Ocean seaweed signals regime change

Scientists report that global floating seaweed blooms are expanding in the oceans, with possible serious consequences. In the past two decades, both microscopic algae, such as phytoplankton, and seaweeds or macroalgae, have increased in certain coastal and open ocean waters.

To address this at the global scale, 1.2 million satellite images were analysed with computer artificial intelligence to quantify macroalgal mats and microalgal scums in global oceans between 2003 and 2022, covering an area of 43.8 million km2. A deep-learning model was employed to detect signals of floating algae. Macroalgae blooms in the tropical Atlantic and western Pacific both expanded at unprecedented rates, amounting to 13.4 percent per year since 2003, with the most dramatic increases occurring after 2008. Before 2008, there were no major blooms of seaweed reported except for sargassum in the Sargasso Sea. The best-known example, the Great Atlantic Sargassum Belt, is visible from space, stretching from the Gulf of Mexico to the mouth of the Congo. Other blooms include a ring around the Chatham Islands off New Zealand and the “red tide” that surfaced off the coast of Florida.

This shift could darken the waters below, changing their ecology and geochemistry, and may also accelerate climate breakdown. The rapid growth of huge mats of seaweed appears to be driven by global heating and excessive enrichment of waters from farming runoff and other pollutants.

While seaweed such as sargassum have thrived in some regions, phytoplankton have not shown similar responses to the changing environment, suggesting seaweed growth may be more sensitive to shifts in temperature and eutrophication. The annual expansion rate of microalgae scums, although slower, is also statistically significant at 1.0 percent per year since 2003. This study provided the first global picture of algae floating in the world’s oceans. Most increases in both floating macroalgae and microalgae scums occurred in the recent decade, in line with the accelerated global ocean warming since 2010. There were tipping points in 2008, 2011 and 2012 for three types of seaweed in different oceans.

Such trends are likely a result of ocean warming and eutrophication, warning of a possible regime shift from a macroalgae-poor ocean to an macroalgae-rich ocean with specialized species of microalgae. These findings have broad implications for ocean ecology, carbon sequestration, environments, and the economy. If a regime shift in oceanographic conditions has already occurred to favour macroalgae, this will have profound impacts on radiative forcing in the atmosphere and light availability in the ocean, as well as on carbon sequestration, ocean biogeochemistry and upper ocean stability.

SOURCES: https://www.theguardian.com/environment/2026/jan/19/scientists-seaweed-…
Original paper: https://www.nature.com/articles/s41467-025-66822-5