Picture football field-sized islands of algae stretched across the open ocean, extending across the entire Atlantic from Western Africa all the way to the Gulf of Mexico. This is the Great Atlantic Sargassum Belt, 5,000 miles of seaweed canopy that provides essential habitat for a host of marine life, from the green sea turtle to the blue marlin, as well as the sargassum fish, a camouflage-coloured frogfish with leafy spines lining its body, so named because they are uniquely adapted to live and feed within the snarls of sargassum beds. By providing shelter, food, migratory pathways, and resting areas for at least 80 marine species, sargassum seaweed ecosystems are essential for the ocean’s biodiversity. World-renowned oceanographer Sylvia Earle characterised sargassum islands as the “golden floating rainforests” of the Atlantic.

Several natural processes promote sargassum growth — foremost among them the upwelling of nutrients from deep ocean waters and the influx of fertilisers like iron, nitrogen, and phosphorus from agricultural runoff. Small quantities of sargassum have always washed up on coastlines, stabilising the shorelines by buttressing the build-up of sand dunes and nourishing the flora that live within them. But since 2011, as sea temperatures rise and new circulation patterns accelerate sargassum growth, the seaweed has bloomed in increasingly large quantities, brought to nearby shores to decompose en masse by changing winds and ocean currents. It’s along the coast where the once golden rainforest turns into a force of destruction — no longer floating in the open ocean, sargassum accumulates, suffocating shores, trapping marine life inside its tangles, smothering seagrass meadows and coral reefs, and ultimately coming between people and their connection with the ocean.

Enter the Dominican Republic, the second largest country in the Caribbean. In 2022 alone, its beachfronts brought in a record 8.5 million island visitors. The country’s fishing industry provides not just local sustenance, but also fuels local economies, bringing in around $93.4 million annually. In some communities, local reports estimate that up to 60% of people directly depend on fishing for a living. But as those millions of visitors set foot on the white sand beaches in the summer of 2022, sargassum accumulation in the Atlantic reached a staggering mass of over 24 million tonnes, blooming larger and spanning farther along the shore than ever before. This influx of seaweed engulfed the islands’ coastal waters and piled up in red blankets across miles of sand. Satellite images taken in March revealed even more massive mats of algae looming in the Caribbean, some of them over half a mile wide, heading towards the coasts of Caribbean islands in unprecedented proportions and arriving earlier in the season than ever before. Sargassum invasion in these quantities raises alarms for both the economies and ecosystems of the Dominican Republic.

Forming a dense layer on the sea surface, sargassum prevents sunlight from penetrating the water column. As a consequence, corals and seagrasses become shrouded in shadow and suffocate. These vital coral and seagrass ecosystems are essential habitats for local populations of reef species and the demersal fish that reside on the sea floor. Once the sargassum dies, it sinks, further stifling marine plants and coral reefs, its decomposition causing ‘dead zones’, areas with low oxygen levels unsustainable for life. “The coast begins to change dramatically upon the arrival of sargassum to the shore, as it decomposes in such a way that it depletes the water of oxygen and kills the fish offspring that reproduce near the shore,” says Carlos Perdomo, president of a fisherman association
in Punta Cana. The devastation doesn’t stop there: Sargassum decomposition acidifies the water, bleaching corals and depleting fish stocks. Because sargassum accumulates in this shallow water, its overgrowth traps dolphins and turtles in deadly natural nets, ensnaring coastal marine life indiscriminately.

Sargassum accumulation creates severe complications for the local fishing community. The thick mats of seaweed entangle fishing gear, hindering the ability of fishermen to cast their nets or even operate their fishing boats at all. Local fishermen like Elías Rodrïguez, president of the Association of Fishermen in Cabeza

De Toro, testify to the technological issues spurred by sargassum. “Our equipment breaks constantly, especially [as] the boat motors get clogged and our propellers sink. In just one week, sargassum has [cost] me four propellers.” When sargassum accumulates, it can extend five metres into the water, not only reducing the catch but also leading to additional maintenance costs and time lost as fishermen struggle to navigate through the dense vegetation. This effect is compounded by the constant friction against the rough seaweed surface that wears out fishing lines and nets, leading to frequent equipment replacements. For small-scale fishermen, who operate with limited financial resources, these additional expenses can become a considerable burden. Eventually, fishermen may be forced to alter their fishing grounds or temporarily suspend fishing operations until the seaweed dissipates, which can lead to significant economic loss and food insecurity for them, their families, and their communities.

The Dominican Republic’s tourism industry has felt devastating effects. Once pristine, sand-covered tropical beaches transform into unsightly mounds of rotting vegetation. As sargassum piles up on the beaches and dries, it produces the notoriously pungent hydrogen sulfide gas. The combined sight and smell of the decaying seaweed not only reduce the aesthetic appeal of the beaches, but also render them unavailable for visitors to engage in water-based activities. Within 48 hours, the decaying sargassum produces significant amounts of other toxic gases like ammonia, endangering anyone nearby and putting people at risk of respiratory and neurological damage. Sargassum overblooms are a public health crisis. The decaying seaweed degrades water quality and harms marine organisms. As a result, popular snorkelling and diving spots in Punta Cana, one of the country’s most visited destinations, have seen a decline in marine biodiversity, threatening underwater tourist experiences. These environmental concerns have also led to much-needed conservation efforts that restrict entry to certain areas, limiting tourist access to beaches and reducing the overall accessibility of the island.