Large lakes of South-Western France are largely impacted by human activities and constitute an important touristic place during summer. Consequently, these lakes have bathing areas regularly monitored for public health. Regulation impose biological monitoring, especially for cyanobacteria that could be potentially toxic. However, analyses carried out in laboratories generate a long delay of response (36 hours) damaging to sanitary safety and leading to important financial costs. The present work, carried out between scientists and managers, proposed to reduce this delay and to limit the costs by modifying the monitoring and alert decision tree with the use of a fluorimetric field probe. Thus, we analyzed 182 water samples from 3 lakes (2012-2013), and compared in situ field probe values with lab's counted cells. We then adapted a decision tree, based on thresholds of cyanobacteria biomass (chlorophyll-a equivalent) instead of the number of counted cells. This new decision tree provides at least the same safety level and is, sometimes, more precautionary than the regulatory method. This new method need however to be tested for other lakes with different cyanobacteria biomass, but provide encouraging results for bathing areas management and is already used for management of lake beaches of the Landes region.