Historical Changes in Nutrient Structure and its Influences on Phytoplantkon Composition in Jiaozhou Bay

Due to the influence of human activities, nutrient concentrations, nutrient ratios and phytoplankton composition have notably changed in Jiaozhou Bay, China since the 1960s. From the 1960s to the 1990s, nutrient concentrations have increased 1·4 times for PO4-P, 4·3 times for NO3-N, 4·1 times for NH4-N and 3·9 times for DIN. The atomic ratio of DIN:PO4-P increased very rapidly from 15·9±6·3 for the 1960s, to 37·8±22·9 for the 1990s. SiO3-Si concentration has remained at a very low level from the 1980s to the 1990s. The high ratio of DIN: PO4-P and low ratios of SiO3-Si:PO4-P (7·6±8·9) and SiO3-Si:DIN (0·19±0· 15) showed the nutrient structure of Jiaozhou Bay has changed from more balanced to unbalanced during the last 40 years. The possibility that DIN and/or PO4-P as limiting factors of Jiaozhou Bay phytoplankton has been lessened or eliminated and that of SiO3-Si limiting has been increased. The changes in nutrient structure may have led to the decrease of large diatoms and a shift of phytoplankton species composition. It is likely that there is a trend from large diatoms to smaller cells in Jiaozhou Bay.