Simulation model of a mesotrophic reservoir (Lac de Pareloup, France): MELODIA, an ecosystem reservoir management model

A five-year research project on Pareloup Lake, an Electricite de France hydroelectric reservoir, increased present understanding of the dynamics of this ecosystem, in both biological and hydrodynamic terms. Simulation models were developed during the study with the aim of providing a tool for water management and decision making. The first phase, described in an earlier paper, was to elaborate the biological model (ASTER) which highlighted the dominant mechanisms in the evolution of the biological variables. The second phase, described here, was the elaboration of the MELODIA model. This model is the result of the coupling of the ASTER biological model with the vertical hydrodynamic and thermal model (EOLE) which had simultaneously been developed to simulate evolution in thermal stratification, taking energy exchanges across the air-water interface and through-flow into account. Considerable processing of data on inflow and its physico-chemical characteristics was required prior to development of this second model. The formulation of certain functions of the biological model, linked to its vertical structure, had to be changed: this was the case for sedimentation and the extinction coefficient. The sizeable data base available enabled calibration of the model and ensuring that it correctly simulated seasonal evolution of each of the variables, as well as their vertical dynamics, over the five-year period. Year-to-year fluctuations were equally well represented. The simulations pointed up the degree to which the ecosystem is sensitive to the hydrodynamic structure. Spring stratification conditions, directly linked to meteorological conditions and hydraulic management, determine the dynamics of the diatom population, essentially made up of Asterionella formosa. This species is not consumed by zooplankton, and disappears by sinking, trapping a large proportion of nutrients until winter isothermia. Because of this, the springtime diatoms in Pareloup play a key role in regulating the ecosystem. As the model was found to simulate well both stratification and the growth and sinking of diatoms, it will later be used to assess different reservoir management modes.