Changes in the microzooplankton assemblages in the northern Adriatic Sea during 1989 to 1992

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Abstract
Microzooplankton were sampled at five fixed stations in the open waters of the northern Adriatic during four cruises in 1989, nine in 1990, six in 1991 and 12 in 1992. Changes in the qualitative-quantitative composition, together with the spatial-temporal distribution of the microzooplankton assemblages in the northern waters of the open Adriatic, were the result of atypical oceanographic and productivity conditions in the entire region of this sea. Owing to low values of ciliated protozoa, which comprised only 26% (1989) and 36% (1991) of the total number of microzooplankton, primary production cannot be controlled through grazing. This may possibly explain the occurrence of marine snow. The presence of the larger mucous aggregates in the later phase causes a significant decrease in the density of the naupliar copepod population. Therefore, the usual dominance of copepods in summer does not occur and their development cycle is displaced to late autumn. Consequently, only Olihona nana, a species of wider ecological tolerance, comprised up to 73% of the total number of postnaupliar copepods during the presence of mucous aggregates in 1989. Thus, this small copepod plays an important role in the processes of remineralization in the northern Adriatic.

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