Sargassum Basics

Sargassum comes ashore in St Maarten on the 27th June 2019. Photo shows Babbitt Point, Oyster Pond, and Dawn Beach through to Guana Bay.

What is it?

Sargassum is a planktonic (free-floating) seaweed found in the tropics. The Caribbean has been inundated with sargassum since 2015, although the unusual blooms started in 2011. In the Caribbean we find both S. natans and S. fluitans – two species which are holopelagic; they reproduce without ever being attached to the sea floor.

While at sea, sargassum patches are a habitat for many sea creatures. Birds, fish, turtles, and crustaceans use the floating mats for breeding, food, and refuge.

Is it good or bad?

When it comes ashore in small amounts, sargassum is also beneficial. It can provide food for shorebirds and crabs, as well as building up the sand to help prevent coastal erosion. Seaweed mixed into the sand provides a healthy environment for beach stabilizing plants like Railroad Vine (Ipomoea pes-caprae) and Sea Purslane (Sesuvium portulacastrum).

However, when sargassum comes ashore in large mats, it has the potential to become an anaerobic stinking mess. As it breaks down it releases hydrogen sulfide gas and deprives the immediate surroundings of oxygen. The anoxic waters around rotting seaweed can lead to large scale fish die-offs (facebook video). The fresh thick mats also create a difficult environment for mammals and sea turtles who can be come entangled and drown. Even strong swimmers should avoid swimming when waters are thick with sargassum.

How can we monitor sargassum?

The University of South Florida publishes satellite imagery showing the density of sargassum blooms. While they caution against using this imagery to make predictions – their bulletins show a clear year-on-year comparison. For example, the May 2019 bulletin shows less sargassum compared to 2018, but still significantly more compared to other years. December 2019 is showing low levels of sargassum in the Caribbean, but warns that the east tropical Atlantic still shows some blooms.

The St Maarten Nature Foundation also monitors the impact of sargassum on our shores, and frequently warns of impending sargassum invasions.

Want to know more about sargassum?

Economic Effects:

The sargassum creates a number of issues: Economically, sargassum is the bane of tourism: the smell has forced properties to close, the constant cleanup has overwhelmed beach crews, and the rotting seaweed in the surf-line has lead to complaints of itchy rashes and respiratory problems.

The large scale die-offs effect both our coral reefs and our fish stocks.

Heavy Metals

Often mentioned, but not heavily document, is sargassum’s ability to bio-accumulate heavy metals. ANSES makes mention of it here, and you can find a study on the potential uses of sargassum to clean up heavy metals here. Most data is from edible seaweed in the pacific ocean, but a report from Ghana (PDF) lists the available nutrients and heavy metals found in the sargassum found along their coast. This PDF lists heavy metal content for sargassum collected in St Maarten, Bonaire, and a number of other places.

What has caused the current inundation?

Sargassum has been noted in unusual quantities in the Caribbean since 2011, with 2015 being the first major inundation. The 2018 season is forecast to be three times worse than 2015 and 2019 is predicted to be another bloom year. ~~The root cause is up for debate~~, and it is likely a combination of factors including climate change, fertilizer runoff, and changes in ocean currents.

2019 update: The consensus is getting pretty clear. Nutrient rich agricultural runoff from the Amazon and Mississippi rivers (amongst many others), higher sea temperatures, and deforestation in the Amazon basin are all frequently blamed. News sources linked below:

Independent – Climate change blamed as huge mounds of rotten seaweed spoil pristine beaches in Mexico

Keys News – Sargassum is snowballing, scientists say

Research Gate – Satellite Images Suggest a New Sargassum Source Region in 2011

Uses of Sargassum

Given the prevalence of heavy metals, chlordecone, and high sodium content, do your own research before eating seaweed, feeding it to livestock, or using it in your garden.

Research into its use is ongoing.

CNRS mentions that sargassum collected in Guadeloupe and Martinique is contaminated with arsenic and chlordecone, a now-banned toxic insecticide which is a persistent organic pollutant. (More on chlordecone here.)

http://www.cnrs.fr/en/research-project-french-west-indies-repurposing-sargassum-seaweed

ADEME advises against its use in agriculture and nutrition, and advises caution with combustion, biogas production, and composting:

https://guadeloupe.ademe.fr/sites/default/files/etat-perspectives-valorisation-sargasses-en.pdf

DCNA recently published the results of an experiment confirming that heavy metal levels (particularly arsenic) are many times more abundant in plants grown in soil amended with 50% sargassum.

https://dcnanature.org/sargassum-fertilizer/

Edible Sargassum? (Nope!)

Seaweed is considered edible in many cuisines. It is often listed as an acquired taste, but there are many different ways to cook and prepare it.

