Large masses of something called sargassum seaweed can clog harbours, interfering with fishing and boating, and in some cases harm marine life. To reduce the impact of these sargassum blooms, the first step is understanding why the seaweed started growing in the first place and what’s sustaining it now. In this new area, now called the Great Atlantic Sargassum Belt, the amount of sargassum surpassed an estimated 37,500 million tons in 2025. These animals collect food around the floating mats and recycle nutrients through their waste and the breakdown of older sargassum. This process can make sargassum blooms very likely to persist in the years ahead.