Cyanobacterial blooms are one of the most serious problems in eutrophic freshwater systems, because of their detrimental effects on fisheries and on the civil, industrial, and recreational uses of water resources(Wu et al., 2010; Ma and Yu, 2013). Dominance and blooms of cyanobacteria, especially M. aeruginosa, in fresh waters have received considerable attention worldwide in the past decades(Dokudil and Teubner, 2000; Cao et al., 2006). With the development of Chinese industry, cyanobacterial blooms caused by eutrophication in shallow freshwater lakes have become an increasingly serious problem in China, such as the intensive and large-scale cyanobacterial blooms in Chaohu Lake, Taihu Lake, and Dianchi Lake(Liu et al., 2006; Chen et al., 2009; Zhang et al., 2011).

Chaohu Lake, located in the Changjiang(Yangtze)River delta, is the fifth largest fresh water lake in Hefei City of China, with a surface area of 770 km², a mean depth of 2.7 m, and a storage capability of 2.1 billion m³(Xu et al., 2005). Chaohu Lake is an important water source of Hefei City. In the 1980s, Tangxi Water Treatment Plant was built in the northwest part of Chaohu Lake; its daily water withdrawals were about 300 000 tons and occupied more than 50% of the total water withdrawals of the urban centers in Hefei City. It was shut down because of water pollution caused by cyanobacteria in the past, but is still an important backup water source for the urban centers in Hefei City. There are still 11 water intakes of water treatment plants along the northern bank of Chaohu Lake, mainly distributed in the northeast of Chaohu Lake, which is known as the water source area of Chaohu City(WSAC). Three water intakes of water treatment plants and three of enterprises are distributed in the WSAC, the most important water source area of Chaohu City, and their total daily water withdrawals is about 190 000 tons.

Since pollution sources are primarily concentrated in the northwest part of Chaohu Lake, and influent rivers are distributed in mainly the west of Chaohu Lake and the effluent river is located in the east of Chaohu Lake, water pollution and eutrophication spread eastward to WSAC from the northwest of Chaohu Lake. Cyanobacterial blooms caused by eutrophication frequently outbreak in the WSAC and seriously influence the quality of water treatment plants, and even lead to the production inhibition of some water treatment plants. To address this problem, the team of Chaohu Lake Project, a part of Chinese National Water Special Project, successfully deployed large bionic equipment to remove cyanobacteria from the water surface. This equipment can clear 3 hectares of lake surface and remove 1 000-ton algae water per hour. The bionic equipment effectively prevents harmful cyanobacterial blooms and has guaranteed the safety of water supply.

Improving the efficiency of cyanobacteria prevention requires an in-depth study on seasonal succession of cyanobacterial blooms during warm seasons in the WSCA. Many studies have been conducted on temporal and spatial variations in phytoplankton in Chaohu Lake to address this situation(Deng et al., 2007; Yang et al., 2013). However, less attention has been paid to the seasonal variation in cyanobacterial biomass and the forms of dominant cyanobacterial populations in Chaohu Lake, especially in the WSAC. Thus, the present study was conducted in the highly eutrophic WSAC, to describe the seasonal variations in cyanobacterial biomass and the forms of its dominant population in relation to environmental factors, and to discuss the possible mechanisms underlying these variations. Our results may provide scientific evidence for preventing, controlling, and removing cyanobacterial blooms from the WSCA and can be used effectively to develop strategies for preventing cyanobacterial blooms in similar lakes and reservoir systems.

WSCA is located in the northeast corner of Chaohu Lake(117.785 555 6°-117.848 333 3°E, 30.575 277 78°- 31.612 5°N), having Guishan hill on the west and Yuxi River mouth on the east. It has a mean surface area of 10 km², a mean depth of 2.5 m, and a storage capability of 2.7 million m³. The westward side of the WSAC is wide, and its water exchange with the open lake is frequent. The eastward side of the WSAC is a Y-shaped fork, its north section is a closed bay, and the south section is the Yuxi River mouth. Chaohu gate is built on the Yuxi River mouth, and it plays an important role in regulating water volume of Chaohu Lake. WSCA is a semi-closed bay if the Chaohu floodgate is closed. WSAC is the only water source area of Chaohu City; it provides water for enterprise production and drinking; the water quality of WSAC has an important impact on the city’s economy and peoples’ livelihood.

Eight sampling sites were selected across the WASC from east to west(Fig. 1).

