Atlantic, Caribbean Sea and the Gulf of Mexico: Major Seaweed Invasion Underway

Over recent years, quantities of Sargassum, a type of seaweed commonly known as sargasso, have continued to wash up on beaches and appear in the waters of the Atlantic, the Caribbean Sea and the Gulf of Mexico. Based on satellite imagery and reports from citizens, this year the impacts can be expected to be the worst ever.

Published 6 years ago, updated 5 years ago

Our thanks to for this interesting article.

Major Seaweed Invasion Underway – What We Can Do About It?

The 2015 season was the heaviest sargasso inundation on record, with the years of 2016 and 2017 showing a downturn. Satellite imagery, new online tracking systems and reports from citizens, however, show that this year the impacts can be expected to be even worse.

This article offers suggestions for how best to clean-up affected beaches, along with ways of detecting the hydrogen sulfide gas given off by the decomposing weed and warnings on possible health impacts.

Major infestations of sargasso were reported in Barbados, in the Grenadines, and in Bonaire as early as February this year, and islands to their north, including St. Lucia and Martinique, are starting to see mats and patches of the weed. Their windward beaches are becoming filled with rotting material off-gassing hydrogen sulfide, which can be a health risk.

The Gas in Sargasso

Since 2015, taking the risk seriously, the Government of Martinique has established an online website with updates on weed volumes and hydrogen sulfide levels, an excellent example for other islands. Two parts of hydrogen sulfide per million can cause spontaneous abortion in a pregnant woman; 100 PPM is deadly, according to Martinique’s website.

The US Occupational Safety and Health Administration states that prolonged exposure to three to five parts of hydrogen sulfide per million may cause effects such as nausea, tearing of the eyes, headaches, and loss of sleep. Asthma sufferers may experience airway problems.

Tarnishing of metals and discoloration of paint has also been reported in areas affected by a sargasso influx and the subsequent release of hydrogen sulfide. One 2015 report cited (“silver tarnishing in days, not months”) as an illustration of how concentrated the gas is, even away from the beaches…. “we lost over $10,000 of electronic equipment solely through hydrogen sulfide damage.”  Another commented about the “appalling, nauseating smell, the damage to health, electronics, metals, and paint that occurred last time there was an inundation of this magnitude.”

The gases in water can destroy reverse osmosis desalination systems’ membranes and filters as well. Virgin Gorda lost its desalination system, located in Spanish Town, in the 2015-2016 sargasso season.

Tourism finds major problems with shorelines being fouled with off-gassing weed, sometimes with a smell strong enough to sicken beach-goers.

Other Problems with Sargasso

Environmentalists find turtle-nesting beaches covered with thick strands of weed, trapping hatchlings and adult turtles alike, and preventing female turtles from getting onto the sand to lay their eggs. Offshore, the weed is friendlier, and fish and sealife travel and live in its strands.

However, yachting and shipping interests recognize the problems this weed causes with propeller fouling, raw-water intake blockage and loss of steerage. Boaters should also be aware that mats of Sargasso accumulate debris, such as floating nylon ropes, nets, and other trash.

Another concern is the potential for this seaweed to accumulate heavy metals or other toxic materials. New testing, currently in progress with the University of South Florida, is obtaining samples and processing for this type of contamination. However, as samples are just now being provided, it will take time to analyze and determine what, if any, issues there are.

Sargasso Tracking and Prediction

The excessive growth of sargasso in 2018 seems grim. Researchers from the University of South Florida and NASA tell us, “The past months of January and February 2018 showed the largest bloom in the central West Atlantic, as compared to the same months in history.” USF researcher Dr. Chuanmin Hu warns of the likelihood of “major beaching events this year as it is more and more becoming a reality — the total amount of sargasso we have seen in February (usually a low month) has now exceeded the maximum month of July/August of 2015.”

To give an idea of the possible impact areas, the above image shows the Sargasso (blues) with projected pathways (yellow) expected for May 2018. The satellite image shows weeds off South America, and you can see Trinidad in the lower left corner, with the Leeward and Windward Islands in the direct path of the projections into the Caribbean Sea and points to the west and northwest.

Over the past several years, several research facilities have been developing technologies to identify the location of weed masses based on satellite imagery. For example, see

Dr. Hu adds “a warning system, similar to tracking weather systems by satellite, a Sargassum Watch System (SaWS), is found at SaWS are now online to distribute daily images with surface currents, allowing viewers and researchers to visually estimate sargasso aggregation and movement direction… if there is a large sargasso aggregation in a ‘hotspot’ in the Atlantic in February, we can predict that there would be major blooms in the Eastern Caribbean in spring and summer. So this prediction gives at least two months of lead time to respond to bleaching events.”

The sargasso’s life cycle and route through the ocean are not yet well understood, but the volume of the weed seems to be increasing, and reports of sargasso sightings are important for researchers. Various theories explaining the faster growth of this floating weed include increasing weather extremes of heat, and/or additional nutrients introduced into the Atlantic that creates or enhance massive blooms. Recent research also suggests that Atlantic seaweeds respond positively to increased carbon dioxide and acidity in seawater. (This is a major change from the former theory of slower growth in a more acidic marine environment.)

