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Sargassum Seaweed: Friend and Foe

If you have visited St. Croix’s beaches in years past you may have found at least a few that were covered in mounds of seaweed. A massive tide of sargassum, an invasive species of brown algae that does not attach to the ocean floor, sometimes washes up on the island’s shores. In fact, in recent years the entire Caribbean has experienced masses of this smelly brown seaweed inundating its coastlines. While the sargassum is a smelly nuisance for local residents and visitors alike, it does offer some ecological benefits that make it a necessary part of the marine environment.

The sargassum that is invading the Caribbean shores originates in the Sargasso Sea (hence the name ‘sargassum’). The Sargasso Sea, also known as ‘the golden floating rainforest’, is located in the open North Atlantic Ocean near Bermuda and is estimated to hold up to 10 million metric tons of sargassum! Most scientists believe that the rise in water temperatures, coupled with low winds, is the cause of the influx of sargassum. Essentially, as the sea temperature increases, sargassum is more likely to make its way along the currents to the shores of Caribbean beaches.

Sargassum floating along the current off the South Shore of St. Croix

Sargassum is not a welcome sight (or smell) on St. Croix’s normally pristine beaches. In fact, it has been a major hit to the occupancy levels of some of the hotels and resorts on St. Croix, and it is a consistent complaint among the tourism sector. With the recent surge in sargassum here in the Caribbean, it has been found that decomposing sargassum releases hydrogen sulfide gas and ammonia, which can cause unpleasant respiratory, skin, and neurocognitive symptoms in both local residents and visitors. 

To help with the issue here in the US Virgin Islands, Kitty Edwards, director of the Division of Territorial Parks & Protected Areas for the Department of Planning and Natural Resources launched a “citizen science and data gathering tool” called the ‘MyCoast’ app. MyCoast allows users to create their own reports of sargassum sightings and track the response progress. Users can submit a photo and add the location and time utilizing satellites to assist in pinpointing the sargassum, along with any specific comments. If you would like to participate, you can download the MyCoast app to your phone or mobile device through the Google Play or Apple App stores, or use it online at:

All that being said, sargassum is a naturally occurring phenomenon and it is actually vital to the ecosystem. Sargassum is a habitat for over 250 species of fish and invertebrates which use it as shelter, as a feeding ground, and for their nurseries. Here is a list of just some of the benefits of sargassum to wildlife and the marine environment:

  • Sargassum is used by sea turtles for shelter and as a food source. When sargassum is traveling in the ocean currents, it acts as both a shelter and food source for turtle hatchlings who are not strong swimmers yet. Green sea turtles eat sargassum as a major food source and will eat copious amounts of it throughout their lifetimes.
  • Sargassum provides food for migrating whales. Endangered and migratory species of whales, such as Humpbacks, find their food amongst the sargassum as they migrate.
  • Sargassum provides food seabirds and shorebirds. When the sargassum, and the organisms living within it, wash ashore it provides food for pelagic seabirds, shore birds, and pelicans.
  • Sargassum protects the beachfront. The brown algae serves as a buffer on the beach by reducing wave and wind erosion. It also protects the sand on the shores, making it more stable and helping to create sand dunes.

For many of the reasons above, mats of sargassum make excellent dive sites, especially because of the many species of creatures living in them, and the predators that hang around the sargassum looking for food. ‘Diving under sargassum is like diving in another world,’ said Billy Causey, Southeast Regional Director for NOAA’s Office of Marine Sanctuaries. ‘If you can’t get out on a reef, it’s just as good.’ Since the mats of sargassum float on the surface, they are easily accessed by snorkelers as well.

In addition to being an integral part of the natural marine environment, sargassum is often gathered from the shore and used for both practical and medicinal purposes. It is often used as nutrient-rich mulch or compost created by allowing the salt to wash out in the rain and then mixing the sargassum with manure and soil. Sargassum is also used as animal feed, fish bait, and insect repellent. The scientific community is researching the use of sargassum for biofuel and making it into bricks for use as a building material. Sargassum even has the potential for use in wastewater treatment before the release of treated waters into rivers or oceans by: 1) reducing the total nitrogen- and phosphorus-containing compounds of sewage and some agricultural wastes; and, 2) removing toxic metals from industrial wastewater.

Medicinally, Asian medicine makes use of various species of sargassum to treat afflictions such as fever, high cholesterol, thyroid problems, and skin ailments. Being a brown algae, sargassum is rich in carotenes (provitamin A) and is used as a source of natural mixed carotenes for dietary supplements. Aside from being eaten as ‘sea vegetables’ all types of kelp, including sargassum, are also sources of alginate (a compound used for thickening and stabilizing in the food industry).

Next time you visit a beach and find it covered with sargassum, or fly over the ocean and see mats of sargassum floating on the surface, remember that while it can be unpleasant it is a necessary part of our ecosystem. Instead of focusing on the unpleasant nature of this influx of seaweed, let’s focus on the possible benefits and solutions offered by the abundance of sargassum. For example, this is a great opportunity for the residents of St. Croix coordinate the collection and rinsing of sargassum for use in local farming and agriculture. If you are visiting St. Croix and are looking for a beach with little or no sargassum, it is worth noting that (due to oceanic currents) the beaches on the West End and North Shore will generally have less sargassum than those on the East End and South Shore. In any case, be sure to keep in mind that this is a temporary and typically seasonal issue, and it will get better with time and the tides.

– Jennie Ogden, Editor

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