Seasonal Dynamics Of Iron And Manganese And Their Effects On Phytoplankton Community Succession In Dashahe Reservoir,Guangdong Province

In order to understand the influence of iron(Fe)and manganese(Mn)concentrations on the succession of phytoplankton communities in Dashahe reservoir,we collected phytoplankton samples and measured environmental variables simultaneously from September,2016 to October,2017 in Dashahe reservoir and 5 inflow rivers.The spatio-and temporal variations of Fe,Mn and phytoplankton in the reservoir and inflow rivers were analyzed.The results showed that the average concentrations of total Fe(TFe)and dissolved Fe(DFe)in Baisha River and Dasha River were lower compared to that in Shuangshi River,Fushi river and Wojiang River.Compared to other inflow rivers,the Fushi river channel is narrow and has a slow water flow,such that the concentrations and fluctuation of TFe and DFe remain significantly high.There was no significant difference in concentrations of total Mn(TMn)and dissolved Mn(DMn)in the 5 rivers.The range of concentration of TMn fluctuated between 0.02 and 0.25mg/L,and the range of concentration of DMn fluctuated between 0.003 and 0.08mg/L.At sampling points DS2 to DS5 there was no obvious difference in concentrations of Fe and Mn,but the concentrations of Fe and Mn were gradually decreasing throughout the year.Fe and Mn exhibited a strong gradient at sampling point DS1 in the stratification period of the water body in summer.There was no significant difference in concentrations of Fe and Mn between the 5 sampling points during the water mixing period(winter and spring).A total of 139 phytoplankton genera belonging to 7 phyla were identified.Specifically,72 genera belong to Chlorophyta,25 genera are members of Bacillariophyta,18 from Cyanophyta,and the other phyla account for only a small proportion,which are from Euglenophyta,Dinophyta,Cryptophyta,Xanthophyta.The variation in species composition generally exhibited a seasonal pattern.Cyanophyta and Chlorophyta dominated in winter while the biomass of Chlorophyta and Bacillariophyta increased in spring.During summer and autumn,Cyanophyta was dominant.The structural difference of phytoplankton community in 5 rivers was obvious.The phytoplankton in Dashahe River and Baisha River were mainly diatoms during the year,and the phytoplankton community structure of the other 3 rivers changed a lot throughout the year.The results of CCA analysis showed that the community structure of phytoplankton was influenced by many environmental factors in 5 inflow rivers.The change of volume,velocity and nutrient input caused by rainfall and human activities were the main factors affecting the succession of phytoplankton community.And the change of DFe concentration within the reservoir area is the main factor affecting the dominant species of water body,and the change of ammonia nitrogen and nitrate concentration is the main factor driving the succession of most of the other phytoplankton community.