Fig. 1 Distribution of sampling sites in the WSAC

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Water samples were collected monthly from eight sampling sites, between November 2011 and October 2012, by using an organic glass tube(OGT; height: 2.1 m, diameter: 8 cm)at the depth of 0-2 m below the lake surface. First, we placed the OGT vertically into the water, ensuring that its upper end was about 5 cm above water level, and then plugged its upper end with a clean rubber plug with about 5 cm of rubber plug into the OGT. Next, we lifted the OGT slowly and vertically until the lower end of the OGT was about 20-30 cm below the water level, and then plugged the lower end with another rubber plug and lifted the OGT out of the lake water, immediately poured the water into a clean plastic bucket(volume: 15 L), and mixed thoroughly. Subsequently, the water was transferred to three polyethylene bottles(volume: 1 L). Water samples for phytoplankton counting and identification were fixed with 10 mL Lugol’s iodine solution. Water samples for physical and chemical analyses were placed in a thermostat and immediately transferred to the laboratory in a cool, dark environment for further analysis.

Water samples were filtered through glass fiber filters(GF/C; Whatman, UK). The filter liquor was used for measuring NO₃^- and NH₄⁺. Total nitrogen(TN) and total phosphorus(TP)were measured using raw water. Three replicate measurements on TN, TP, NO₃^-, and NH₄⁺ concentrations of subsamples of each sample were performed.

Physical and chemical parameters such as water pH and dissolved oxygen(DO)were determined using a portable instrument pH/ORP/Conductivity/ DO meter(SX751; Shanghai San Xin Instruments Co., China). The field test data of pH and DO were measured at 1 m depth. TP and TN were determined using Mo-Sb antispetrophotography method and persulfate oxidation spectrometry(EPAC, 2002), respectively. Nitrate(NO₃^-) and NH₄⁺ were analyzed by Ultraviolet spectroscopy method and Nessler’s reagent Spectrophotometric method, respectively(EPAC, 2002). A weather station located on WSCA provided 2-h measurements of wind speed and direction, air temperature(T), and photosynthetically active radiation(PAR).

A sedimentation method was used for counting and species identification. To optimize the counting process, the samples preserved with Lugol’s solution were checked for high densities of phytoplankton, and the subsamples of water were settled over a period of 48 h. And then 0.1 mL concentrated samples were counted on an inverted microscope(Nikon TS100F, Japan)under(40-200)× magnification after mixing(EPAC, 2002). Phytoplankton species were identified according to Hu and Wei(2006). Biomass was estimated from cell numbers and cell size measurement, assuming that 1 mm³ of algal volume equals 1 mg of fresh weight biomass(Niu et al., 2011).

The measurement software(Software version: NIS-Elements D)of the inverted microscope, (DSFi1)was used to measure the colonial diameter of M. aeruginosa. By using the two-dimensional projection area(S) and major-minor axis of ellipse fitting(A axis, B axis)measurements, the software could automatically generate the equal-area-circle diameter, which is called colonial diameter in this study. The single cell’s diameter could be measured by the same way.

Considering that M. aeruginosa mainly existed in colonial form in summer, we counted the cell numbers by taking into account the volume relationship between the colonial and unicellular M. aeruginosa. The following formula(Joung et al., 2006)was used for the calculation of the approximate number of cells in a colony:

The average data of the eight sampling sites were used as those for WSCA while analyzing the annual variation in cyanobacteria and its dominant species.

All the data are expressed as mean±st and ard deviation(SD)of three individual sample analyses. One-way analysis of variance(ANOVA; LSD test) and two-tailed Spearman correlation analysis were performed using SPSS for Windows(Version16.0; USA).

The nonlinear fitting of the variation in cyanobacterial biomass was performed using logistic function and computed by Origin8.5 software. The logistic function is as follows: y = A /(1+e^{(b - kx)}), where x is month; y is biomass(mg/L); and A is environmental capacity(the maximum of population in specific environment, mg/L); b is integration constant, and k is the instantaneous growth rate of cyanobacteria.

Figure 2 shows the annual variation in temperature and PAR from November 2011 to October 2012. The annual variations were two sinusoids and showed similar changing trends. The peak values of temperature and PAR were recorded in July, and the lowest values were recorded in January. Temperature and PAR increased rapidly from March to April. Between April and August 2012, Temperature was in the range of 15-30°C. PAR was greater than 1 500 μmol/(m² ·s)during April and October 2012.

