Abstract:

Since the initial filling of Three Gorges Reservoir (TGR) in 2003,serious algal blooms have occurred in its tributary bays.The number of tributaries involved showed significant increase,as well as the intensity and frequency of algal blooms.Cyanobacteria blooms have been observed in a numbers of tributary bays and threatened the drinking water security of residents in the TGR region.It was obvious that algal blooms dominant species tended to diversification and seasonal dynamics indicated a trend of changes from the lake type to the river type.TGR water environmental problems have aroused widespread concern recent years.
Reynolds believed that energy utilization and nutrient uptake of algae had a close relationship to its physiological characteristics of morphology,growth characteristics,which decided its growth strategy in the habitat; while similar adaptability of algae species could coexist and form a particular species collection,growths succession accompanied with changes in habitats and present specific ecological model.
Because the tributaries of TGR are deep backwater bay,the mainstream and tributaries of water temperature are different and the difference of particle concentration leads to stratification of density.Significant water temperature stratification exists in tributary bays during spring and summer but the density current may modify the natural condition because the layer receiving the density current intrusion is well mixed.The continuing increase of water level in TGR during the late fall impoundment period influenced plunging depth of the density current and the vertical light and heat conditions of the water body,inducing the succession of the phytoplankton community structure.
This study was carried out based on algae ecological functions and aimed to explore the tributaries of the bay,the phytoplankton community structure in successional trends and the impact factor through monitoring water temperature,phytoplankton and other environment factors from January to December in 2010.The results showed that 18 functional groups were classified based on the phytoplankton species data.B,X1,P,Y,X2,D,Lo,J,MP,G were the common species in Xiangxi Bay.Seasonal development and disappearance of the water temperature stratification were the main factors affecting succession of phytoplankton community structure.In winter,due to the limit of water temperature and light intensity,community structure presented mainly the CS/S strategy.As weak stratification in spring,nutrients was adequate,CR/R strategy species could take advantage.In summer flood season,the CR/R/C/CS strategy algae mixed state was showed,for the reason that on the condition of water temperature stratified,a large number of nutrients were consumed which exacerbated species competition,and the community structure succession to the direction of tolerance nutrient limitation environment,therefore,suitable for the relative stable environment for the growth of S strategy to take advantage.In fall,the increase of disturbance broke the stratification,benefiting R strategy with   more tolerance to the frequent fluctuations in shortterm environment for the growth advantage.