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Water and quantitative zooplankton samples were collected at approximately monthly intervals from June 2000 to June 2002 in the middle section of the Po River (Northern Italy). Highly variable hydrological conditions were observed in the 2 years, with river flow varying approximately between 400 and 9300 m 3 s À1 . Dissolved inorganic nitrogen (DIN) usually decreased at higher river discharges. Autotrophic production was limited by phosphorus availability during the growing period (May-October); no significant adverse effect of zooplankton grazing upon algal biomass was detected. A total of 161 zooplankton taxa were found. The records of the rotifers Lecane incospicua, L. cornuta, Proales sigmoidea, Cephalodella limosa, and of the cladoceran Phrixura leei were new for Italy. Rotifers strikingly were the most abundant group over the entire sampling period. Zooplankton abundance was positively correlated to water temperature and chlorophyll-a concentration, and only in warmer months inversely correlated to river flow. Species of the genus Brachionus were responsible for the two density peaks (>1000 ind L À1 ) of the rotifer taxocoenosis observed in July 2000 and August 2001, coinciding with chlorophyll-a peaks higher than 50 mg L À1 and minima of Shannon equitability index. Water temperature, chlorophyll-a, and soluble reactive phosphorus (SRP) were found to be the most important variables in explaining the observed rotifer assemblages by Canonical Correspondence Analysis. A simple descriptive model is proposed to represent the combined influence of abiotic constraints and biotic interactions on the zooplankton abundance and diversity in the middle reach of the Po River.

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Zooplankton samples from the middle reach of the Po River were collected daily from 27 July to 24 August 1988 from a station located near Viadana. Changes in the biocoenosis structure were analyzed in relation to variations in flow rate. Rotifers accounted for more than 99% of the total zooplankton (protozoans excluded) in every sample. The dominant species were Brachionus calyciflorus, Brachionus bennini, Brachionus budapestinensis and Epiphanes macrourus. Under scanty flow conditions, the taxocoenosis showed marked stability. An increase in flow rate acts as a disturbance factor leading to a significant decrease in both total density and dominance.