Use of process-based coupled ecological-hydrodynamic models to support lake water ecosystem service protection planning at the regional scale.

Protection plans of lake waters are based on ecological and/or chemical targets, often simplified in terms of total phosphorus (TP) concentrations, customarily the depth-averaged ones at spring mixing for temperate environments. These target lake TP concentrations are then commonly employed to determine target external loading through reverse use of Vollenweider-OECD-type steady-state empirical models. Such models are also adopted in their direct form to estimate lake TP concentrations following hypothetical external load reductions. However, such approaches suffer from extreme parameterisation and often give inaccurate results. Process-based coupled ecological-hydrodynamic models offer a much wider flexibility and produce an extensive set of information, solving many of the issues of Vollenweider-OECD-type models. However, their application has been up to now restricted to single lakes due to calibration effort and data availability burdens. To overcome these obstacles, in this study we developed a simplified application of the process-based coupled model QWET over 9 lakes in Northern Italy, making use of the ParSAC automatic calibration tool and feeding the models only with general data available from public monitoring. QWET models were calibrated over past observations, simulating nutrient reduction scenarios for the near-future decades. The advantages over traditionally employed models for lake water protection planning at the regional scale were hence identified through a practical application, determining the strengths and limits of the herein-adopted simplified process-based approach over lakes with different features. Obtained results were also analysed considering the specific case study.