Eleven years of time-series analysis on the net-zooplankton community in the Gulf of Trieste

Net-zooplankton data collected monthly at one coastal station of the Gulf of Trieste (Northern Adriatic Sea) from 1970 to 1980 were analysed in order to test the existence of any successional trend among the copepod community. No periodicity was evident for single species. However, a clear periodicity was found for two species groups obtained by cluster analysis. One group was characterized by Acartia clausi and Temora longicornis and showed a regular appearance in late spring–summer. The other group was characterized by Temora stylifera and Oncaea sp. and showed a regular appearance in autumn–winter. Time-series analysis of the diversity of the copepod community showed a clear 12-month periodicity with winter maxima corresponding to the highest level of stability. However, a decreasing trend of the copepod biomass appears to have originated in the mid-1970s. The biomass decrease was probably caused by Noctiluca miliaris and Pelagia noctiluca blooms.