## 4 Discussion
### Factors Affecting Cadmium Uptake by Hydrocharis morsus-ranae  
The most important factor affecting the availability of cadmium is considered to be pH [88]. At a pH value around 7, this element exists as a free ion (Cd\({}^{2+}\)). This form is easily taken up by living cells and is hence the most toxic. For pH values above 7, the metal forms complexes (oxides and hydroxides) that are inaccessible to cells [89]. Field studies of aquatic ecosystems [9; 24; 43; 44; 90; 91] have found _Hydrocharis morsus-ranae_ in waters where the pH ranged from 6.44 to 8.27. In the present study, the water pH ranged from 7.03 to 7.40 (Table 8), which ensured that Cd\({}^{2+}\) was available for the plant, especially for the samples at the lower nutrient level after three weeks of cadmium exposure (pH = 7.03). In natural aquatic ecosystems, frogbit occurs in waters with varying concentrations of biogenic compounds. In Poland, the plant was recorded in stagnant and stagnant waters with the following parameters: N\({}_{\text{min}}\): 0.054-1.086 mg dm\({}^{-3}\) and N\({}_{\text{Kjeh}}\): 1.12-2.10 mg dm\({}^{-3}\), PO\({}_{4}\)[3]-: 0.009-0.919 mg dm\({}^{-3}\), and K: 0.70-7.63 mg dm\({}^{-3}\)[9; 90]. It was also found in a lake in New York under the following conditions: total phosphorus 19-36 ug/L in the coastal zone with frogbit, PO\({}_{4}\)[3]-: 0.24-0.324 mg/L, and NO\({}^{3-}\) 2.712-15.147 mg/L [16; 92]. Unfortunately, the results of field studies of aquatic ecosystems do not confirm whether these concentrations of mineral nutrients promote or inhibit the development of frogbit. In the present study, in the samples with a lower level of plant nutrition, the initial concentrations of the elements were N\({}_{\text{min}}\) (N\({}^{-}\)NO\({}_{3}\)- + N\({}^{-}\)NH\({}_{4}\)): 1.8 mg dm\({}^{-3}\), PO\({}_{4}\)[3]-: 0.54 mg dm\({}^{-3}\), and K\({}^{+}\): 4.56 mg dm\({}^{-3}\); these levels are similar to those in the natural environment. At a higher level of supplementation (N\({}_{\text{min}}\): 9.3 mg dm\({}^{-3}\), PO\({}_{4}\)[3]-: 2.73 mg dm\({}^{-3}\) \begin{table}
\begin{tabular}{c c c c c c} \hline \multicolumn{2}{c}{**Conditions**} & \multicolumn{4}{c}{**Linear Correlation**} \\ \hline high & I & d.w. = 27.02 \(-\) 0.12 \(\times\) EC & r = \(-\)0.5280 & \(p\) = 0.0777 & - \\  & II & d.w. = 20.62 \(-\) 0.07 \(\times\) EC & r = \(-\)0.3426 & \(p\) = 0.2757 & - \\ \hline \end{tabular}
\end{table} 
Table 12: _Cont._and K\({}^{+}\): 22.8 mg dm\({}^{-3}\)), higher concentrations of biogenic compounds were observed compared to those in natural water reservoirs. Three weeks after the introduction of mineral nutrients into the water, a significant increase in the fresh weight of plants was noted (from 19.9 to 25.8) due to the plant having greater access to these compounds (Table 5). However, after six weeks, a decrease in the fresh weight was observed in the samples with the high nutrient content; this was probably due to the significant decomposition of leaves of older rosettes and the depletion of oxygen dissolved in the water (Table 7). The availability of water-soluble metal cations and anions of acid residues affects the EC value. In natural aquatic ecosystems, the frogbit inhabits waters with EC values ranging from 70 to 996 \(\upmu\)g\(\cdot\)cm\({}^{-1}\)[9; 24; 43; 44; 90; 91]. In our present findings, these values depend on the time of exposure to cadmium and the level of plant nutrition. Its value was not affected by the introduction of cadmium salts into the water. The total EC score ranged from 168.2 to 202.0 \(\upmu\)g\(\cdot\)cm\({}^{-1}\), slightly lower than the typical value observed in natural water bodies (467 \(\upmu\)g cm\({}^{-1}\)). The concentration of cadmium in natural waters in Poland, where the frogbit is present, ranges from 0.0 to 22.0 \(\upmu\)g dm\({}^{-3}\)[9; 91]. In the present mesocosm experiment, cadmium was introduced into the water at a concentration of 100 \(\upmu\)g dm\({}^{-3}\).