Wetlands are important ecosystems: they are habitats for many key species and act as natural regulators of the environment. They filter water and remove pollutants with the help of plants. Constructed wetlands are artificial wetlands built to treat mainly wastewaters. These artificial ecosystems can also be used for freshwater treatment, such as river water. The Kis-Balaton Water Protection System (KBWPS) is one of them and the main goal of the system is to treat the River Zala water before its entrance in the Lake Balaton. The two mains objectives are to avoid the eutrophication of the Lake Balaton and improve the wetland ecosystems. It is a vast constructed wetland of 71 km2, situated at Southern part of Lake Balaton, in Hungary. The goal of this thesis was to analyze the loads, water quality and biological production of the KBWPS. Interpretation of three different sets of data and field work was carried out. The first set was about inflow and outflow data, from which the removal rates of different nutrients on a long time period were computed. The second data series contained regular water quality monitoring data along the flow path and the goal was to study the inner processes happening in the KBWPS. The last data set was collected on the field and included basic water properties and some biological and physico-chemical measurements, such as primary production or tecton analysis. It allowed to differentiate the water types of the system and their properties and to better understand the inner processes, particularly the biological characteristics of the system. The interpretation of the data series gave background to understand the processes occurring within the system and it helped to characterize the actual different water groups. It drew the main lines for possible more detailed analysis. This work was partly connected to a project aiming at the development of a monitoring system that serves a better management of the KBWPS. Several conclusions can be drawn from the results found in this thesis: • General conclusions on the system, which could apply to other systems of the same type. • Specific conclusions about the operation of the system, which would refer to this system only. • Some management and monitoring suggestions for the future system.