Ammonia, nitrite and nitrate nitrogen, dissolved oxygen, pH, temperature, total hardness, transparency and Zooplankton abundance were determined in lake water samples in order to monitor the eutrophication process. Multivariate analysis methods, such as principal factor, cluster and stepwise regression analysis, were used to verify the associations among variables and to separate factors ... [Show full abstract] responsible for the observed increase of the eutrophication. Nitrate fertilizers from surrounding farmland appear to be the main source of pollution. A secondary factor is constituted by the whole biomass, which is both origin and effect of the increasing productivity of the lake: the anaerobic decomposition of the organic debris in the hypolimnion is relevant. Seasonal trends of the variable associations are also observed.