Worldwide the current environmental situation is preoccupying: the water needs for humans increase while the quality of the available resources is deteriorating due to pollution of various kinds and to hydric stress. In particular, the eutrophication of lentic ecosystems due to excessive inputs of nutrients (phosphorus and nitrogen) has become a major problem because it promotes cyanobacteria blooms, which disrupt the functioning and the uses of the ecosystems.

For 40 years, some measures of reduction of nutrient inputs, especially of phosphorus, have been applied in developed countries to fight against these blooms whereas a reverse tendency of increase of the nutrient loading is observed in other countries. At the same time, the temperature and the atmospheric CO₂ concentration have been increasing with significant consequences on the dynamics of phytoplankton communities in lakes. All these local and global changes finally lead to significant modifications of the C/N/P ratio in ecosystems (i.e. the modification of the availability of the three major elements: C, N & P). The project ANSWER will evaluate the consequences that these variations can have on the dynamics of cyanobacterial blooms, by using modelling approaches and numerical simulations which will be based on existing and specific experiments and field data.

To represent the complexity of the functioning of lentic ecosystems, the use of 3D dynamical models, obtained by coupling hydrodynamic and ecological models, is essential. Because the development, the calibration and the simulation of such models require a wide range of skills, researchers of various fields (computer science, applied mathematics, modelling, physical limnology and microbial ecology) will work together to develop an integrative platform for lake ecosystems modelling, which will include tools of data management and knowledge representation, models and statistical methods for model calibration and validation. The models will be used for:

1. the 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;
2. comparative simulations of the dynamics and the composition of phytoplankton communities during an annual cycle following the observed trends for the C/N/P ratio in France and in China;
3. the test and optimization of control strategies whose combined impact with climate changes results in a modification of the C/N/P ratio.

The processes of recycling of organic matter by bacteria in the water column and of release of nutrients from the sediments will be studied experimentally to improve the model ability. Indeed, although these processes are considered as essential for the dynamics of cyanobacteria, they are often ignored or poorly represented in existing models.

One of the difficulties of the implementation of these modelling approaches often comes from the lack of data, or from their poor quality, which makes the validation and the calibration of the models difficult. Thanks to the diversity of the French and Chinese teams gathered in our consortium, we will have access to a rich database composed of good quality data acquired on diversified ecosystems (the large shallow Lake Taihu in China, the long and deep Villerest reservoir and the small and shallow urban Lake Champs-sur-Marne in France). The broad skills of our consortium, ranging from microbial ecology and limnology to modelling, will allow us to take into account in the construction, the validation and the analysis of the models, all the key processes involved in the ecosystem functioning.