This paper reports the rate of change of mesozooplankton biomass on seasonal, inter-annual and decadal
time scale in the Gulf of Trieste (NE Mediterranean, Northern Adriatic). We measured variability in
mesozooplankton dry weight (mg DW m 123), organic carbon and nitrogen contents of the DW in relation to
mesozooplankton taxonomic structure and some environmental parameters. The study is based on data
obtained from mesozooplankton samples collected monthly by a vertical WP2 net (200 \u3bcm) from January
1986 to September 2005 at one monitoring station, a coastal site in the Gulf of Trieste. We considered
mesozooplankton DW in relation to copepods, phytoplankton taxonomic structure, water temperature and
North Atlantic Oscillation. For further analyses we counted also on data for DW for the period 1972\u20131980,
monthly data for organic carbon (mg C m 123) and nitrogen contents (mg N m 123) of the DW for the period
1991\u20132005, determined by a CHN Elemental Analyzer.We explored statistically our high temporal resolution
time series data picking out the main features: seasonal components and trends. Mesozooplankton DW
ranged from only 1 mg m 123 (January 1977) to 95 mg m 123 (March 1990) in the coastal ecosystem of the
Northern Adriatic during the period 1972\u20132005. The annual cycle of the DW was found to be bimodal with
maximum in late winter\u2013spring and a weaker one in late summer. Maximum DW were regularly recorded
when Copepods prevailed the mesozooplankton community structure. Similarly, high organic carbon and
nitrogen contents were detected when copepods dominated, although wide-ranging on a seasonal scale.
Irregular intra- and inter-annual fluctuations were typical mostly during the 1990s. Mesozooplankton DW
sharply shifted since 2001\u20132002 to the level exceeding the observed one during the regime of the 1980s. Our
results indicate substantial changes in the seasonal timing of mesozooplankton DW, which together with
decadal inter-annual fluctuations match significantly with shifts in copepods, phytoplankton taxonomic
structure, temperature and NAO index. The discussed zooplankton descriptors (DW, C and N contents and
C:N ratio) being to a certain degree predictive of phyto- and mesozooplankton composition, in essence
might foretell the phenological response of plankton communities to environmental changes in the coastal
ecosystem, including climate change