since 2012, Brest’s wrap has been subject to administrative closures for the marketing of shellfish and collection due to the toxicological effects of the development of the harmful microalgae Alexandrium minutum. The field monitoring that existed since 2009 at the Castle Pointe (Mignonne Estuary, Daoulas) was reinforced in 2016 as part of this project (AlexBreizh) to identify the growth dynamics of this microalgae in relation to environmental factors and the dynamics of other phytoplankton species. This 12-year weekly monitoring made it possible to understand the seasonal dynamics of this environment and its inter-annual variability. Alexandrium minutum was observed only in the period from April to September and the abundance maximums were in June and July. Depending on the years, the bloom starts in May (hot spring as in 2014) or June (2013, cold spring). The temperature of 15 °C is the indicator of the possible start of a bloom, which has also been verified at other Breton sites. The maximas are linked to summer flow conditions, with high flows resulting in significant inputs of nitrates and phosphates that stimulate the growth of A. minutum and enable it to dominate phytoplankton flora. This was the case in 2012, when the summer was exceptionally rainy. The phytoplankton community in winter is at rest. From March, there is an ecological succession, with the development of the microphytoplankton, while the concentrations of nitrogen and phosphorus in the medium are high (strong winter flows) followed by nanophytoplankton and then picophytoplankton in summer. On summer floods, it is once again the microphytoplankton that is developing. These measurements of the whole phytoplankton community could be carried out in AlexBreizh by the use of in situ cytometry. The set of measured data, integrated into the Quadrige2 database and available to all, allowed different analyses to be tested to identify links between environmental parameters and Alexandrium minutum development. Ecological niche analyses show Alexandrium minutum’s preference for summer conditions (high temperatures and light) with tolerance for more estuarine conditions and more favourable environmental conditions in Brittany for the development of A. minutum. The functional analysis on microphytoplankton taxa identified 3 periods 2009-2011, 2011-2015 and 2015-2018. The former is associated with the dominance of Chaetoceros, which subsequently declines, the second is represented by the dominance of A. minutum and the third is the decline of A. minutum and a higher early microphytoplankton blooms. Applied to the Daoulas Estuary, a numerical model of A. minutum dynamics in competition with 72 other phytoplankton phenotypes identified and prioritised the control factors of A. minutum. The latter is controlled first by temperature, then by phosphates and finally by nitrates in summer. This graduated model simulates the 10 years measured and reproduces the seasonal and inter-annual variability of A. minutum and the whole phytoplanktonic community, but underestimates the concentrations of phosphate in summer and thus the abundances of the microphytoplankton, which are severely limited by phosphates. This bias reduces its ability to simulate nitrogen and phosphorus reduction scenarios, as phosphorus is significantly more limited than nitrogen. However, it highlights the strong trophic imbalance in the Brest beach background, where excessive nitrogen inputs from the Mignon and Aulne rivers (and the others) cause extremely high nitrogen/phosphorus ratios. Several ways of developing the model have been tested, such as the inclusion of cysts (dormance phase of A. minutum and other species), mixotrophy (ability to absorb organic matter in addition to photosynthesis) or the injection of cells from the wrap. This work did not improve the model and thus it can be said that A. minutum’s bloom is essentially linked to its photosynthetic and local development capacity. The extension of the model to the entire wrap is ongoing but complicated by the management of open borders with the Iroise Sea, which requires more information on the 72 simulated phenotypes. A comparison of the monitoring of the Château Pointe with the existing tracks, the Rade centre (Lanvéoc) and the entrance to the rade (Ste Anne) shows the particular appearance of the Daoulas estuary with local development and abundances of nanophytoplankton, picophytoplanancton and A. minutum well above the centre and entrance to the wrake. Finally, an alert system is tested over 12 years, based on daily monitoring of flows, temperature and tidal coefficient. It is an opening to a more robust system to be improved and a possible operational set-up.