Assessing water-quality data: The relationship between the water quality amelioration of Lake Balaton and the construction of its mitigation wetland

Lakes are sensitive to environmental changes and anthropogenic effects; and Lake Balaton, the largest shallow freshwater lake in Central Europe, is no exception. To protect the lake (primarily Keszthely Bay) against elevated nutrient loads, the Kis–Balaton Water Protection System (KBWPS) was constructed as a mitigation wetland at the mouth of the River Zala, the largest tributary of the lake. This research demonstrates the effect of the KBWPS on Keszthely Bay's oligotrophication processes. The concentration values of KBWPS and Keszthely Bay's measured parameters and their stochastic relations were analyzed by applying cluster- discriminant analysis, Wilks' lambda statistics, confirmatory principal component- (PCA), and input–output analyses to 19 parameters from 14 monitoring stations. First, the sampling sites were clustered for each year, showing that when the runoff in the system was low, Keszthely Bay behaved as a separate entity, while in the years with high runoff it rather displayed a similarity to the wetland area of the KBWPS. Secondly, PCA showed that, from the perspective of stochastic relations, changes in the background processes of Keszthely Bay were observed only after the settling of the KBWPS; and that the runoff in the examined years did not have any significant influence on it. Furthermore, the eutrophication processes moved upstream from Keszthely Bay to the hypertrophic area of KBWPS. This study confirms that the effects of the KBWPS – together with other load reduction measures – had a beneficial effect on the oligotrophication of Lake Balaton.