DyLEM-1D: a 1D physical and biochemical model for planktonic succession, nutrients and dissolved oxygen cycling: Application to a hyper-eutrophic reservoir

DyLEM-1D, a numerical 1D-vertical model of the phytoplankton succession and of the nutrients and dissolved oxygen cycling for a reservoir has been developed. The physical part of the model allows describing the appearance and disappearance of the vertical thermal stratification, taking into account change in the water level and several depths for outflows. It is therefore well-suited to reservoir modelling. The biochemical part is based on a stoichiometric composition of the organic compounds which allows a rigorous computation of the elemental mass balance from the stoichiometric coefficients of the biochemical transformation processes. The model was applied to a reservoir over a 10-year-long period. During this period a restoration process of the reservoir was initiated mainly thanks to a substantial reduction of the phosphorus inputs. After having selected the identifiable set of parameters, the model was calibrated using trial and error and a Monte Carlo analysis using 6 months of data (bi-monthly vertical profiles of phytoplankton biomass, nutrients and dissolved oxygen) and was validated over two other study periods (about 1 year with similar available data and 3 years of data collected 6 years after the calibration period). The results show that despite the difficulties in identifying the “true” values of parameters, the model was able to reproduce the data even in different nutrients loading conditions and enables to evaluate the progress of the restoration process. The model results show that the reduction of phosphorus loading into the reservoir had a significant influence on the calculated oxygenation conditions, but only had a slight impact on Microcystis aeruginosa dominance in the reservoir.