Aquatic Environment And Its Relevance To Algal Community And Growth In Permanent Backwater Area In Pengxi River Of Three Gorges Reservoir

After impounding, water environment of the total Three Gorges Reservoir (TGR) has been significantly changed due to dramatically increased water level and longer retention time in the tributaries. Due to water environment in the tributary permanent backwater area, the situation of algal blooms has been so serious that it has attracted heavy social concern. In order to identify environmental factors involving in the growth of algae in tributaries in fluctuating backwater area water, and key environmental factors that affect their algal bloom outbreak, in April 2013 to June 2014 we carried out routine fixed-point sampling, as well as intensive surveillance during algal bloom on Gaoyang Lake in Pengxi River, the largest river on the northern bank of TGR catchment. By annual routine surveillance found that water column stratification regularity on backwater area, along with which the algal community structure changed accordingly. The main influence factors of algal bloom at backwater area were also discussed in order to provide some theory evidence for algal bloom prevention and control in the area. The main conclusions are as follow.(1) Results show that there was clear thermal stratification at the vertical profile of water column in spring, summer and autumn; however, thermal stratification disappeared during winter in the sampling site. Moreover, the mixing layer depth (Zmix) ranged from 3.8m to 10m. The annual water euphotic layer depth (Zeu) fluctuated from 1.35m to 9.45m, and stayed around 3m from May to September. In the stratified water, it is noticeable that in eutrophic water like in Gaoyang Lake, stratification and high ratio of Zeu/Zmix strongly controlled the activities of algal community.(2) In late spring when algal bloom happened, the algal community consisted of 9 genera of 4 divisions, while the average cell density in the surface water was 2284×104 cell/L, mainly composed by cyanophyta and chlorella groups, and the first dominant genus was composed by Microcystis, Pandorina and Eudorina. However, as the seasons altered, algae structures increased to 52 genera of 7 divisions in autumn. Cyclotella and Synedra became the first dominant genus, and the algal density dropped as high as 14×104 cell/L. Furthermore, the difference in the average cell abundance at multiple layers disappeared. Consequently, a conclusion that water stratification affects the growth and distribution of algal community on Gaoyang Lake is proposed.(3) Before the algal bloom happened at the permanent backwater area on Pengxi River, the weak thermal stratification was observed. Since the beginning of March the depth of 10m water cannot exchange with the range of the upper body of water, and the aqueous layer’s water temperature decreased. As temperatures continue to rise, water mixed layer, thermocline, stagnation layer formed gradually, different layers can be identified obviously. As the Gaoyang Lake affected by upstream runoff, subsurface flow layer appeared at 2.8-7.5m depth range. Before the outbreak of water bloom, as the water environment changes, algae community also will changed, and rapid decline in its diversity, with cynobacteria and green algae increasing from 46% to 89%, and 38% to 63% respectively. The algal structure on backwater area was affected by the combination of in situ aquatic environment and upstream water.(4) Before and after the algal bloom, vertical water column chlorophyll a concentration did not have too much change, only the case to the naked eye to determine whether the algal bloom happened is not accurate manner, chlorophyll concentration of algae in water suspension is the main factors of water bloom degree. Through field monitoring during the algal bloom on Goyang Lake, chlorophyll a concentration just had small fluctuations under normal circumstances within the depth range of 0-5m. Algal blooms happened under normal circumstances is not the reproduction of the algae in a short time, and it is the already existing, suspended algal populations in the water column to gather into groups, floating up under certain hydrological conditions.(5) Based on the contour mapping analysis to the chlorophyll concentration during algal bloom period, the underwater algae growth gathered at 1-2m depth range, formed a high chlorophyll a concentration, and moved to the surface under certain hydrological environment. Under the condition of the external environment disturbance, high chlorophyll a concentration layer disappeared, algae diffused and diluted in 5m depth accompanied by the disappearance of visible blooms.(6) The set-up surveillance during the algal bloom period result showed that these long term and high frequency two monitoring points had mixed algal blooms of cyanophyta and dinoflagellates blooms. CANOCO statistics showed that the growth of the dominant algal species in the middle of Goyang Lake was positively related with the water environmental factors including pH, ORP, Zeu/Zmix and Chlorophyll a, and was significantly affected by the water acidity and water stability. On the other hand, the joining point of Dongxi River, the tributary of Gaoyang Lake, to Gaoyang Lake, the growth of the dominant algae was positively related with Temperature, ORP, pH, Chlorophyll a and Turbidity.