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

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... orophyll "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