## 3 Results and Discussion
### Changes in Littoral Plant Waste
#### 3.1.2 Heavy Metal Content  
The experiment revealed an increase in the concentration of most heavy metals in vermicompost, that may be a result of a reduction in weight and volume of the final product [34; 35] (Table 2). Similar observations concerning an increase in heavy metal concentration in vermicompost composed of various types of organic waste were made by [PERSON] and [PERSON] [36] as well as [PERSON] and [PERSON] [37].  
Lead is the element whose content decreased by 50% (\(p<\) 0.05) in (I) and (C) as a result of the vermicomposting process, whereas a decrease in Mn content was less pronounced and amounted to 31.8% in (T) and 40% in (C) (\(p<\) 0.05) (Table 2). A reduction of Pb concentration in the final product of vermicomposting may be caused by the fact that _E. ftida_ earthworms can accumulate high Pb concentrations in the form of non-toxic compounds [38]. However, such a concentration in the vermicomposts does not exclude them from use as fertilizers, because the permissible Pb content in composts in EU countries is 45 mg kg\({}^{-1}\)[39].  
A significant increase in Zn concentration was observed; in (I) from 8.4 \(\pm\) 0.1 to 143.1 \(\pm\) 4.9 mg kg\({}^{-1}\) (d.m.) (1700%; \(p<\) 0.05), in (C) from 32.4 \(\pm\) 0.9 to 110.2 \(\pm\) 12.9 mg kg\({}^{-1}\) (d.m.) (340%; \(p<\) 0.05), whereas in vermireactor (T) from 11.3 \(\pm\) 0.1 to 35.5 \(\pm\) 0.1 (300%; \(p<\) 0.05) (Table 2). Permissible Zn content in fertilizer composts in EU countries is 200 mg kg\({}^{-1}\)[39].  
There was also a significant increase in Cu concentration: in (I) from 1.6 \(\pm\) 0.1 to 28.8 \(\pm\) 5.6 (1800%; \(p<\) 0.05), in (C) from 6.4 \(\pm\) 0.1 to 13.5 \(\pm\) 0.9 (210%; \(p<\) 0.05), whereas in vermireactor (T) from 1.7 \(\pm\) 0.0 to 5.2 \(\pm\) 0.4 (300%; \(p<\) 0.05). As demonstrated by [PERSON] et al. [40] in the studies on vermicomposting of mixed organic waste, Cu concentration decreased under the effect of earthworm activity. Copper concentration in littoral plant vermicompost that was shown in the present study does not exclude it as a fertilizer, because the mean permissible concentration of this element in composts in EU countries is 70 mg kg\({}^{-1}\)[39].  
The study results have also shown an increased Cd concentration in the vermicomposts from (T) and (I), an increase from \(<\)0.06 to 0.09 \(\pm\) 0.0 and from \(<\)0.06 to 0.1 \(\pm\) 0.0 respectively, whereas in (C) a rise from 0.1 \(\pm\) 0.0 to 0.15 \(\pm\) 0.0 mg kg\({}^{-1}\) (d.m.) was noted (Table 2). These observations are not in line with the results by [PERSON] et al. [40], who showed a decreasing tendency in Cd concentration during the process of vermicomposting of various organic waste. In turn, [PERSON] [41] demonstrated a negative effect of Cd on the number and biomass of _Dendrohaena octaedra_ earthworms and their cocoons. This author also indicated that Cd content in soil prolonged the maturation time of earthworms.  
As shown by [PERSON] and [PERSON] [21], Zn, Cu, and Mn are essential for the proper development of plants, but their high concentrations may be toxic for living organisms. On the other hand, Pb and Cd are not vital for the growth of plants, because there are no known physiological processes in plants in which these elements participate. Even a small amount of these heavy metals may be harmful for living organisms, but their toxicity depends on their form. Instead, the bioavailability of these elements depends on the features of soil, e.g., pH or the content of organic matter and clay [42; 43].  
A negative effect of heavy metals on the selected characteristics in earthworms was demonstrated in the following studies: [PERSON], [PERSON] [44]--growth, [PERSON] et al. [45]--survival, [PERSON] [46]--cocoon production. By contrast, there are no observations concerning the effect of heavy metals on the rate of processing organic matter by earthworms.