Predicting future effects from nutrient abatement and climate change on phosphorus concentrations in Lake Bourget, France

Like many temperate European lakes, Lake Bourget (France) has suffered from eutrophication during the second half of the last century. Due to a remarkable policy restoration program, the lake has been recovering for the past 25 years after a massive decrease in total phosphorus (TP) loading. TP concentrations have decreased from about 100-120 to 20-25 µg/L. Additional efforts are, however, still required to obtain a perennially sustainable good ecological status and model parameterisation of fluxes can assist in predicting future outcomes, especially in the context of global warming. In this paper, a dynamic model (MeroLakeMab) was developed and tested with the purpose to reconstructthe loading history of Lake Bourget and to predict TP concentrations during scenarios of increased temperature, decreased water runoff and decreased P loading. Simulations suggested that the historical TP loading decrease may have been as extensive as 90%. Decreases in water discharge to Lake Bourget at magnitudes forecasted by the Intergovernmental Panel on Climate Change would not affect TP concentrations notably, but marked concentration changes could, however, occur if decreases in runoff would have a strong impact on the TP loading. Increasing temperature effects on yearly mean TP concentrations in the water column would be very small compared to effects from changes in the TP loading. Predictions such as these could be instrumental for future successful lake management.