Applicability of a one-dimensional coupled ecological-hydrodynamic numerical model to future projections in a very deep large lake (Lake Maggiore, Northern Italy/Southern Switzerland)

One-dimensional coupled ecological-hydrodynamic numerical models of lakes require extensive calibration of their chemical and biological parameters. Application of these models to future projections relies on the time invariance of the calibrated model parameters and of the adopted schematisation. This is mere speculation for real ecosystems, so that it is relevant to explore the limits of coupled models over extended periods. To date, almost all applications in literature have been calibrated over a couple of years at most, with comparable validation periods, if present. Furthermore, past studies mostly concerned shallow to moderately deep small lakes, so that reproducing the hypolimnetic chemical evolution of very deep large lakes has generally been overlooked. Last, most works did not compare with observations or even model the succession of phytoplankton species, but only dealt with total Chlorophyll-alpha. Here, the GLM-AED2 (General Lake Model - Aquatic EcoDynamics) coupled model was calibrated and validated for an overall 16.75-year period for the 370-m deep and 213-km(2) wide Lake Maggiore (Northern Italy/Southern Switzerland), focusing on the reproduction of both deep-water chemistry and phytoplankton biomass and succession. Despite the modelling simplifications needed for this complex basin, the resulting performances are comparable to those in literature for shallower and smaller lakes over shorter periods. Still, extreme care must be put when interpreting the results of coupled ecological-hydrodynamic models for long-term projections, especially regarding the evolution of phytoplankton.