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Temporal and spatial variations of the phytoplankton in a subtropical lake of ArgentinaSpecies composition, biomass and diversity of the phytoplankton in a subtropical lake (27°29'S; 58°45'W) were studied from February 1995 to March 1996. One or two samples were taken monthly in the limnetic and the littoral area. Cell counts were done by the Utermöhl method (1958 Cyanophyta was the dominant group all the year contributing among 25% and 83% of the total biomass. Cylindrospermopsis raciborskii, Aphanizomenon sp. and Microcystis aeruginosa were the dominant species. The Chlorophyta, second group in importance, contributed among 8% and 50% being Botryococcus braunii the dominant species. Bacillariophyceae, Xanthophyceae, Euglenophyceae and Cryptophyceae they did not surpass 20% of total biomass. Chrysophyceae and Dinophyceae they did not surpass 5% of total biomass. Chlorophyll a content ranged between 0.7 and 8 µg.L -1 reflecting the annual variation of phytoplankton biomass. Phytoplankton density diversity varied among 0.34 and 3.69 bits.ind -1 . Biomass diversity varied among 1.04 and 2.92 bits.µm 3 , with a similar variation in the depths of the lake.The lowest biomass and the highest diversity were always found in the littoral zone. C-Strategists, with a high SA/V ratios prevailed in density (Monoraphidium contortum, M. tortile, Chlorella vulgaris and other small flagellates of Chlorophyta) while S-Strategists (with a low SA/V ratios) such as Microcystis aeruginosa and Botryococcus braunii prevailed during the greater part of the year. Cylindrospermopsis raciborskii and Aphanizomenon sp. were dominant in density and biomass when the temperatures were superior to 20°C and the solar radiation surpassed 20 MJ.m -2 . The R-Strategists (Aulacoseira italica and Gymnodinium sp.) prevailed during great unstability of water column.Phytoplankton biomass was positively correlated with rain falls 10 days before each sampling (r = 0.772; p < 0.01; g.1. = 30). During the period that prevailed the filamentous Cyanophyceae an increase in the heterocysts frequency was observed in coincidence with a period of notorous drought.

The toxic effects of the insecticides chlorpyrifos and cypermethrin, and the herbicide glyphosate on the growth, biovolume and ultrastructure of the green microalgae Selenastrum capricornutum were evaluated. Concentrations between 9.37-150 mg L -1 of chlorpyrifos, 3.12-100 mg L -1 of cypermethrin and 4.7-60 mg L -1 of glyphosate were assayed along with a control culture. The assayed concentrations were prepared using commercial formulations. After 48 h all tested concentrations of the three pesticides reduced signifi cantly the population growth. The 96 h effective concentration 50 (EC 50 ) was 14.45 mg L -1 for chlorpyrifos, 12.37 mg L -1 for cypermethrin and 15.60 mg L -1 for glyphosate. Cells exposed to the three pesticides showed an increase in the cellular size related to the increase in pesticide concentration and exposure time. The most significant damages observed on the ultrastructure of cells exposed to the three pesticides included thylakoids and mitochondria disruption, formation of electrodense bodies, accumulation of lipids and increase in the size and number of starch granules.The present study demonstrates that the effects of pesticides also extend to non-target organisms having signifi cant ecological implications.