## 2 Materials and Methods  
### Experimental Materials  
To verify the hypothesis given above, a vegetation experiment was conducted using the containers method over three consecutive years in the vegetation hall of the West Pomeranian University of Technology in Szczecin (53.447838; 14.530589). Containers with frogbit grown in hydroponic conditions during the plant growing season (18 June-7 August 2006, 1 st growing season (gs); 17 June-6 August 2007, 2 nd gs; 22 June-11 August 2008, 3 rd gs) were placed in the open air under a roof (3 m high) made of transparent plastic cover sheets. The plant material for the study was obtained from the water reservoir at Lake Glebokie in Szczecin. The collected frogbit rosettes had five developed leaves with a diameter of about \(5\pm 1\) cm and leafless stolons (2 pieces). As frogbit occurs in clay-pit and on riverbanks, a soil with a granulometric composition corresponding to sandy loam, and a similar composition to river bottom sediments was chosen as the substrate [48]. The soil was composed predominantly of 60.4% by mass sand (0.05-2 mm), 35.6% dust (0.002-0.05 mm), and 4.0% colloidal clay (<0.002 mm). It was characterized by a slightly acid reaction (5.6 < pH \(\leq\) 6.5) and low levels of assimilable forms of phosphorus (23-44 mgP\(\cdot\)kg\({}^{-1}\)), potassium (63-104 mgK\(\cdot\)kg\({}^{-1}\)), and magnesium (21-30 mgMg\(\cdot\)kg\({}^{-1}\)) [49]. It can be considered fertile soil (C:N = 8), indicated by a carbon-to-nitrogen ratio of 12.8, i.e., a pH\({}_{\rm KCl}\) reaction of 5.9; C\({}_{\rm org}\) 9.6 g\(\cdot\)kg\({}^{-1}\), humus 16.6 g\(\cdot\)kg\({}^{-1}\), organic substance 37.6 g\(\cdot\)kg\({}^{-1}\), and N\({}_{\rm gen}\) 0.75 g\(\cdot\)kg\({}^{-1}\); the concentrations of assimilable forms (mg\(\cdot\)kg\({}^{-1}\)) were 28 P, 83 K, 24 Mg, and saline 0.37g NaCl dm\({}^{-3}\). The salt content in the soil was within natural limits [50]. The total content of selected metals in the soil used in the study corresponded to that of the bottom sediments of lakes and rivers [51; 52], i.e., 185 Mn, 64.5 Zn, 15.4 Cu, 24.2 Pb, and 0.3 Cd (all values ug\(\cdot\)g\({}^{-1}\) d.w. soil); in addition, there was 11.6 Fe mg\(\cdot\)g\({}^{-1}\) d.w. soil. Three phosphorus, nitrogen, and potassium (PNK) compounds, viz. K\({}_{2}\)HPO\({}_{4}\), NH\({}_{4}\)NO\({}_{3}\), and KNO\({}_{3}\), were added to diversify the trophic properties of the water environment; of these, P and N have the greatest impact on the water eutrophication process. The K\({}_{2}\)HPO\({}_{4}\) (low level of NKP: 0.98 g in 1 dm\({}^{3}\) distilled water; high level of NKP: 4.95 g in 1 dm\({}^{3}\) distilled water), NH\({}_{4}\)NO\({}_{3}\) (low level of NKP: 0.98 g in 1 dm\({}^{3}\) distilled water; high level of NKP: 4.98 g in 1 dm\({}^{3}\) distilled water), and KNO\({}_{3}\) (low level of NKP: 10.65 g in 1 dm\({}^{3}\) distilled water; high level of NKP: 53.21 g in 1 dm\({}^{3}\) distilled water) solutions were introduced into the water at two concentrations characteristic of water purity classes: a low concentration (mg dm\({}^{-3}\)) PO\({}_{4}\)\({}^{3-}\): 0.54 (water purity class III), NO\({}_{3}\)\({}^{-}\): 7.14 (II), NH\({}_{4}\)\({}^{+}\): 0.22 (I), and K\({}^{+}\): 4.56 (\(-\)); and a high concentration (mg dm\({}^{-3}\)) PO\({}_{4}\)\({}^{-}\): 2.73 (V), NO\({}_{3}\)\({}^{-}\): 36.7 (IV), NH\({}_{4}\)\({}^{+}\): 1.12 (III), and K\({}^{+}\): 22.8 (\(-\)). The water in the containers was adjusted to a cadmium concentration of 0.1 mg Cd\(\cdot\)dm\({}^{-3}\), corresponding to class V water purity, by the addition of a cadmium salt solution (3 CdSO\({}_{4}\) 8H\({}_{2}\)O: 0.43 g in 1 dm\({}^{3}\) distilled water) [53]. Szczecin and its surroundings are characterized by high variability of meteorological conditions during the summer. The mean annual air temperature in Szczecin in the years 1991-2021 was about 9.4 \({}^{\circ}\)C, and the hottest month is July, with a mean long-term (1991-2021 years) temperature of 18.5 \({}^{\circ}\)C [54].