Sargassum whale

Seven miles off the shore of Dominica, in the Martinique Straight, I lay below a thick layer of sargassum seaweed that stretched as far as I could see. Almost two feet thick, the sargassum cloud blocked out almost all the sunlight of the Caribbean midday sun. Only a few rays could penetrate through, guiding a path of light into the depths of the sea.

Words & photographs by Luca Malaguti

I was in complete awe of the sunlight peaking through the small holes among the sea of sargassum. Before me, playing among the sargassum, her features barely visible, a female sperm whale clicked and rolled around amicably. I wasn’t sure if she was seeking shelter beneath the sea of sargassum seaweed, a moment away from the sunlight or distancing herself from the whale-watching boats. Maybe none of those reasons, maybe all of them.

Shortly after that momentous dive, I wanted to find out more. As I lay there, underwater on one breath, I wondered what role the sargassum played in the lives of the resident sperm whales of Dominica. Watching her, her eyeing me, aware of my presence, intrigued and knowledgeable of everything in my human composition.

The sargassum seaweed belt is a vast expanse of floating seaweed that stretches across the Atlantic Ocean. It is named after the Sargasso Sea, a region in the North Atlantic Ocean where the seaweed accumulates due to ocean currents and wind patterns. Sargassum seaweed belongs to a genus of brown algae, and it plays an essential role in marine ecosystems. It provides habitat, shelter, and food for various marine organisms, including fish, turtles, and birds. The seaweed belt is particularly known for its significance as a nursery area for many species, offering protection for the early stages of their life cycles.

In recent years, there has been an increase in the size and frequency of sargassum seaweed blooms, leading to concerns and challenges for coastal communities and ecosystems. The excess seaweed can wash ashore, accumulating in large amounts and causing problems such as beach erosion, hindered tourism, and negative impacts on marine life when it decomposes. The factors contributing to the increased blooms are still being studied, but some potential causes include nutrient runoff from land, changes in ocean temperature and currents, and shifts in atmospheric conditions. These factors can promote the rapid growth and spread of the seaweed.

Efforts are underway to better understand the sargassum blooms and find ways to mitigate their impacts. Research institutions, governments, and local communities are collaborating to develop strategies for monitoring, removing, and repurposing the seaweed. Some innovative ideas include converting the collected sargassum into useful products like fertilisers, biofuels, or even building materials. A recent article, for example, discusses how researchers convert this seaweed into a biofuel.

By studying and addressing the challenges posed by the sargassum seaweed belt, we can strive for a better understanding of our oceans’ intricate ecosystems and work towards sustainable solutions that balance the needs of both human communities and marine life.

A recent Nasa report from March 2023 highlighted satellite imagery of a massive ‘belt’ of sargassum stretching from the east coast of Africa to the Caribbean. This mass of sargassum is considered by researchers to be the largest ever record in a season. In 2018, a record amount of 20 million tons of sargassum was estimated to have washed up on the shorelines.

Now, this 2023 season, an even larger amount is heading towards the Caribbean Sea and tropical Atlantic. What will this mean for marine wildlife and and coastal areas here?

As noted in the NASA study, “in patchy doses in the open ocean, Sargassum contributes to ocean health by providing habitat for turtles, invertebrates, fish, and birds and by producing oxygen via photosynthesis.“ However, too much of this sargassum seaweed can hinder the migration, movement and habitat of many species.  As with everything in nature, a fine, delicate balance is always needed.

Furthermore, when “sargassum sinks to the ocean bottom in large quantities, it can smother corals and seagrasses. On the beach, rotten Sargassum releases hydrogen sulfide gas and smells like rotten eggs.” This has major impacts on local tourism, but also marine ecology on the Caribbean. For example, an important concern is for many sea turtle species that hatch their eggs on the sand and heavily rely on beaches for the survival of their young. As is stands, sea turtles already have a very low survival rate as they reach adulthood, and having tons of sargassum seaweed smothering the beaches will only lower that survival rate.

As with everything in nature, a balance always exists and one can find beauty even among a devastating disaster waiting to happen. Photographing a young female sperm whale off the shores of Dominica and watching her ‘dance’ and move under the sargassum gave me perspective. It gave me motivation to study, learn more and write this article. It gave me incentive to head back to this same spot next December 2023 and continue photographing them.

She seemed content under the thick bed of sargassum that blocked out all sunlight. It seemed like she wanted repose from the strong Caribbean sun. Around her, smaller fish also found refuge under the thick pancake of sargasso that spread as far as the eye could see. At one stage, I noticed that she nudged a small opening with her head to allow a beam of sunlight to shine through as if she knew that me, as an underwater photographer, needed a bit more sunlight for the shot.

Watching her, I couldn’t help but wonder about the potential benefits of these sargassum blooms for sperm whales and other cetaceans in the open sea. Most certainly it will damage the habitat of many a species like sea turtles, but in that moment, the calmness of the female sperm whale made me forget about that.

I believe we now need to observe the behaviour of these marine mammals as they interact with large sargassum blooms in the Caribbean Sea. We need to jump in the water with the right questions in mind and watch and record.

Do they seek shelter from boats and flocks of whale watching tourists under the sargassum?

After a deep dive for foraging, will they seek these sargassum shelters for rest and digestion?

Could large blooms decrease the sightings of sperm whales or increase their danger to being struck by vessels?

Sperm whales ‘fear’ other cetaceans such as pilot whales. How will their presence change among more sargassum in the Caribbean Sea?

These are questions on my mind, and I want to find out more. 


Current issue

Back issues

Enjoy so much more from Oceanographic Magazine by becoming a subscriber.
A range of subscription options are available.