The Kis-Balaton reservoir system, consisting of the upper and lower reservoirs, is located near to the mouth of the Zala River. It was established for the protection of Lake Balaton against high nutrient loads. In accordance with the original plan prepared at the end of 70s the aim was that before entering the lake, nutrients - primarily phosphorus - will be removed by macrophytes. The Upper Reservoir started to operate in 1985. In contrast to the predictions, it became an open lake dominated by algae. Until 1991 about 50% of the external phosphorus load has been removed yearly. However, the retention efficiency has decreased considerably after reduction of the external load upon phosphorus removal at the wastewater treatment plant of the largest town of the Zala catchment. This observation can be explained by the increased contribution of the internal loading. To analyze the phosphorus removal mechanisms and to understand the behavior of the sediment, different water quality models were applied. The results show that the P retention is characterized by different processes acting spatially differently. Essentially, abiotic processes like settling of inorganic particulate P and the adsorption of dissolved inorganic P are responsible for the P retention in the reservoir, mostly in the neck of the Upper Reservoir. Simple models indicated that the internal loading increased after the external load reduction. In fact, it was not that the internal loading increased, but the adsorption decreased.

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