On The Trail Of Explosive Seaweed Blooms

You’ve probably heard of the Sargasso Sea – it is well-known for the floating seaweed called Sargassum that provides a habitat for baby sea turtles and many other sea critters. Floating in the Atlantic Ocean just off the east coast of North America, it’s also the region where the European and American eels mate, a process that scientists still don’t fully understand after centuries of research.

For the last 10 years, a phenomenon has occurred in the Atlantic where never-ending masses of Sargassum inundate beaches after uncharacteristically large blooms occurred. The Sargassum originates from the nutrient-poor waters of the North Equatorial Recirculation Region off the west coast of Africa and spreads throughout the Atlantic Ocean and adjacent ocean basins, affecting the Caribbean, states surrounding the Gulf of Mexico, South America and even Africa. Tom Theirlynck  is a marine biologist, currently working on his PhD at the Royal Netherlands Institute for Sea Research (NIOZ) and University of Amsterdam (IBED-FAME), and he as part of the Amaral-Zettler (NIOZ/UvA) research group is studying the excessive Sargassum blooms in detail. 

Eva Paulus: First things first: What is Sargassum, and why is it currently a hot topic?

Tom Theirlynck, Royal Netherlands Institute for Sea Research (TT):

Sargassum is a genus of brown algae with over 350 species, of which only two are free-floating during their whole life cycle: Sargassum natans and Sargassum fluitans. Floating Sargassum can be split into multiple morphotypes that can be differentiated genetically. Since 2011, we are noticing a huge increase in Sargassum blooms throughout the Atlantic and adjacent ocean basins, which ends up inundating beaches along the coasts of the Gulf of Mexico, Florida, the Caribbean Islands, the western coast of Africa, and northern Brazil. Sargassum is asexual and reproduces rapidly by fragmenting – it breaks apart and the two halves grow independently from one another. While small accumulations of Sargassum have always been there – even Columbus made a note of this in his journals – the current extent of the blooms is something we’ve never seen before.  

EP: How come this is happening now?

TT: Sargassum is really opportunistic, and the blooms seem to happen when sea surface temperatures are high and there are excess nutrients in the water, but this is not fully understood yet. Climate change definitely has an impact, but deforestation also causes nutrient runoff from land that ends up directly in the oceans or in rivers, such as the Amazon or Congo rivers, and gives Sargassum a perfect opportunity to bloom. In addition, researchers at the NIOZ are monitoring Saharan dust which gets blown over the Atlantic, and is also hypothesized to be an important factor relevant for these blooms.

EP: The Sargasso Sea is well-known for being a nursery for many species, such as sea turtles. Are there any positive effects of these blooms?

TT: Sargassum has definitely received a lot of negative PR recently, especially when it piles up on beaches. But in the open water, it is a very unique ecosystem, and a lot of biodiversity is found within the floating Sargassum carpets. More than just baby sea turtles live there! During a recent research cruise, we used a GoPro to look underneath the floating masses of seaweed, and just by eye you can see a huge diversity of fishes, crustaceans and other pelagic organisms that thrive in this environment. Biodiversity-wise, they’re similar to coral reefs actually, as hotspots of biodiversity in marine environments. In addition, the Sargassum in the open ocean is a very effective carbon sink, which is relevant in the face of climate change. When it dies off, it slowly sinks into the deep sea, where it is an important part of the food web.

EP: Tell us more about the research cruise, what did you do exactly?

TT: On the RV Pelagia in 2019, at the beginning of my PhD, we started our trip in the Azores Islands about 1500 km off the coast of Portugal and followed the path of Sargassum – we first encountered it in the subtropical regions right in the middle of the Atlantic, close to Cape Verde, and then went all the way to Guadeloupe in the Caribbean, where the Sargassum ends up stranded on the beaches. It was really cool, since we were able to spontaneously follow the Sargassum, using a combination of lookouts directly on the ship and advice from our collaborators in Florida who were tracking the Sargassum with remote sensing to help us identify Sargassum accumulations.

We were also able to set up a small molecular biology lab on the ship, so we could extract DNA directly and determine which of the species of Sargassum we were dealing with. They are very difficult to distinguish morphologically, even when using a microscope, so having the genetic backup information was very important for my experiments on the ship.

EP: What are some of the effects of this Sargassum in the shallower waters?

TT: So once the Sargassum spreads to the coastal regions, it actually becomes very harmful. It’s well adapted to the open water ecosystem, but in the coastal waters it decays rapidly and drowns out the sun for coral reefs, seagrass beds and other organisms that rely on the sun for photosynthesis. Coral reefs are already under a lot of stress and adding Sargassum to the mix has caused huge die-offs of coral already, most recently in Mexico. When it decomposes, it also causes deoxygenation events in the water, which literally suffocates all other living organisms in the water. The decaying Sargassum also causes coastal decay on the beaches it ends up on, which in turn can be harmful for animals and also humans.

Coral reefs like this one could be threatened by the vast sheets of Sargassum which cut their access to light off (Image Credit: National Marine Sanctuaries, Public Domain Mark 1.0)

EP: So once these strandings occur and the Sargassum ends up on the beaches, is there anything useful we can do with it?

TT: There is some research that we are involved with, for instance with Climate Cleanup, a Dutch company. They are looking into how we can use Sargassum in a range of different ways, for instance as animal feed, biofuel or for other biotechnological applications. There are some issues, such as the high amounts of arsenic that are found in Sargassum, but it’s definitely something to research and use to our advantage. It would be better to intercept and utilize the Sargassum before it reaches the beaches and the shallow waters though, as it has already caused a lot of harm once it is there.

EP: Which parts of the Sargassum blooms are you focusing on in your research?

TT: I’m lucky that I get to do both fieldwork and lab work: as part of the research cruise, I was able to do experiments directly on the ship – such as testing which nutrients are responsible for the intense growth. We had tanks set up during the cruise where we could vary certain nutrients levels in the water. We are interested in seeing whether elevated nitrate and phosphate concentrations are linked to higher growth rates.

Another interesting research project I’m currently working on is disentangling the different species and morphotypes of Sargassum. It’s very difficult to tell them apart morphologically, but also the genus Sargassum is extremely similar genetically. We are hoping to find more defined genetic differences when working with whole genomes instead of just using certain markers (smaller fragments of the DNA that act like barcodes or fingerprints). So far, using only single mitochondrial and nuclear markers to differentiate between species of Sargassum, even free-floating and benthic ones, have proven difficult.

Within our research group, we are also collaborating with physical oceanographers that model how Sargassum spreads through surface ocean currents using GPS tracking devices. A future goal is to combine these multidisciplinary approaches to better understand the life cycle of Sargassum

Read More: Dispersion of Surface Drifters in the Tropical Atlantic


Eva Paulus is an evolutionary marine biologist who just started her PhD at the University of Konstanz. Follow her on Twitter @Deep_Sea_Dirndl, or read more of her writing on her Ecology for the Masses profile.

Title image Credit: Vera Kratchovil, CC0 Public Domain

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