CONCENTRATION,DISTRIBUTION AND ANNUAL FLUCTUATION OF CHLOROPHYLL-a IN THE JIAOZHOU BAY

The Jiaozhou Bay is a typical temperate bay that provided with harbor,traveling,mariculture,etc.During the past decades,aquaculture,sea farming and changes in inputs of freshwater,nutrients and sediments have resulted in changes in structure and function of the marine ecosystem.In order to improve our understanding of the marine ecosystem of this area,the concentrations and distributions of chlorophyll-a were investigated monthly during June 2003 to May 2004 in the Jiaozhou Bay and its adjacent area.The results showed that the annual average concentration of chlorophyll-a was 2.81mg/m~(3) and the monthly mean concentrations ranged between 0.73 to 8.44 mg/m~3 in the study area.The concentrations of chlorophyll-a exhibited two-peaks during the study period.Phytoplankton reached annual highest concentrations in February,and then decreasing gradually.From May,the concentrations of chlorophyll-a increasing again with the water temperature increasing,and reached their second peak is in August,but the values were only half of those in February.The seasonal changes of chlorophylla are larger in the northern shallow area than those in the southern and out of the bay.The highest value occurred in February(monthly average concentration was 15.52mg/m~3) was nearly 30 times higher than that in December(0.55mg/m~3) in the north of bay.The horizontal distribution patterns showed that the concentrations decreased from northern to southern in the Jiaozhou Bay,and higher inside than out of the bay.Comparing the chlorophyll-a concentration with nitrate,ammonium,phosphate and silicate,there was no obvious correlation between them,although nutrient salts present a similar distribution pattern as that of chlorophyll-a.Only the generally decline of the nutrients with phytoplankton increasing indicates that the phytoplankton growth has an important impact on the change of nutrients.The concentrations of silicates declined to nearly 0 when the phytoplankton blooming occurred in winter.On the other side,the following decline of phytoplankton biomass also suggested that the low silicate limited the further developing of the winter phytoplankton bloom.The fluctuation of zooplankton biomass is generally synchronous to that of phytoplankton in the study area,and the only exception is that zooplankton biomass still keep at a low level while phytoplankton increase in the northern area in summer.The culture of Ruditapes philippinarum increased dramatically in the Jiaozhou Bay during last decade.Although our survey area outside the shallow,dense culture area,the distribution of high concentrations of chlorophyll-a shifted from northeast of the bay(mainly culture area of Ruditapes philippinarum) to the west area during this shellfish fast growth season,which indicated the influence of human activity on the nature ecosystems.Analysis on the variations of the nutrient salt,chlorophyll-a,and zooplankton,top-down control mechanism play an important role on regulating the fluctuation.