Changes in the cycling of phosphorus in the Upper Kis-Balaton Reservoir following external load reduction

I. The Upper Kis-Balaton Reservoir was opened in 1985 to control eutrophica tion in Lake Balaton. High external TP load of the reservoir halved in 1991 due to improved sewage treatment. Annual TP retention efficiency of the re servoir dropped immediately from 50 to 20%, while the hypertrophic algal bi omass started to decrease after 1995. 2. We examine mass balances of the main forms of P in three consecutive are as (1, 2 and 3) of the reservoir. Biologically available P (BAP) includes s oluble reactive P and algal P, whereas the rest of TP is considered to be n on-available (nBAP). 3. Important pathways for P cycling varied in both space and time. A reduct ion in external load induced an enhanced net internal P load in the reservo ir, and a variety of changes in the three areas. Thus, in the smallest Area 1, BAP retention dropped almost to zero, and the net sedimentation of nBAP decreased significantly. In Area 2 net BAP retention was characteristic be fore management, and net internal P load was observed afterwards. In Area 3 net internal P load has been substantial in some years ever since the rese rvoir was filled. The OECD model, corrected for internal P load, reasonably predicted the measured TP concentration in each area both before and after reduction in load. 4. The external load-algal biomass curve differed in the three areas. An im mediate response occurred in the upstream Areas 1 and 2, while a delay of 4 -5 years was observed in Area 3. Different long-term sediment dynamics may underlay these load-response relations in the three areas. Intense sediment redistribution, however, sets a serious constraint to the simple calculati on of a mass balance. An evaluation of the long-term sediment dynamics is n eeded in order to predict future performance of the reservoir.