The bacterial community, both in terms of community structure (denaturing gradient gel electrophoresis fingerprinting) and activity (exoenzymatic hydrolysis of proteins, polysaccharides and phosphorylated molecules and leucine uptake), was investigated seasonally for 3 years (2004-2006) in a large-scale grid in the northern Adriatic Sea. A high variability characterized the spatial structure of bacterial assemblages and a scarce seasonality was found in all the nine studied stations. Bacterial communities were substantially diverse in the same season of the 3 years, in contrast to what was reported previously for oceanic sites. Assemblages were in general strongly affected by river inputs, especially in spring, when freshwater loads were higher. Finally, a close relationship was found between given assemblages and their patterns of degradation/production activities by applying a multivariate analysis (linear discriminant analysis) to the dataset. The high variability of bacterial community structures and patterns of activity may indicate an ecological response to the high dynamism that characterizes the basin both on a physical and on a biological basis.