Sargassum is a brown macroalgae (seaweed), with hundreds of different species found in different tropical and temperate ocean environments. In the Atlantic Ocean, the two dominant species are Sargassum fluitans and Sargassum natans (Imageset 1), both reproducing vegetatively and living in the surface ocean in their entire life cycle. Because of this, Sargassum in the Atlantic is also called pelagic Sargassum. Each species also has several subtypes. They all float on the ocean surface because they have grape-like air bladders, and the name “Sargassum” may have come from the Spanish name “Sargacinha” which means grape. In the ocean, Sargassum appears in different forms, from small clumps, surface mats, to large rafts and windrows that can be several kilometers long and hundreds of meters wide (Imageset 2). Sargassum can live in both nutrient-poor and nutrient-rich waters but prefer warm (temperature range of 18 – 30°C) and salty (salinity > 24) waters. Depending on the environment, Sargassum can double in 10-50 days. In nutrient-poor waters Sargassum tends to have yellowish colors, and large rafts moving with currents and winds are often called “golden tides.” In nutrient-rich waters their colors often turn brownish, thus the name “brown tides.”

Sargassum in the Atlantic was first reported in 1492 by Christopher Columbus during his voyage in the North Atlantic Ocean, which possibly led to the name of Sargasso Sea, the only sea on earth that has no physical boundary.

Before 2011, Sargassum was mainly found in the Sargasso Sea and Gulf of Mexico (Imageset 3, top), although small amounts have also been sighted in the Caribbean Sea and off the Brazilian coast.

Since 2011, however, although the Sargasso Sea and Gulf of Mexico continued to show abundant Sargassum, most Sargassum was found in a continuous “belt” in the tropical Atlantic extending from the west Africa to the Gulf of Mexico (Imageset 3, bottom). Because of the large scale (about 5,000 miles or 8,000 kilometers long) and high abundance of Sargassum, the belt was termed the Great Atlantic Sargassum Belt. On average, during the peak months, Sargassum density within the belt is about 0.1%, equivalent to 3 metric tons per square kilometer or 3 grams per square meter.

Analysis of satellite data along with other data indicates that the GASB may be a new normal in the future. This means that, depending on the time of the year, large quantities of Sargassum can be found in most waters of the Sargasso Sea, Gulf of Mexico, Caribbean Sea, tropical Atlantic, and around the peripheries of the GASB, such as coastal waters off west Africa and Brazilian coast.

Because Sargassum moves with water and because of variable ocean circulation patterns, it is difficult to pinpoint their origins. From the large-scale distributions and from the changes of timings in peak months, it appears that two regions may provide seed populations in their respective seasonal cycles: one in the western Gulf of Mexico, and the other in the tropical Atlantic and Sargasso Sea.

Although Sargassum can be found in most waters of the above regions, the timing can be different. Based on satellite observations, before the GASB era, the Sargasso Sea typically had more Sargassum in the fall and winter months than in other months, and the western Gulf of Mexico typically had more Sargassum in the spring months. In the GASB era (since 2011), because of the transport from the tropical Atlantic, large quantities of Sargassum can also be found in these two regions in summer months as well. Within the GASB, Sargassum typically peaks in June or July, with minimum quantity during the winter months. In some years, however, Sargassum quantity in the GASB, and especially between the Lesser Antilles islands and west Africa, can be very large even during the winter months (e.g., December 2022 – February 2023). This often suggests a major Sargassum year in the following year.

The occurrence of the GASB represents a new normal since 2011. Although the size and location of the GASB remained to be about 5 million square kilometers in the tropical Atlantic during peak months, there appears an increasing trend in Sargassum abundance (or areal density) within the belt. The total amount of Sargassum in the last 6 years (2017 – 2022) actually doubled the amount in the previous 6 years (2011 – 2016). During the peak month of June, the two record-high years are 2018 and 2022. Whether this increasing trend will continue in the future is under research.

As with any other plants, Sargassum requires enough sun light, nutrients, and an optimal environment (temperature and salinity) to grow. In the tropical Atlantic, all these conditions are met in favor of Sargassum growth. Compared with the “traditional” location of the Sargasso Sea, the tropical Atlantic is warmer, receives more sunlight, and contains more nutrients from multiple sources. Surface waters in open oceans are typically nutrient poor, but coastal and open-ocean upwelling can bring the nutrient-rich bottom waters to the surface to fuel the macroalgae growth. Deposition of the Saharan dust and smoke ashes can bring micronutrients such as iron to the surface ocean. Large river plumes such as those from the Amazon and Congo can also carry a large amount of nutrients (nitrogen and phosphorous) to coastal oceans. Recent increases in fertilizer use and deforestation in the Amazon basin may have contributed more nutrients than in the past.

One theory supporting the sudden appearance of the GASB in 2011 is that during 2010, stronger-than-usual winds and ocean currents brought large seed populations of Sargassum from the Sargassum Sea to the tropical Atlantic, thus representing a “tipping point” to result in the recurrent GASB in the following years. If this is true, the 2011 GASB is a result of climate variability. Then, both climate variability and human activities contributed to the inter-annual changes of the GASB. The continuous ocean warming in the tropical Atlantic, may be another factor.

However, the ocean works in a very complex way, and different parts of the ocean are connected through ocean circulations, meaning that changes in one location may actually be due to changes in another. What caused the development of the post-2011 GASB and what contributed to the inter-annual changes are still under active research.

