Zhelin Bay is characterized by intensive aquaculture along the coast of Southeast China, with approximately half of the water area occupied by either cultured oysters or caged-fish farms. As one of the most important bays for large-scale mariculture in Guangdong Province, aquaculture production in Zhelin Bay has made significant contributions to the local economy. However, development of intensified caged-fish farms during the last decade has accelerated eutrophication process of the bay, and massive fish kills due to harmful algal blooms such as Phaeocystis pouchetii were reported in 1997 and 1999. Diatom blooms such as Skeletonema costatum and Thalassiosira diporocyclus have also been reported in the recent years. Since May 2000, our research group has initiated an ecological investigation around Zhelin Bay, where samples were collected monthly or quarterly for analysis of phytoplankton, zooplankton, microorganism, water temperature, salinity, nutrients and other environmental parameters. In the present study, we described the population dynamics of S. costatum in 9 sampling stations from May 2000 to November 2004.
Samples were collected at high tide ( 1.5 h) weekly (March to November) or biweekly (December to February) at eight stations (except for S₃) between May 2000 and June 2001, and monthly at all nine stations between July 2001 and December 2003, and quarterly at all stations in 2004. For quantitative phytoplankton analysis, one liter of water samples were collected with 5L HQM-1 sampling bottles 0.5 m under the surface and 0.5 m above the bottom at each station. Immediately after collection, the water samples were fixed with Lugol’s iodine solution with a final concentration of 15‰. Water samples were then transferred to graduated glass cylinders and concentrated to 30-100 ml gradually through sedimentation over a period of 3 days. Species identification and density counting was performed with an inverted microscope (Zeiss Axiovert25, magnification 400 ) using a 1ml Sedgewick-Rafte counting frame, and a minimum of 400 cells were counted for each sample. Water temperature, salinity, turbidity, pH value, dissolved oxygen (DO) and water depth were determined in situ with a portable instrument for water quality analysis (YSI6600-02, USA). Additional water samples (250ml) were collected (filtered in situ) for nutrient and chlorophyll a analysis, and they were measured with an injection water quality analyzer (SKALAR, Netherlands) and a spectrophotometer (UV-2501PC, Japan) respectively.
A total of 1045 phytoplankton samples were collected in the present study, and among which S. costatum was found to be present in 1020 samples, with its population density ranged from 0 to 1.4×10⁷ cells/dm3 and an overall average of 3.3×10⁵ cells/dm³. In relation to the algal bloom density, 65 samples were found with S. costatum density larger than 10⁶ cells/dm³ and 4 samples were larger than 10⁷ cells/dm³. The percentage of S. costatum accounting for the total phytoplankton ranged from 0% to 99% with an overall average of 67.1％.
Spatially, total averages of S. costatum density at each station ranged from 1.9× 10⁵ to 6.5 × 10⁵ cells/dm3, and there were no significant differences among the sampling stations. Temporally, monthly averages of S. costatum density in the whole bay ranged from 0.03× 10⁵ to 67.42 × 10⁵ cells/dm³, and was characterized by a typical bimodal annual pattern with the first peak observed in May or July and second peak in August or September. Percent monthly averages of S. costatum accounting for the total phytoplankton density ranged from 6.5% to 94.5%.
The grey incidence-regression analysis (ρ＝0.1) placed the importance (with the decreasing order) of effect of 13 environmental factors on S. costatum population dynamics as follows: water temperature > pH > zooplankton > salinity > DO > NH₄-N > DIP > DIN > NO₂-N > Turbidity > NO₃-N > Fe > SiO₃-Si. Thus, water temperature appeared to be the most important factor on S. costatum population dynamics. In addition, significant linear correlation was found between logarithmic values of S. costatum density and water temperature. Samples with S. costatum at high density (>10⁶ cells/dm3) showed a range of water temperature of 24.5~32.0℃, corresponding to the warm seasons (May to September). Since the peak of zooplankton population also occurred in warm seasons, it seems reasonable to deduce that zooplankton placed a predation pressure on S. costatum population. The population density of S. costatum and its dominance in total phytoplankton were found to decrease rapidly in 2004, which were suspected to relate to the variation of nutrients structure and Fe content. Results in the present study indicated that long-term investigation are necessary for a comprehensive evaluation of estuary ecology, and also will facilitate the understanding of the mechanism of algal bloom formation.