Analysis of long-term water quality changes in the Kis-Balaton Water Protection System with time series-, cluster analysis and Wilks' lambda distribution

Lake Balaton is the largest shallow freshwater lake in Central Europe. Its water quality is mainly affected by the supplying rivers and other water sources. The primary source is the Zala River. Its water used to be filtered by the Kis-Balaton Wetland (KBW) before entering Lake Balaton. During the nineteenth century. as a result of artificial water level modifications, the KBW disappeared and the Zala River's waters became partially unfiltered. It is for this reason that the Kis-Balaton Water Protection System (KBWPS) had to be constructed as a mitigation wetland. The aim of the study is to examine the available physical, chemical and biological parameters to get a more comprehensive picture of the processes evolving in the functioning of the KBWPS, and to make suggestions concerning the management and preservation of the system's wetland habitat. The central concept of the present study was to group the sampling points of the KBWPS and to determine which parameters had the greatest effect on the groups, and where. Multivariate data analysis was applied to the data concerning 25 chemical, biological and physical parameters for the time period 1984-2008 from 13 monitoring stations. The sampling locations were clustered then grouped. The groups were formed annually. The change of alignment of similar sampling points shows how the border between the determining groups (covering the eutrophic pond and wetland habitats) changed over the years. This change followed the transition from macrophyte vegetation to an open water area which took place as a result of the water level being kept artificially constant, and which did not therefore follow the weather conditions (rain, drought, etc.). Using Wilks' lambda distribution it was possible to determine that the parameters responsible for eutrophication were primarily responsible for forming the groups of the sampling points. The next most important factors determining the groups were the variables in close relation with the parameters characteristic of eutrophication. The inorganic chemical components affected the conformation of the groups the least. Finally, by examining the phosphorous forms and chlorophyll-a we tried to show the milestones in the history of the mitigation wetland, the KBWPS. The result of this research was that it points out changes in the KBWPS over a long time period, which had not been done previously. This research could hopefully help scientists to gain a broader perspective on processes evolving in the KBWPS. When it comes to finishing the second phase of the reservoir system, more knowledge will be available on what can be expected regarding the quality of the water entering Lake Balaton, and the conservation of the nature preserve wetland area. (C) 2011 Elsevier B.V. All rights reserved.