## 3 Results and Discussion
### Biomass Characteristics
The conducted analyses demonstrated a carbon to nitrogen ratio (C/N) appropriate for methane fermentation, i.e., from 10 to 20, in 74% of the samples [PERSON] et al. (2018). The TN concentrations ranged from 16.2 mg/gTS to 36.0 mg/gTS with a mean of 26.1 \(\pm\) 11.2 mg/gTS, whereas TOC concentrations ranged from 172 mg/gTS to 456 mg/gTS. The concentrations of nitrogen and phosphorus determined in the analyzed samples were comparable to those found in grass silage. For heavy metals, the threshold values set for biowaste were exceeded by nickel, zinc, and cadmium in three samples. There were no significant seasonal differences in the characteristics and quality of the hydrophyte biomass. The VS content in the biomass of hydrophytes harvested by hacking the bottoms of water reservoirs reached 87.9 \(\pm\) 6.5%TS in V1 and 86.3 \(\pm\) 5.2%TS in V2. The content of TN and TP were 26.2 \(\pm\) 4.2 mgN/gTS and 13.3 \(\pm\) 2.7 mgP/gTS in V1, respectively, and 27.4 \(\pm\) 2.1 mgN/gTS and 13.2 \(\pm\) 1.6 mgP/gTS in V2, respectively (Table 2). Significantly higher TOC content was determined in V1, i.e., 449 \(\pm\) 55 mg/gTS in V1 vs. 420 \(\pm\) 28 mg/gTS noted in V2. The differences in TOC content directly influenced C/N values, which reached 17.2 \(\pm\) 2.0 in V1 and 15.3 \(\pm\) 1.9 in V2. The silage of submerged hydrophytes also had a lower pH value, reaching 6.25 \(\pm\) 0.11, compared to the pH value of 7.23 \(\pm\) 0.12 measured in fresh biomass (Table 2). The biomass analyzed in V2 also had lower sugar content, accounting for 15.4 \(\pm\) 2.5%TS. In a study by [PERSON] et al. (2018) [PERSON] et al. (2018), the TS content of submerged hydrophyte biomass ranged from 5 to 22%, whereas the mean VS content was 83.4%TS. The C/N ratio ranged from 10.8 to 31.8. In most samples, the content of mineral impurities was lower than 1% dry matter of the whole material. The major species identified in the analyzed biomass were those belonging to the _Elodea_ and _Myriophyllum_ genera. Figure 4: Species composition of hydrophyte biomass harvested during research works ((**a**) biomass tested in S1; (**b**) biomass tested in S2). The hydration ratio of substrates used in a study conducted by [PERSON] and [PERSON] [35] ranged from 63.52% (common need) to 89.51% (hornwort). The organic dry matter of broadleaf cattail and sweet flag were similar and were approximately 23%. Among the analyzed substrates, hornwort had the lowest VS content in dry matter, approximately 71% compared to the VS content levels of 84%, 92%, and 91% determined for common reed, broadleaf cattail, and sweet flag, respectively [35]. These values are similar to those determined in the present study. The advisability of ensilaging the hydrophyte biomass was confirmed in studies addressing the fermentation of _Eledea nuttallii_ and _Eledea canandensis_. These are invasive neophytes spreading at a fast and uncontrolled pace, thereby posing problems in hydropower plants, water transport, tourism, and recreation as well as in transport. It should be emphasized that this species had the largest share in the biomass of submerged hydrophytes harvested in this study. The study results demonstrate that _Eledea nuttallii_ and _Eledea canandensis_ can be effectively stored by ensilaging their biomass, which ensures high-efficiency biogas production. The present study proves that high hydration, reaching up to 95%, significantly complicates the preservation process of this type of biomass; therefore, in some cases, the addition of other typical energy plants should be considered. The pH values of seaweed silage ranged from 4.9 to 6.5, and the lipid content was up to 1.9%TS [36].