Cluster and time series analyses were applied to a 1970–1999 data set of zooplankton
collected from the Gulf of Trieste (Northern Adriatic Sea). Two time periods were compared:
1970–1980 and 1986–1999. Associations of copepod species were identified and an attempt was
made to reveal their long-term dynamics and significant patterns over the study period. Two main
groups with 2 and 3 subgroups each were identified. We observed a seasonal alternation of the 2
main groups, albeit with remarkably different patterns during the 2 periods. Time-series analyses
suggested recent changes in the periodicity of the associations. The long-term annual mean copepod
abundance showed an increase in the late 1990s in comparison to the 1970s; however, this increase
was not constant. Small-sized copepods increased in the second period, indicative of changes in the
phytoplankton size spectrum. Shifts in the 1990s appeared to be related to climate change in the
Northern Hemisphere that appeared abruptly after 1987. Climate change is thus recognized as the
most important factor (more so than anthropogenic change) affecting inter-annual and inter-decadal
variability of plankton dynamics. The strong signal recorded in the zooplankton community seems to
reflect this, identifying the Gulf of Trieste as one of the most sensitive areas in the Mediterranean.
