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 zooplankton were studied in the Ligurian Sea at a station sampled two times a month between 1985 and 1995. As a break of 2 years (April 1989-December 1990) occurred in the time series, the STATIS method was chosen instead of time series analysis. Each of the nine sampled years was a single table of monthly or seasonal average densities of 26 plankton taxa. STATIS allowed (i) estimation of similarity between each yearly table, (ii) visualization of the trajectories of both species and observations (seasons) from one year to another, and (iii) associations of particular species, which showed similar temporal variations, to be determined. A strong seasonal variation was evident for most species, and years 1987, 1992 and 1994 were different from the others. Trajectories indicated which species were stable and which were characterized by small or large fluctuations during the nine years. Five different taxa associations were detected. For each association, the most representative period was identified, where each period was a group of months obtained by clustering on species abundances. Taking into account hydro-climatic factors in the representative periods, a contingency discriminant analysis allowed us to identify and characterize the most discriminant environmental parameters associated with each group of species. Environmental factors that best discriminated the different representative periods were atmospheric pressure, current speed and direction, and water temperature.