Seaweed can be very nutritious, and is rich in vitamins A, B, C, and E. It also contains high levels of minerals.

However, reports of heavy metals suggest it is best to stick to established sources of culinary seaweed, not sargassum.

Mulch

Seaweed has be used as mulch, helping to suppress weeds and control moisture evaporation. Apply 6-8″ of rinsed seaweed around salt-tolerant plants. The seaweed will shrink considerably as soon as it dries, so 2-3 applications might be necessary to suppress weeds. However, the heavy metals present in sargassum make it an unattractive mulch.

Fertilizer

Sargassum has an N-P-K reported around 1-10-3. Although low in nitrogen, sargassum still works well as a fertilizer because it is rich in nitrate (NO3) and ammonia (NH4). Some concerns have been raised about the high salt content (and heavy metals), but it has been made into fertilizer in both Barbados (YouTube) and St Lucia (Visit Algas Organic’s facebook page here).

Compost

Seaweed is usually listed with a C:N ration of 19:1 making it a low-nitrogen green. It can be added to compost at roughly the rate of one part seaweed to one part carbon (cardboard, wood chips). Compost made with seaweed reaches active temperatures quickly, but the nitrogen is often used up before the carbon source has even begun to break down. This isn’t an issue with slow composting, but can delay the hot-compost process.

Composting seaweed should not smell or produce excessive hydrogen sulfide gas as long as there are sufficient browns to keep the pile well aerated.

Some regional tests didn’t show significant rise in heavy metal levels when used at amounts less than 10% of the total feed-stock volume.

Keep in mind that any heavy metals introduced to compost can transfer to food grown in that compost, so any rise could have significant negative environmental impacts.

Biogas

Sargassum has a methane potential (PDF) ten times lower (+/- 40) than food waste (+/- 400). It is inefficient compared to other biogas feedstocks. A further hindrance to using sargassum to generate biofuel is collection. Heavy machinery can damage beaches and many countries have bans on harvesting sargassum in open water.

However, anaerobic decomposition of sargassum should not be overlooked as both a way to manage large amounts of sargassum and as a way to generate biofuel. Sargassum, while unpredictable, reaches St Maarten’s shores in significantly larger amounts in summer (the low tourist season) compared to the winter (high tourist season). The seasonal nature of both feedstocks (sargassum and food waste) could potentially balance each other out nicely.

Biogas production may be particularly useful on Caribbean islands when used as a way to reduce organic food waste reaching the landfill. It can also help fragile small island developing states (SIDS) reduce their dependence on fossil fuels and make them more resilient in the face of climate change. Research has also been done combining seaweed with glycerol and waste frying oil.

Care must be taken not to introduce contaminants to the environment where sargassam is processed, as mentioned in the PDF on the valorisation of sargassum.

Small DIY anerobic digesters are not difficult or expensive to make. There are many instructional videos and tutorials available on line.

What else can sargassum be used for?

Researchers in Mexico have developed shoes made from recycled water bottles and seaweed.

Another has made Adobe bricks using sargassum and clay. The bricks have been praised for their insulating properties and the costs they save. Fiber reinforced adobe is not a new concept. A small two bedroom house uses up two tons of fiber, making sargassum an affordable option.

Sargassum is harmful to our tourism product, but innovative ways to use it as a resource are being developed.

How Can We Prevent Future Sargassum Blooms?

Diet:

Research suggests that nutrient rich rivers flowing into the sea is the primary fuel of this Sargassum bloom. The is not a problem only in Brazil, but also a worldwide problem. As the world population grows and demand for food rises, larger areas of land are being deforested and turned over to intensive food production. Our current methods of agriculture, specifically animal agriculture, are not sustainable.

Animal agriculture accounts for 14.5% of global greenhouse gas emissions and uses 70% of agricultural land (source) while only producing a fraction of the food.

For example, 1 acre of land can produce only 150 lbs of beef. That same acre can produce 6,000 lbs of grains, 1,800 lbs of tree nuts, or 19,000 lbs of fruit (source).

The IPCC (Intergovernmental Panel on Climate Change) suggests a shift towards a flexitarian diet for those not willing to give up meat entirely.

Whatever you eat, choose locally grown using sustainable practices wherever possible.

Water Runoff:

Another way we can help keep nutrients in our soil instead of the ocean is by composting our organic waste, and using that compost to enrich the soil. Green spaces planted with compost can filter storm-water pollutants by 60-95%. Compost holds 5x it’s weight in water, helping to prevent flash floods.

Gray water, black water, and waste must also be handled correctly on land and NOT allowed into the ocean.

We are not helpless, but the problem of sargassum is much bigger than us as individuals. Collectively, we can make a change.

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