Understanding and forecasting the proliferation of holopelagic Sargassum in the tropical Atlantic requires progress in modeling the transport and biology of the macroalgae. Current models focus mostly on the transport part and base the biological part on relatively outdated knowledge for Sargassum fluitans or S. natans indistinctively. However, there is now strong evidence of the presence of three co-occurring morphotypes of Sargassum, with possibly different physiologies, and that the nutrients controling their growth may vary considerably among regions. Therefore we need to gain knowledge in the morphotype-specific response to varying environmental conditions and on the origin of nutrients in order to improve our capacity to simulate and forecast Sargassum biomass. The main objectives of BIOMAS are to acquire the necessary knowledge to build an individual-based model of Sargassum growth, to include that model into a drift model, and finally perform simulations of the integrated growth-drift model in order to forecast Sargassum morphotypes proliferation at seasonal scale. These objectives will be achieved through a strong synergy between laboratory experiments and modeling. The project is structured under : WP1, development of a growth model of Sargassum morphotyes based on the Dynamic Energy Budget (DEB) theory; WP2, laboratory experiments for calibrating the DEB model; WP3 in situ monitoring focusing on morphotypes and elemental composition in the Western and Eastern Atlantic; WP4, integration of the DEB model into a drift model and simulations. The consortium has expertise and experience in Sargassum biology, laboratory and field works, DEB and drift modeling, and is already involved in Sargassum research. Pan-Atlantic sampling will occur through the involvment of Brazilian, Mexican and African partners. The general public, including schools, will be engaged through a smartphone app that will serve both citizen science and results dissemination.