Decision support for the selection of an appropriate in-lake measure to influence the phosphorus retention in sediments.

Many in-lake measures which aimed to influence the phosphorus retention in lake sediments have failed to improve the trophic state of the lakes. The present paper introduces a systematic approach to select an appropriate in-lake measure. Before selecting an in-lake measure the goal of the measure should be defined, the problems of the lake must be identified, and the probability of success must be estimated. The proposed decision support consists of two parts. Part A, pre-selection, excludes inappropriate measures. It uses six criteria, which are mainly based on a simple mass-balance model, and the targets of restoration. The criteria describe the magnitude of the external versus the internal phosphorus loading, the dynamics of the internal load, and the lake morphometry. Each measure is weighted differently with respect to importance and suitability by specific quantified limits. Part B, selection, uses qualitative criteria, which are specific to the measure in question. Checking these criteria will help to select a measure with a low risk of failure. The suggested decision support is illustrated in flow charts and exemplified by Lake Arendsee in Germany.