Sargassum in the ocean serves as an important habitat for many marine animals including fish, shrimp, crabs, and turtles. Because of this, the U.S. NOAA prohibits harvesting of Sargassum in U.S. waters without authorization. Sargassum may be used as fertilizers for sand dunes and thus protects shoreline stability. It is also a marine resource for other uses such as biomass for food, fuel, and as a possible source of pharmaceutical materials. Additionally, through photosynthesis at the surface and sinking to the ocean floor, Sargassum can fix and sequester carbon, thus reducing carbon dioxide (CO₂), the primary gas responsible for global warming. There are currently projects being implemented or planned to sink Sargassum (and thereby atmospheric carbon) to the ocean floor as part of the Carbon Dioxide Removal (CDR) effort.

However, excessive amounts of Sargassum in nearshore environments can cause environmental and economic problems (Imageset 4). Large and thick Sargassum mats on beaches can smother turtle nesting sites. Under Florida and Caribbean sunshine, Sargassum can decompose in 1-2 days, and then emit stinky gases (hydrogen sulfide and ammonia), attract insects and grow bacteria, and thereby pose threats to humans. Large quantity of sinking Sargassum in coastal waters can smother corals and seagrass, among other benthic fauna. Dead Sargassum in coastal waters can produce leachates which deteriorate water quality (Imageset 4) and can consume oxygen to cause a local “dead zone”. Large Sargassum mats in the ocean can bock the waterways for boats and tangle boat propellers. In July 2022, the U.S. Virgin Island and FEMA declared a state of emergency due to Sargassum clogging of two water intakes of a desalination plant.

Large Sargassum inundation events have not only caused environmental problems, but also exerted economic burden to local governmental and management agencies. In 2015, Mexico employed its navy to physically remove Sargassum at sea. In Quintana Roo (along the Mexican Yucatan coast), during the peak months of 2018 and 2019, local hotels removed 1,000 – 10,000 tons of Sargassum per kilometer of beach in 7 beaches. In Florida, the Miami-Dade county alone spends millions of dollars every year to remove Sargassum from beaches. The tourism industry around the Caribbean Sea has particularly suffered from major Sargassum beaching events in the past decade.

Mostly no. Unlike red tides in the Gulf of Mexico, Sargassum itself is not toxic, and people can swim around it. Sargassum on beach is not toxic either unless toxic gases (hydrogen sulfide and ammonia) are released from rotten Sargassum. Even in this case, primarily those with underlying respiratory issues (e.g., asthma) may be affected. Beach goers can simply choose to avoid large quantities of accumulated Sargassum. Small quantities on beaches may be simply ignored.

Sargassum might be edible by humans and animals, but there are some reports showing relatively high arsenic levels in Sargassum tissues.

Although satellite-based monitoring can provide timely information on the general situation of Sargassum abundance and distribution in a relatively large region, predicting beaching events in future months for individual beaches is difficult or impossible. This is because even though a large quantity of Sargassum is floating near a certain beach, whether beaching events occur depends on winds and tides as well as on regional ocean circulation, most of which are difficult to predict. However, this doesn’t mean one cannot predict the likelihood of potential beaching events for a local region. For example, based on historical data, the west coast of Florida (from the Big Bend region to Naples) is mostly Sargassum free regardless of the Sargassum amount in the Gulf of Mexico. In contrast, once a large quantity of Sargassum is observed along the Loop Current or in the Straits of Florida, beaching events in the oceanside of Florida Keys and along the east coast of Florida are likely to occur in the coming weeks.

Based on satellite observations of Sargassum location and its proximity to shore, the NOAA CoastWatch (Atlantic node) has worked with the USF Optical Oceanography Lab to develop statistics-based Sargassum Inundation Reports (SIRs) with weekly updates, which predicts likelihood of beaching events. In the future, high-resolution satellite imagery, in combination with high-resolution ocean circulation models, can be used to form a forecast system to predict possible beaching events for individual beaches.

In the field, Sargassum can be measured in different ways. Recordings of sightings represent a crude indicator of abundance. Since the 1970s, the Sea Education Association (SEA) conducted systematic surveys of Sargassum in the western Atlantic (including the Caribbean Sea and Gulf of Mexico) several times a year, during which Sargassum abundance was measured using a towed net. A total of 7000 net tows were collected between 1992 and 2015. Sargassum samples are also collected this way or using a dipping net, and then stored and analyzed in the laboratory following community-accepted protocols to determine the tissue contents (particularly nitrogen, phosphorous, iron, carbon, arsenic, among others).

Since 2006, satellite data has been used to observe and quantify Sargassum. Sargassum has different colors from the background ocean water, and such color changes are quantified in every pixel of satellite images, from which Sargassum biomass in every pixel is estimated. Field measurements are used to validate such satellite-based estimates.

The advantage of satellites is their synoptic and frequent observations. There are currently several satellites orbiting around the earth, providing daily observations of every location in the ocean. There are also higher-resolution satellites that provide “zoomed in” views for particular locations, but these satellites typically cover smaller areas (particularly around the coasts) and do not provide daily coverage.

Based on satellite observations, the USF Optical Oceanography Lab has developed and operated a Sargassum Watch System (SaWS) to generate information on Sargassum abundance and location in the tropical Atlantic, Caribbean Sea, and Gulf of Mexico. A user can select a region of interest through a “clickable” map (Imageset 5), and then follow the instructions under “Where the Images are Found”. The SaWS provides daily updates on satellite-observed Sargassum and other ocean variables (e.g., surface temperature, chlorophyll concentration). A user can bring the selected images together with modeled ocean currents to the user’s Google Earth environment to help navigate the images.

Based on the SaWS, the USF OOL also generates Sargassum monthly outlook bulletins and distributes the bulletins to subscribers by the end of the month. All past bulletins can be found under the SaWS page “Bulletins”. Those interested in subscribing to future bulletins may send a request to Dr. Brian Barnes (bbarnes4@usf.edu).

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