Long-term copepod dynamics in the Gulf of Trieste (Northern Adriatic Sea): recent changes and trends

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