Annual dynamics of the mesozooplankton communities in a highly variable ecosystem (North Adriatic Sea, Italy).

Mesozooplankton community composition at the species ⁄ taxa level was investigated from January to December 2001, at eight stations in the northern Adriatic Sea. Annual dynamics, taxonomic composition and spatial diversity in relation to different trophic conditions are discussed as related to previous studies and to different conditions, such as the presence of mucilage events. Zooplankton communities all over the northern sub-basin were dominated by the cladoceran Penilia avirostris in summer, and by the calanoids Paracalanus parvus, Acartia clausi and the poecilostomatoids Oncaea spp. during the rest of the year. Whereas coastal communities were more variable with time and location, we were able to identify a group of offshore stations with a similar species ⁄ taxa composition and annual dynamics. Comparing the present results with previous studies, we observed significant changes in community composition in the time scale of 20 years, mostly due to a general decrease of A. clausi as the dominant species, being replaced by P. parvus. Also, P. avirostris swarms appear to have extended their temporal occurrence, and were present for longer periods of time when compared to past records. These changes might be related to the observed general increase of the average sea water temperature in the northern Adriatic Sea.