Phytoplankton Community Characteristics And Driving Environmental Factors Of Regulating Lakes In The East Route Of South-to-North Water Diversion Project

The South-to-North Water Diversion Project (SNWDP) in China is a strategic measure relieving water shortages fundamentally in northern China, and the water quality in long-distance water diversion is the key factor to determine the smooth implementation of the project. Hongze Lake, Luoma Lake, Nansi Lake and Dongping Lake are important regulating lakes and water channels in the east route of SNWDP. Therefore, it is necessary to confirm the water quality condition, distribution of phytoplankton community and the risk of water blooms in the implementation process of this project. So far, there have been more studies with regard to the water quality and distribution of phytoplankton community in the regulating lakes of SNWDP. However, most studies did not last for long and the sampling frequencies were low, and generally these studies about the four regulating lakes were not carried out simultaneously or intending to figure out the differences of phytoplankton community among these four lakes. Besides, the studies concerning to the distribution of harmful cyanobacteria species and the risk of water bloom in the regulating lakes were also less.The water quality and distribution characteristics of phytoplankton communities in regulating lakes before the implementation of east route in SNWDP (from March 2011 to May 2013) were analyzed in this study.11,5,5 and 3 sampling sites were selected in Hongze Lake, Luoma Lake, Nansi Lake and Dongping Lake, respectively. Water quality parameters, composition of phytoplankton community, variation of phytoplankton abundance and dominant species were studied in each regulating lakes. Thus, seasonal succession pattern of phytoplankton community were determined. The similarity degree of phytoplankton community of four regulating lakes and the major diagnostic genera for phytoplankton community dissimilarities were figured out by cluster analysis, non-metric multidimensional scaling and similarity percentage analysis. The driving environmental factors influencing the variation of phytoplankton community in four regulating lakes were determined by redundancy analysis. Harmful cyanobacteria speices and their distribution features were identified in each regulating lake. The grading early warning plan and risk assessment for water blooms were accomplished, which could provide advices for aquatic environment protection and water bloom control in regulating lakes of the east route in SNWDP. Moreover, this study can supply important background data for the evaluation of the influence to aquatic ecology brought by the implementation of large-scale water diversion project, which is of great realistic significance. The main conclusions in this paper are as follows.There were no significant differences on water temperature among the regulating lakes in the east route of SNWDP, while there were significant differences on dissolved oxygen, transparency, pH and water depth. There were significant differences on nutrient materials among the regulating lakes in the east route of SNWDP, while there were no significant differences on Chlorophyll a among these four lakes. According to the evaluation criterion of TSIM trophic state index, Hongze Lake and Nansi Lake were at the state of light eutrophic while Luoma Lake and Dongping Lake were at the state of mesotrophic to light eutrophic.There were 310 phytoplankton species belonging to 127 genera in 8 phyla (including Chlorophyta, Bacillariophyta, Cyanophyta, Euglenophyta, Xanthophyta, Cryptophyta, Pyrrophyta and Chrysophyta) identified in the regulating lakes in the east route of SNWDP. Among these 8 phyla, Chlorophyta accounted for 45.8% of the total number of phytoplankton species and its occurrence frequency was the highest (44.2%), followed by Bacillariophyta (22.9%), Euglenophyta (13.2%) and Cyanophyta (12.3%), while the other phyla had relatively low occurrenc frequencies. The number of detected phytoplankton species in Nansi Lake was the highest, followed by Luoma Lake, Dongping Lake and Hongze Lake.The variation ranges and average values of total abundance of phytoplankton community in Hongze Lake, Luoma Lake, Nansi Lake and Dongping Lake were 247.0～3403.0×104cells/L (1142.4×104cells/L),116.7～2233.1×104cells/L (830.2×104 cells/L),83.0～2997.5×104 cells/L (1139.1×104 cells/L),56.3～3613.9×104 cells/L (997.1×104 cells/L), respectively. The variation of phytoplankton community abundances showed an apparent seasonal trend that they were higher in summer and autumn than those in winter and spring. The ratio of Chlorophyta to total phytoplankton community was relatively high in every season, while the ratio of Bacillariophyta to total phytoplankton community was relatively high in spring. The ratio of Cryptophyta to total phytoplankton community was relatively high in spring and autumn, while that of Cyanophyta to total phytoplankton community was relatively high in summer and autumn. The seasonal succession pattern of phytoplankton community in Hongze Lake, Luoma Lake and Nansi Lake showed many similarities, but the most dominant phyla in spring of Dongping Lake was Bacillariophyta and the most dominant phyla in summer of Dongping Lake was Cyanophyta, which was not the same with other