Dyllan Furness, College of Marine Science
A new study led by researchers at the University of South Florida’s College of Marine Science has found that certain populations of the seaweed sargassum have experienced a significant decline over the past decade. This was true even as an increased abundance of sargassum in the tropical Atlantic caused large mats of the seaweed to inundate beaches across the Caribbean and Gulf regions.
The abundance of sargassum in the Atlantic’s north Sargasso Sea has plummeted since 2015, according to the paper published this week in Nature Geoscience. Sargassum from the Gulf, which annually supplies the Sargasso Sea, has also decreased substantially.
The findings point to ocean warming as a possible cause of the decline and suggest a dramatic shift in sargassum’s distribution, which could affect the health of marine ecosystems.
“What is fascinating is that two opposite patterns occurred in the Atlantic Ocean,” said Chuanmin Hu, professor of oceanography at the USF College of Marine Science and senior author of the study. “The tropical Atlantic has seen a significant increase in sargassum in the past decade, but at the same time the Sargasso Sea has a lot less sargassum than it used to.”
A floating clump of healthy Sargassum natans var. wingei, one of three common holopelagic morphotypes in the North Atlantic. Credit: Jeff Schell.
Much like rainforests, large floating mats of sargassum support high levels of biodiversity. They serve as an important habitat for turtles, crabs, shrimp, fish, and seabirds, some of which are specially adapted to inhabit the mats of seaweed.
However, once sargassum mats wash ashore and decay, they can emit a foul odor, harm marine life, and disrupt coastal communities. Cleanup efforts in the United States alone have cost businesses and governments millions of dollars annually.
Hu has used satellites to study sargassum since 2006 and spearheaded the discovery of the Great Atlantic Sargassum Belt in 2019. His Optical Oceanography Lab is a global leader in sargassum research, providing satellite imagery and monthly bulletins that inform stakeholders about sargassum biomass seen from space.
Read more: USF experts lead on sargassum research, monitoring, and prediction
For the recent study, Yingjun Zhang, then a postdoctoral researcher, and Brian Barnes, assistant research professor at the USF College of Marine Science, analyzed satellite data gathered by NASA to assess changes in sargassum biomass. The lab partnered with researchers at Sea Education Association and Eckerd College, who observed similar declines using field collected data, and provided historical records, temperature tolerance information, and variety-level sargassum distribution insights that were unavailable by remote sensing. Researchers from Woods Hole Oceanographic Institution and Florida Atlantic University also contributed to the study.
Samples collected from the field were key to informing the findings of the new study in Nature Geoscience. Here, Samuel Bunson, a graduate student at the University of South Florida’s College of Marine Science, uses a net to gather sargassum in the Gulf in May 2023. Courtesy of Samuel Bunson.
The Gulf typically experiences a spring bloom of sargassum, which is carried north by currents to the Sargasso Sea, where peak season occurs during late fall or early winter. Lower levels of healthy sargassum in the Gulf result in a decreased abundance of healthy sargassum in the Sargasso Sea.
“These findings suggest we may be witnessing the early stages of a basin-scale regime shift in sargassum distribution,” said Zhang, now a postdoctoral scholar at the Scripps Institution of Oceanography. “Since a wide range of marine life relies on pelagic sargassum ecosystems, this could really make a difference.”
Barnes said, “The regime shift also includes changes in sargassum seasonality, as the once fall and winter peaks are now replaced by summer peaks in the north Sargasso Sea.”
By analyzing three ingredients all plants depend on — light, nutrients, and temperature — the researchers posit that record high temperatures in the Gulf, including more frequent marine heat waves and possible nutrient competition by sargassum transported from the Caribbean Sea, may have stunted the region’s population of sargassum. The result is weakened sargassum that struggles to survive once it arrives in the Sargasso Sea.
Sea Education Association’s SSV Corwith Cramer passes a clump of Sargassum natans var. natans while transiting the Gulf Stream. For almost 50 years, research vessels operated by the group have documented the abundance of three common sargassum varieties, information essential for tracking different regional populations and understanding reasons for observed long-term changes. Credit: Jeff Schell.
Studies have shown that waters in the Gulf warmed approximately 0.19°C (0.34°F) per decade between 1970 and 2020, about twice the rate of the global ocean. While sargassum in the Gulf prefer temperatures between 20 and 28°C (68 to 82°F), summer water temperatures in the Gulf have recently exceeded 30°C (86°F).
Even the Great Atlantic Sargassum Belt, which stretches 5,000 miles across the Atlantic, hasn’t helped compensate for the decline in the Gulf. Sargassum from the belt may be acclimated to warmer conditions or in poor health upon arrival and thus does not tolerate the colder waters of the Sargasso Sea.
“It's a complex story and challenging to unravel due both to the spatial scale and the fact that each variety of sargassum responds to ocean environmental conditions in different ways,” said Deb Goodwin, chief scientist at Sea Education Association and a co-author of the study. “Long-term data identifying and quantifying sargassum varieties provided critical context to the satellite observations.”
Looking ahead, the research team aims to better understand how sargassum’s shifting population dynamics could impact marine ecosystems, including whether competition from the Great Atlantic Sargassum Belt could drive further declines of sargassum in the Gulf.