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Two Sargassum Species
Two Sargassum Species
Caribbean Sargassum: Sargassum natans (left) and Sargassum fluitans (right).
University of South Mississippi
Beaching Event
Beaching Event 2015
Sargassum accumulates along Bathsheba Beach on the east coast of Barbados
(courtesy of Romel Hall/Barbados Today)

   SaWS Clickable Map

The Sargassum Watch System (SaWS) is designed to use satellite data and numerical models to detect and track pelagic Saragassum in near-real time.

Pelagic Sargassum seaweed (Image courtesy of Tracy Villareal) is a brown macroalgae floating on the ocean surface. Comprised primarily of two species, S. natans (Image courtesy of Amy Siuda) and S. fluitans (Image courtesy of Amy Siuda), it is abundant in the Intra-Americas Sea (IAS), the Atlantic, and along the coasts of British Isles and mainland Europe. In the ocean it serves as an important habitat for many marine animals as it provides food, shade, and shelter (from predators) to fish, shrimp, crabs, and turtles. Sargassum may serve 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.

However, excessive amounts of Sargassum on beaches in populated areas can cause a lot of problems and they must be physically removed. Sargassum decomposition on beaches smells bad, attracts insects, and causes many environmental problems (e.g., smothering turtle nesting sites, sea turtle mortality, fish kills) and economic problems (e.g., diminished tourism). The beaches along the Texas coast have experienced Sargassum inundation events annually, and since 2001 beaching events have also occurred on many Caribbean shores in nearly every spring and summer. Sargassum beaching events have also been reported in western Africa and northern Brazil. Monitoring Sargassum distribution and abundance in the ocean in a timely fashion is of great importance for studying ocean ecology, helping fishery management, and forecasting Sargassum beaching events. The SaWS is meant to provide such a function through producing and sharing customized satellite imagery in near real-time in a user-friendly way.

You may be interested in these links:

Sea turtles victims of Sargassum seaweed

Sargassum: The What, Where, and Why of this Seaweed

Sargassum is a genus of large brown seaweed (a type of algae) that floats in island-like masses

Gov’t allocates initial $5 million for removal of sargassums

   SaWS Clickable Map

The table below shows the four satellite sensors that are currently used in SaWS, with the first three sharing similar characteristics.

Sensor Revisit Frequency Spatial Resolution (m) Spectral bands for
floating algae (nm)
Spectral bands for
color index
Period
MODIS/Terra Near daily 1000 667, 748, 869 469, 555, 645 2000 — present
MODIS/Aqua Near daily 1000 667, 748, 869 469, 555, 645 2002 — present
VIIRS Daily 750 671, 745, 862 443, 551, 671 2012 — present
Landsat 8 OLI 16-day 30 655, 865, 1609 443, 561, 655 2013 — present

The raw satellite data are provided by the U.S. NASA and USGS. These data are downloaded and processed through a Virtual Antenna System using both standard and customized algorithms. Of particular importance are the two customized data products, namely the floating algae index (FAI) to detect floating algae and other materials on the ocean surface, and the color index (CI) to trace ocean circulation features. Floating algae often appears in images as slicks over the relatively homogeneous background in the FAI imagery, while ocean circulation patterns may be inferred from the various eddy and plume features in CI imagery. Two examples are given below.

Examples of Satellite Imagery used in SaWS
Floating Algae Index Ocean Circulation CI
Floating Algae Index (left) reveals algae and Color Index (right) shows circulations patterns

   SaWS Clickable Map

SaWS covers the entire Intra-Americas Sea. To facilitate visualization and navigation, the imagery products are divided into different geographic regions, for example Central Atlantic, Eastern Caribbean, Central East Atlantic, Gulf of Mexico, etc. The "SaWS Clickable Map" at top of this paragraph shows the current coverage, where any region can be clicked to open a separate page unique to that region. A user may then select a specific date on the calendar, or use the “animate” function to browse through the image sequence quickly to determine the date of interest. The most useful image type may be the “FA_UNET_DENSITY”, which shows the Sargassum density over the past week. All images can be brought to the user’s Google Earth, together with ocean surface currents as a separate data layer. A screenshot of a typical web interface is presented below.

More information can be found in this README file.

