Plankton samples of rivers, lakes, pools and minor water bodies were taken in the Patagonian Argentine provinces of Rio Negro, Chubut, NeuquCn and the Southern part of Buenos Aires. The relationship between physical and chemical characteristics of these waters and their rotifer fauna was analyzed. Sixty taxa were identified, and their value as indicators of temperature, pH, conductivity, total alkalinity and chloride determined. The use of rotifers as indicator species of these variables is discussed. Lecane (MJ closterocerca closterocerca showed a rather wide tolerance to the 5 analyzed factors, and is considered to be euryokous. A classification of water bodies based on conductivity, chloride and alkalinity is proposed.

Twelve taxa belonging to the genus Brachionus recorded from Reconquista River, Argentina, are reported with descriptions and figures . Several taxa are new to Argentina, and three (B . moronensis nov . sp ., B . postcurvatus nov . sp . and B . urceolaris reductispinis nov . subsp .) are new to science. Morphological variability in the species of Brachionus identified is discussed . Biogeographical information and a key to the recorded species are also given .

Aquatic environments associated with human settlements are mostly deteriorated by untreated or poorly treated discharges. Buenos Aires city and surroundings constitutes one of the largest urbanizations of the world. High levels of pollution have been detected for its related watercourses, showing the occurrence of a diversity of pathogenic bacteria in surface waters as well in biological members of the ecosystem. In the present paper the finding of new records for the area is reported and discussed. Yersinia enterocolitica and two subspecies of Klebsiella pneumoniae were isolated from the Reconquista River, while Citrobacter freundii, Pseudomonas aeruginosa, Pseudomonas luteola, Vibrio fluvialis, and Aeromonas hydrophila were detected in the nearby Matanza and Luján rivers.

On the present study, the nourishment and the digestive system of a population of the spotted pimelodid Pimelodus maculatus Lacepède 1803 from a polluted urban river in Argentina were analyzed (Reconquista River, Buenos Aires province). The specimens are characterized by having benthic habits and an omnivore diet. In their intestinal content, a diversity of components was identified. Such components were grouped into ten alimentary items, with dietary variations according to body size while the longitude of the intestine remained proportional to standard length. Microbiological analysis in the gills, stomach and intestine showed the occurrence of Staphylococcus aureus, Escherichia coli and Salmonella sp. Fungi and yeast were also detected. Bacterial accounts were very high both in the analyzed organs as well in water samples. The ecological and sanitary significance of the identified bacteria is discussed.

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