Changing drivers of the Great Atlantic Sargassum Belt from physical forcing to ecological control.

Inundations of pelagic Sargassum plague the tropical Atlantic, with size and impacts steadily increasing to surpass 30 million tons in 2025. Understanding the drivers of Sargassum growth in the so-called Great Atlantic Sargassum Belt is fundamental to developing effective mitigation strategies for affected nations. We present a nonlinear regression model that both explains the seasonal and interannual variability observed between 2011 and 2022 and predicts Sargassum concentrations in 2023 and 2024. The growth of Sargassum, initiated by a prolonged negative phase of the North Atlantic Oscillation, is initially enhanced through winter mixed layer deepening in response to stronger winds. An additional overlooked driver is the recycling of nutrients within the mixed layer, carried out by the community of organisms associated with Sargassum and aging Sargassum mats. This contribution increases over time to become dominant in recent years, offsetting the increase in stratification in 2023 and 2024.