Virtual Antenna Web Interface
Virtual Antenna Web Interface
The information of each image type can be obtained by clicking on the “Information” link

   SaWS Clickable Map

Under each images icon in the VAS (previous tab) there is a small GE button in the lower right corner. When this is clicked, a KML file is generated for that image. In addition, surface currents from the Hybrid Coordinate Ocean Model (HYCOM — from the National Ocean Partnership Program (NOPP)) are obtained, updated nightly, and made available via the VAS as a layer within Google Earth. Thus, both the image and surface currents can be displayed in Google Earth with a simple mouse click (see example below), thereby facilitating visualization and navigation. Within Google Earth, once a Sargassum raft is identified with latitude and longitude, current speed and direction (available through a mouse click over the current vector) near the raft can be used to predict the movement of the raft and a possible beaching time, in essence, forming an early warning system.

Ocean Currents Vectors Integrated with Satellite Imagery in Google Earth
Google Earth Interpretation
A KML file is created to provide Google Earth the image and ocean current data for visualization

   SaWS Clickable Map

Sargassum has impacted the Caribbean in so many ways. Here we show a few photographs that reveal the serious nature of the problem.

Capesterre Guadeloupe
Capesterre
Airborne survey during spring and summer 2015
(photo courtesy of Jean-Philippe Maréchal)
Desirade Guadeloupe
Desirade
Airborne survey during spring and summer 2015
(photo courtesy of Jean-Philippe Maréchal)
Martinique
martinique_2015a
Sargassum Beaching Event 2015
(photo courtesy of Jean-Philippe Maréchal)
Martinique
martinique_2015b
Sargassum Beaching Event 2015
(photo courtesy of Jean-Philippe Maréchal)
Sailing Vessel Skipping Stone
Capesterre
Stranded boat in a Sargassum bloom off Admiralty Bay, Bequia, Grenadines
June 2015 (Photo credit: Sailing Vessel Skipping Stone)

  SaWS Clickable Map

Brooks MT, Coles VJ, Hood RR, Gower JFR (2018) Factors controlling the seasonal distribution of pelagic Sargassum. Mar Ecol Prog Ser 599:1–18. https://doi.org/10.3354/meps12646

Dierssen, H. M., A. Chlus, and B. Russell (2015). Hyperspectral discrimination of floating mats of seagrass wrack and the macroalgae Sargassum in coastal waters of Greater Florida Bay using airborne remote sensing. Remote Sens. Environ., 167:247-258. https://doi.org/10.1016/j.rse.2015.01.027

Feagin, R. A. and A. M. Williams (2010). Sargassum: Erosion and Biodiversity on the Beach, Spatial Sciences Laboratory, Dept. Ecosystem Science & Management, Texas A&M University, pp23.

Franks, J., D. R. Johnson, D. S. Ko, G. Sanchez-Rubio, J.R. Hendon, and M. Lay (2011), Unprecedented Influx of pelagic Sargassum along Caribbean island coastlines during summer 2011, Proc. Gulf Caribb. Fish. Inst., 64:6-8. https://nsgl.gso.uri.edu/flsgp/flsgpw11001/papers/006.pdf

Godínez-Ortega et al. (2021). A Natural History of Floating Sargassum Species (Sargasso) from Mexico. https://doi.org/10.5772/intechopen.97230

Gower, J., C. Hu, G, Borstad, and S. King (2006), Ocean color satellites show extensive lines of floating Sargassum in the Gulf of Mexico, IEEE Trans. Geosci. Remote Sens., 44, 3619–3625. https://doi.org/10.1109/TGRS.2006.882258

Gower, J., E. Young, E., and S. King (2013), Satellite images suggest a new Sargassum source region in 2011, Remote Sens. Lett. 4, 764–773. https://doi.org/10.1080/2150704X.2013.796433

Hardy, R. F. (2014). Assessment of surface-pelagic drifting communities and behavior of early juvenile sea turtles in the Northern Gulf of Mexico. MS Thesis, University of South Florida, 118pp. https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=6561&context=etd

Hu, C (2009), A novel ocean color index to detect floating algae in the global oceans, Remote Sens. Environ., 113, 2118–2129. https://doi.org/10.1016/j.rse.2009.05.012

Hu, C., B. B. Barnes, B. Murch, and P. Carlson (2014), Satellite-based virtual buoy system (VBS) to monitor coastal water quality, Optical Engineering, 53, 051402. https://doi.org/10.1117/1.OE.53.5.051402

Hu, C., L. Feng, R.F. Hardy, and E. J. Hochberg (2015), Spectral and spatial requirements of remote measurements of pelagic Sargassum macroalgae, Remote Sens. Environ., 167, 229-246. https://doi.org/10.1016/j.rse.2015.05.022

