Carlyn Scott and Dyllan Furness, College of Marine Science
A record amount of sargassum has inundated beaches across Florida, the Caribbean, and Gulf Coast this summer, leaving communities scrambling to clean up.
The annual bloom, which began in 2011, has earned a name: the Great Atlantic Sargassum Belt. Seasonal peaks of the yellow-brown seaweed reach up to .
In the open ocean, sargassum provides essential habitat for marine organisms. However, when it washes ashore, the decaying biomass harms people, marine life, and economies. Coastal communities that rely on fishing and tourism are particularly hard hit. Analyses from the Environmental Protection Agency put the annual economic cost for U.S. businesses and governments in the millions of dollars.
The 杏吧性世界 has positioned itself as a leader in the study of sargassum. At the USF College of Marine Science, experts are hard at work uncovering the causes of the Great Atlantic Sargassum Belt, measuring its impacts, and helping vulnerable communities prepare.
Measuring sargassum's impact on coastal communities
While remote sensing is successfully used to detect masses of sargassum floating in the open ocean, researchers are developing methods to determine the amount of the seaweed that washes up on the shore.
involving researchers from the USF College of Marine Science used satellite images mixed with transport models to estimate the amount of sargassum reaching the coast. Published in Scientific Reports, the study relied on converting the satellite images of sargassum into wet biomass, or the amount that could potentially wash ashore.
The researchers then built upon the existing Coastal Vulnerability Index, which uses geomorphological criteria to determine a region鈥檚 vulnerability to sea level rise, to include the impact of sargassum and identify regions that are most susceptible to significant ecosystem and socioeconomic pressures from these inundations.
The results were significant. Estimates showed that some months saw more than two million tons of sargassum arrive on the coastlines of the Caribbean between 2018 and 2023. In 2018 alone, 10 million tons of sargassum inundated these beaches, according to the model.
IMAGE ABOVE: Sargassum has had a significant negative impact on coastal communities, which must contend with massive amounts of the seaweed washing ashore during the summer months. Image credit: Robyn Wishna.
The distribution was not uniform, though, and the offshore biomass was not a strong predictor of how much sargassum made it to shore. Instead, the strandings were dictated by orientation of the coast, wind direction, and currents.
Using the updated vulnerability index, the study determined that the Mexican Caribbean coasts and the northern Lesser Antilles and eastern Greater Antilles were the most vulnerable regions, estimated to have received over 100,000 tons of sargassum per year since 2018. As a result, these countries face significant economic and ecological pressure from the influx of sargassum, mainly due to loss of tourism.
鈥淭here is still so much to understand about the socioeconomic impact of sargassum on these coastal communities at a local level,鈥� said Frank Muller-Karger, USF Distinguished University Professor and co-author of the study.
Read more: Researchers identify 鈥榯ipping point鈥� that caused Sargassum inundations in the Caribbean
Most impact reports rely on reporting from citizens and local governments for observations but these are uncertain and highly variable. This study builds upon satellite imagery and incorporates models to better estimate the sargassum reaching beaches.
鈥淲e hope that these results can improve the forecasting tools to help coastal communities better understand and prepare for the drastic and unbalanced impact sargassum has on their regions,鈥� said Muller-Karger.
Uncovering how eddies influence sargassum
Researchers are still trying to determine the factors that influence sargassum offshore. Ocean currents, wind, and nutrients are all known to play a role.
in the journal Geophysical Research Letters led by researchers from the College of Marine Science found that eddies 鈥� small-scale circular water masses that pinch off from currents 鈥� may be a good place to start.
鈥淭he Atlantic Ocean is so vast, we wanted to narrow down locations where you should first start to look for sargassum,鈥� said Chuanmin Hu, USF Distinguished University Professor and director of the .
Eddies are swirling water masses that can transport floating seaweed, such as sargassum. Eddies can vary in size, lifespan, and location.
鈥淲e had been looking at the larger currents and wind patterns that could control sargassum, but we often overlook the importance of eddies in this process,鈥� said Hu.
IMAGE ABOVE: Sargassum can become entrained in small ocean masses called cyclonic eddies, which are lower than the sea surface. This figure from the recent publication demonstrates how the seaweed is moved by eddies in the Caribbean. Image from Zhang et al., 2024.
The authors used satellite altimetry data to infer the location of large eddies via sea surface height. Anticyclonic eddies raise above the sea surface while cyclonic eddies pull water down, lowering the sea surface.
Using observations from 2011 to 2023, the researchers found that sargassum was up to 47% more likely to be found in cyclonic eddies than in surrounding water.
They also found that larger eddies contained greater amounts of the seaweed.
鈥淭here is the physical transport 鈥� a cyclone will drag materials inside since it is lower than surrounding water 鈥� and the biological component, inside the upwelling center of an eddy there are more nutrients which could lead to more sargassum growth,鈥� Hu said.
Read more: Study identifies nutrients as driver of the Great Atlantic Sargassum Belt
Hu is teaming up with the Yonggang Liu, associate professor and director of the college鈥檚 Ocean Circulation Lab, to incorporate satellite altimetry into the model and monitor selected coastal regions for the presence of eddies.
Determining the location of eddies may point researchers to the areas with more sargassum, since the algae is entrained in the swirling mass. This may be helpful as more coastal regions in the Caribbean prepare to mitigate how much sargassum reaches the shore.
鈥淲e hope this study can lead to better mitigation strategies for countries impacted by sargassum,鈥� said Hu.
Monitoring sargassum and forecasting inundation
When summer arrives, coastal communities in Florida, the Caribbean, and Gulf Coast brace for sargassum. Many turn to guidance from the satellite-based Sargassum Watch System.
Developed by the Optical Oceanography Lab at the USF College of Marine Science, the system offers interactive maps that details sargassum abundances in near real time. A monthly bulletin released by the lab offers a report about the present state of sargassum and prediction about potential abundances in the months ahead.
鈥淭he information we provide through the Sargassum Watch System has proven to be a powerful resource coastal resource managers throughout the Gulf Coast and the Caribbean,鈥� said Brian Barnes, assistant research professor at CMS, who has been monitoring the seaweed since the 2011 bloom.
In 2023, Barnes was tapped to lead a five-year, $3.2-million grant from the . The funding is being used to improve the resolution of satellite images of sargassum slicks and help forecast how much of the seaweed will wash ashore in each region.
By using high-resolution satellite data at a scale of just 50 meters, researchers can now monitor smaller slicks of sargassum and make more accurate predictions about where it will end up.
IMAGE ABOVE: The improved resolution of the Sargassum Watch System allows researchers to detect relatively smaller slicks of sargassum. Courtesy of Brian Barnes.
鈥淥ur updated system has vastly improved capabilities over the prior iterations of the Sargassum Watch System,鈥� said Barnes. 鈥淲e are now able to see potential impacts on individual beaches, which means local communities can prepare for inundations and mobilize resources to address the problem.鈥�
A partnership with the Ocean Circulation Lab will combine the new, higher-resolution satellite imagery with coastal ocean circulation models, allowing researchers to forecast the movement of sargassum patches days in advance. The Caribbean Coastal Ocean Observing System recently launched for Puerto Rico and the U.S. Virgin Islands based on coarse resolution data and forecast trajectories.
Additional partners on this project include , , and .
For more information and to sign up for the monthly bulletin, .