Abstract: Based on the observed data from 1991 to 2004,in north-eastern of Jiaozhou Bay,the variations of months,ten-day period and long-term in nutrient concentration and their molar ratios,and the relations between them and the chlorophyll a content are discussed in this paper.The various nutrient concentrations slowly increase in a wave in spring and summer which are closel related with the input of the outer-source nutrients.Big variations in a ten-day period in nutrient concentrations and their molar ratios mainly reflect instability of the input of the outer-source nutrients.Among various nutrients,the obvious peak value is found in August only for SiO₃-Si and there are similar variations to SiO₃-Si/DIN and SiO₃-Si/PO₄-P ratios related to rainy season which brings a lot of SiO₃-Si.After autumn,the nutrient concentrations rapidly increase,particularly SiO₃-Si and DIN,as high as 97.7 and 173.4μmol/L respectively,and lead to the bloom of Lauderia annulata Cleve in winter which is found first time in winter in Jiaozhou Bay.During the peak of bloom,nutrients are heavily consumed,and SiO₃-Si is almost exhausted(0.49μmol/L) and SiO₃-Si/DIN and SiO₃-Si/PO₄-P ratios rapidly drop down to 0.01and 1.04 respectively.SiO₃-Si becomes the limiting factor of phytoplankton growth and the bloom is finally controlled.The study shows that there are close relations of growth and decay between the season variations of nutrients concentrations and phytoplankton.The long-term variations of nutrients and their structure show that,compared the 1999-2004 period with the 1991-1998,the average DIN and SiO₃-Si concentrations increase by 1.0 and 2.2 times,respectively,and the average DIN/PO₄-P and SiO₃-Si/PO₄-P ratios in-crease by 0.4 and 0.9 times,respectively,and the PO₄-P concentration and the SiO₃-Si/DIN ratio increase a little.All the molar ratios of nutrients are much different from the Redfield ratios(N/P=16,Si/P=16,Si/N=1).It is close related to the developments of a society and its economy of Qingdao city.