Sargassum, a type of free-floating brown seaweed, forms expansive mats across the Atlantic Ocean. These mats are increasingly washing ashore, causing widespread impacts on coastal environments and human populations.
What Exactly is Sargassum?
Sargassum is a genus of brown macroalgae belonging to the class Phaeophyceae. Unlike most seaweeds that attach to the seafloor, certain sargassum species are holopelagic, floating freely in the ocean. The two main species in Atlantic blooms are Sargassum natans and Sargassum fluitans. These species have berry-like, gas-filled bladders that provide buoyancy, forming dense mats on the ocean surface.
Sargassum natans has long-stalked, narrow leaves and gas bladders often tipped with a spike. Sargassum fluitans features short-stalked, broad leaves with bladders usually lacking spikes. These floating aggregations, which can stretch for miles, serve as ecosystems in the open ocean. They provide habitat, food, and shelter for a diverse array of marine species, including crabs, shrimp, sea turtles, and commercially important fish like mahi-mahi and tuna. Sargassum is designated as “Essential Fish Habitat” in certain regions.
The Expanding Great Atlantic Sargassum Belt
Historically, sargassum concentrated in the Sargasso Sea in the western North Atlantic. Since 2011, sargassum has expanded, forming the “Great Atlantic Sargassum Belt” (GASB). This bloom, extending over 8,800 kilometers from West Africa to the Gulf of Mexico, is the largest seaweed bloom globally. Its size has increased over a hundred-fold since 2011, with record amounts reported recently.
The GASB’s growth links to several factors. Increased nutrient runoff from major rivers, like the Amazon, fuels sargassum growth. Research shows an increase in sargassum tissue nitrogen over four decades, indicating more land-based nutrient inputs such as agricultural runoff, wastewater discharge, and atmospheric deposition. While the Amazon River’s role was initially emphasized, recent research suggests ocean changes may be a primary nutrient source, with riverine input accounting for about 10% of the total biomass.
Changes in ocean currents and warmer ocean temperatures also contribute. A negative North Atlantic Oscillation event in 2009-2010 likely shifted sargassum into the tropical Atlantic, where it found suitable growth conditions. Ocean warming influences sargassum growth rates. The combination of nutrient availability, ocean dynamics, and shifting weather patterns has transformed sargassum into a widespread challenge.
Coastal Consequences and Human Impact
When large quantities of sargassum wash ashore, it affects both ecosystems and human well-being. Environmentally, dense mats smother coastal habitats like coral reefs and seagrass beds, blocking sunlight and disrupting ecosystems. Light reduction can reach 73% under sargassum mats, preventing photosynthesis. As sargassum decomposes, it depletes oxygen, creating hypoxic conditions that lead to fish kills. Decomposition also releases nutrients, contributing to eutrophication.
Economically, sargassum influx poses challenges, especially for tourism-dependent regions. Unsightly seaweed and foul odors deter tourists, causing revenue losses. Fishing industries are impacted as sargassum entangles nets and obstructs fishing areas. Coastal infrastructure can also be affected, with mats clogging water intake pipes for facilities like desalination plants.
Human health is a concern due to decomposition. Rotting sargassum releases hydrogen sulfide gas, which has a distinct rotten egg smell. Exposure can irritate eyes, nose, and throat, and cause respiratory issues, particularly for individuals with asthma. While sargassum itself is not harmful to skin, tiny organisms within the mats, such as jellyfish larvae, can cause skin irritation or rashes. Prolonged exposure to hydrogen sulfide and ammonia can lead to severe symptoms like headaches, nausea, and vertigo, though high concentrations are typically associated with enclosed spaces.
Strategies for Managing Sargassum Blooms
Managing sargassum blooms requires a multi-faceted approach, including preventative measures and direct management strategies. Preventing blooms at their source focuses on reducing nutrient runoff into the ocean. This involves managing agricultural fertilizer use, improving wastewater treatment, and mitigating other land-based pollution. However, the global scale of the problem makes immediate preventative solutions difficult.
Direct collection and removal strategies are used once sargassum reaches coastal areas. Methods include deploying offshore barriers to divert seaweed from sensitive areas or beaches, and mechanical beach cleaning. Manual removal by local communities is also practiced. These methods present challenges like high costs, potential environmental impact from heavy machinery compacting beaches or removing sand, and limited capacity to handle the volume of seaweed.
Research explores alternative uses for collected sargassum. It can be processed into organic fertilizers, used in biofuel production, or incorporated into construction materials. Investigation into its potential for pharmaceutical applications is also underway, given its unique biochemical composition. These alternative uses offer avenues for sustainable management, potentially offsetting removal costs, creating economic opportunities, and reducing the environmental burden of decomposing biomass.