Changes Of Phytoplankton Community Structure And Its Affecting Factors In The Guangzhou Sea Area

Seawater samples were collected from 13 designed stations in the Guangzhou sea area once a month during March,2004 to December,2004.And analysed their nutrients content,ecological characteristics of phytoplankton and the relationship between them.According to stoichiometric limitation standard(Dortch et al,1992 and Justic,1995), the plots of atomic DIN:P,Si:P and Si:DIN ratios displayed the stoichiometric limitation of 0.00%in DIN,89.23%in phosphorus and 9.23%in Si.These results showed that the phosphorus was the main limitation in the Guangzhou sea area.Among 213 taxonomic species identified in the Guangzhou sea area,121 species (56.8%of the total species) belongs to Bacillariophyta,followed by Chlorophyta(62 species).In the Guangzhou sea area,phytoplankton species decreased from the inner bay to the outer bay.And its seasonal changes were distinct with most species in summer, followed by in spring,in autumn and in winter in turn.The results showed that the species number of phytoplankton extremely positive correlated with water temperature(r=0.580, p＜0.0001) in the Guangzhou sea area,and extremely negative correlated with salinity (r=-0.688,p＜0.0001),but did not evidently correlate with nutrients.In the Guangzhou sea area,phytoplankton cells decreased from the inner bay to the outer bay,which were different in each season.During the period studied,the mean density of phytoplankton were 53.9×104 cellsl/L in the Guangzhou sea area.And its seasonal changes were double-peak with higher peak in spring than in autumn.The results showed that phytoplankton cells evidently positive correlated with water temperature(r=0.327, p＜0.001) and silicate(r=0.292,p＜0.001) in the Guangzhou sea area,and evidently negative correlated with salinity(r=-0.675,p＜0.001) and phosphate(r=-0.277,p＜0.05),but did not evidently correlate with DIN.During the period studied,with the mean numbers of 9.4×10⁴ cells/L and 5.9×10⁴cells/L,cells of Melosira grannlata and Skeletonema costatum were respectively accounted for 25.8%and 15.3%of the total cells of phytoplankton in the Guangzhou sea area,which were extremely positive correlated(r=0.725 and 0.385,respectively) to the total cells.Therefore,their horizontal and seasonal distribution patterns dominated the patterns of total phytoplankton.The diversity index(H′),evenness(J) and dominance(D₂) of the phytoplankton community were 2.72,0.71 and 0.53,respectively,in the Guangzhou sea area.Compared with other estuaries,the diversity index and evenness in both bays were lower,whereas the dominance was higher.This finding suggests that the community structure of phytoplankton became retrogression,and that the biodiversity and evenness decreased in the Guangzhou sea area.