Abstract: Concentrations and ratios of nutrients in Jiaozhou Bay, China, have changed much in the past decades, with trends indicating an increase in nitrogen and a decrease in silicate. Statistical analysis has shown that the long-term variations of nutrients are associated with agricultural activities, precipitation, and anthropogenic factors. Stoichiometric calculations indicate that the nutrient structure has become more and more unbalanced. There has been almost no possibility for nitrogen limitation since the 1980s, the probability of P limitation has increased, and the probability of Si limitation has also increased markedly from the 1980s to the 1990s. As a consequence of changes in nutrient structure, a decrease in the abundance of net phytoplankton was evident, whereas total chlorophyll a levels have remained roughly unchanged at around 3.55 mg/L. Thus, it is likely that smaller species have taken the niche vacated by the larger species. Changes in phytoplankton size and species composition may ultimately lead to various functional and structural changes at the system level.

Key words: anthropogenic factor, diatoms, Jiaozhou Bay, nutrient structure, silicon limitation, stoichio metric calculations.