Variation Of Phytoplankton Community And Its Driving Factors In Hongze Lake

Hongze Lake, as the fourth largest lake in China, has been employed in the east route of the South-to-North Water Diversion Project as one of the significant storing lakes and water channels, and the water quality of this lake is very important for the project. The South-to-North Water Diversion Project of China is the strategic measure relieving water shortages fundamentally in north China, while the water quality in long-distance water diversion is the key factor to determine the successful implementation of the project. It is necessary to confirm the possibility of the occurrence of water bloom during the implementation process of this project. Study of phytoplankton community and spatial distribution can improve water quality and avoid the occurrence of water bloom effectively due to the sensitivity of the phytoplankton to water environmental changes. The project was completed and trial run on the end of May 2013. This project intend to conduct a long-term study before the implementation process, focusing on the water quality, phytoplankton community distribution of regulating lakes on the east route of the South-to-North Water Diversion Project. The environmental factors influencing the cyanobacteria population distribution will be determined in Hongze Lake. Meanwhile, the possibility of occurring water bloom in Hongze Lake was also be discussed accordingly with the survey concerning local hydrometeorological conditions and cyanobacteria growth requirements, which provides advices for aquatic environment protection and prevention for water bloom. In addition, the comparison study for the changes of phytoplankton population distribution before and after the implementation process of the project may provide theoretical basis and database for the evaluation of the influence to aquatic environment brought by the implementation of the South-to-North Water Diversion Project.The thesis represents the research progress on phytoplankton, relationship between phytoplankton variation and environmental factors and phytoplankton studies in Hongze Lake at first, and then put forward the research purposes and methodology for this study.The phytoplankton distributions and the community structures were analyzed in the thesis. From March 2011 to May 2013,201 phytoplankton species which belong to 101 genera in 8 phyla (including Chlorophyta, Bacillariophyta, Cyanophyta, Euglenophyta, Xanthophyta, Cryptophyta, Pyrrophyta and Chrysophyta) have been identified in Hongze Lake. Chlorophyta accounting for 45.8% of the total phytoplankton species was dominant, followed by Bacillariophyta (21.4%) and Cyanophyta (13.9%). The temporal dynamics of phytoplankton were as follows: Chlorophyta-Bacillariophyta- Cryptophyta (spring), Chlorophyta- Cyanophyta-Bacillariophyta (summer), Chlorophyta- Cryptophyta- Cyanophyta (autumn), Chlorophyta- Bacillariophyta- Cryptophyta (winter). The total phytoplankton abundance in winter was less than other seasons. The phytoplankton abundance ranged from 172.2×104 to 3633.3×104 cells/L in all regions, and had an apparent increasing trend in spring. Phytoplankton abundance was apparent difference between three regions:was the highest in North Region (with mean of 1209.7×104 cells/L) due to the slowly water flow rate; was the lowest in Eastern Region (with mean of 981.7×104 cells/L) which including water channels and Jiangba Bay due to the fast water flow rate. In Hongze Lake, Chlorophyta was dominant both in species richness and abundance, and the rate between Chlorophyta and total abundance was high in summer and autumn than in winter and spring. Chlorella vulgaris, Cyclotella comta and Chroomonas acuta were dominant in most months during study period, and Microcystis wesenbergii was dominant only in several months in summer.The functional groups approach proposed by Reynolds has been proved to be a more efficient way to analyze variation in phytoplankton biomass during the last decade, and was applied in Hongze Lake. Due to the turbid condition of the shallow lake, the dominant FGs were those tolerant of light deficiency. The predominant FGs in the succession process were D (Cyclotella spp. and Synedra acus), T (Planctonema lauterbornii), P (Fragilaria crotonensis), Xl(Chlorella vulgaris and C. pyrenoidosa), C (Cyclotella meneghiniana and C. ocellata) and Y (Cryptomonas erosa). Groups C, D and X1 were dominant in the whole lake region during winter