Read: After last year's hurricanes, Caribbean lizards are better at holding on for dear life
So, a quartet of factors—strong Amazon discharge, strong West African upwelling, moderate temperatures, and the presence of a seed population—could potentially explain the Sargassum Belt, including why it appears every summer, and why it was especially thick in 2015 and 2018. But such factors “have impacted the central Atlantic Ocean for decades, if not centuries,” says Siuda “Why are we only seeing sargassum bloom in this region now? What tipped the balance? There is clearly still more to learn.”
Chuanmin Hu, who led the study, agrees. “I have to emphasize that we have no direct evidence to prove any of this,” he says. “These are our speculations, some educated and some hand-waving.” They’ve been forced into that because many of the factors they identified aren’t regularly measured. For example, they could only find data on the nutrients in the Amazon for two years: 2010 and 2018. The latter levels were much higher, which might explain why sargassum blooms were so big that year. Or it might not. The river might have more nutrients due to increased fertilizer use, and stronger runoffs due to deforestation. Or it might not. “I don’t think there’s enough data,” Hu says. “It takes a huge amount of money to go there and take measurements.”
Of the four factors that the team identified, only sea surface temperatures are regularly measured. And while many scientists have suggested that hot water could speed the growth of sargassum, “we found the opposite,” Hu says. That’s not to say climate change is irrelevant, he cautions: Changing patterns of rain and wind could, for example, influence the strength of the West African upwelling. Nor should the Caribbean count on rising temperatures to solve its sargassum woes, because the pace of change is likely too slow to make a difference in the near future.
Hu adds that other factors could be behind the rise of the Sargassum Belt, including nutrient-rich dust blowing in from the Sahara and changes in ocean currents. And several aspects of the blooms still don’t make sense. “If I were you, I would ask: If you have so much nitrogen and phosphorus, why do other [algae] in the ocean not grow as fast?” he says. “I can’t answer that.”
Goodwin adds that “scientific understanding of Sargassum growth and bloom dynamics under natural, open ocean conditions is extremely limited,” since scientists have only addressed these questions in lab experiments. And the sargassum itself is changing, too. Siuda says that the recent blooms have included “a previously rare and genetically distinct form of sargassum” that comes from the south, differs from those in the Sargasso Sea, and harbors a slightly different community of organisms.
Little is known about this strain, or how the bloom is affecting the ecology around it, which makes it hard to predict how it will react to future conditions. And since it likely evolved in relative isolation from its northerly relatives, its northward expansion suggests that “environmental conditions and ocean circulation patterns in the central Atlantic may have been shifting, undetected, for longer than the time interval examined by [Wang and her colleagues],” says Goodwin. “A critical larger question emerges: What drove such an ecological transformation at unprecedented scale?”