Seaweed species that grew to twice the width of US also found in Singapore waters

SINGAPORE – Monster seaweed alert: a continent of free-floating algae twice the width of the United States has begun its blitz on Florida, with the earliest wave washing up thickets onto the state’s shores as early as last Tuesday.

If uncleared, the sargassum that accumulates on the beaches rots, releasing hydrogen sulphide – which smells like rotting eggs – and may leach toxic arsenic into the groundwater. 

Experts have noted that the seasonal sargassum blooms have been starting earlier and growing more massive over the years, and attributed this explosive growth to fertilisers from farms that have leached into the sea and warmer sea surface temperatures.

With sargassum also floating in waters around Singapore, should Singaporeans be concerned about an apocalypse by seaweed? Journalists Gena Soh and Gabrielle Chan wade in.

Q: What is sargassum?

A: Sargassum is a type of seaweed found in tropical and subtropical seas worldwide. Unlike other seaweed that grows on the ocean floor, this type is rootless and has air-filled “berries” that help it to float on the surface – often in large masses that can extend for hundreds of kilometres.

The Sargasso Sea in the Atlantic Ocean is so named for the patches of seaweed that, when in season, form an oasis for many fishes and reef organisms. Many animals, including endangered species, eat and shelter in these “golden rainforests”, forming a richly biodiverse habitat that bluefin tuna visit during their open ocean migrations.

Sargassum is also a source of blue carbon in the ocean, storing carbon dioxide captured from the atmosphere during photosynthesis. When it dies, the seaweed’s organic carbon-rich tissues eventually become part of the ocean floor sediment.

However, the seaweed can also wreak environmental havoc. Runaway propagation can see “rafts” of sargassum blocking sunlight from coral reefs and even draining oxygen from the waters they are in, resulting in “dead zones” where other marine life cannot survive.

And when deposited on the shore and left to rot, the noxious hydrogen sulphide fumes emitted are toxic to humans, causing respiratory, skin and neurocognitive symptoms if people come into close contact with the decaying seaweed.

Vast mounds of seaweed can also block endangered sea turtles from laying their eggs on shore and prevent their hatchlings from moving to the ocean. 

Assistant Professor Adriana Lopes dos Santos from the Asian School of the Environment at Nanyang Technological University (NTU) said the continent of sargassum swallowing the Floridian coast could be due to “the excess of nutrients coming from the Amazon basin from factors like farming, deforestation and livestock possibly causing the intensification of sargassum blooms”.

Q: Where can we find sargassum in Singapore?

A: Sargassum occurs naturally in most subtropical and tropical seas, and the waters off Singapore are no exception. It is usually found in shallow waters where light is not limiting photosynthesis, said Dr Karenne Tun, director of the coastal and marine branch at the National Biodiversity Centre of the National Parks Board (NParks).

A combination of factors like temperature, nutrients and sunlight may influence sargassum’s seasonality, she said. “In Singapore, this season happens to be between August and March.”

While rafts of seaweed have extended across hundreds of kilometres in the Atlantic Ocean, the seaweed does not stretch so far in Singapore, said Associate Professor Rebecca Case from the Singapore Centre for Environmental Life Sciences Engineering at NTU.

Prof Case said: “Sargassum is the main seaweed in Singapore, but it does not form these floating rafts like it has off Florida… It can grow to several metres, but here in Singapore, I’ve seen it closer to half a metre off St John’s Island.”

She added that the seaweed often washes up on intertidal habitats when in season, cloaking the rock pools with brown carpets of algae. 

More intense rain due to climate change could lead to unmanageable blooms of sargassum and other unwanted microbial species, said Prof Lopes dos Santos.

Q: Should Singaporeans be worried?

A: The short answer, from experts, is “no”. 

Dr Tun said there has not been any change in sargassum populations that would cause alarm. 

The NParks intertidal watch programme has also been keeping a watch on the intertidal habitats where sargassum can be found growing. It has found no major fluctuations in water or habitat quality on Coney Island, where algae like sargassum is more dominant.

While Singapore does not have a foreseeable sargassum bloom, Prof Case said that other blooms have been reported in waters in the region.

For instance, bioluminescent blooms were found in the Java Sea and even in Singapore in 2022. 

Insufficient data means the cause of these blooms is yet unknown, said Prof Case. “It is important to continue monitoring our waters so that we can detect any signals that may affect the ecological balance.”

This can be done through the Marine Environment Sensing Network buoys that will all be deployed by 2025, she said.

The first of three buoys was deployed off St John’s Island in 2022 to measure key indicators in the water like temperature, salinity and nutrient levels. 

Prof Case, whose team of researchers is looking into such phenomena to better understand their exact causes, said: “Certainly climate is at the forefront of our investigations, but further work is needed to determine the causes.”