Succession Of Phytoplankton Assemblages In Xiangxi Bay Of Three Gorges Reservoir

Since the impoundment of the Three Gorges Reservoir(TGR), the eutrophicationproblem of the XiangXi Bay (XXB) has become increasingly serious and the waterenvironment has significantly changed. The algal blooms in XXB have become a hotissue studied by many researchers. In order to know the succession of phytoplanktonassemblages and mechanism of algal blooms,we carried out detailed field monitoring ofphytoplankton community structure and habitat conditions in XXB; discussed the mainenvironmental factors that affect the succession of the phytoplankton community structure;summarized the succession of phytoplankton community structure model, divided the bayinto different ecological sections and discussed the response mechanism of phytoplanktoncommunity structure to reservoir scheduling. Through monitoring and analysis, the mainresults are obtained as follows:(1) Spatial and temporal distribution characteristics of phytoplanktoncommunity structureA total of87genera representing7phylum [note: phyla (singular), phylum (plural)and genus (singular), genera (plural)] were monitored in XXB in2010,and it was72genera representing7phylum in2011. The phytoplankton community was dominated byChlorophyta and Bacillariophyta,and they were more in spring and summer while less inautumn and winter.There were little differences among each monitoringsection.Dinoflagellates generally occupied a large proportion in JAN. andFEB.,Chlorophyta and Bacillariophyta were dominated in spring, Cyanobacteria generallyappeared in summer and all the species occupied a certain proportion in winter.Algal celldensity, biomass levels in2011were significantly lower than those in2010. On the spatialdistribution, Dinoflagellates generally occupied a large proportion at upstream,Chlorophyta and Bacillariophyta distributed throughout the bay.Algal blooms outbroke throughout the year in2010,however it outbroke only inspring and summer in2011.The intensity of algal blooms in2011was weaker than that in2010.The Dinoflagellate bloom was observed at the end of XXB in2010,and thePeridiniopsis sp. was identified as the predominant species. In the spring algal bloom,phytoplankton displayed an obvious succession, from Dinophyta to Bacillariophyta andChlorophyta. Dinoflagellate, Chlorophyta and Bacillariophyta blooms also outbroke insummer and autumn,besides, the Cyanobacterial bloom was observed in the whole XXB in summer. Chlorophyta and Bacillariophyta were the predominant species in spring andsummer in2011and it displayed an obvious succession, from Bacillariophyta toChlorophyta.The frequency of algal blooms in2011was less than that in2010.(2) Phytoplankton habitat characteristics and variation in XXBIn winter,the water of midstream and downstream mixed evenly while upper streamstratified weakly. In summer, upstream stratified strongly and downstream stratifiedweakly and the stratified area expanded gradually. After impounding in autumn, waterwas well-mixed, with stratification increasing from downstream to upstream,and itstratified weakly at the end of XXB in the end of impounding.Water in the Yangtze riverwas in mixed state all the year round while it was stratified at the end of XXB.The tendency of surface water temperature changes was consistent between2010and2011. Surface water temperature was the lowest in winter and the highest insummer.It rebounded significantly in spring and decreased obviously inautumn.Temperature difference between surface water and bottom water increasedgradually from downstream to uptream.Water stratified obviously at uptream and lastedfor a long time throughout the year.The water stratification state at downstream andmidstream were seldom affected by upstream flow and water depth but easily affected bythe main stream.The euphotic depth was smaller in spring and summer and larger inautumn and winter,besides,it was smaller at upstream and midstream and larger atdownstream.Mixing depth was the biggest in winter and smallest in summer anddecreased obviously in spring,besides,it was bigger at downstream than upstream.Theratio(Zeu/Zmix) of euphotic depth(Zeu) to mixing depth(Zmix) was less than1duringwinter,it increased in spring with the rebounding of water temperature and decreased inautumn with the impoundment. Water transparency was the lowest in summer and thehighest in winter, with transparency in spring and autumn at intermediate values. Thetrend of seasonal distribution of water transparency was opposite with Chlorophyll a.Mean values of TN was1.10mg/L in2010and0.77mg/L in2011.Concentration ofTN and NO3-N were lower in summer and higher in winter and spring. On the spatialdistribution,concentration of TN and NO3-N reduced gradually from estuary to the end ofXXB.Concentration of TP and PO4-P were lower in spring and at intermediate values inautumn and winter. TP and PO4-P concentration decreased significantly duringimpoundment and with small volatility in dry period. On the spatial distribution,concentration of TP and the proportion of PO4-P in Phosphorus nutrient reduced gradually from the end of XXB to estuary. Concentration of D-Si was lower in spring and summerand higher in autumn and winter,it was higher at estuary and lower at othersections,besides,the space difference in2011was smaller than2010.(3)The model of succession of the phytoplankton community structure in XXBAccording to the ecological functional group-environment response relationship inthe model of CCA,concentration of N, P didn’t significantly effect phytoplanktoncommunity succession and the concentration of D-Si has a limited influence onphytoplankton community succession,which only for Bacillariophyta.Seasonal variationof light and temperature and hydrodynamic conditions were the primary determinants inphytoplankton community succession.Phytoplankton displayed an obvious succession,from stress-tolerators(S) to ruderals(R) and competitors(C) in spring.In summer,itdisplayed an opposite succession,from competitors(C) to ruderals(R) andstress-tolerators(S).Phytoplankton composed of lots of algae in autumn andstress-tolerators(S) was the predominant species in winter.(4)Ecological sections and the response mechanism of phytoplanktoncommunity structure to reservoir scheduling in XXB.Ecological sections were different in XXB with the seasonal variation and reservoirscheduling.XXB was divided into three areas in the pre-flood falling stage, CJXX-XX03,XX04-XX07and XX08-XX10.In the flood stage,it was also divided into three areas,CJXX,XX00-XX03and XX04-XX09. It was divided into three areas in the impoundmentperiod, CJXX-XX05,XX06-XX08and XX09-XX10.In the dry period, It was divided intotwo areas, CJXX-XX07and XX08-XX10. Phytoplankton community structure wasmainly affected by stratification of water column and the ratio(Zeu/Zmix) of euphoticdepth(Zeu) to mixing depth(Zmix) near the upstream.It was affected by stratification ofwater column and nutrient concentration at midstream.All factors had a certain influenceon phytoplankton community structure near the downstream.