Effect of hydrological patterns of tributaries on biotic processes in a lowland reservoir - consequences for restoration

Relationships between river hydrology and chemistry were considered as the key point of ecohydrological approach for a shallow, eutrophic reservoir (2 500 ha) restoration. By comparison of data from the two main tributaries of the reservoir it was demonstrated that nutrient concentrations and consequ ently load supplying the reservoir are significantly positively correlated with hydrological characteristics of the spring spate of the rivers and dep ends on theirs catchment characteristics. Concentrations of total phosphoru s (TP), which is the main factor responsible for reservoir eutrophication, are positively correlated with spring spate dynamics (Pilica River, r = 0.6 19; P = 0.05; Luciaza River, r = 0.443; P = 0.05) but not with discharge. T he above findings also indicate that a high nutrient load has been entering the reservoir in relatively small water volume. As biota react with the nu trient load, a statistically significant correlation was found (r = 0.769; P < 0.05) between hydrological characteristics of the tributaries' spring s pates and biomass of diatoms in the reservoir. The understanding of tempora l variability of nutrient supply in relation to the hydrology of tributarie s indicates that a fundamental first step of reservoir restoration should b e the reduction of the external nutrient load transported into the reservoi r by rivers at the raising hydrograph limb. This may be achieved by convert ing part of the natural river floodplain in the upper part of the reservoir into constructed wetlands. On the basis of the data obtained, maps and are a photographs, the necessary area of treatment wetlands has been estimated. For a reduction of 20% of the TP load of an average spring flood, up to 5 km of the Pilica River valley has to be converted into constructed wetlands (area, 150 ha; depth, 1 m). However, for a reduction of 25% of the load, a re-construction of up to 25 km (area, 500 ha; depth, 1.5 m) of the river i s necessary. (C) 2000 Elsevier Science B.V. All rights reserved.