Sargassum is a genus of free-floating brown seaweed that historically formed an ecosystem in the North Atlantic’s Sargasso Sea. Since 2011, massive, open-ocean blooms have emerged, spanning the tropical Atlantic. This unprecedented growth has led to large-scale coastal inundations across the Caribbean, the Gulf of Mexico, and the West African coasts. These accumulations have transformed what was once a beneficial marine habitat into an environmental and economic burden.
Identifying the Peak Season
Sargassum accumulation is worst from late spring through early autumn, most intensely between April and September. Although the seaweed is present year-round, the volume washing ashore during this window is significantly higher. The peak months for the heaviest beaching events are typically June and July, when the Great Atlantic Sargassum Belt is fully developed and moving westward.
This seasonality is driven by the algae’s life cycle and the consistent ocean currents and winds that transport it. The severity and exact timing of landfall vary annually and geographically across the Caribbean basin. For instance, the Gulf of Mexico may experience a local peak in April or May, distinct from the larger summer influx originating in the tropical Atlantic. Coastal areas generally see an increase in late spring, reaching a maximum during the warmest months before declining in the late fall.
The massive scale of the Great Atlantic Sargassum Belt means that some regions may still receive significant amounts outside the peak season. However, the period from late spring to early fall represents the primary window of concern. The increase in volume during this time overwhelms local cleanup efforts and maximizes negative impacts on beaches.
Environmental Drivers of Severe Blooms
The massive growth of Sargassum is fueled by environmental factors in the tropical Atlantic. A primary driver is the increase in sea surface temperatures (SST), which accelerates the algae’s growth rate. Warm waters near the equator provide the heat necessary for the Sargassum to multiply rapidly.
Nutrient availability is also important, acting as fertilizer for the blooms. While some nutrient input comes from land-based sources, such as river runoff from the Amazon or Orinoco rivers, deep-ocean processes also play a substantial role. Equatorial upwelling, where deep, nutrient-rich water is brought to the surface, is a major source of phosphorus. This phosphorus, combined with nitrogen supplied by symbiotic organisms, provides the necessary elements for explosive growth.
These drivers create optimal conditions for the Great Atlantic Sargassum Belt to form and expand. Sustained warm temperatures and the continuous supply of nutrients allow the seaweed to reach high biomass levels annually. Changes in atmospheric pressure and wind patterns can also influence the initial movement of Sargassum into this fertile region, triggering the subsequent bloom.
The Origin and Movement of the Great Atlantic Sargassum Belt
The source of the massive inundations is the Great Atlantic Sargassum Belt (GASB), the largest macroalgae bloom globally. This belt stretches over 5,000 miles across the central Atlantic, from the coast of West Africa to the Gulf of Mexico. The GASB first appeared in large volumes in 2011.
The biomass begins to aggregate near the equator, often starting in January or February. Major ocean currents and prevailing winds transport this quantity of seaweed westward. The North Equatorial Current and the Caribbean Current push the Sargassum toward the Caribbean islands and the Gulf of Mexico.
The belt’s movement is influenced by seasonal shifts in wind and current patterns. By June, the GASB is typically fully formed and positioned to deliver large quantities of algae to the eastern Caribbean islands. This westward transport explains why coastlines facing the open Atlantic are generally the first and most severely impacted by the annual influx.
Immediate Impacts of Sargassum Accumulation
When Sargassum washes ashore and decomposes, the effects on the environment and human health are significant. The most noticeable impact is the release of hydrogen sulfide gas, produced as the seaweed decays. This gas has a foul odor and can cause irritation to the eyes, nose, and throat.
Environmental and Physical Barriers
The physical accumulation of the seaweed creates a barrier, making beach access difficult for tourists and marine life. Thick layers of Sargassum can smother nesting beaches, impeding sea turtles from reaching suitable nesting sites. In the water, the decaying biomass consumes dissolved oxygen, leading to localized low-oxygen zones, or hypoxia, in shallow coastal areas.
Oxygen depletion can lead to the mortality of near-shore marine life. The thick mats of algae also block sunlight from reaching the seafloor, negatively impacting seagrass beds and other light-dependent ecosystems. The volume of material poses a challenge for local authorities responsible for cleanup and disposal.