As the cyanobacteria blooms of lakes mainly come from the nutrient over-enrichment of the water (in particular in P and N) we will limit our study to eutrophic systems. In such environments, the cyanobacteria dynamics is mainly driven by the bottom-up control feedbacks. The grazing (zooplankton) and competitive (macrophytes such as water bottom plants) pressures are limited and only competition with floating plants can occur. The huge amounts of organic matter generated by cyanobacterial blooms (and sometimes by floating plants) makes the recycling of organic matter of great importance for the ecosystem functioning and for the dynamics of cyanobacteria. However, the microbial loop is often greatly simplified in the models. The mineralization is generally represented as a simple chemical reaction with a rate either constant or dependent on the bacteria concentration at best, whereas the recycling of organic matter is a very complex process, which is moreover influenced by global changes. For example, it is known that rising temperature increases the nutrient loading in water ecosystems by increasing the rate of mineralization in catchment soils [26], [27] and by increasing deoxygenation at the surface of lake sediments, which promotes the release of nutrients in water [28].

In this project, we intend to improve the existing models of lake ecosystems. Our modelling work will be conducted at two different scales, which will be merged during the project: the scale of the biochemical/physical processes and the scale of the whole ecosystem.

1. First scale: the recycling of organic matter by bacteria
   The objective of the first part will be to evaluate the impact of changes in the C/N/P ratio on the recycling of the organic matter by bacteria and the consequences for the population dynamics of Cyanobacteria
   For that, we will use experimental and modelling approaches:
   • specific experiments will be conducted to study the mineralization process both in the water column and in the sediments, from which the nutrients are released.
   • models of the recycling process will be developed based on the experimental results and some field measurements. These models will then be integrated in the model of the whole ecosystem dynamics developed in the second part.
2. Second scale: cyanobacteria dynamics as a part of the whole ecosystem
   The objective of the second part is to simulate the temporal and spatial distribution of cyanobacteria population by means of specific 3D dynamical models of water ecosystems.
   These models will be obtained by coupling a hydrodynamic model to an ecological one:
   • for the hydrodynamic model, we will use the software programs FVCOM and Delft3D-FLOW;
   • the ecological models will combine the advantages of two types of existing dynamical models: (1) the complex dynamic models used as virtual reality (FVCOM-WQM, DELWAQ-BLOOM), which are both complex and poor in terms of description of some processes; (2) the minimal dynamicmodels (including those developed in the first part), which try to capture the essence of a process;
   • to better use and share the produced information, data and knowledge, we will propose data and knowledge management tools; statistical methods will be developed for the data and models analysis.

This ensemble will form an integrative platform for lake ecosystems modelling (see Figure 1).

Long-term monitoring data series of several lakes of France and China will be used for the model calibration. After validation by comparison with data, the coupled hydrodynamic-ecological models will be used to conduct 3 numerical studies:

• Study 1: simulation of the short-term (weeks) impact of sudden changes of C/N/P ratio (due to weather events) on the initiation of blooms and the evolution of their spatial distribution;

• Study 2: comparative simulation of the dynamics and composition of phytoplankton communities during an annual cycle following the observed and predicted values of the C/N/P ratio in France and in China;

• Study 3: test and improvement of strategies for the control of cyanobacteria blooms whose combined impact with climate changes results in a modification of the C/N/P ratio.

The modifications of the C/N/P ratio due to anthropogenic environmental changes include: (1) the increase in nutrients loading and atmospheric CO 2 due to the urbanization and the intensification of agricultural/industrial activities (allochtonous inputs); (2) the variation of internal (sediment resuspension, nutrient release) and external (river discharge) loadings in response to weather events such as rainstorms, typhoons or monsoon; (3) the changes induced by lake management strategies based on the reduction ofnutrient loadings (P reduction paradigm, or dual reduction of N and P).