Sargassum blooms: the Atlantic’s algae nightmare

Last summer a record bloom of sargassum seaweed stretched almost 9000km across the entire Atlantic Ocean. It normally plays an important role in helping protect marine life but a drastic increase is now threatening the species it used to support. But what is it? Why is there so much of it? And what’s the big deal?

A lone clump of sargassum floats on the surface, unfortunately there has been a lot more of it around in recent years

It may look like your everyday seaweed, but sargassum is quickly becoming a big problem for ocean life and coastal communities across the Atlantic. It is normally an important habitat and food source for a diverse array of marine species. However drastic increases in the size of its seasonal blooms over the last decade are changing the effect it is having on the ocean. This was made evident last summer when the largest bloom yet formed, stretching almost 9000km from the west coast of Africa to the Gulf of Mexico. The bloom, so large it was clearly visible from space, blocked out sunlight to coral reefs, smothered juvenile animals and was eventually left to rot on beaches causing major damage. Unsurprisingly it is human activity that is radically altering the sargassum blooms and changes need to be made before the problem inevitably gets worse in the future.

Super seaweed

Although sargassum has been making headlines for all the wrong reasons recently it actually plays a vital role in the lives of many marine species. In fact it may be one of the most important types of seaweed there is. The brown floating algae looks very ordinary at first glance but it is actually something of a super seaweed.  Famed ocean activist Sylvia Earle once described sargassum as a ‘golden rainforest’ for marine life. A very accurate metaphor because of how it provides shelter and protection to the juveniles of lots of species including fish, turtles and sharks. But it is also an important food source and makes up a large percentage of the base of the food web for the Atlantic. Marine biologist Brian Lapointe told National Geographic that he believes “there’s nothing like it, in any other ocean” in terms of supporting life in the open ocean.

The ‘golden rainforest’ – juvenile fish use sargassum as protection against predators and as a tasty snack

Sargassum has always accumulated on mass on the surface of the ocean. In fact part of the North Atlantic Gyre, a ‘vortex’ created by ocean currents in the mid Atlantic, is even named the Sargasso Sea after the large islands of the algae that form in its centre. Here the seaweed has historically been so thick that ships can get stuck within it, including Christopher Colombus during his expedition in 1492. Blooms of the seaweed like most algae also occur naturally every year when in spring time nutrient levels rise and sunlight increases allowing for increased photosynthesis and growth. The trouble is that those blooms are now much larger and there is now much more sargassum than there should be in our oceans.

Bloomin mad

The record sargassum bloom, at its peak in June of last year, covered an astounding 8,850km stretch of the Atlantic. The belt stretching from the west coast of Africa to the Gulf of Mexico was comprised of an estimated 20 million metric tonnes of seaweed making it the largest mass yet by a significant margin. Whilst it was surprising that there was so much of the algae in the Atlantic it was not a complete anomaly. A team led by Mengqiu Wang at the University of South Florida has recently released a paper showing a significant trend of increased bloom growth in the last decade. Using a 19 year old satellite imagery database they have shown a gradual trend in larger sargassum blooms since 2011 which peaks with the record summer of 2018.

This graph from Wang et al 2019 shows the increased growth of the sargassum blooms since 2019

They believe that the shift in 2011 coincides with increased nutrients in the oceans allowing for unnaturally increased growth. The most likely factors are an increased run-off of fertilisation chemicals in the amazon basin and greater upwelling of nutrients on the African coast caused by climate change. Both a direct result of human activity. It is no real surprise that we are to blame for the change in sergassum behaviour considering how much we are changing ocean systems. But it is something that we need to start considering more due to the effect it is having on the ocean. Wang and her team believe that ‘the algal blooms are likely to be the new normal’ going forward and so we need to understand just how big the effects will be in the future.

Too much of a good thing

So if sergassum is so beneficial to so many species and there is lots of it, what’s the big deal? The problem is that whilst in its natural quantities it helps marine species, in these supercharged blooms it smothers them. That quite literally happens to juvenile fish and turtles that can get trapped within the algae and either starve or drown as a result. But it can also prevent larger marine mammals from reaching the surface to breathe. The next issue is that it blocks out sunlight to the waters below. This means it can prevent phytoplankton and coral reefs from accessing the sunlight to produce the vital energy they need by photosynthesis. It can also stop air mixing with the seawater at the surface meaning large patches of anoxic water, containing low or no oxygen, which kills of all fish and other water breathing organisms below. So when a belt of it stretches across an entire ocean it becomes a very serious issue.

Large quantities of Sargassum seaweed lay ashore at the "Anse au Bois" beach in Martinique
It may not look like it but underneath all this seaweed is a popular Caribbean tourist beach 

As well as in the ocean sergassum is also creating havoc on the coastlines when it washes ashore. Particularly in Mexico and the Caribbean where entire beaches are left covered in knee high thick brown seaweed. This is not only expensive and hard to remove, but also creates an unbearable smell as it rots which is scaring away tourists and depriving the regions of much needed income. Considering the impacts the sergassum blooms are having both ecologically in the ocean and economically on land it is becoming clear that they will pose a serious long term problem in the future. As we speak this year’s bloom is likely already well underway to becoming as big if not bigger than last years. More work is urgently needed to see how nutrient run-off and changing ocean currents effect the blooms and what can be done to avoid even larger blooms in the coming years.

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