Recent trophic changes in lake pusiano (northern Italy) with particular reference to the influence of hydrodynamics on the zooplankton community

Lake Pusiano (northern Italy) has been subjected for more than 20 yr to increasing eutrophication which caused hypereutrophic conditions in the mid-1980s. Due to the introduction of a sewage-treatment network in 1986, the total P concentration in the lake water dropped from 185 μg1-1 in 1985 to 74 μg1-1 in 2003. Although there was a reduction in nutrient load, cyanobacteria blooms were observed from 1994 onwards. The aim of this study was to compare physical and chemical parameters and the zooplankton community between two investigation periods (February-December 2002 and January 2003-February 2004) that were separated by an extreme flood event in November 2001 affecting the total plankton community. From the analysis of the data sampled in 2002-2003, an increase in transparency (from 3.5 m to 5.7 m) and a decrease in chlorophyll α concentration (from 6 μg1-1 to 1.5 μg1-1), both measured during the circulation period (December-February), were recorded. Within the phytoplankton, a shift from cyanobacteria to chlorophytes was detected. The zooplankton community changed in the composition of species, that is a higher number of species known to be less tolerant to toxic cyanobacteria were observed. The results emphasise that the flood event at the end of 2002 accelerated the process of reoligotrophication in the lake and contributed to an increased diversification of the zooplankton community. The generally increasing frequency of occurrence of flooding events following heavy rainfalls might be considered a major factor in regulating plankton community composition in the future.