Sargassum is spreading rapidly across the Atlantic, reshaping coastlines and worrying many communities. Normally helpful to ocean life, it has recently grown to record levels in some areas while declining in others, a shift linked to changing climate conditions. When huge amounts wash ashore, the seaweed rots, creates strong smells, disrupts tourism, affects businesses, and can cause breathing issues. Scientists say this year’s changes are some of the most dramatic they have seen.
A Giant Belt of Seaweed Is Growing Faster Than Ever
One of the biggest changes is happening along the Equator, where a massive band of sargassum has formed. This stretch is called the Great Atlantic Sargassum Belt. It now reaches about 5,500 miles across the ocean and weighs around 38 million metric tons. That is a huge increase—around 40% more than the previous record.
Before 2011, scientists did not see sargassum growing in this equatorial region at all. But over the last decade, winds and currents have carried these floating mats westward toward coastal areas each spring. Now, countries throughout the Caribbean, Mexico’s Yucatán Peninsula, and parts of Florida regularly face heavy seaweed landings that pile up on beaches.
These piles can release hydrogen sulfide gas as they decompose. This gas smells like rotten eggs and can irritate the eyes, nose, and throat. Last year, some Florida beaches saw millions of tons of this seaweed come ashore before it broke down in the summer heat.
Researchers studying this sudden growth believe the bloom does not have a single cause. Instead, it appears to be driven by several factors happening at once. Dust from the Sahara Desert and smoke from wildfires in Africa drift across the ocean, carrying tiny particles that act like fertilizer for the sargassum. These nutrients help the seaweed grow faster and spread wider.
Large rivers are also adding to the problem. The Amazon, Orinoco, and Congo Rivers send massive flows of nutrient-rich water into the Atlantic Ocean. After periods of drought, these rivers can suddenly flood, carrying even more nutrients into the sea. When that happens, the sargassum belt often grows quickly soon afterward.
Warmer temperatures, stronger rainfall events, and shifting winds add another layer of change. These conditions can create ideal environments for sargassum to spread, making it harder for experts to predict when the next large bloom will appear.
A Centuries-Old Seaweed Patch Is Rapidly Fading
While the southern belt is growing larger, a very different trend is unfolding farther north. For centuries, the Northern Sargasso Sea has been home to a stable patch of sargassum. This region has long been a natural nursery where seaweed gathers and drifts gently in swirling currents.
But researchers have noticed that this northern patch has been shrinking quickly since around 2015. One of the main reasons appears to be rising ocean temperatures. In the past 20 years, the Gulf of Mexico, an important source of seaweed for this region, has warmed by almost half a degree Celsius on average. Warmer waters create stressful conditions for native sargassum, especially during summer when temperatures are already high.
Marine heat waves have become more frequent too. These sudden bursts of extreme ocean warmth can make the waters even harsher for seaweed survival. Some research shows that sargassum has difficulty growing in these overly warm waters, which may explain why the northern patch is disappearing while the southern belt is expanding.
Even so, not all studies agree on how sargassum reacts to warmer oceans. Some experiments show it grows well in warm conditions, while others suggest the opposite. This makes it harder for experts to pinpoint a single explanation. Many say there are simply too many variables—nutrients, weather patterns, heat waves, rainfall, river outflows, and ocean currents—to study all at once.
Communities Face Unpredictable Seaweed Surges
As these seaweed patterns shift, the impact on people is becoming more noticeable. In many Caribbean countries, sargassum has become a yearly struggle. Piles sometimes reach several feet high, covering beaches and creating unpleasant smells that discourage tourists. Governments often pay millions of dollars to clear the seaweed, protect boats, and keep beaches open.
For travelers, choosing a beach destination during the spring months has become more challenging. Some coastal areas may experience heavy seaweed landings, while others stay relatively clean. This year, experts say places like the Bahamas may face fewer problems than regions such as the Yucatán Peninsula.
Scientists continue watching these changes closely using satellite images and ship surveys collected over the past 25 years. The patterns show a rapidly shifting ocean environment where some regions are overwhelmed with seaweed and others are losing it. The unusual size of this year’s bloom along the Equator highlights how quickly these changes can happen.
Even though the causes are complex, one thing is clear—the Atlantic’s sargassum is behaving in new and surprising ways. The combination of warmer waters, changing rainfall, shifting winds, and increasing nutrients is reshaping the ocean’s seaweed landscape. Many coastal communities now face unpredictable conditions year after year, as these giant seaweed blobs continue to transform the Atlantic in unexpected ways.
