Dissolved oxygen in the water presented high values (> 80% sat) in all of the flasks after 24 hours of incubation (Table 1). In all the flasks containing S. auriculata, the values were above 100% saturation. In the flasks with E. crassipes, there was an oxygen consumption after 24 hours of incubation. In the flasks concerning the S1 and S2 treatments, there was an increase in the chlorophyll-a values, while in the flasks of S3 treatment a small reduction related to the initial concentration (To) was detected.
In all of the treatments, higher percentages of nutrient reduction in E. crassipes compared to S. auriculata flasks were observed (Table 1) after 24 hours of incubation.
In the S1 and S2 treatments, the individuals of E. crassipes exhibited high percentages of reduction (around 90%) of the nutrients (except for TN). High percentage of ammonium reduction (97%) in S1 treatment was observed in the flasks with S. auriculata. The great enrichment of the initial concentrations of nutrients in the water (S3 treatment) was not followed by a corresponding increase in the percentage of absorption in both species.
No significant correlation between the chlorophyll-a and nutrient values was found in the enriched flasks (S1, S2 and S3 treatments) containing the macrophytes (Table 2).
By the other hand, the ANOVA analyses (Table 3) showed that: a) the nutrient uptake in the three treatments in the water containing E. crassipes was significantly different that from S. auriculata; b) there are significant difference, in both macrophytes, comparing the treatments (S1, S2 and S3 treatments) and the initial concentration of nutrients on the rates of nutrient reduction; c) the controls flasks always had decreasing concentrations of nutrients that were significantly different from the flasks with plants.