## 3 Results
### Changes in Water Quality Induced by Restoration Treatments  
The restoration of the Kwacza River induced the most pronounced changes in water quality in the vicinity of the by-pass (K2), semi-palisade (K1), and groynes (K5 and K6). The greatest changes were noted in chloride concentrations (Figure 3), which were significantly higher in T+6 than in T+0. After restoration, the concentrations of Chl-a and ammonia nitrogen peaked in the insolated area (K7), whereas pH value and EC were significantly influenced by the single semi-palisade (Table 5). As a result of the restoration, the mean concentration of NO2-N decreased 1.5-fold between sites K1 and K10, but it remained 5-fold higher than in the Slupia River (Tables 4 and 6). Similar observations were made in the mean concentration of nitrate nitrogen (Table 6). Only the concentration of NH4+-N was higher in the Kwacza than in the Slupia. The differences in nutrient loads carried by the Kwacza River were compared in the T+6 period after the completion of the restoration project. The comparison revealed that altered hydrological conditions and decreased nutrient inputs contributed to a decrease in the amount of substances transported by the river (Table 6). After the restoration of the Kwacza River, the amount of total dissolved matter transported annually by the river at site K10 decreased by 7%, from 7.3 tonnes-year \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Structure** & **Parameters** & **U** & **Z** & **P** \\ \hline Single semi-palisade & EC & 1489.5 & \(-\)5.33 & 0.0001 \\  & pH & 401.0 & 11.55 & 0.0001 \\  & NO3–N & 987.4 & 6.47 & 0.001 \\ \hline Bank protection with trunks & pH & 307.6 & 8.04 & 0.001 \\  & EC & 298.2 & \(-\)7.24 & 0.05 \\ \hline River bar & NH4+-N & 941.5 & \(-\)2.74 & 0.01 \\  & pH & 321.0 & 9.55 & 0.001 \\  & DO & 1362.0 & \(-\)5.79 & 0.0001 \\ \hline Single groynes & Cl– & 1644.5 & \(-\)4.77 & 0.0001 \\  & pH & 334.6 & 9.87 & 0.001 \\  & EC & 254.4 & \(-\)6.87 & 0.0001 \\  & PO43–P & 372.0 & 3.37 & 0.05 \\ \hline Double groynes & pH & 355.0 & 9.99 & 0.001 \\  & NO3–N & 1616.5 & 4.87 & 0.0001 \\  & NH4+-N & 957.2 & \(-\)2.99 & 0.01 \\ \hline Stone islands & DO & 1324.0 & \(-\)5.11 & 0.0001 \\  & EC & 888.7 & \(-\)4.44 & 0.001 \\ \hline Double semi-palisade & NO3–N & 1616.5 & 4.87 & 0.00 \\  & NH4+-N & 1018.4 & \(-\)3.99 & 0.0001 \\  & EC & 844.4 & \(-\)4.14 & 0.001 \\  & Chl-a & 944.5 & \(-\)2.32 & 0.01 \\ \hline By-pass & pH & 387.1 & 10.21 & 0.0001 \\  & EC & 688.7 & \(-\)1.44 & 0.05 \\  & DO & 1205.8 & \(-\)5.09 & 0.001 \\  & NO3–N & 1316.5 & 2.87 & 0.0001 \\  & NH4+-N & 1106.0 & 3.79 & 0.0001 \\  & PO43–P & 387.5 & 3.55 & 0.05 \\ \hline Insolation & DO & 1302.6 & \(-\)5.91 & 0.0001 \\  & Chl-a & 1105.5 & \(-\)3.87 & 0.01 \\ \hline No treatment & - & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The effects of the applied hydrotechnical structures on the monitored parameters (Mann–Whitney \(U\) test). The calculations are based on a comparison of pre-restoration (T+0) and post-restoration (T+6) periods. to 6.8 tonnes-year\({}^{-1}\). However, the annual loss of chemical elements per unit area from the Kwacza catchment remained higher even after the restoration period, and amounted to 80 kg\(\cdot\)year\({}^{-1}\cdot\)km\({}^{-2}\). A comparison of the annual loads carried by the river in K10 before and after restoration revealed that the amount of nitrate-N decreased by 64%, NH\({}_{4}\)\({}^{+}\)-N by 50%, K\({}^{+}\) by 12%, and Na\({}^{+}\) and Cl\({}^{-}\) by 9%. In contrast, the newly created habitats did not influence the amount of P carried by the river at site K10 (Table 6). Calcium was the only studied macroelement whose loads increased (by 11%) after restoration. Six years after installation, the restoration system contributed to the removal of 12 kg NO\({}_{3}\)\({}^{-}\)-N per year (57%), and it increased K\({}^{+}\) and Mg\({}^{2+}\) loads by 1 kg between up-stream (K1) and down-stream (K10) segments of the Kwacza River.