Facts First: Explaining Sargassum

Sargassum washed up on Crandon North earlier this summer (Key News/Rumya Sundaram)

If you’ve walked on the beach at all this year, you know what sargassum looks like. But beyond the unsightliness and the smell, what is really happening here? 

What is sargassum?

Sargassum is a type of macroalgae that floats at the ocean’s surface. Each stipe (that is, stalk) and frond can be several meters in length. Many species create small berry-like attachments filled with air, which help them stay afloat.

There are two species of sargassum to consider in the Atlantic and the Sargasso Sea (although there are hundreds of species worldwide):  Sargassum fluitans and Sargassum natans. S. fluitans tends to be the more common species. 

The Sargasso Sea is a vast area of about 2 million square miles off the East Coast of the United States and is a unique habitat for dozens, if not hundreds, of species of marine animals including fish, shrimp, crabs, baby sea turtles, and many others. The sea itself is distinctive in that it has no land barriers and instead is contained solely by ocean currents.

The sargassum problem

Sargassum is not new to beachgoers in South Florida and the Caribbean. It has always been a part of the scenery, and it plays an important role in beach ecology. 

As early as 2012, researchers in the Caribbean started seeing an increase in the amount of sargassum washing up. In 2011 satellite imagery used for spotting algal blooms had revealed a marked increase in sargassum.

By 2015 sargassum was piling up feet deep on Caribbean beaches. This continued to a nadir in 2018, when financial strain for many countries in the Caribbean, Mexico, and cities or counties in Florida, both through potential loss of tourism and the cost of hiring companies to remove it.

Satellite imagery shows sargassum blooms stretching 5,500 miles from the Gulf of Mexico all the way to the western coast of Africa. Although it has been observed for the last 8 years, the size and scale of the bloom, the known as Great Atlantic Sargassum Belt, has been growing. Imagery studied from 2000 through 2018 showed more of the macroalga showing up in new areas in 2011, before which the influx of these large amounts of sargassum were not observed in the Caribbean.

A research team who discovered the bloom, with members from University of South Florida and Georgia Institute of Technology, also determined that its seasonal formation has both natural and unnatural contributing factors. Specifically, it is naturally fed in the winter by upwelling from deep in the ocean off the eastern African coast which feeds nutrients to the sargassum. However, increased nutrient runoff and discharges from the Amazon River from deforestation and fertilizer use, predominantly nitrogen and phosphorus, have been unnaturally feeding the bloom also. Both increased deforestation and fertilizer use in 2010 coincide with the increase in sargassum observed by satellite the following year.

But the problem might be even more complex. 

In July of this year, University of Miami Rosenstiel School of Marine and Atmospheric Science released a study which showed that aerosolized phosphorus from smoke in Africa is also unnaturally feeding nutrients to the Amazon. This smoke comes from biomass burning including “land clearing, brush fires and industrial combustion emission.” The transport of aerosolized African dust from the Sahara Desert depositing nutrients in the Amazon, tropical Atlantic Ocean, and Southern Ocean has been studied for decades. The study shows these newer aerosolized particles from African fires may now make up a major part in the normal movement of airborne nutrients from the African continent (though it did not specifically link the increase in aerosolized phosphorus to the increase in Atlantic sargassum). 

Effects on the environment

Sargassum washing on shore is a natural process. The movement onshore adds to beach stabilization and transport of marine nutrients to terrestrial environments.

The floating mats of sargassum in the water, particularly in the Sargasso Sea, are key habitats and nursery grounds for a number of species. For example, loggerhead sea turtles hatch from their native beaches and make the journey out to the Sargasso Sea where they live, feed and gain protection from sargassum as they grow. A frogfish known as the Sargassum fish, (scientific name Histrio histrio) has evolved to live in floating mats of these macroalga.

There has been some evidence that sargassum buildup on beaches may cause problems in nesting or hatching of sea turtles. During large-scale buildups on nesting beaches in Antigua, fewer or no nests were observed, indicating that the large amounts of sargassum may impede or deter females from nesting in those areas.

Once a nest is hatched, large amounts of sargassum may prevent or hinder baby sea turtles from reaching the sea. They may also get mired in or confused by floating mats which can cause them to wash back to shore or starve. However, not enough research has been done to provide definitive conclusions of the effects on sea turtles yet. So far, in some areas such as Bill Baggs State Park, which does not manually remove or bury its sargassum, there appears to be little effect on hatchlings making their way to the sea.

In other areas of Florida and the Caribbean, there have been reports of large amounts of sargassum causing the deaths of many animals (including sea turtles) by preventing them from coming up to breathe or trapping and entangling them.

One major problem which has been documented are changes in the Caribbean in seagrass meadows. Large mats of macroalga can block out the light, causing photosynthesizing seagrasses to die out and be replaced by other species, changing the seascape entirely. As the sargassum dies and decomposes, it removes oxygen from the water, which can cause fish and other organisms in the area to suffocate.  All of these issues have been shown to change the water quality, negatively impacting some coral species.

Most governments have relied on removal techniques using bulldozers, trucking to landfills, composting when possible, or burying it. However, the costs of these actions can quickly build up into the millions, making them financially prohibitive.

How do you solve a problem like sargassum? 

The current solution of burying it comes with its own pitfalls and may possibly be responsible for some of the high bacteria counts lately – a point made by Kelly Cox of Miami Waterkeeper at the Nov. 5 workshop. 

When the sargassum is exposed to the sun, it dries out and decomposes relatively quickly (although the process is slower when it is piled several feet deep). When it is buried, it decomposes more slowly and remains damp and warm longer, creating a possible breeding ground for enterococci bacteria, which could then leach back into the water, and may exacerbate the bacteria problem that Key Biscayne beaches have been experiencing.

Differences in coastal currents can also dictate how and where sargassum lands. In the case of Key Biscayne, the southern end around Bill Baggs has more flow through, possibly preventing very large amounts from accumulating. The northern end, on the other hand, tends to accumulate quite a bit more (this may also account for the difference in bacterial counts along the East Coast of the Key).

As of now, no removal technique has proven better than any others, and the environmental impacts are still being investigated and researched. 

As many scientists are now saying, sargassum blooms may become the new normal.