Adaptation measured based on smart nutrient management at the catchment scale for future-proof water quality

Global changes such as nutrient pollution and climate change are strongly affecting aquatic ecosystems worldwide 4, being some of the biggest pressures on freshwaters. Global changes decrease the state and functioning of aquatic ecosystems causing degradation of their ecosystem services such as nutrient retention, drinking water, flood protection or recreation uses. Water quality across the world has declined in the last decades due to anthropogenic activities causing eutrophication of freshwater ecosystems as the result of excessive nutrient inputs. The increase of nutrient concentrations in aquatic ecosystems promotes phytoplankton development. It generates an imbalance between algal production and consumption resulting in algal blooms and impacting the whole aquatic communities through changes in habitat quality and the trophic web 12. This phenomenon can trigger a shift of the water systems from a clear stable state to a turbid stable state deteriorating the ecological quality 13. In addition, climate change impacts many ecosystems worldwide 14 through changes in temperatures, precipitation patterns, and the increase of extreme events affecting ecosystems' resilience and stability. Several studies demonstrated a synergy between eutrophication and climate change resulting in an even larger deterioration of lakes and reservoirs and their ecosystem services than based on the separate pressures 5 4. Ecosystem services of water systems are vital for the well-being of the human population. Those are divided into four categories: (i) the supporting services represent key ecosystem functions such as primary production or nutrient retention. (ii) The provisioning services are resources obtained from the ecosystems like drinking water or food (fish). (iii) The regulating services represent the ecological benefits of the ecosystems such as climate regulation or flood protection. (iiii) Lastly, the cultural services gather social activities such as education or recreation (fishing) 15. Lakes and reservoirs play an important role by providing ecosystem services which are essential for the United Nations Sustainable Development Goals (SDGs). This applies mainly to SDG 6, corresponding to “ensure availability and sustainable management of water and sanitation for all”. Therefore, it is important to mitigate the impact of global changes on freshwater ecosystems to improve or protect lake/reservoir ecosystems states supplying crucial ecosystem services. To achieve the sustainable protection of lake/reservoir ecosystems, it is crucial to quantify the different sources of nutrients (point and non-point) through data monitoring at different time/spatial scales but also to understand their processes and effects on freshwater ecosystems through modelling tools. Therefore, the combination of modelling tools is necessary to define and evaluate the current and future environmental status of an ecosystem to finally propose management options for land uses such as agricultural practices and water management. Previous studies have applied this method to support ecosystem restoration. For example, the combination of a nutrient load model (MARINA) and an aquatic ecosystem model (PClake) has determined loading thresholds for lakes to provide the critical nutrient load corresponding to the tipping point of an ecosystem state shift 8,9,16. Those studies were mostly applied in different lakes in China at the sub-basin scale and further studies need to be developed to enlighten the applicability for study cases in Europe. The main research objective of this study is to assess the sources of nutrients at the catchment scale and to understand lake ecological processes in response to eutrophication and lake management. The aim is to develop adaptation measures based on smart nutrient management from land into water systems, taking the Rappbode Reservoir System in Germany as a case study. This project will start with a systematic review of the different modelling tools to assess the effect of nutrient pollution and climate change on water quality and their evaluation through different selection criteria. Secondly, this research will apply and integrate two existing models: (i) the MARINA model which assesses river inputs of nutrients. It quantifies the export of dissolved nitrogen and phosphorus by sources at the sub-basin scale from land to water 17 (ii) the PCLake model which is designed to simulate the main nutrient and food web dynamics of a lake in response to nutrient loading and related restoration measures 16. This will be applied to a typical European reservoir: the Rappbode Reservoir in Germany. In addition, this research will combine those models with climate models (e.g. ISIMIP) to forecast the future impact of climate change18 and eutrophication on this reservoir. Finally, a new version of the PCLake model will be further developed under the name of Meta-PCLake. This new version consists to apply and integrate several PCLake models on different zones of a lake/reservoir to evaluate their effect on the whole system. To achieve the objectives, three main research questions were formulated: • RQ1: What are the current modelling tools assessing the nutrient flows from land to surface water and assessing the lake ecological processes; how are they evaluated? • RQ2: What are the current sources of nutrient pollution and effects on lake stability for the Rappbode Reservoir System, in Germany? • RQ3: What are the impacts of (smart) management and future trends of nutrient flow and climate change on the lake ecosystem quality in the Rappbode Reservoir?