and spring seasons, characterized by low temperature and available light. In monsoonal summer with low water level and limited transparency, group T absolutely dominated the Northern region, while group P dominated the Western region. In autumn, characterised by low dissolved nitrogen concentrations, group M (Microcystis wesenbergii) was dominant in water column stable region (Northern region) in 2012.The eutrophic status of Hongze Lake was evaluated by modified trophic state index method (TSIM) and trophic state index method. The results indicated that the lake was between lightly eutrophicated and meso-eutrophicated. Redundancy analysis (RDA) was applied to examine the relationships between the environmental variables and phytoplankton assemblage and to select the best variable describing the functional group distribution. The results of RDA demonstrated that water temperature and water depth were significant environmental variables influencing dominant phytoplankton groups in Hongze Lake. The monsoonal climate and artificial drawdown of water level, which alters the hydrological conditions and influences light and nutrient availability, were the significant driving factors affecting phytoplankton succession in Hongze Lake.The cyanobacteria variation and its driving environmental factors were studied in this thesis. Based on analyzing physical, chemical, biological and other factors causing water blooms, the possibility of water blooms in Hongze Lake was discussed. From March 2011 to May 2013,28 phytoplankton species which belong to 16 genera in cyanobacteria have been identified in Hongze Lake. Cyanobacteria abundance ranged from 172.2×104 to 3633.3×104 cells/L and mean abundance in Northern, Western and Eastern regions were 166.4×104,118.1R104 and 80.8×104 cells/L, respectively. The cyanobacteria abundance of each monitoring station and region was higher in summer and autumn than other seasons, and was higher in 2012 than 2011. Microcystis, the main dominant genus, has a similar variation trend with cyanobacteria in Hongze Lake. The abundance of Microcystis was much higher in Northern region than that of other regions with mean abundance of 108.4×104cells/L. Microcystis wesenbergii dominated at the whole lake scale, with predominant indices ranged from 0.11 to 0.31. A total of 9 genera of cyanobacteria including Microcystis, Aphanocapsa, Anabaena, Pseudanabaena, Raphidiopsis, Oscillatoria, Phormidium, Cylindrospermum and Aphanizomenon, which have been proved to produce toxin, were identified in Hongze Lake. Microcystis wesenbergii was the main toxin-producing cyanobacteria, and often found with high abundance in summer and autumn in Hongze Lake. The highest abundance of Microcystis wesenbergii was found in July 2011 at site Xisunhe with the value of 1293.9×104 cells/L. Microcystis incerta was often found but with low biomass. Other toxin-producing cyanobacteria were occasional identified with low biomass in Hongze Lake.RDA results suggested that temperature, chemical oxygen demand, water level, TP and N:P were key drivers of the cyanobacterial assemblage in Hongze Lake. Temperature was the most important factor affecting cyanobacterial abundance. CODMn was also considered to be the one important factor driving the change in cyanobacterial community composition, indicating that these preponderant species were easy affected by general water quality. Water level fluctuations caused by monsoonal climate and artificial drawdown were also affected cyanobacteria successions in Hongze Lake, since they alter the hydrological conditions and influence light and nutrient availability. Phosphorus was the limited factor for cyanobacteria growth in Hongze Lake. M. wesenbergii was preponderant species of cyanobacteria in the high concentration of total nitrogen and total phosphorus environment in summer. In Hongze Lake, it is possibility to occur water boom in low water flow rate regions such as north region and sluice gate areas at the special hydrometeorological conditions, i.e. slowly water flow, continuously low precipitation, long periods of high temperature and beneficially wind for clustering of cyanobacteria. With the characteristic of short water exchanged time, the lake water could be replaced about 11 times one year, so Hongze Lake did not have necessary conditions for occurring large squares of water blooms. Low possibility of occur water blooms in Hongze Lake also due to the continuously improved water quality in the past few years.