Hu, C., et al. (2016). Sargassum watch warns of incoming seaweed, Eos, 97(22):10-15. https://doi.org/10.1029/2016EO058355

Hu, C. et al. (2021). On the Atlantic pelagic Sargassum's role in carbon fixation and sequestration. Science of the Total Environment, 781, 146801. https://doi.org/10.1016/j.scitotenv.2021.146801

Huffard, C. L., S. von Thun, A. D. Sherman, K. Sealey, and K. L. Smith Jr. (2014). Pelagic Sargassum community change over a 40-year period: temporal and spatial variability. Mar. Bio, 161:2735-2751. https://doi.org/10.1007/s00227-014-2539-y

Johns, E. et al. (2020). The establishment of a pelagic Sargassum population in the tropical Atlantic: Biological consequences of a basin-scale long distance dispersal event. Progress in Oceanography, 182:102269. https://doi.org/10.1016/j.pocean.2020.102269

Lapointe, B. E., L. E. West, T. T. Sutton, and C. Hu (2014), Ryther revisited: nutrient excretions by fishes enhance productivity of pelagic Sargassum in the western North Atlantic Ocean, J. Exp. Mar. Bio. Ecol. 458:46-56. https://doi.org/10.1016/j.jembe.2014.05.002

Lapointe et al. (2021). Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin. Nature Communications. 12:3060. https://doi.org/10.1038/s41467-021-23135-7

Maurer, A. S., et al.(2021). The Atlantic Sargassum invasion impedes beach access for nesting sea turtles. Climate Change Ecology, 2, 100034. https://doi.org/10.1016/j.ecochg.2021.100034

Milledge, J. J., B. V. Nielsen, and D. Bailey (2015). High-value products from macroalgae: the potential uses of the invasive brown seaweed, Sargassum muticum. Reviews in Environmental Science and Bio/Technology. 15:67-88. https://doi.org/10.1007/s11157-015-9381-7

Modestin, E., et al. (2022). Arsenic in Caribbean bivalves in the context of Sargassum beachings: a new risk for seafood consumers. Environ. Monit. Assess. 194, 1–20. https://doi.org/10.1007/s10661-022-10230-5

Olguin-Maciel, E., et al. (2022). Environmental impact of Sargassum spp. landings: an evaluation of leachate released from natural decomposition at Mexican Caribbean coast. Environ. Sci. Pollut. Res. 29, 91071–91080. https://doi.org/10.1007/s11356-022-22123-8

Parr, A.E. (1939), Quantitative observations on the pelagic Sargassum vegetation of the western North Atlantic, Bull. Bingham Oceanog. Coll., Peabody Museum of Natural History, Yale University, 6(7): 1-94. Oceanography 28(3):8–10

Resiere, D., et al. (2021). Sargassum seaweed health menace in the Caribbean: clinical characteristics of a population exposed to hydrogen sulfide during the 2018 massive stranding. Clin. Toxicol. 59, 215–223. https://doi.org/10.1080/15563650.2020.1789162

Rodríguez-Martínez et al. (2021). Element concentrations in pelagic Sargassum along the Mexican Caribbean coast in 2018-2019. PeerJ 8:e8667. https://doi.org/10.7717/peerj.8667

Rodriguez-Martinez et al. (2022). Spatio-temporal variability of pelagic Sargassum landings on the northern Mexican Caribbean. Remote Sensing Applications: Society and Environment. 27, 100767. https://doi.org/10.1016/j.rsase.2022.100767

Sargasso Sea Commission (link: http://www.sargassoseacommission.org/publications-a-news/atlantic-sargassum-belt)

Skliris, N., et al. (2022). Physical drivers of pelagic sargassum bloom interannual variability in the Central West Atlantic over 2010–2020. Ocean Dynamics, 72:383-404. https://doi.org/10.1007/s10236-022-01511-1

Smetacek, V., and A. Zingone (2013). Green and golden seaweed tides on the rise, Nature, 504:84-88. https://doi.org/10.1038/nature12860

The Journey of the Sargassum forum (link: https://ambergriscaye.com/forum/ubbthreads.php/topics/557467/the-journey-of-the-sargassum.html)

Trinanes, J., et al. (2021). Monitoring pelagic Sargassum inundation potential for coastal communities. J. Operational Oceanography, 16(1), 48-59. https://doi.org/10.1080/1755876X.2021.1902682

van Tussenbroek, B.I., et al. (2017). Severe impacts of brown tides caused by Sargassum spp. on near-shore Caribbean seagrass communities. Mar. Pollut. Bull. 122, 272–281. https://doi.org/10.1016/j.marpolbul.2017.06.057