Fig. 2 Annual variation in T and PAR

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With regard to the trophic conditions of Chaohu Lake, TN and TP covered a wide range of concentrations(TN range, 0.45 to 3.35 mg/L; TP range, 0.03 to 0.22 mg/L; Fig. 3). TN and TP concentrations exhibited a similar trend, with high concentrations occurring in summer and low concentrations in spring and autumn. This type of fluctuation led to a wide variation in the N:P ratio(from 7.17 to 52.53)in the Chaohu Lake.

Fig. 3 Annual variation in TN and TP

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Figure 4 shows that the annual variations in NO₃^- and NH₄⁺ were similar in WSCA. The contents of NO₃^- and NH₄⁺ increased rapidly from March 2012 and reached the maximum concentration in June 2012. From July 2012, the concentrations of NO₃^- and NH₄⁺ became low and reached the valley value in September.

Fig. 4 Variation in NO3- and NH4+

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Figure 5 shows the seasonal variations in all types of phytoplankton. The first peak value(17.67 mg/L)of phytoplankton biomass appeared in December 2011, the first valley value(6.38 mg/L)appeared in February 2012, and the second peak value(17.12 mg/L)appeared in August 2012. Two peaks of cyanobacterial biomass occurred in December 2011 and August 2012, respectively; the valley value appeared in March 2012. Biomass of different types of phytoplankton is in the order of Cyanophyta > Chlorophyta > Bacillariophyta > Cryptophyta > others. The annual average biomass of Cyanophyta occupied about 80% of phytoplankton biomass.

Fig. 5 Annual biomass variation of phytoplankton

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The variation in cyanobacteria biomass in WSAC from March to August 2012 was fitted using logistic nonlinear fitting. The fitting equation was y =16.911 2/(1+e^{(7.650 9-1.199 8 x)}), R² =0.951 9; the inflection point was S(6.376 9, 8.455 6). The measured values are located on both sides of the fitting curve and close to it. The fitting curve shows that the growth rate of cyanobacteria was slow in March and April, became faster in May, reached the maximum in June, and thereafter declined(Fig. 6).

Fig. 6 Fitting curve of cyanophyta biomass from March to August

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Unicellular biomass of M. aeruginosa reached the lowest value in February 2012, began to increase from March, reached the maximum in July, gradually decreased thereafter, and began to increase again in October. The colonial biomass of M. aeruginosa was zero in February and March 2012 and gradually increased from April and reached the maximum value in August. M. aeruginosa mainly existed in the unicellular form from February to April, and colonial biomass of M. aeruginosa began to exceed unicellular biomass from May. The colonial biomass of M. aeruginosa was markedly higher than the unicellular biomass in June, July, August, and September(Fig. 7). The colonial biomass of M. aeruginosa decreased significantly in October, and there was considerable amount of both the types of biomass of M. aeruginosa in October.

Fig. 7 Annual variation in unicellular and colonial biomass of M. aeruginosa

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The diameter of M. aeruginosa colonies varied remarkably. The M. aeruginosa colonies with a small diameter consisted of several single cells, and colonies with a large diameter were noted. The diameter range of colonial M. aeruginosa was 18.37-237.77 μm; most of the colonies were distributed in the range of 20-200 μm, occupying 95.5% of the total numbers of samples. The colonial diameters of M. aeruginosa of less than 20 μm and more than 200 μm occupied 1.32% and 3.18% of the total numbers of samples, respectively. From January to March 2012, the main form of M. aeruginosa was unicellular; colonial M. aeruginosa was not found. The maximum diameter of colonial M. aeruginosa was recorded in July; there was no significant difference in the colonial diameter between July and August 2012(P >0.05). The colonial biomass began to decrease significantly from September(P <0.05).

The seasonal variation of phytoplankton differed across different years. Deng et al.(2007)showed that the minimum value of Chl-a content in Chaohu Lake is in December and the valley value is in May. Chen et al.(2003)concluded that the Chl-a content of the estuary of Nanfei River, an input river of Chaohu Lake, reached its maximum value in August and the minimum value in May. In our study, the results were different from the above studies, the peak values appeared in December and August, and the valley value appeared in February. Our results are similar with those of the study by Jiang et al.(2014)in Chaohu Lake. The peak value of cyanobacteria appeared in December 2011 probably because of the favorable weather conditions that prompted cyanobacteria to float on the water surface; in addition, the wind frequency of this month was mainly westerly, and the wind speed was about 3 m/s, which provides favorable conditions for cyanobacterial migration. Therefore, high concentrations of cyanobacteria migrated from west to east of Chaohu Lake, leading to a high content of cyanobacteria in the WSCA.

