| Harmful Algal Blooms (HABs) have been plaguing the development of coastal fisheries and tourism. In order to explore the mechanism of HAB outbreaks, a one year survey in the area of the Pearl River Estuary was carried out in each season from December,2007 to study the controlling environmental factors on the growth of phytoplankton. A bloom of Cochlodinium geminatum Schutt, which occurred during October-November,2009 in this area,was studied in details. The environmental factors in the bloom event were compared with those in the same season in November,2008, without an HAB event, to explore the possible factors regulating the bloom of C. geminatum. The conclusions were as follows:1. The seasonal survey results of the background values in the area of the Pearl River Estuary:(1) In the Pearl River Estuary, the dominant species of phytoplankton were diatoms, followed by dinoflagellates. The average concentration of Chlorophyll a (Chla) in spring, summer, autumn and winter was (0.81±0.43), (1.15±0.68), (11.05±7.83) and (0.73±0.42)μg/L respectively. Water temperature in summer and autumn were higher with values of 26.68 and 27.1℃, while temperature in spring and winter were 23.16 and 20.47℃respectively. The salinity was highest in spring and winter with 25.6 and 27.5, respectively, while the lowest was found in summer with value of about 5.3. The salinity in autumn was 18.8. In the area of the Pearl River Estuary, the average pH of sea water in Spring, summer, autumn and winter were 8.05±0.12,7.92±0.12,8.21±0.08 and 8.04±0.17, respectively. The concentration of dissolved oxgen (DO) ranged between 2.37～8.32 mg/L with the average value of (6.19±1.50) mg/L, in which the highest was detected in winter and the lowest was found in summer.(2) The range of dissolved inorganic nitrogen (DIN) concentrations in four seasons were 0.089～0.774 mg/L,0.633～1.222 mg/L,0.311～1.724 mg/L and 0.201～0.806 mg/L, with the average values of (0.440±0.257), (0.942±0.201), (1.100±0.623) mg/L and (0.425±0.227) mg/L, respectively. Whereas the range values for dissolved inorganic phosphorus (DIP) concentrations were 0.008～0.025 mg/L,0.012～0.029 mg/L,0.013～0.078 mg/L and 0.010～0.044 mg/L, with the average values of (0.016±0.007), (0.021±0.005), (0.032±0.020) and (0.022±0.011) mg/L in spring, summer, autumn and winter, respectively. In the spring, the average of N/P, Si/P and Si/N ratio were 66,350 and 7 respectively; In the summer, the average of N/P, Si/P and Si/N were 103,30,1 respectively; In the autumn, the average of N/P, Si/P and Si/N ratio were 89,91 and 1 respectively; While in the winter, the average of N/P, Si/P and Si/N ratio were 42,36 and 1. PO4-P might be the potential limiting factor for the growth of phytoplankton in the whole year in this area.(3) The results of the principal component analysis and correlation analysis of the major environmental factors for the growth of phytoplankton in the Pearl River Estuary revealed that nutrients and temperature were the most important factors for phytoplankton growth. The importance of various factors on primary production were in an order of NO3-N, sea salinity (SS), sea temperature (ST), N02-N, PO4-P, DO, pH, SiO3-Si and NH3-N.(4) The multiple linear regression between the Chla and the major environmental factors showed that five variables of ST, DO, SS, NO3-N and PO4-P were followed by the regression equation, and were the most significant factors. 2. The results of the C. geminatum bloom:(1) During the bloom of C. geminatum, the central blooming area was not the waters with higher nutrient concentrations. In the course of the bloom, the concentrations of NO3-N, NO2-N and SiO3-Si decreased at the first beginning and then recovered gradually. While the concentrations of NH3-N and PO4-P increased with the development of the bloom. High concentrations of the dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were found in the HAB zone. Values of SS, DO and pH during the bloom period were comparable to those during the non-bloom period in 2008. However, temperature was significantly higher and might be the main factor for regulating the outbreak and dissipation of the C. geminatum bloom.(2) The results of the principal component analysis indicated that nutrients, salinity and temperature were the most important factors for the outbreak of the C. geminatum bloom in the Pearl River Estuary. The factors contributed to the outbreak of HABs were in an order of SiO3-Si, SS, NO3-N, NO2-N, ST, DOP, PO4-P and DON.(3) The results of the correlation analysis between Chla and various environmental factors in the C. geminatum bloom event showed that Chla was positively correlated with PO4-P, SiO3-Si, DOP, DON, ST, DO and NO2-N, while was negatively correlated with SS and NH3-N.(4) By comparing of the HAB core area with that of the edge zone, DON, DOP and SS in the core area were significantly different from those in the edge region of HABs, while DIN, DIP, SiO3-Si, ST, pH and DO showed no significant difference.(5) The results of the stepwise multiple linear regression analysis between Chla and the major environmental factors in the HAB event showed that the four variables DON, ST, DOP and PO4-P coincides with the regression equation and influenced Chla significantly.(6) The comparative study of the environmental factors during the C. geminatum bloom in November,2009 with that in the non-bloom period in November,2008 revealed that high nutritent concentrations, plus the suitable temperature might be important factors for the outbreak of, the C. geminatum bloom, while the temperature drop and bad weather conditions were the main reasons for the bloom dissipation.
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