ANALYSIS OF THE SPATIAL AND TEMPORAL CHANGES OF CHLOROPHYLL A CONCENTRATION IN XIANGXI BAY IN SPRING AND ITS IMPACT FACTORS

The investigations on chlorophyll a of the water body、total nitrogen、total phosphorus and other environmental factorswere carried out at 14 sampling sites of Xiangxi Bay of Three Gorges Reservoir fromMarch to April 2005.We have also analyzedthe spatial and temporal variation of chlorophyll a and its relationship with environmental factors.Chlorophyll a was extracted and measured according to procedures of UNESCO’s spectrometer methods under 750nm,665nm,645nm and 630nm wavelengths.Dissolved oxygen and nutrient data were supplied by the chemical working group of theresearch project. The result showed that fromthe upper reaches to the lower reaches of the Xiangxi Bay,there was evident horizontal variation of chlorophyll a concentrations in the water column.One bloom event occured at March 17th at a very large scale.Then bloom went forward fromthe lower reaches to the upper reaches along with the time in April. In detail,bloom event occurredat April 11th at the sampling sits of group Ⅰ,then about five days later it occurred at the sampling sits of group Ⅱ.Bloom eventoccurred at the sampling sits of group Ⅲbetween April 16th and April 22th,while it occurred at April 22th at the sampling sitsof group Ⅳ.Obviously,ecological factors,including total nitrogen、total phosphorus、water temperature 、transparency and dissolved oxygen,had significant impact on the concentrations of chlorophyll a directly and indirectly. Generally speaking,total phosphorus increased from the lower reaches to the upper reaches of the Xiangxi Bay,while total nitrogen decreased first,then increased. Theresult obtained showed that on the one hand total phosphorus and total nitrogen were enough for the growth of phytoplankton,onthe other hand the ratio of total nitrogen to total phosphorus was very low,which ranged from 0157 to 21114. The relationship between chlorophyll a and total nitrogen、total phosphorus was negative to some extent. The correlation coefficent between chlorophyll a and total nitrogen was higher than that between chlorophyll a and total phosphorus. Considering the distribution of the nutrients and their correlation with the chlorophyll a,total nitrogen was possibly the main limited factor of phytoplankton growth,while total phosphorus.was not the main limited factor of phytoplankton growth. The relationship between chlorophyll a and transparency was a curve of antihyperbola. This result was similar to the research result of the Japan and North America’s experts.The relationship between chlorophyll a and dissolved oxygen was positive. It was due to the increase of oxygen which was releasedduring photosynthesis.