## 4 Discussion
### Factors Affecting Cadmium Uptake by Hydrocharis morsus-ranae
The effect of cadmium pollution is well illustrated by the pollution index, which depends on the level of plant nutrients and the length of exposure to cadmium (Figure 1). Regardless of the level of plant nutrition, after three and six weeks, the concentration of cadmium in the water steadily fell, ranging from 2.9 to 37.6 \(\upmu\)g\(\cdot\)dm\({}^{-3}\) (Table 1). In natural water bodies, the cadmium content of frogbit ranged from 0.0 to 2.08 mg kg\({}^{-1}\)[9; 93; 94; 95; 96; 97; 98; 99; 43; 90; 91; 92; 93; 94; 95; 96]. However, cadmium levels dozens of times higher were recorded by [PERSON] et al. [70] after five days of exposure to cadmium at a concentration of 0.25 mg/L. In the present study, the cadmium content in the plants depended on the level of supplementation and the length of exposure to cadmium, but only at the lower level of supplementation (Table 2). It is possible that at the higher level of supplementation, the high phosphorus concentration could have limited the efficiency of cadmium uptake (Figure 3). Interestingly, the plant cadmium contamination index did not appear to be affected by the level of cadmium supplementation. This effect may result from the method of calculation, where cadmium uptake in variable trophic conditions is compared with uptake in cadmium-free conditions. This may influence the use of the plant in the phytoremediation of cadmium-polluted waters. [PERSON] and [PERSON] [43; 96] report that, while cadmium was not found in the water, based on the employed research methods, it was nevertheless present in frogbit. Other field studies of aquatic ecosystems have also confirmed higher plant cadmium levels in contaminated water than in clean waters; this is reflected in a high bioconcentration factor (BCF) of up to 30,000, calculated as the ratio of plant cadmium concentration to water cadmium concentration [91]. For comparison, an unpolluted environment with frogbit typically has a BCF value of 5000 [97]. In the present study, the highest BCF value (12,655) was obtained at the low nutrient supplementation level after three weeks of exposure; this value was 10 times higher than in the remaining variants, suggesting that the plant reduces its metal uptake in the presence of higher nutrient content. [PERSON] [98] indicates that phosphorus has a varied influence on the uptake of cadmium by the plant: it can increase or decrease cadmium content in the plant depending on the species and the physical and chemical properties of the soil. In contrast, in studies conducted in natural aquatic ecosystems, [PERSON] and [PERSON] [96] report a positive correlation between the weight of frogbit and the concentration of _inter alia_ phosphorus. A similar measure to BCF is the correlation between the concentrations of cadmium in water and in the plant. However, only half of the experimental variants in the present study demonstrated a positive relationship in this regard (Table 3). In turn, [PERSON] and [PERSON] [9] did not report any such linear correlation in field studies of aquatic ecosystems. These findings may be due the influence of many factors that could limit the uptake of metal by the plant, and their possible antagonistic and synergistic effects [98]. [PERSON] and [PERSON] [44] determined that frogbit, similar to _Ceratophyllum demersum_, can be considered a bioindicator of water pollution with trace metals. The Nemerow Pollution Index (NPI) values for frogbit exceeded 3 at some points of a polluted river, indicating strong contamination of the aquatic ecosystem with these metals. [PERSON] and [PERSON] [96; 43] found the cadmium content of the Odra River sediment to range from 0.01 to 2.86 mg kg\({}^{-1}\). These findings indicate that the presence of smaller amounts of cadmium in the aquatic ecosystem creates better conditions for the growth and development of the plant. In the present study, cadmium was found to be present at 0.3 mg kg\({}^{-1}\) in the soil material. Part of the added cadmium (100 ug dm\({}^{-3}\)) was taken up by the frogbit, and the rest remained in the water.