Thanks to supporting research by Jim Franks of the Gulf Coast Research Laboratory of the University of Southern Mississippi, and the forecasting of sargasso movements developed by oceanographer Dr. Don Johnson, the migration paths and weed patterns are becoming better known; ongoing research will allow a better understanding of variation and aggregation patterns.

The University of South Florida is now generating weekly algae density maps. See:

What Can We Do?

What can be done in the case of a major influx of Sargasso?

Areas in the impact zones should monitor the situation daily, track hydrogen sulfide levels, and use best practices for cleaning target beach areas. Reports to researchers should be made. This should all be part of an overall approach to protect citizens, visitors, wildlife and environments affected by this overgrowth of seaweed.

Report Sargasso Sightings

Developing sargasso predictions has required on the ground, and in this case in the sea, knowledge of where volumes of weed were by providing the location (latitude and longitude), date and time, and photographs for comparison of affected waters. This allowed comparison to satellite data to develop suitable applications. For several years, private citizen scientists and cruisers have reported their sargasso findings to the Gulf Coast Research Laboratory’s Sargassum research group.

Continuing reports are critical to these efforts, and cruisers, as well as concerned citizens, can report their findings.

To report sargassum sightings to visit

Use Gas Monitors for Safety

Because of the dangers of hydrogen sulfide gas coming from the Sargasso as it rots, governments, employers, and individuals could use personal gas monitors to chart gas levels. These monitors cost about US$100 and are available online. For example, see BW Technologies BW Clip-BWC2-H-Single-Gas H2S Monitor.

Employ Best Beach-Clearing Practices

Employ best practices for keeping beaches clear of large volumes of Sargasso. Clearing beach areas take an organized approach to clear the weed and dispose of it safely. In several places, the business sector is attempting to keep harbors and beaches free of weed, using a daily pickup approach. An information paper by Emma Doyle and Jim Franks (2015, Sargassum Fact Sheet, Gulf, and Caribbean Fisheries Institute) describes the best ways to clear beaches, with monitoring for sea life; care for turtle and bird nesting areas, and without the use of heavy machinery. Key provisions from the Fact Sheet follow.

• Beach cleaning should be done only in the presence of monitors who check for wildlife prior to any cleaning, and operators must respect no-go areas such as sea turtle or bird nests;

• Patience is required, and be aware that it’s not necessarily desirable to clean beaches that are already facing a precarious erosion situation, that is essential habitat for sea turtle nesting, or where grooming will increase wind-blown sand and worsen erosion;

• Removal of sargasso should be from and to agreed areas only, and equipment should use the same route onto and off the beach to prevent harming dunes, destroying dune vegetation and turtle or bird nests;

• There is a difference between achieving a naturally clean beach and an over-sanitized beach — constant grooming of the beach for regular maintenance or for aesthetic purposes is discouraged due to very real risks of worsened beach erosion from physical damage of machinery and unintended removal of sand;

• Least intrusive practices are preferred — hand raking is preferable to machinery, beach-raking equipment with a perforated conveyor belt is preferable to heavy construction equipment, and heavy tracked equipment like road graders are prohibited. Front loaders must utilize a bucket-level control indicator/float mechanism to prevent gouging of the beach;

• A mechanized beach rake can remove moderate quantities of sargasso on dry sand. When exceptional amounts occur (i.e. in excess of three feet deep) then removal of just the upper layers of sargasso first with a front loader, without touching sand, can be followed by mechanized beach raking in order to reduce sand loss;

• Cleaning should always occur at low tide and heavy equipment should stay on wet sand in the tidal zone. Adjust cleaning schedules to when wind and storms are less likely to immediately bring new influxes;

• Consider public safety and avoid mechanical beach cleaning in the presence of fishers or beach-goers;

• In embracing the challenge of Sargasso, good communications between agencies and the private sector, with the press, and with locals and visitors is essential. Make sure everyone knows where clean or less-affected beaches can be found.

For more information contact: [email protected].

Key information on sargasso can also be found at as well as regional cooperation efforts at online forums and meetings at

Related content

The Atlantic Ocean – The Sargassum Phenomenon is a Serious Issue for Atlantic sailors

Atlantic Sargasso Weed: On-going Research Update

Sargasso Weed Infestation Research – Citizen Science for Transatlantic Sailors

Reporting Site: Pelagic Sargassum in the Caribbean – 2018

Caribbean: Tracking Sargassum Seaweed ‘Invasions’ via Satellite

Read and Post Related Comments

Related to the following Cruising Resources: , , ,

You must Login or Register to submit comments.

  1. April 30, 2018 at 9:00 AM
    Data Entry5 says:

    s/v Full Monty on Twitter – @svFullMonty

    ‏Apr 29, 2018
    We saw 100’s of miles of sargasso as we sailed from Panama to Jamaica. It wreaks havoc on engine intakes, makes fishing difficult, and stains the deck where it washes onboard. Have read that this is the worst it’s ever been.