Global seaweed boom – is more always better?

Over the past two decades, seaweed blooms have expanded by a staggering 13.4% a year in the tropical Atlantic and western Pacific, with the most dramatic increases occurring after 2008, according to researchers at the University of South Florida.

In a new paper, researchers say this shift could darken the waters below, changing their ecology and geochemistry, and may also accelerate climate breakdown.

“Before 2008, there were no major blooms of macroalgae [seaweed] reported except for sargassum in the Sargasso Sea,” said Chuanmin Hu, a professor of oceanography at the USF College of Marine Science and the paper’s senior author.“On a global scale, we appear to be witnessing a regime shift from a macroalgae-poor ocean to an macroalgae-rich ocean.”

However, while seaweed such as sargassum had thrived in some regions, phytoplankton have not shown similar responses to the changing environment, suggesting their growth may be more sensitive to shifts in temperature and eutrophication.

“If this is the case, we believe that a regime shift in oceanographic conditions has already occurred to favour macroalgae, which 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,” the researchers wrote.

Furthermore, the unexpected increase of a species such as sargassum not only destabilizes marine ecosystems and biodiversity, but can also adversely affect fishing communities as they get tangled in nets, prevent boats from reaching fishing grounds, and reduce catches of key species. These blooms can cause significant economic losses, damage tourism, and threaten food security in coastal areas, particularly in the Caribbean and West Africa. 

Rapid algal blooms, like other destabilizing phenomena caused by climate change, are often the first domino in a series of unexpected changes and impacts. The collapse and destabilization of ecosystems is not always dramatic and intense—sometimes it occurs slowly, beneath the surface of the water. Effective protection of the sea and ecosystems thus requires holistic, preventive, and creative solutions that take into account ecosystems, wildlife, and the people who depend on the sea for their survival. 

Photo: A beach in Playa Caribe, Dominican Republic, blanketed in sargassum, a floating macroalgae that forms vast colonies and drifts with the ocean currents. Photograph: Orlando Barría/EPA