Changes of zooplankton communities in the Gulf of Tigullio (Ligurian Sea, Western Mediterranean) from 1985 to 1995. Influence of hydroclimatic factors

Year-to-year variations in abundance and composition of zooplanktonwere studied in the Ligurian Sea at a station sampled two timesa month between 1985 and 1995. As a break of 2 years (April1989–December 1990) occurred in the time series, the STATISmethod was chosen instead of time series analysis. Each of thenine sampled years was a single table of monthly or seasonalaverage densities of 26 plankton taxa. STATIS allowed (i) estimationof similarity between each yearly table, (ii) visualizationof the trajectories of both species and observations (seasons)from one year to another, and (iii) associations of particularspecies, which showed similar temporal variations, to be determined.A strong seasonal variation was evident for most species, andyears 1987, 1992 and 1994 were different from the others. Trajectoriesindicated which species were stable and which were characterizedby small or large fluctuations during the nine years. Five differenttaxa associations were detected. For each association, the mostrepresentative period was identified, where each period wasa group of months obtained by clustering on species abundances.Taking into account hydro-climatic factors in the representativeperiods, a contingency discriminant analysis allowed us to identifyand characterize the most discriminant environmental parametersassociated with each group of species. Environmental factorsthat best discriminated the different representative periodswere atmospheric pressure, current speed and direction, andwater temperature.