The seaweed mystery stretching across an ocean

I honestly had never heard of sargassum until I started working on Blue Planet III. When I first look it up, I'm not exactly blown away. It's brown, it floats around at sea, and fish seem to hang around it. Fascinating... but this is Blue Planet III! We are filming some of the most spectacular wildlife stories on Earth. What could possibly be interesting about floating seaweed?

As it turns out, quite a lot.

What begins as mild scepticism quickly becomes an obsession. As I dig deeper, I find reports from across the Caribbean of beaches buried beneath seaweed, boats struggling to leave harbour and turtle nesting grounds becoming blocked. I find myself asking a simple question: how could a simple seaweed cause so much disruption?

A few months later, I am looking down from the window of an aircraft as we descend towards Puerto Rico. Below us, stretching across the Atlantic, is a vast golden-brown ribbon on the ocean surface. The Great Atlantic Sargassum Belt.

After landing, we load a small speedboat with cameras, dive gear and drones and head into the open ocean. As we bounce across the waves, I can't help wondering whether the belt would really live up to its reputation, and whether I've remembered to charge the camera batteries. Several hours later, guided by satellite imagery and local fishermen who can spot the slightest change in colour on the horizon, we find it.

What looks from a distance like scattered patches of seaweed reveals itself to be something much more remarkable. Long floating rafts of sargassum stretch across the sea surface. Beneath them is a thriving community of life. Juvenile fish shelter amongst the tangled fronds, seeking protection in an otherwise featureless ocean. Crabs and other invertebrates cling to the weed, while larger fish patrol the edges.

Drone footage revealed just how extensive these golden rafts are, while underwater cameras show the wildlife depending upon them. It's easy to see why scientists often describe sargassum as a floating nursery in the middle of the Atlantic. The scale is extraordinary.

The Great Atlantic Sargassum Belt stretches roughly 5,000 miles from the coast of West Africa to the Caribbean. Today it contains tens of millions of tonnes of floating seaweed and forms one of the largest marine habitats on the planet.

What makes it so intriguing is that, until relatively recently, this belt did not exist in its current form. When researching the story, I speak with Professor Chuanmin Hu from University of South Florida's Optical Oceanography Lab, who has spent years studying sargassum using satellites. His team were among the first researchers to identify the extraordinary scale of the blooms appearing across the Atlantic. "I first heard reports of unusual sargassum inundation in the Caribbean in 2011," he told me. "When we looked at the satellite imagery, we found features we'd never seen before."

To understand where the belt comes from, we first need to understand the Sargasso Sea. Located in the North Atlantic, the Sargasso Sea is defined not by land but by a ring of ocean currents that trap floating sargassum. This region is famous for its abundance of sargassum. Early sailors, including Christopher Columbus, record vast floating mats there during voyages across the Atlantic.

Prior to 2011, most Atlantic sargassum was largely concentrated within the Sargasso Sea and nearby Gulf of Mexico. As Chuanmin explains: "Sargassum used to occur mainly in the Sargasso Sea and Gulf of Mexico. The scale we see today in the Great Atlantic Sargassum Belt is unprecedented.".

Then, in 2011, something changes.

Millions of tonnes of sargassum appeared across the tropical Atlantic. The scale is unlike anything previously recorded. Since then, the belt returns every year, repeatedly overwhelming beaches across parts of the Caribbean, and creating challenges for wildlife and coastal communities alike.

While sargassum is a vital habitat offshore, huge beach strandings can bury coastlines, block turtle nesting grounds and cost Caribbean communities millions each year in clean-up efforts. This is why scientists are so interested in understanding where the belt came from. Solving the mystery is not simply about explaining the past. It may help predict what happens next.

One of the leading hypotheses, centres on changes in ocean currents and winds around 2010. Scientists believe unusually strong currents and winds may have transported a seed population of sargassum out of its traditional North Atlantic stronghold. Chuanmin explains: "Stronger-than-normal currents and winds in 2010 may have transported sargassum from the Sargasso Sea into the tropical Atlantic." Once in the tropical Atlantic, the seaweed encounters warmer, nutrient-rich conditions that allow it to thrive and expand into the vast belt we see today.

This hypothesis offers an explanation for how the belt becomes established. However, Chuanmin is careful to stress that the story is unlikely to be that simple. "The evidence supports this transport theory," he explained, "but that doesn't mean it is the complete explanation." One challenge is that some of the dominant sargassum types now found in the belt were not commonly recorded in the Sargasso Sea before 2011.

There are still unanswered questions. Other researchers have therefore explored alternative explanations. Some examine whether changes in nutrient availability, major oceanographic events, or even iron-rich dust blowing across the Atlantic from the Sahara may have helped fuel the blooms.

More recently, researchers investigate evidence suggesting significant sargassum accumulations may have existed along parts of the West African coastline before 2011. This has led to further discussion about whether the belt's origins might be more complex than any single explanation.

The more scientists learn, the clearer it becomes that several factors may have contributed simultaneously. Chuanmin believes that possibility should not be overlooked. "Multiple theories can all be true," he said.

That is the biggest lesson I take away from filming this story. The Great Atlantic Sargassum Belt is not a solved story but a scientific investigation still unfolding today.

As researchers continue studying its origins, they are also searching for ways to reduce its impact on coastal communities. From offshore barriers and improved forecasting to large-scale removal projects, efforts are underway throughout the Caribbean.

For me, the biggest surprise is how completely my view of seaweed has changed. What sounds like one of the least exciting stories in the series turns out to be one of the most fascinating. Not because we have all the answers, but because we didn't. Sometimes the most compelling wildlife stories are not about what we know, but about what we're still trying to figure out.