Wang, M., and C. Hu (2016). Mapping and quantifying Sargassum distribution and coverage in the Central West Atlantic using MODIS observations. Remote Sens. Environ., 183:356-367. http://dx.doi.org/10.1016/j.rse.2016.04.019

Wang, M., and C. Hu (2017), Predicting Sargassum blooms in the Caribbean Sea from MODIS observations, Geophys. Res. Lett., 44, 3265–3273. https://doi.org/10.1002/2017GL072932

Wang, M. et al. (2019) The Great Atlantic Sargassum Belt. Science, 365: 83 – 87. https://doi.org/10.1126/science.aaw7912

Webster, R. K., and T. Linton (2013), Development and implementation of Sargassum Early Advisory System (SEAS), Shore and Beach, 81(3): 1 – 6. http://www.sargassoseacommission.org/storage/Webster_et_linon_2013_1.pdf

Witherington B., H. Shigetomo, and R. Hardy (2012), Young sea turtles of the pelagic Sargassum-dominated drift community: habitat use, population density, and threats, Mar Ecol Prog Ser 463: 1-22. https://doi.org/10.3354/meps09970

   SaWS Clickable Map

Disclaimer

The information bulletin is meant to provide a general outlook of current bloom condition and future bloom probability for the Caribbean Sea and Gulf of Mexico. By no means should it be used for commercial purpose, or used for predicting bloom conditions for a specific location or beach. The authors of this bulletin, as well as USF and NASA, take no responsibility for improper use or interpretation of the bulletin. Any use of the image, data, or graph from this bulletin page in reports or publications should obtain permission from the USF OOL group.

This page provides links to Sargassum forecast or outlook bulletins (see bottom of this page), which are updated routinely. The bulletins started in early February 2018, and they are meant to give an outlook of Sargassum bloom probability in certain regions in the upcoming months, based on bloom conditions in previous months, and previous years.

Sargassum blooms in the Caribbean Sea

Since 2011, large amounts of Sargassum seaweed appeared in the Caribbean Sea every summer except 2013, creating many environmental, ecological and economic problems in many regions. The seaweed originated from the tropical Atlantic, and is believed to be a result of climate variability and other natural and unnatural processes. Based on satellite observations and statistics of historical events, in early February 2018 the Optical Oceanography Lab developed the first 1-page Sargassum outlook bulletin for the Caribbean Sea. Since then, the bulletins have been generated and distributed to subscribers by the last day of the month. These monthly bulletins are also made available at the bottom of this page. Further readings are also available under the Reference page.

In February 2024, the Caribbean Sea and Gulf of Mexico continued to be largely Sargassum free, but the eastern Caribbean Sea will likely experience increased Sargassum amount in March. Further details can be found in the attached bulletin.

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Sargassum Outlook Bulletin, February 2024