Spring and summer showed rapidly increasing levels of cyanobacterial biomass(Deng et al., 2007). In this study, the variation in cyanobacterial biomass from March to August 2012 complies with the logistic growth model. The fitting curve showed that cyanobacteria grew rapidly in June and reached the maximum level in August. Our study results are inconsistent with those of Deng et al.(2007)who indicated that the biomass of cyanobacteria was the maximum in June in the WSAC. The reason for this discrepancy could be that, in the study of Deng et al.(2007), the hottest month was June 2007, and the rainfall began in June and July 2007. The rainfall was very heavy and remarkably reduced the concentration of cyanobacteria in the natural water body(Deng et al., 2007). Unlike the above study, the maximum temperature was recorded in July, and heavy rainfall mainly began in August; thus, the climate variation across different years led to the difference in cyanobacterial growth.

The variation in colonial diameter and M. aeruginosa biomass were basically the same(P <0.01; Table 2). However, the maximum M. aeruginosa biomass was recorded in August, and the peak value of colonial diameter of M. aeruginosa was noted in July. Temperature and PAR peaked in July, which is in accordance with the colonial diameter of M. aeruginosa ; moreover, temperature and PAR correlated significantly with M. aeruginosa colonial diameter(P <0.01; Tables 2, 3). Thus, temperature and PAR may be the main determining factors of the colonial diameter of M. aeruginosa(Walsby et al., 2006; Imai et al., 2009). The maximum M. aeruginosa biomass and colonial diameter were recorded in different months; this might be due to the variations in weather condition. From late July to early August, the impact of heavy rains fragmented the bigger M. aeruginosa colonies into more small colonies(Deng et al., 2007); thus, the colonial diameter in August was smaller than that in July. Since the weather condition was suitable for M. aeruginosa growth in August, its biomass continued to increase and reached a peak value in this month.

Table 1 Correlation between Cyanophyta, M. aeruginosa, and environmental factors(November 2011 to October 2012)

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Table 2 Correlation between Cyanophyta, M. aeruginosa, and environmental factors(March-August 2012)

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Temperature is a necessary factor for M. aeruginosa photosynthesis, and it could determine the reaction rate of desmoenzyme, which is closely correlated with plant anabolism, respiratory intensity, and physiological activity of heterotrophic bacteria in water(Shi et al., 2004; Wu et al., 2008; Davis et al., 2009; Yang et al., 2012). Since seasonal temperatures increased from 10 to 30.8°C in freshwater ecosystems, the phytoplankton group with the highest growth rates generally shifted from diatoms to green algae to cyanobacteria(Canale and Vogel, 1974; Reynolds, 1997). Reynolds et al.(1973)reported that Microcystis populations reached the upper parts of the water column and started to show accelerated growth at the water temperature of 15°C. Krüger and Eloff(1978)detected a sharp decline in the growth rate below 15°C in four strains of Microcystis. From the findings of the above studies, we could conclude that 15-30°C is suitable for the growth of cyanobacteria. In the present study, the temperature ranged from 15-30°C in late spring and summer. Furthermore, cyanobacteria and M. aeruginosa biomass was significantly influenced by Temperature(P <0.05; Table 3). Thus, the temperature, ranging from 15-30°C, is suitable for cyanobacteria(especially M. aeruginosa)growth in temperate shallow lakes.

PAR is a key factor that affects the seasonal variation in cyanobacteria and has a direct effect on cyanobacterial growth(Moreno et al., 1998; Staats et al., 2000; Granum et al., 2002; Otero and Vincenzini, 2003; Magaletti et al., 2004; Yang et al., 2012). Most cyanobacteria(particularly laboratory strains)prefer low irradiance and are extremely susceptible to photoinhibition(Walsh et al., 1997). Zhang et al.(2011)reported that M. aeruginosa could grow rapidly under low light environment. In this study, cyanobacterial biomass and M. aeruginosa biomass were significantly influenced by PAR(P <0.05; Table 2)from March to August. Biomass of cyanobacteria kept increasing in July when PAR reached the peak value, indicating no evidence of photoinhibition, inconsistent with the findings of the study by Walsh et al.(1997) and Zhang et al.(2011). The discrepancy in the results might be because our study was conducted in the field and included several influencing factors. Moreover, most cyanobacteria(especially M. aeruginosa)can adjust their position in accordance with light in the water, thereby protecting cyanobacterial cells from intense PAR(Kromkamp and Walsby, 1990). Thus, the cyanobacterial biomass continued to increase.

