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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.

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Soto marsh is a non forested inland freshwater wetland with a humid subtropical weather and fed by rains and domestic wastes through the Soto lake. Main hydrological characteristics and the composition of vegetation and soil through which the water circulates are described. Chemical and biological conditions of water entering and leaving the marsh during dry and rainy periods were compared. Passage through the marsh reduces BOD 5 by 64%, producing an average effluent of 2.2 mg.l -1 . Average chlorophyll "a" concentration was reduces by 76% in relation to value recorded at inflow. During the rainy period, the concentration of chlorides, calcium, potassium and sodium was lower in the leaving of the marsh while in the dry period the concentration of anions and cations was very variable. Our results indicate that Soto marsh functions as a source of nutrients (phosphorus and ammonium) when the concentration of organic matter was high and the concentration of dissolved oxygen was low. In the Soto lake and at inflow of the Soto marsh, phytoplanton was dominated by cyanophyte, specially colonies of Microcystis aeruginosa while at the marsh outlet there were higher proportion of other algae. After passage through the marsh zooplankton abundance decreased specially rotifers populations characteristic of eutrophic environments. Comparing the same vegetation (Salvinia biloba), the number of invertebrates associated with the plants was higher in the lake than in the marsh. This decrease was more marked during the rainy period