Sargassum, a brown macroalgae or seaweed, is increasingly present in the Atlantic Ocean and coastal areas. This free-floating organism, forming large, buoyant mats, has garnered significant attention due to its rapid proliferation. Its expanding range impacts marine environments and human activities across a wide geographical area.
Understanding the Great Atlantic Sargassum Belt
The Great Atlantic Sargassum Belt (GASB) represents the largest known macroalgae bloom globally. Historically, pelagic sargassum concentrated within the Sargasso Sea, an area in the North Atlantic defined by circulating currents. Since 2011, a distinct and massive aggregation has emerged, stretching thousands of miles from West Africa across the central Atlantic to the Caribbean Sea and the Gulf of Mexico.
Sargassum possesses air-filled structures called pneumatocysts, allowing it to float on the ocean’s surface. These floating mats provide a rich, open-ocean ecosystem, offering crucial habitat, food, and refuge for hundreds of marine species. Fish, sea turtles, crabs, and shrimp utilize these mats as breeding grounds and nursery areas, making them biological hotspots in nutrient-poor waters.
Factors Driving Its Recent Expansion
The expansion of the Great Atlantic Sargassum Belt is attributed to a combination of environmental factors. Increased nutrient runoff from land is a major contributor. Rivers such as the Amazon, Mississippi, Congo, and Orinoco carry elevated nitrogen and phosphorus into the Atlantic, primarily from agricultural fertilizers and deforestation.
Changes in ocean currents and wind patterns also play a role in sargassum’s proliferation and distribution. Shifting atmospheric pressures, such as the North Atlantic Oscillation, can transport sargassum from its traditional grounds into the tropical Atlantic. In these new, nutrient-rich regions, the seaweed rapidly multiplies, forming the expansive belt observed today.
Rising sea temperatures can influence sargassum growth. While sargassum can thrive in warm waters, optimal growth rates occur when sea surface temperatures are normal or slightly cooler than average, with growth decreasing at very high temperatures (around 31°C). Saharan dust clouds, carrying iron, nitrogen, and phosphorus, also fertilize these blooms as they settle on the ocean surface.
Impacts of Sargassum Blooms
Large-scale sargassum blooms have negative consequences for marine ecosystems and human societies. Dense mats can block sunlight, smothering coral reefs and seagrass beds that depend on light for photosynthesis. The decomposition of large quantities of sargassum consumes oxygen, creating localized low-oxygen or anoxic zones that can result in fish kills and harm other marine life.
Massive sargassum influxes can impede marine animal movement, potentially entangling them and blocking sea turtle access to nesting beaches. The decaying seaweed can also release accumulated pollutants, including heavy metals like arsenic and microplastics. The mats themselves can increase water temperature within their dense formations, further impacting local ecosystems.
Economically, sargassum influxes disrupt coastal tourism, a major industry for many affected regions, due to the unsightly appearance and foul smell of decaying seaweed. Fishing and shipping industries also face challenges, with boat propellers becoming fouled and coastal infrastructure, such as desalination plant intake pipes, getting clogged. These events necessitate costly cleanup operations.
Regarding human health, decomposing sargassum releases gases such as hydrogen sulfide (which produces a rotten egg odor) and ammonia. Exposure to these gases can cause respiratory irritation, nausea, headaches, and other symptoms, particularly for sensitive individuals. Contact with the seaweed can also cause skin irritation, often from tiny organisms like jellyfish larvae that live within the mats.
Addressing the Sargassum Influx
Communities and researchers are exploring various approaches to manage and mitigate the sargassum influx. One common strategy involves physical collection and removal of the seaweed from coastlines. This typically occurs as quickly as possible after it washes ashore to prevent extensive decomposition. Collection efforts can also take place in near-coastal waters using barriers, nets, or barges to intercept the mats before they reach beaches.
Large-scale removal poses challenges, including high operational costs, potential for beach erosion, and the risk of disturbing marine life. Research into early detection and forecasting is underway, utilizing satellite monitoring and modeling to predict the movement and density of sargassum blooms. These forecasts help communities prepare for potential inundation events.
Scientists are also investigating potential beneficial uses for collected sargassum, aiming to transform a problem into a resource. Applications include its use as an organic fertilizer due to its rich nutrient profile, a feedstock for biofuel production, or as a component in building materials. Efforts also focus on understanding the carbon sequestration potential of sinking sargassum. However, the presence of heavy metals, such as arsenic, requires careful consideration for its safe utilization.