Wind speed and direction can cause considerable heterogeneity in the horizontal distribution of plankton populations(Webster, 1990; Webster and Hutchinson, 1994; Kanoshina et al., 2003). Wind has also been reported to affect the waxing and waning of phytoplankton in Taihu Lake(Chen et al., 2003; Marcé et al., 2007). Marcé et al., 2007 and Wu et al.(2010)indicated that wind affected phytoplankton by producing advective movement of superficial water masses in the downwind direction, and formation of water blooms is easier when the wind speed is about 3 m/s. In this study, Chaohu gate was opened during the flood season only, and hence, water flows were not noted at ordinary times. Therefore, the horizontal distribution and migration of cyanobacteria were caused by wind mainly. The wind direction of the WSCA was mainly easterly during the whole investigation period, and the wind speed(about 3 m/s)was suitable to form water blooms. Thus, the cyanobacterial blooms were apt to migrate from the WSCA to open lake; as a result, the water quality of the WSCA was hardly influenced by the cyanobacteria originating from the open lake during most of the time.

Cyanobacterial blooms are generally associated with water eutrophication(Philipp et al., 1991; Carmichael, 1994; Rapala et al., 1997; Oliver and Ganf, 2000; Paerl et al., 2001). Nitrogen and phosphorus are considered in general as the main nutrients for algal growth in temperate and subtropical lakes(Shaprio, 1990; Yang et al., 2012). When the nutrient concentration of lake water is high, especially when the phosphorus content is high, cyanobacteria dominate the phytoplankton communities(Smith, 1986; Trimbee and Prepas, 1987; Watson et al., 1997; Paerl and Huisman, 2008). The outbreak of cyanobacterial blooms in eutrophic lakes is usually accompanied by an increase in phosphorus content in water(Osgood, 1988; Pettersson et al., 1993). In our study, the correlations between cyanobacterial biomass and nitrogen and phosphorus contents were superficial(P >0.05); the correlation between annual variation in M. aeruginosa biomass and TP was significant(P <0.05; Table 1). This could be because there were many types of Cyanophyta species, and the nutrientabsorbing abilities differed across species. However, M. aeruginosa, the dominant species in the WSCA, could strongly absorb phosphorus(Kim et al., 2007; Lehman et al., 2008); therefore, the annual variation in M. aeruginosa biomass significantly correlated with the concentration of TP.

Formation and development of M. aeruginosa colonies is a complicated process that is influenced by many factors(Walsby et al., 2006; Naselli-Flores and Barone, 2007). Of the factors required for the formation of M. aeruginosa colonies, we mainly focused on the abiotic factors. Naselli-Flores and Barone(2007)indicated that light intensity has a positive influence on algal morphogenesis in natural water bodies. Walsby et al.(2006)investigated the relationship between light and M. aeruginosa colony formation and found that the average volume of cells cultured under high light intensity is two times greater than that cultured under low light intensity. In the present study, correlation analysis showed that the annual variation in the diameter of M. aeruginosa colonies correlated significantly with PAR(P <0.01; Table 1), and diameter of colonial M. aeruginosa was the greatest in July when the PAR reached the peak. Thus, high PAR might be helpful for increasing M. aeruginosa colonies. Temperature was another important abiotic factor that could influence the formation and development of M. aeruginosa colony(Imai et al., 2009). In our study, the colony diameter correlated with temperature significantly(P <0.01; Tables 1, 2)both in the investigation period and the growth seasons. Thus, PAR and temperature were the critical factors for the development and formation of M. aeruginosa in the WSCA.

Our monthly sampling from eight stations from December 2011 to October 2012 in the WSCA revealed the seasonal variations in cyanobacterial biomass and the forms of its dominant population in relation to environmental factors. Cyanophyta occupied the maximum proportion of phytoplankton biomass in August. The variation of cyanobacterial biomass in spring and summer in WSAC fitted logistic growth model well. The growth rate of cyanobacteria was considerably faster in June, and the biomass of cyanobacteria reached the maximum in August. From January to March 2012, the main form of M. aeruginosa was single cell form. M. aeruginosa colonies began to appear from April, and the maximum diameter of the colonies was recorded in July; the colonies gradually decreased from August. The diameter range of M. aeruginosa colonies was 18.37- 237.77 μm. Temperature and photosynthetically active radiation may be the most important factors that influenced the seasonal variation in M. aeruginosa biomass and forms. The suitable temperature for cyanobacterial growth was in the range of 15-30°C. In a natural water body, PAR has a positive influence on colonial diameter of M. aeruginosa.