Sargassum Outlook Bulletin, January 2024

Sargassum Outlook Bulletin, December 2023

Sargassum Outlook Bulletin, November 2023

Sargassum Outlook Bulletin, October 2023

Sargassum Outlook Bulletin, September 2023

Sargassum Outlook Bulletin, August 2023

Sargassum Outlook Bulletin, July 2023

Sargassum Outlook Bulletin, June 2023

Sargassum Outlook Bulletin, May 2023

Sargassum Outlook Bulletin, April 2023

Sargassum Outlook Bulletin, March 2023

Sargassum Outlook Bulletin, February 2023

Sargassum Outlook Bulletin, January 2023

Sargassum Outlook Bulletin, December 2022

Sargassum Outlook Bulletin, November 2022

Sargassum Outlook Bulletin, October 2022

Sargassum Outlook Bulletin, September 2022

Sargassum Outlook Bulletin, August 2022

Sargassum Outlook Bulletin, July 2022

Sargassum Outlook Bulletin, June 2022

Sargassum Outlook Bulletin, May 2022

Sargassum Outlook Bulletin, April 2022

Sargassum Outlook Bulletin, March 2022

Sargassum Outlook Bulletin, February 2022

Sargassum Outlook Bulletin, January 2022

Sargassum Outlook Bulletin, December 2021

Sargassum Outlook Bulletin, November 2021

Sargassum Outlook Bulletin, October 2021

Sargassum Outlook Bulletin, September 2021

Sargassum Outlook Bulletin, August 2021

Sargassum Outlook Bulletin, July 2021

Sargassum Outlook Bulletin, June 2021

Sargassum Outlook Bulletin, May 2021

Sargassum Outlook Bulletin, April 2021

Sargassum Outlook Bulletin, March 2021

Sargassum Outlook Bulletin, February 2021

Sargassum Outlook Bulletin, January 2021

Sargassum Outlook Bulletin, December 2020

Sargassum Outlook Bulletin, November 2020

Sargassum Outlook Bulletin, October 2020

Sargassum Outlook Bulletin, September 2020

Sargassum Outlook Bulletin, August 2020

Sargassum Outlook Bulletin, July 2020

Sargassum Outlook Bulletin, June 2020

Sargassum Outlook Bulletin, May 2020

Sargassum Outlook Bulletin, April 2020

Sargassum Outlook Bulletin, March 2020

Sargassum Outlook Bulletin, February 2020

Sargassum Outlook Bulletin, January 2020

Sargassum Outlook Bulletin, December 2019

Sargassum Outlook Bulletin, November 2019

Sargassum Outlook Bulletin, October 2019

Sargassum Outlook Bulletin, September 2019

Sargassum Outlook Bulletin, August 2019

Sargassum Outlook Bulletin, July 2019

Sargassum Outlook Bulletin, June 2019

Sargassum Outlook Bulletin, May 2019

Sargassum Outlook Bulletin, April 2019

Sargassum Outlook Bulletin, March 2019

Sargassum Outlook Bulletin, February 2019

Sargassum Outlook Bulletin, January 2019

Sargassum Outlook Bulletin, December 2018

Sargassum Outlook Bulletin, November 2018

Sargassum Outlook Bulletin, October 2018

Sargassum Outlook Bulletin, September 2018

Sargassum Outlook Bulletin, August 2018

Sargassum Outlook Bulletin, July 2018

Sargassum Outlook Bulletin, June 2018

Sargassum Outlook Bulletin, May 2018

Sargassum Outlook Bulletin, April 2018

Sargassum Outlook Bulletin, March 2018

Sargassum Outlook Bulletin, February 2018

Sargassum Outlook Bulletin, January 2018

   SaWS Clickable Map

Frequently Asked Questions

Because of increased inquiries on Sargassum, the Optical Oceanography Lab at the University of South Florida College of Marine Science prepared the following FAQ page to address common questions in simple language. By no means is it meant to be comprehensive. Interested readers are encouraged to read the references at "Further Readings".

What is Sargassum?

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.”

Two Sargassum Species

Imageset 1. The two dominant pelagic Sargassum species in the Atlantic Ocean (photo credit: Amy Siuda).

Sargassum raft, mat, and clumps

Imageset 2. Different forms of Sargassum in the ocean. Top left: Sargassum raft in the Gulf of Mexico (photo credit: Tracy Villareal). Top right: Sargassum mat and clumps in the Gulf of Mexico, together with a 1m x 1m quadrate to estimate the size of the mat (photo credit: Sam Bunson and Sarah Sullivan). Bottom: video from a cruise survey in June 2022, eastern Gulf of Mexico (video credit: Sam Bunson and Sarah Sullivan)

Where?

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.

Sargassum maps before and after 2011

Imageset 3. Sargassum maps showing in a typical year before (top) and after (bottom) 2011, which marks the first year of the Great Atlantic Sargassum Belt. The color scale is from 0.00‰ to 0.10‰.

When?

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.

Why do we have so much Sargassum in recent years? Is it due to climate change or human activities?

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.

Is Sargassum good or bad?

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.

Sargassum on beaches and in coastal waters
Sargassum on beaches and in coastal waters

Imageset 4. Sargassum on beaches and in coastal waters. Smathers Beach, Key West, Florida, March 5, 2023 (top, photo credit: Brian Lapointe), July 20, 2021 (middle left, the brownish water is due to Sargassum leachate, photo credit: Rachel Brewton). Middle right: Orange Beach, May 25, 2022, Alabama (video credit: Debbie Williams). Bottom: Cayman Islands (video credit: Sandy Hill).

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.

Is Sargassum toxic or dangerous?

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.

Which beaches will be impacted the most and when?

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.

How is Sargassum measured?

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.

Where can I get the most recent information on Sargassum locations?

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.

Screenshot of clickable map

Imageset 5. Clickable map in the Sargassum Watch System (SaWS, https://optics.marine.usf.edu/projects/saws.html). A mouse click over a boxed region will launch the Website for that region, from which Sargassum maps can be displayed in a Web browser or in Google Earth.

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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