## 3 Results
### Trophic Status  
The calculated Carlson index indicates that the water within the Wroclaw I and Wroclaw II hydropower plants are mesotrophic or eutrophic due to the average total phosphorus concentration in the analyzed period (Table 11) [22; 23; 24; 25]. The differences between the points are not large. Downstream of the Wroclaw II hydropower plant, the Carlson index value was lower by 1.67% than the point upstream of the hydropower plant. For the Wroclaw I hydropower plant, the downstream value was lower by 0.60% than the point upstream of the hydropower plant. Downstream of the hydropower plants, less enrichment with phosphorus compounds were found than upstream.  
The TLI (Table 12) [26; 27; 28] noted hypertrophic status at each point due to high phosphorus and nitrogen concentrations. Higher concentrations of total nitrogen had a greater negative effect on the final result. Downstream of the Wroclaw II hydropower plant, the trophic status improved, and downstream of the Wroclaw I hydropower plant, it worsened (by 0.38% and 0.30%, respectively). Considering the overall results, the hydropower plants marginally improved the trophic status expressed in TLI, i.e., by 0.04%.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{3}{c}{**Comparison of the Results Upstream and Downstream of HPs (Meeting Standards—Limit Values)**} \\ \cline{2-7}  & \multicolumn{3}{c}{**Wroclaw II Hydropower Plant (HP)**} & \multicolumn{3}{c}{**Wroclaw I Hydropower Plant (HP)**} \\ \cline{2-7}  & **Improvement** & **Deterioration** & **No Changes** & **Improvement** & **Deterioration** & **No Changes** \\ \hline EC & 22.22\% & 72.22\% & 5.56\% & 52.78\% & 36.11\% & 11.11\% \\ \hline Temperature & 30.56\% & 38.89\% & 30.56\% & 44.44\% & 22.22\% & 33.33\% \\ \hline NH\({}_{4}\)-N & 52.78\% & 30.56\% & 16.67\% & 38.89\% & 30.56\% & 30.56\% \\ \hline NO\({}_{3}\)-N & 44.44\% & 55.56\% & 0.00\% & 58.33\% & 36.11\% & 5.56\% \\ \hline NO\({}_{2}\)-N & 27.78\% & 25.00\% & 47.22\% & 30.56\% & 13.89\% & 55.56\% \\ \hline PO\({}_{4}\)-P & 47.22\% & 19.44\% & 33.33\% & 22.22\% & 38.89\% & 38.89\% \\ \hline DO & 47.22\% & 38.89\% & 13.89\% & 58.33\% & 27.78\% & 13.89\% \\ \hline BOD\({}_{5}\) & 55.56\% & 41.67\% & 2.78\% & 44.44\% & 44.44\% & 11.11\% \\ \hline \hline \end{tabular}
\end{table}
Table 10: Detailed summary of the results of the meeting standards against parameter limit values within the Wroclaw I and Wroclaw II hydropower plants.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Point**} & \multirow{2}{*}{**Carlson Index** (TSI\({}_{\text{P}}\))} & \multicolumn{3}{c}{**Classification**} \\ \cline{3-5}  & & **[PERSON]** & **[PERSON]** & **EPA Survey** & **Carlson and Simpson** \\  & & **(1965) [23]** & **(1966) [24]** & **(1974) [25]** & **(1996) [22]** \\ \hline
1 & 69.47 & & & & \\ \hline
2 & 68.31 & & & & \\ \hline
3 & 68.90 & & & & \\ \hline
4 & 68.49 & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 11: Trophic status at the analyzed research points on the Odra River (Poland)—Trophic Status Index.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Point**} & \multirow{2}{*}{**TLI\({}_{\text{P}}\)**} & \multirow{2}{*}{**TLI\({}_{\text{N}}\)**} & \multirow{2}{*}{**Trophic Level Index (TLL)**} & **Classification** \\ \hline
1 & 5.05 & 5.96 & 5.51 & \\ \hline
2 & 5.11 & 5.86 & 5.48 & \\ \hline
3 & 5.01 & 5.91 & 5.46 & \\ \hline
4 & 5.08 & 5.88 & 5.48 & \\ \hline \hline \end{tabular}
\end{table}
Table 12: Trophic status at the analyzed research points on the Odra River (Poland)—Trophic Level Index.  
The observed average PO4-P level for the multi-year period is higher for the presented results (0.086-0.093 mg/L) than for the estuary section of the Odra River (0.070 mg/L) [56].