Time series (> 20 years) of bacterioplankton, phytoplankton and zooplankton abundances and of phytoplankton production in the Adriatic Sea were analysed. Multivariate methods were used to extract the main patterns of year-to-year changes in abundances (Principal Component Analysis), and to discriminate between years and sites (Multidimensional Scaling). Increasing long-term trends were established for planktonic abundances and phytoplankton production in both coastal and open sea areas, presumably as a result of eutrophication processes. In the open sea, natural factors (water exchange between the Mediterranean and Adriatic Seas, and temperature) were dominant in controlling year-to-year fluctuations of plankton. On the other hand, long-term fluctuations of plankton in the coastal area were chiefly controlled by man-induced factors. Thus, besides natural factors, the dominant pattern of planktonic fluctuation was correlated with the fluctuation of nutrients coming from the land as a result of human activities.