Sargassum is a genus of brown macroalgae, commonly known as seaweed, found throughout the world’s oceans. These plant-like organisms are particularly abundant in tropical and subtropical waters, forming extensive floating mats or growing attached to the seafloor. The genus encompasses a wide array of species, each with distinct characteristics and ecological roles within marine environments.
The Two Worlds of Sargassum
Sargassum exists in two primary forms. Most of the over 350 recognized Sargassum species are benthic, meaning they grow attached to hard surfaces on the ocean floor in coastal areas. These attached forms often create underwater forests, providing complex structures in their habitats. Their holdfasts anchor them to rocks, coral, or other stable substrates.
In contrast, two species, Sargassum natans and Sargassum fluitans, are holopelagic, meaning they live their entire lives free-floating in the open ocean. These species reproduce asexually through fragmentation and do not attach to the seafloor. They form large rafts that drift with ocean currents across vast expanses of the Atlantic Ocean.
These free-floating species are characterized by buoyant structures, including gas-filled bladders called pneumatocysts, which help them stay afloat near the surface. This buoyancy allows them to access sunlight for photosynthesis in the open ocean, where nutrients might otherwise be scarce. Their unique adaptation enables them to thrive in the high seas.
Ecological Importance
Sargassum, in both its benthic and pelagic forms, provides benefits to marine ecosystems. Benthic sargassum creates complex underwater habitats, offering shelter and nursery grounds for a variety of fish and invertebrates. These submerged forests support biodiversity by providing refuge from predators and abundant food sources.
The free-floating sargassum also forms expansive floating ecosystems in the open ocean. These mats offer shelter, foraging grounds, and breeding sites for a diverse array of marine life, including juvenile sea turtles, crabs, shrimp, and over 120 species of fish. Many species are uniquely adapted to live within these mats, camouflaging themselves among the seaweed.
Sargassum contributes to the broader ocean food web, serving as a primary producer that converts sunlight into energy. This energy then becomes available to herbivores that consume the seaweed, and subsequently to carnivores higher up the food chain. The mats also act as migratory pathways for various marine animals, offering resting points and sustenance during long journeys across the ocean.
Understanding Mass Sargassum Blooms
Since 2011, there has been an increase in the size and frequency of sargassum blooms. These massive accumulations have grown to form the Great Atlantic Sargassum Belt, stretching thousands of miles from the coast of West Africa to the Gulf of Mexico. This belt represents the largest macroalgal bloom globally, with biomass estimates reaching millions of tons.
Several factors are thought to contribute to the formation and expansion of these large-scale blooms. Increased nutrient runoff from land-based sources, such as agricultural fertilizers and sewage discharge, is a suspected driver. For example, higher nutrient loads from the Amazon River outflow into the Atlantic are believed to fuel sargassum growth. These added nutrients act as a fertilizer for the seaweed.
Changes in ocean currents and rising sea temperatures also play a role in the proliferation and transport of sargassum. Warmer waters can accelerate the growth rate of the seaweed. Shifting current patterns can then concentrate these growing mats into larger belts, guiding them towards coastal regions where they eventually wash ashore.
Widespread Consequences
The large-scale stranding of sargassum on coastlines presents numerous negative impacts across ecological, economic, and human health domains. Ecologically, thick mats of sargassum can smother coastal ecosystems, blocking sunlight from reaching and suffocating coral reefs and seagrass beds. As the seaweed decomposes, it depletes oxygen in the water, creating anoxic or hypoxic conditions that harm or kill marine life.
Economically, these massive beaching events affect coastal communities that rely on tourism and fishing. The unsightly and malodorous piles of decaying sargassum deter tourists, leading to reduced hotel occupancy and fewer visitors to beaches. Fishing activities are also disrupted as the dense mats interfere with boat navigation and fishing gear, impacting local livelihoods and seafood supply chains.
Human health concerns arise from the decomposition of sargassum on beaches. As the seaweed breaks down, it releases hydrogen sulfide gas, which produces a strong, unpleasant odor often described as rotten eggs. Exposure to high concentrations of this gas can cause respiratory irritation, headaches, and nausea in coastal residents and visitors, making affected areas less habitable and enjoyable.