⛱️ Why are brown algae invading beaches? The mystery of sargassum

For several years, certain beaches in the Caribbean, Mexico, West Africa, and even French Guiana have been invaded by massive carpets of brown algae. These algae are called sargassum. They wash ashore in large quantities, emit a rotten egg smell, and worry both locals and scientists. But where do they come from, and why are they arriving in such huge amounts?

What exactly is sargassum?

Sargassum is a type of brown algae that naturally floats in the ocean. It originates from an area called the "Sargasso Sea," located in the North Atlantic. There, it lives on the surface, never attaching to the seabed, and forms a useful ecosystem: it provides shelter for fish, turtles, and seabirds.


But since 2011, a strange phenomenon has been observed: a massive belt of algae has formed over thousands of miles (kilometers), stretching between Africa and America. This belt drifts and regularly deposits tons of sargassum on coastlines.

Why has this phenomenon exploded?

Scientists believe several factors are at play:
- Warmer waters, caused by climate change, promote rapid sargassum growth.
- Nutrient runoff (such as nitrogen and phosphorus) from agriculture and rivers (notably the Amazon) feeds these algae, acting like fertilizer.
- Ocean currents can carry these algae over very long distances.

Result: uncontrolled and increasingly frequent proliferation.

A threat to ecosystems... and humans

When sargassum washes ashore, it quickly rots. This releases hydrogen sulfide, a toxic gas in high concentrations, which smells like rotten eggs and can cause headaches, irritation, or nausea.

It also harms tourism, smothers marine life, and requires enormous cleanup efforts.

Research is underway to better predict their arrival, improve collection at sea, and even... repurpose them! Some companies are exploring ways to turn sargassum into compost, paper, or biofuel. But for now, the phenomenon remains difficult to control.