## 3 Results and Discussion  
### Biomass Characteristics  
In S1, during which biomass was harvested by moving the banks of water reservoirs, biomass species composition was predominated by common reed (_Phragmites australis_--62 \(\pm\) 9% wt. ), reed canary grass (_Phalaris arundinacea_--19 \(\pm\) 4% wt. ), broadleaf cattail (_Typha latifolia_--7 \(\pm\) 3% wt. ), and sweet flag (_Acorus calanus_--5 \(\pm\) 2% wt.). The remaining species of hydrophytes accounted for 7 \(\pm\) 4% wt. together (Figure 4a). In S2, during which the biomass was harvested by hacking the bottoms of water courses, biomass species composition was predominated by Canadian pondowed (_Elodea canadensis_--34 \(\pm\) 8% wt. ), hornwort (_Ceratophyllum demersum_--23 \(\pm\) 7% wt. ), pondwored (_Potamogeton_--17 \(\pm\) 6% wt. ), parrot's feather (_Myriophyllum aquatic_--11 \(\pm\) 4% wt. ), and longroot smartweed (_Polygonum amphibium_--6 \(\pm\) 3% wt.). The other species of hydrophytes accounted for 9 \(\pm\) 5% wt. together (Figure 4b). The aquatic biomass harvested during the shoreline maintenance of water reservoirs, recreational sites, and hydrotechnical installations is treated as waste not intended for further economic or energy use and is usually disposed of [30]. In areas with little tourist pressure, it is often left on the banks of water reservoirs, which causes its uncontrolled decomposition, rotting, and, consequently, secondary contamination of the reservoir with biogenic and organic compounds as well as the emission of unpleasant odors [31]. This state of affairs is influenced by the high content of organic and nitrogen compounds in the biomass and the fact that hydrotechnical treatments related to the removal of aquatic vegetation are performed during the period of rapid vegetation, i.e., in the summer, when high temperatures promote and accelerate the processes of biomass degradation [32]. The characteristics of the hydroophyte biomass harvested while moving the banks of water reservoirs (S1) was similar regardless of whether fresh substrate or substrate silage was tested. The content of TS (%FM) and VS (%TS) was 27.2 \(\pm\) 2.9%FM and 84.3 \(\pm\) 3.9%TS in V1, respectively, and 26.7 \(\pm\) 3.2%FM and 83.7 \(\pm\) 4.2%TS in V2, respectively (Table 1). In both variants, nitrogen concentration in the biomass approximated 32 mg/gTS, and that of TAC was nearly 400 mg/gTS. The C/N ratio, being important in the methane fermentation process, reached 12.9 \(\pm\) 3.2 in V1 and 12.3 \(\pm\) 2.4 in V2. A significantly lower pH value was measured in the biomass of emergent hydrophytes subjected to ensilaging, i.e., 6.34 \(\pm\) 0.09 \begin{table}
\begin{tabular}{c c c} \hline
**Indicator** & **Unit** & **Computation Method** \\ \hline Net heat production & GJ/year & gross heat production \\  & & -heat consumption for process purposes \\ \hline Gross electric energy production & kWe/year & theoretical electric power (kW) \\  & & \(\times\)operation time (8000 h) \\ \hline Electric energy consumption for process purposes & kWe/year & gross electric energy production \\  & & \(\times\)0.09 \\ \hline Net electric energy production & kWe/year & gross electric energy production \\  & & -electric energy consumption for process \\  & & -purposes \\ \hline Biogas plant power & kWe & net electric energy production \\  & & : 365 days \\  & & : 24 h \\ \hline \end{tabular}
\end{table} 
Table 5: _Cont._in V2 vs. 7.12 \(\pm\) 0.11 noted in V1. In both variants, the protein content of biomass accounted for ca. 20%TS, that of lipids reached nearly 13%TS, and that of sugars was 21%TS (Table 1). Similar biomass characteristics to the hydrophytes harvested by moving were reported by [PERSON] et al. (2018) [PERSON] et al. (2018). They analyzed the biomass derived from 39 sites located across Germany and found that its TS content fit within a very broad range, i.e., from 5.0%FM to 35.8%FM, and that its VS concentration ranged from 47.2%TS to 93.9%TS, depending on the species composition and collection site.