| Hong Kong is one of the coastal areas with frequent red tide events, and long-term monitor for phytoplankton is the foundation for marine environment investigation, water quality control and the red tide studies. In the present studies, various phytoplankton samples were collected and analyzed from the coastal waters of Hong Kong, in four seasons of 2000, 2002 and 2004. Through the species identification of these phytoplankton samples, our studies have been focused on investigating phytoplankton species composition and diversity, population distribution, community structure and dynamics in the relationship with the environmental factors of Hong Kong coastal waters. Based on these studies, we proposed from a view of ecology the main mechanism of phytoplankton community structure as well as the dynamics in Hong Kong coastal waters. The results are as follow:1. There are significant geographical differences in the horizontal distribution of salinity in Hong Kong coastal waters. Hong Kong coastal area can be divided into eastern, central, southern and western regions, based on different impact from the freshwater of Pearl River. The salinity is gradually weakened from west to east, leading to the formation of limnetic western region and oceanic eastern region. Therefore the different groups of phytoplankton changed in their structural characteristics according to the different water regions.2. The overall level of nutrients in Hong Kong coastal waters is relative low. Total nitrogen and total inorganic nitrogen varied from 0.01-0.96 mg/L,0.06-1.17 mg/L, respectively. Phosphate and phosphorous varied from 0.002-0.079 mg/L,0.02-0.16 mg/L, respectively. The silicon content is in the range of 0.05-3.3 mg/L. The nutrient level was dramatically reduced compared to that of the end 1980s, but is relative stable compared to that of 1999.3. Totally, 254 species (including varieties) of phytoplankton were identified. They are including 177 diatom species belonging to 51 genus, 64 dinoflagellate species belonging to 22 genus, 4 silicoflagellate species belonging to 3 genus, 1 englenoid species belonging to 1 genus, 2 rhaphidophyte species belonging to 2 genus, 1 chrysophyte species belonging to 1 genus, 1 cryptomonad species belonging to 2 genus, 1 Cyanophyta species belonging to 2 genus, 1 Cholophyta species belonging to 1 genus. In the waters of Hong Kong, diatoms is the major component of phytoplankton, accounting for of 69.69% of the total species numbers of phytoplankton. Among these, the most frequently appearance diatom genus is Chaetoceros (27 species, representing 10.63%); while according to the cell densities, Skeletonema is the largest , about 44.69% of total phytoplankton cell densities.4. Comparing to literature records, the dominant phytoplankton species of Hong Kong coastal waters changed to some extent. Diatom had been reported as the most predominant group in Hong Kong coastal waters. In this study it was shown that in certain seasons, Dinoflagellate or Cryptomonad appeared to be dominant in some sea areas. In the spring of 2000, the dominant species was Plagioselmis prolonga (Cryptomonad), while in summer, autumn and winter, the diatom dominated Hong Kong coastal waters, with the dominant species of Skeletonema spp., Leptocylindrus minimus, and Asterionellopsis glaciali. In the spring and summer of 2002, the dominant species was Skeletonema spp., while in autumn it was Prorocentrum minimum, and in winter the dominant species were Asterionellopsis glaciali and Cylindrotheca closterium. In the spring, summer and autumn of 2004, diatom still dominated in Hong Kong coastal waters, with the dominant species of Chaetoceros curvisetus, Skeletonema spp., Pseudo-nitzschia spp. and Rhizosolenia fragillissima, while in winter Dinoflagellate was the dominant group and the dominant species was Prorocentrum minimum.5. The abundance of phytoplankton showed significant season dependent variation. Generally cell density of phytoplankton appeared lower in spring and winter, but higher in summer and early autumn. In 2000, 2002 and 2004, the annual average of cell density of phytoplankton increased, with the average cell density of 2.65×105cells/L, 6.90×105cells/L, and 7.73×105cells/L, respectively. The horizontal distribution of phytoplankton cell density varied in different sea areas according to the extent of effect exerted by fresh water from Pearl River. Cell density did not fluctuate obviously by seasons in eastern waters, while in central, southern and western waters, the seasonal fluctuation was significant. It reached the peak in summer when large amount of fresh water were pouring in.6. According to phytoplankton community structure studies, the percentage of Dinoflagellate and other algae were higher in eastern waters than in other sea areas. In central, southern and western waters diatom was the dominant group in most seasons. The percentage of other algae was higher than Dinoflagellate。7. In 2000, 2002 and 2004, the diversity index of phytoplankton was annually slightly decreased, with the actual value of 2.56, 2.54 and 2.17 respectively. According to this study, the value of average biodiversity index and evenness index were moderate, which were 2.42 and 0.54, respectively.8. Studies were carried out to conduct the relationship between phytoplankton cell density, concentration of chlorophyll a and various environmental factors: including temperature, salinity, concentration of nitrate, nitrite, amino nitrogen, total nitrogen, total phosphorus, and Si, TN/TP, Si/TN, Si/TP. The result showed that in Hong Kong waters, both phytoplankton cell density and concentration of chlorophyll a significantly related to salinity, concentration of Si and TN/TP ratio (P<0.01). It is concluded that the above three environmental factors are the main factors affecting the biomass of phytoplankton in Hong Kong coastal waters.
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