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Sargassum bloom: A new enemy for Caribbean reefs

- Announcement
The beaches of Puerto Morelos, Mexico, have been hit again by an extraordinary bloom of Sargassum weed this summer, causing major problems for ongoing reef restoration work and research. In the following interview, biologist and coral expert Dr. Anastazia Banaszak explains how coral reefs are suffering due to this mysterious phenomenon.



Photos: Top) Elkhorn corals (Acropora palmata) growing in a reef along the Caribbean coast of Mexico. (Paul A. Selvaggio) Bottom: left) Coral expert Dr. Anastazia Banaszak (Vincent Lavigne), mid and right) Huge Sargassum fields washed ahore along the beach of Puerto Morelos. (Amador Hernández Gomez); 


SECORE: Puerto Morelos has been hit really hard by the Sargassum bloom this summer. Could you first describe what you see when you look at the beach in front of UNAM’s Marine Research Station in Puerto Morelos?

Dr. Anastazia Banaszak: The view is devastating. As far as one can see, the beach is again covered by a thick brownish carpet of decaying seaweed. Massive rafts get washed ashore with each incoming tide. The water close to the beach is brownish too and the stench is awful as the seaweed and associated biota rot in the hot sun. Sargassum gives off Sulphur gas so the smell of the rotting seaweed is very potent like rotten eggs. Under normal circumstances, Sargassum has an important ecological role such as being a haven for species of fish, seahorses, crabs, and others as well as being a resting place for baby turtles as they swim on their long journey out to sea from their nest. Every year in the summer, Sargassum can be found floating in the ocean and the currents, wind, and tides bring it to shore where it decomposes. However, in 2015 and again this year the Sargassum has bloomed to extraordinary proportions causing major problems here in Puerto Morelos.


Could you give us an example?

Sargassum floating in the ocean per se is not much of a problem – although in major amounts it can be hazardous for ship propellers, as a navy captain once told me. However, once the seaweed reaches shore, it gets trapped and all of the organisms associated with the Sargassum die and rot. As a result, the water becomes anoxic and flora (seagrasses) and fauna (fish etc.) in the water column also die. At the same time, large amounts of nutrients are released so the water becomes eutrophic – exactly the opposite of the water quality that is optimal for healthy coral reefs.

How does the Sargassum bloom affect your coral restoration work and research?

Sargassum blooms have a serious impact on our restoration work and research. The water that is laden with nutrients causes macroalgae to bloom and these successfully compete with baby corals for space. Baby corals are slow growing in comparison to macroalgae, so they usually lose the fight as algae take over. In addition, for rearing the larvae in the laboratory we need to have good quality water and because our seawater intake is not that far away from shore, the water we pump in is not high quality. Lastly, because we go into the field a lot or work on our aquaria, the stench reaches us, and it is not pleasant to work near the ocean.



Photo: Fish and crabs that presumably died due to the lack of oxygen in the water. (Veronica Monroy)

Is there any way to protect your aquarium system and the corals growing in your tanks?

We need to bring water in from outside while we find a way to fund the cost of getting bigger pumps to pump ocean water from further out. We also have to find alternative sites to outplant our coral recruits so that they have a better chance of survival. In times like this we see much more macroalgae and cyanobacteria in the reefs, both of which are terrible for coral health.

What do you think, are those extraordinary blooms becoming the “new normal”?

We had a Sargassum bloom in 2015 and again in 2018, although this year is much, much bigger than 2015. Those big rafts of seaweed started to arrive in February. But yes, I believe it will be a recurring problem and if we do not take serious action then it will keep coming.

However, in order to know what actions to take we need to know what is causing the blooms. Research done by different groups of biologists and oceanographers has shown that the Sargassum fouling Caribbean beaches in 2011 came from the tropical Atlantic Ocean, east of Brazil – and not from the Sargassum Sea that is known for its huge Sargassum fields and that had been expected to be the main source. Since then, those tropical Sargassum blooms have recurred nearly every year with seaweed being swept up the Brazilian coast toward the Caribbean in spring. I read that this year, Sargassum blooms visible in satellite images are even bigger than the ones in the past. But up until now no one really knows what is causing them.


Dr. Anastazia Banaszak is head of the Integrative Reef Conservation Research Laboratory at the Universidad Nacional Autonoma de Mexico (UNAM) and SECORE’s local lead for restoration and conservation efforts along the Yucatan Peninsula, Mexico.

If you want to know more about the Sargassom bloom we recomend the following articles:

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