Sargassum, a type of free-floating brown seaweed, has become an increasingly noticeable presence in Atlantic coastal areas, particularly since 2011. This proliferation poses significant environmental and economic challenges for many regions. Understanding its biology, the factors contributing to its blooms, and its impacts is key to addressing this concern.
Understanding Sargassum
Sargassum is a genus of large brown algae that can exist in two forms: attached to the seafloor or free-floating. The free-floating species, primarily Sargassum natans and Sargassum fluitans, are responsible for the large mats observed in the open ocean. These species reproduce vegetatively, meaning they do not need to attach to the seabed at any point in their life cycle.
Historically, sargassum has been a natural component of the Sargasso Sea, an area in the North Atlantic Ocean. Within this traditional habitat, sargassum creates a floating ecosystem. It provides food, refuge, and breeding grounds for a diverse array of marine animals, including fish, sea turtles, crabs, shrimp, and marine birds. This natural accumulation acts as a “golden floating rainforest.”
The Great Atlantic Sargassum Belt
The “Great Atlantic Sargassum Belt” (GASB) has emerged, stretching across the tropical Atlantic Ocean. This mass of sargassum extends from the coast of West Africa to the Gulf of Mexico, spanning approximately 8,000 kilometers (5,000 miles). The GASB is the largest macroalgae bloom globally, weighing an estimated 5.5 million metric tons as of 2023.
This belt differs from the traditional Sargasso Sea, where sargassum is contained by circulating currents. The GASB is a seasonal occurrence that has appeared every year since 2011, with the exception of 2013. Its formation is influenced by wind and ocean circulation patterns, pushing these vast quantities of seaweed westward across the Atlantic.
Factors Driving Sargassum Blooms
The increased sargassum blooms since 2011 are attributed to environmental factors. Increased ocean temperatures create favorable conditions for sargassum growth. Changes in ocean currents influence the transport and accumulation of the seaweed.
Increased nutrient runoff from land-based sources is a factor. Agricultural fertilizers and wastewater discharge from major river systems, such as the Amazon River, introduce elevated levels of nitrogen and phosphorus into the ocean. These excess nutrients accelerate sargassum growth in areas like the North Equatorial Recirculation Region, off the northeast coast of Brazil. Saharan dust plumes, carried across the Atlantic, are considered a potential source of nutrients like iron and phosphorus.
Ecological and Human Impacts
When sargassum mats wash ashore, they have negative consequences for both marine ecosystems and human populations. Ecologically, these dense mats can smother coral reefs and seagrass beds by blocking sunlight. The decomposition of sargassum in coastal waters depletes dissolved oxygen, creating hypoxic conditions that can stress or kill fish, bottom-dwelling organisms, and corals. The mats can also entangle marine life and potentially introduce heavy metals and pollutants as they decay.
The economic impacts are significant, particularly for tourism-dependent regions. The unsightly appearance and foul odor of decaying sargassum, which releases hydrogen sulfide gas, deter tourists, leading to decreased revenue. Fishing industries suffer, as the mats can entangle fishing gear, impede vessel access, and alter fish behavior, resulting in reduced catches. Infrastructure, such as desalination plants, can be affected as sargassum clogs intake systems. Human health concerns include respiratory issues, headaches, nausea, and skin irritation from exposure to hydrogen sulfide gas.
Managing Sargassum Accumulations
Dealing with large-scale sargassum accumulations presents management challenges. Strategies involve collection at sea, nearshore, or directly on beaches. Collecting sargassum before it reaches the shore is preferred to minimize negative impacts and yield higher quality material for potential uses. However, large-scale removal operations are costly, requiring significant labor, specialized equipment, and transport.
Despite the challenges, collected sargassum offers several potential beneficial uses. It can be processed into agricultural products such as fertilizer, compost, or mulch, providing valuable nutrients for plants. Research also explores its use in producing biofuels, animal feed, and even building materials like concrete blocks due to its thermal properties. However, the variable chemical composition of sargassum, including potential heavy metal accumulation, requires careful analysis to ensure product safety, especially for consumables.