Abundance and composition of microzooplankton were studied over a 15 years period (from March 1986
to December 1990 and from July 1998 to December 2010) in the Gulf of Trieste (Adriatic Sea, NE
Mediterranean Sea). Sampling was conducted biweekly-monthly at the surface at the historical station
C1, 200 m offshore (bottom depth 17.5 m). Aloricate ciliates dominated in both periods (median 117 and
243 ind. L1 in the first and second period respectively) while tintinnids were more abundant in the first
period (median 55 ind. L1vs 16 ind. L1). For heterotrophic dinoflagellates there are no data during the
first period and in the second one they represented the second major group. Micrometazoans remained
almost constant over time. In the first period all microzooplankton groups showed a maximum in April,
while in the last period the peak has shifted to September. This is particularly evident for both aloricate
ciliates and micrometazoans. Tintinnids, that in the past had the absolute maximum in spring, in the
second period maintained the only, lower peak in October. Tintinnids in the first period were constituted
by 27 species and dominated by the genera Tintinnopsis, Stenosemella and Salpingella. In the last 10 years
we registered a dramatic decrease in abundance, paralleling an increase in species (40) with some “new
entries” as well as the almost complete disappearance of genera Helicostomella, Favella, Coxiella and
Steenstrupiella. The observed changes of the seasonal dynamics of microzooplankton abundance, as well
as of the tintinnids composition over the long period considered in our study, might suggest a climatic
forcing together with the known anthropogenic oligotrophication of the entire North Adriatic.
