Lake Champs-sur-Marne is one of the study sites of the ANSWER project. Lake Champs-sur-Marne is a small and shallow polymictic peri-urban lake located in the Ile-de-France region. N₂-fixing and non-fixing cyanobacteria species alternate during summer and early autumn. The main drivers of this competition are nutrient stoichiometry and the alternation of stratification and mixing episodes during which the nutrients are released from the sediments. The data set collected on the lake is used to develop a model of the dynamics of phytoplankton community composition.