SPIEL—a model for phosphorus diagenesis and its application to lake restoration

The internal phosphorus (P) cycle of a lake can delay the effects of external P load reduction. Thus, some in-lake measures are used to speed up the recovery process by influencing the P retention in the sediment. The P retention is determined by the quantity and composition of settling seston and the P diagenesis. Existing diagenetic models do not include all important processes of the P diagenesis, or the control parameters of in-lake measures as input variables. Therefore, a deterministic P diagenetic model (SPIEL) has been developed for lake sediments of eutrophic lakes, to predict the development of the P release after an in-lake measure has been carried out. This tool helps to analyse the complex biological and chemical processes in the sediment, and to determine the appropriate measure for a lake. The model is structured in modules, reflecting the control variables of the measures and their effects on the steering factors of the P diagenesis. The main module calculates the P cycle within the sediment; other sub modules can be added to calculate the aerobic layer thickness, the iron–sulphur cycle, the P sorption capacity of aluminium, the change in porosity, or the P concentration in the overlying water body. Thus, it is possible to create measure-specific models by combining different modules, for example, for oxidation with oxygen or nitrate, precipitation with iron or aluminium, capping, hypolimnetic withdrawal and measure combinations. The model has been calibrated and verified for Lake Sempach, Switzerland. Scenarios have been calculated to show the effects of different in-lake measures on the P retention. The model results indicate that the most suitable measures for Lake Sempach regarding the P retention would be a P precipitation with aluminium or iron. Aluminium has the advantage of a longer lasting effect on the P release than iron.