Young individuals of E. crassipes and S. auriculata were manually collected in the mouth of the largest sewage channel (Fig. 1). The plants were washed in flowing water in the laboratory and incubated for 24 hours. They were exposed to sunlight in a plastic pool where the water temperature was maintained between 27 and 31oC. The light intensity was measured with a radiometer (Li-Cor LI-185B).
The macrophytes and controls (flasks without plants), were incubated in plastic vials containing 700 ml of a solution previously prepared with water from the Imboassica lagoon enriched by ammonium nitrate (NH4NO3), ammonium chloride (NH4Cl) and dibasic potassium phosphate (KH2PO4). Three different solutions of increasing concentrations, S1 (with approximately 0.8 mg NO3/L, 1.0 mg NH4/l, 0.6 mg PO4/L); S2 (4.0 mg NO3/L, 5.0 mg NH4/L, 3.0 mg PO4/L) and S3 (8.0 mg NO3/L, 10.0 mg NH4/L, 6.0 mg PO4/L) were tested.
A young individual of Eichhornia crassipes, having an average of 3 leaves with their sizes close to 4 x 7 cm, was incubated in each flask. Another series of flasks received an individual of Salvinia auriculata, each one with approximately 6 rosettes of 1.0 cm (diameter). Before and after the incubation period, the temperature, salinity and electrical conductivity of the water were measured (digital thermo-salinometer-conductivimeter, LabComp SCT), as well as the concentrations of nitrogen and phosphorus. At the end of the experiment the plants were removed from the flasks and dried at 70oC in order to obtain dry weight.
After a filtration using filters GF 52/C (Schlücher & Schüll), the chlorophyll-a concentrations were obtained according to Nusch & Palme (1975). Total nitrogen and nitrate (µg/L) were determined according to Mackereth et al. (1978) and the ammonium (µg/L) according to Grasshoff (1976). Total and reactive soluble phosphorus (µg/L) were determined as suggested by Golterman et al. (1978). Dissolved oxygen concentrations were measured using Winkler method (Golterman et al., 1978).
The analysis of variance (ANOVA) was applied to verify the significance different among the uptake rates (E. crassipes and S. auriculata) and the treatments (S1, S2 and S3 enrichment). Pearson's Linear Correlation between chlorophyll-a values and nutrient concentrations were also obtained.