Cyanobacteria blooms frequently disturb the functioning of freshwater ecosystems and their uses, due to the toxins dangerous to health that cyanobacteria are able to synthesize. Therefore, many countries have implemented monitoring programs aimed at reducing the risk of human exposure to these toxins. The main limitation is related to the heterogeneity of the spatial distribution of cyanobacteria. In the vertical dimension, these organisms can stay in different layers in the water column and in the horizontal scale, the cells may accumulate in some area of the water body, under the action of winds or currents. In an attempt to improve monitoring, many research projects have been undertaken in order to develop new tools, like buoys developed during the program PROLIPHYC (ANR PRECODD). This tool is highly relevant but it does not allow assessing the horizontal distribution of cyanobacteria and its cost remains expensive. In addition, if satellite remote sensing can be considered very useful for estimating biomass and horizontal distribution of cyanobacteria in a water body, the cost of this technology and the lack of satellite availability make it unaffordable for routine monitoring. In this context, our OSS-CYANO project aims to develop and validate a new, low-cost aerial sensor, to be used in a fixed single location, or deployed in network, to detect the presence of cyanobacteria in a water body. In addition, OSS-CYANO also aims to implement a drone capable of carrying the sensor to perform spatial measurements on large water bodies or river sections, and other instruments for water sampling or for performing underwater measurements. Our project is organized into five tasks. Task 1 is dedicated to (i) the coordination of the project, (ii) the scientific animation and dialogue with end-users through the formation of a monitoring committee and (iii) dissemination of results, including the organization at the end of the project of an international workshop dedicated to new monitoring tools. The second task will help to identify potential end-users and identify their expectations for the developed tools. Task 3, a key task of the project, will involve technical development of the sensor (wavelength selection, influence of natural processes on the measurements ...) and of the drone system (implementation of an adaptive platform for supporting the measuring equipments). These developments will largely benefit on the facilities offered by the support center PLANAQUA (French label "Investissement d’avenir ") which provides all the required facilities to carry out tests of the sensor on a range of aquatic systems, from microcosm to macrocosm. The fourth task will test in real conditions of application, and in the long term, the sensor and the drone on different lakes and river systems impacted by representative cyanobacteria. Finally, the last task will relate to (i) data processing methodologies and data integration, (ii) the implementation of a three-dimensional hydrodynamic model using data from the sensor and / or drone to forecast short-term changes of the spatial dispersion of cyanobacteria in a water body, and (iii) to define the characteristics of a future warning system. This project relies on the participation of six laboratories and a private company. All these teams have the required skills for all planned work and have already collaborated on previous research projects.