lakes. Common dominant phytoplankton species in regulating lakes in the east route of SNWDP included Chroomonas acuta of Cryptophyta, Chlorella and Ankistrodesmus of Chlorophyta, Cyclotella, Fragilaria and Synedra of Bacillariophyta, Pseudanabaena limnetica, Microcystis wesenbergii and Aphanizomenon issatschenkoi of Cyanophyta. According to the results of diversity index and evenness index, the regulating lakes in the east route of SNWDP were at the state of slight pollution to mild pollution, which also indicated the stability of phytoplankton community was relatively high.The phytoplankton community structures of regulating lakes in the east route of SNWDP showed significant differences, and the similarity degrees of phytoplankton samples from different lakes ranged from 40% to 90%. In Dongping Lake, the phytoplankton community samples had the highest dispersion degrees than other lakes; in Hongze Lake, the phytoplankton community samples had the highest similarity degrees than other lakes. The phytoplankton community samples from Nansi Lake and Luoma Lake showed relatively higher similarity degrees, while those samples from Hongze Lake and Dongping Lake had relatively lower similarity degrees. The major diagnostic genera for phytoplankton community dissimilarity among the regulating lakes was Pseudanabaena of Cyanophyta, which contributed the most to the dissimilarity degrees. Other common diagnostic genera for phytoplankton community dissimilarity among the regulating lakes included Cyclotella, Chlorella, Chroomonas, Chlamydomonas, Synedra, Dinobryon, Aphanizomenon and Microcystis. The differences on geographic latitude of regulating lakes in the east route of SNWDP might be the main reason leading to the phytoplankton community dissimilarities among these lakes.The main driving environmental factors affecting the variation of phytoplankton community in Hongze Lake were water temperature, soluble reactive phosphorus and permanganate index of chemical oxygen demand; the main driving environmental factors affecting the variation of phytoplankton community in Luoma Lake were water temperature, nitrite nitrogen, permanganate index of chemical oxygen demand and total phosphorus; the main driving environmental factors affecting the variation of phytoplankton community in Nansi Lake were water temperature, total nitrogen, dissolved oxygen and pH; the main driving environmental factors affecting the variation of phytoplankton community in Dongping Lake were water temperature, total nitrogen, oxygen demand and total phosphorus. With the variation of water temperature, the abundance of phytoplankton community was much higher in summer and autumn than that in winter and spring when the water temperature was lower, and the dominant phytoplankton species also changed regularly. The variation of nutrients had an effect on the distribution features of phytoplankton community. Besides, the variation of hydrological and hydrodynamic features of the water body in regulating lakes also influenced the growth and succession of phytoplankton community. In summer, the variation of phytoplankton community in Hongze Lake and Luoma Lake were affected by water-level fluctuation to some extent.The numbers of cyanobacteria species detected showed no significant differences among the regulating lakes in the east route. There were more species detected in summer and autumn than in winter and spring, and the variation of cyanobacteria community abundance displayed a similar seasonal pattern. The variation ranges and average values of cyanobacteria community abundance in Hongze Lake, Luoma Lake, Nansi Lake and Dongping Lake were 2.1～914.9×104 cells/L (127.1×104 cells/L), 0.8-1091.5×104 cells/L (274.5×104 cells/L),1.0-1494.0×104 cells/L (238.1×104 cells/L),0.2～2093.5×104 cells/L (448.0×104 cells/L), respectively. The dominant cyanobacteria species in Hongze Lake was mainly Microcystis wesenbergii, while the dominant cyanobacteria species in Luoma Lake, Nansi Lake and Dongping Lake was mainly Pseudanabaena limnetica, besides, Aphanizomenon issatschenkoi was dominant only in Dongping Lake. Main toxigenic cyanobacteria species in the regulating lakes of east route included Microcystis wesenbergii, Microcystis aeruginosa, Anabaena circinalis and Aphanizomenon issatschenkoi. Microcystis wesenbergii accounted for the largest proportion among the toxigenic cyanobacteria species in Hongze Lake and Luoma Lake. Anabaena circinalis and Microcystis accounted for relatively large proportion among the toxigenic cyanobacteria species in Nansi Lake. Aphanizomenon issatschenkoi accounted for the largest proportion among the toxigenic cyanobacteria species in Dongping Lake. Based on the grading early warning plan for cyanobacterial blooms, in summer and autumn, Hongze Lake was at the level of medium risk while some sampling sites could reach the level of high risk. The early warning level for cyanobacterial bloom in Luoma Lake and Nansi Lake was low risk in most months. The early warning level for cyanobacterial bloom in Dongping Lake was also low risk; however, the toxigenic cyanobacteria species-Aphanizomenon issatschenkoi might have the ability to form cyanobacterial bloom as Aphanizomenon flos-aquae. Thus, the monitoring work for Aphanizomenon issatschenkoi should be enhanced and further study regarding this cyanobacteria species should be introduced.