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The Reconquista river is one of the most polluted watercourses in Argentina. More than 3 million people and over 10,000 industries are settled on its basin. The available data show that pollution is mainly related to the discharge of domestic and industrial liquid wastes that are poured into the river almost untreated. At present no site of the river can be characterized as free of pollutants. Samples were taken monthly from 5 sites of the river; some 18-20 physicochemical parameters were determined in each sample. Analyses revealed significant differences in the degree of deterioration between sites. Two of them, close to the source of the river (Cascallares-S(1) and Paso del Rey-S(2)) resulted less polluted than the two points located close to the mouth of the river (San Martin-S(4) and Bancalari-S(5)). The worsening of the water quality in S(4)-S(5) was attributed to the discharges of the Moron stream, a tributary that flows into the main course of the river a complex mixture of non treated waste waters. PCA was used in the ordination of samples (sites, season and physicochemical parameters). In the PCA performed using all variables, the first principal component showed positive correlation with N-NH(4) (+), conductivity, orthophosphate, BOD(5), COD and alkalinity, and negative correlation with DO. The second principal component was positively correlated with pH, temperature and chlorophyll a and negatively with phenols and hardness. In respect to the spatial distribution, the plot of the scores for the first two components of samples taken in each sampling station showed S(1) and S(2) values displayed farthest at the left side of the X axis with high DO. In contrast, S(4) and S(5) values stayed at the right side of this axis with high N-NH(4) (+), conductivity, orthophosphate, BOD(5), COD and alkalinity; data of S(3) were "intermediate". In order to identify seasonal trends in the concentration of contaminants scores of cases labeled by season were plotted. The line drawn on the ordination plane showed that summer samples tended to converge to the upper right portion of the graph where pollution variables had more importance on the first axis, and along the second axis with high correlation with pH, temperature and chlorophyll a. Winter scores were settled in the left lower part of the plot with minor contribution of pollution parameters and more importance of DO on the first axis. Samples of autumn and spring did not fit a clear cut pattern.

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Quatre bioessais in vitro ont été réalisés pour évaluer la réponse des communautés phytoplanctoniques naturelles à différentes concentrations de zinc (2,5; 10; 20 - 25 et 40 - 50 mg.l-1). Les essais réalisés au cours des quatre saisons , ont été menés en conditions contrôlées pendant un mois avec des échantillonnages tous les 2-3 jours.La réponse algale fut variable selon la saison et dépendante des concentrations en Zinc, des espèces algales, de la densité de l' inoculum et de la température. 2,5 et 10 mg Zinc.l-1 dans le milieu de culture ont stimulé la croissance de certaines diatomées; à une concentration de Zinc de 20-25 mg.l-1Chlorella vulgaris (Chlorophyceae) s'est particulièrement développée, et, dans des systèmes contenant 40-50 mg Zinc.l-1 une mortalité importante a été généralement trouvée, à l'exception de Chlorella vulgaris et de quelques espèces de diatomées qui se sont montrées tolérantes au zinc: Cyclotella meneghiniana, Gomphonema parvulum, Navicula sp., Nitzschia palea, Nitzschia sp., Pinnularia biceps, Synedra acus et Synedra ulna var. amphirrhynchus. Les Cyanophyceae, les Euglenophyceae, les Tribophyceae, les Chrysophyceae, les Zygophyceae et les Dinophyceae ont été particulièrement sensibles. En présence d' inoculums abondants, la sensibilité au Zinc a été observée à partir de 25 mg.l-1 (automne-20°C, printemps-20°C et été-25°C); par contre, en présence d'inoculums peu denses, elle est apparue dès 10 mg.l-1 (hiver-15°C). De manière générale, à mesure que la concentration de Zinc augmentait, Chlorella vulgaris Biej. est devenue graduellement, le taxon dominant; ce qui a eu pour conséquence une diminution de la diversité du système algal testé. Des communautés plus simples se sont alors développées, dominées presque toutes par des espèces tolérantes au Zinc.In order to study the response of native phytoplanktonic communities to different zinc concentrations (2.5, 10, 20 - 25 and 40 - 50 mg.l-1), four bioassays in vitro were carried out. They were conducted in autumn, winter, spring and summer under controlled conditions during a month and the samplings were performed each 2-3 days.The algal responses were variable according to the season and zinc-concentration, species, inoculum density and temperature dependent. 2.5 and 10 mg.l-1 of Zinc in the test medium caused a stimulatory effect on the growth rate of certain diatoms; with 20-25 mg.l-1 of Zinc the development of Chlorella vulgaris (Chlorophyceae) was stimulated, and with 40-50 mg.l-1 of Zinc it was generally found important mortality rates except for Chlorella vulgaris and for some tolerant diatom species: Cyclotella meneghiniana, Gomphonema parvulum, Navicula sp., Nitzschia palea, Nitzschia sp., Pinnularia biceps, Synedra acus and Synedra ulna var. amphirrhynchus. The Cyanophyceae, Euglenophyceae, Tribophyceae, Chrysophyceae, Zygophyceae and Dinophyceae were particularly sensitive to Zinc. In presence of abundant inoculums, the sensivity to Zinc was observed up to 25 mg.l-1 (autumn-20°C, spring-20°C and summer-25°C) while in presence of s...

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