What Is Invasive Sargassum, and What Does It Have To Do With Climate Change?
invasive sargassum on a beach

What Is Invasive Sargassum, and What Does It Have To Do With Climate Change?

By Rachel Jaeger

If you explored the ocean, you would find everywhere a large, leafy, brown seaweed called sargassum. There are many kinds of sargassum, but all of them have little round structures called pneumatocysts attached to their leafy branches that are filled with oxygen, giving the seaweed buoyancy. It floats on the surface of the ocean, stretching for miles. There are lots of different types of seaweed in the ocean, but most of it grows from the bottom. Sargassum is the only kind that is holopelagic—it floats in the open ocean for its entire life cycle.

sargassum seaweed

Where can Sargassum seaweed be found?

Though different types of Sargassum seaweed are found throughout every ocean, the area with the most Sargassum is roughly two million square miles of sea east of Bermuda. There is so much Sargassum there that sailors named it the Sargasso Sea and put it on maps. This is also where the Bermuda Triangle myth comes from! So many ships sailed through the Sargassum and got stuck and lost at sea that people began to believe there were mythical monsters in the area.

Sargasso Sea
The Sargasso Sea

The Sargasso Sea

The Sargasso Sea, located in the Atlantic Ocean, is the only sea that is entirely locked by ocean currents rather than land. It has always been a part of the ocean and is a beautiful natural habitat for thousands of marine animals. Dr. Sylvia Earle, an American marine biologist and oceanographer, calls it “the golden rainforest of the ocean.” Endangered turtles and American and European eels, thirty different species of whales and dolphins, apex predators like swordfish, and countless other animals all live, feed, or migrate to the Sargasso Sea because of the rich abundance of the Sargassum seaweed.

The Great Sargassum Belt
Image from Smithsonian Ocean

The Sargassum Belt

Over the past decade, though, sargassum has begun to appear outside of the Sargasso Sea in the Caribbean, the Gulf of Mexico, and the Atlantic Ocean. Scientists believe it stretches from an area between Brazil and Africa, what they have now dubbed the “New Sargasso Sea.” It is the largest bloom of microalgae in the world—the Great Atlantic Sargassum Belt. Scientists first noticed the bloom in 2011, and it has only grown since then. In September 2022, the ocean sargassum count totaled 8.5 million tons, compared to two years before, where it was only 3.5 million tons. The worst year so far was 2018, where the belt reached up to 22 million tons of sargassum spread all the way from the Gulf of Mexico to the coast of Africa.

What is Causing the Invasive Sargassum?

The belt fluctuates based on two nutrient inputs: the Amazon River and the west coast of Africa. Discharge from the Amazon River has always added nutrients to the ocean, but in recent years, increased logging and fertilizer have raised the nutrient levels that the river sends into the ocean. The sargassum uses the fertilizer and grows, spreading into the western Caribbean.

In the winter, upwelling—rising seawater—from the west coast of Africa draws nutrients up from the deep seas to the surface of the water, sending them straight to the sargassum. The dust from the Sahara deposits into the water as well, adding even more nutrients.

There are several other variables aside from the nutrient inputs that fluctuate the sargassum belt as well, such as shifting ocean currents, wind patterns, and warming water temperatures.

Invasive Sargassum’s Negative Effects

Invasive sargassum is not “the golden rainforest of the ocean.” While it is beautiful when it stays where it belongs, too much of the seaweed results in clogged bays and dirty waters—negatively impacting people, plants, and animals.

On people and economies

The sargassum belt negatively impacts people due to its dirtiness and bad smells. It transforms beaches—beautiful, turquoise waves and white sands turn into mucky, brown water and seaweed strewn, dirty stretches. Many of the places where the sargassum beaches—the Caribbean islands, Mexico, and Florida—heavily rely on tourism for their country’s revenue. Beachfront hotels can spend up to sixty-thousand dollars a year cleaning up the sargassum from their beaches. If they don’t put in the money, time, and effort, however, fewer tourists may come to the area. Barbados declared a national emergency in 2018 because of how much the seaweed affected their tourism.

On other species and ecosystems

It also infects mangroves and reefs, as well as clogging habitats so animals cannot move or breathe. It drains oxygen from the water and blocks sunlight, killing meadows of seagrass native to the Caribbean. Also, because of its short life cycle, the large mats of sargassum seaweed will slowly die and sink to the bottom of the sea, smothering and killing the native seagrass underneath.

Removal Techniques

The countries affected by the sargassum belt are trying several different techniques to remove it and regain their pristine beaches. There is no one way that works best; techniques will be better or worse based on the affected area, sargassum amounts, and available funds. Since the sargassum comes from the sea and washes up on the shore, scientists have developed both land and sea removal systems.

Land removal techniques

The simplest way to remove the sargassum from the beaches is to clean it off with manpower. In Mexico, the city councils and hotels hire thousands of temporary workers to clean off the sargassum every time there are large surges in sargassum numbers. Hand raking is cheaper, less intrusive, and less likely to harm wildlife than mechanical or chemical solutions. However, it is also strenuous on the workers and requires hundreds of volunteers or employees to clean even small areas, rendering it inefficient for large mats of sargassum. Also, when the sargassum is removed from the beach it is generally taken to a landfill where it emits methane gas.

Another option for land cleanup is mechanized cleanup. This involves using machinery like tractors, diggers, or specific sargassum collectors to collect the sargassum . Machinery is helpful in cleaning large swaths of the seaweed, and it is easier than manual labor. It is also more dangerous to nature. The machines cause erosion on the coastlines and risk crushing turtle and bird nests. These machines are much better to use for mass sargassum strandings, where manual labor is always preferable for smaller areas.

Water removal techniques

Where possible, it is better to remove the sargassum seaweed from the water close to the shore than it is to clean it from the shore. It avoids removing sand from the shoreline and harming animals that build nests there. Sargassum can also rot when it is in the water, so removing it before it rots keeps the water clean. This technique does have many limitations, though. Boats can only collect the seaweed in very specific places, like enclosed bays, lagoons, or marinas. The currents and ocean swells also disrupt the sargassum collection, so the boats can only go out at certain times. And similarly to land cleanup, when sargassum is removed from the water and taken to landfills it continues to harm the environment.

Sinking Sargassum for Carbon Removal

Another way to remove the invasive sargassum is to sink it to the bottom of the ocean. Not only does this get rid of the seaweed, it helps with climate control, too! As the sargassum seaweed sits on the surface of the water, it takes in CO2 from the atmosphere to use in its photosynthesis. When the sargassum sinks to the bottom of the ocean the carbon in the sargassum is stored there. There, some will flow beneath the currents and stay out of the atmosphere until the currents bring it back up again in hundreds of years. The rest will settle into the silt, forming rocks, locking the carbon out of the atmosphere for hundreds of thousands of years.

Pull To Refresh is working towards climate control and carbon removal by sinking sargassum! We send out solar powered, unmanned vessels into the ocean and sink the carbon to the bottom of the sea, where it will no longer be a concerning part of the carbon cycle. Our vessels help both in controlling the carbon growth issue and in removing the invasive sargassum.

With your help, we can reverse emissions and make Earth sustainable for future life.      

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