton in the lower Yangtze River:Bacillariophyta were the dominant species of phytoplankton in the lower Yangtze River:Melosira varians,Aulacoseira granulate and its variaties,Cyclotella were the most dominant while Skeletonema costatum was dominant in the Yangtze River estuary.The biomass of Chroomonas acuta and Cryptomonas erosa were relatively large.Pseudanabaena and Scenedesmus were also abundant.The dominant species in river bends,tributaries and periphery were Bacillariophyta as well:Aulacoseira granulate and its variaties,Melosira varians,Nitzschia were the most dominant.Chroomonas acuta,Cryptomonas erosa showed dominance and Crucigenia tetrapedia,Scenedesmus were dominant.Such as Pseudanabaena,Oscillatoria and Aphanizomenon flos-aquae were the dominant species in tributaries and peripheral.Especially in summer,Microcystis,Anabaena,Merismopedia minima and Pediastrtlm simplex showed dominance.4.Phytoplankton diversity index in the lower reaches of the Yangtze River:Shannon-Wiener index of phytoplankton in the lower reaches of the Yangtze River ranged from 1.50 to 3.08,with an average of 2.49.Simpson index ranged from 0.57 to0.93,with an average of 0.84.Pielou evenness index ranged from 0.45 to 0.90,with an average of 0.71.Shannon,Simpson and Pielou indices had the same distribution pattern:highest in Pengze and lowest in the Yangtze River Estuary.The diversity of phytoplankton was not significantly different among sections except in the Yangtze River Estuary.The situation of Shannon,Simpson and Pielou indices in river bends,tributaries,peripheries were consistent.The diversity of river bends,tributaries and peripheries showed a decreasing trend.5.Factors affecting the phytoplankton community in the lower reaches of the Yangtze River:The species number and biomass of phytoplankton in the tributaries and lakes on both sides of the Yangtze River were larger than those in the mainstream,which were closely related to the flow rate and discharge.The increase of water temperature could promote the species of phytoplankton as Chlorophyta,Cyanophyta,Cryptobacteria and Euglenophyta,among which the Euglenophyta was the most significant.The increase of turbidity(total dissolved solids,TDS)reduced phytoplankton biomass by affecting photosynthesis and sediment deposition.Salinity was the main environmental factor affecting the growth and distribution of algae in the Yangtze River Estuary.Nutrient was not the key limiting factor in the mainstream.6.Functional Groups:By comparing the relationship between taxonomical method,functional groups and environmental factors in Zhenjiang section,it was found that the main environmental factors were the same with the two methods in autumn,spring and summer indicating that both methods reflected objective facts,and the community characteristics were clear,while the latter did not show more superior effectiveness.