Main shortcomings of eutrophication models are related to the uncertain cause-effect relations of mostly shallow, hypertrophic, blue-green algae and large internal load dominated systems. Additional difficulties emerge when these tools are used to tackle complex decision making problems, of which eutrophication may be only one element. The Kis-Balaton Reservoir (in Hungary) is characterized by both features. The system, consisting of an Upper and Lower Reservoir was designed to reduce the phosphorus load of agricultural non-point source origin to control the eutrophication of Lake Balaton. The original assumption in the late seventies was that macrophytes would play the decisive role in nutrient removal. Operational experiences and the ongoing unique monitoring program demonstrated that this assumption was not met: the Upper Reservoir quickly developed into a hypertrophic phytoplankton system. The P retention of the inundated reed area of the Lower Reservoir is not satisfactory and the large, intact reed stand of high nature conservation value started to die off due to the water level increase and algal-rich inflow. Thus, the need for revision of the original design (more than half of which was implemented) became evident. The paper summarizes the approach adopted, the project alternatives and criteria of evaluation used, details of phosphorus cycle models applied for the Upper Reservoir, and their transfer to the lower one to derive estimates for the future P retention. We also discuss dilemmas related to the estimation of the sorption isotherm of the soil/sediment system which appear in the models and which are crucial to ‘extrapolate the future’. Finally, we outline the multi-criteria evaluation, the recommendations prepared for modifying the original design of the Lower Reservoir and the outcome of decision making.

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