Sargassum, a brown macroalgae, floats freely across oceans. Known for its leafy appendages and gas-filled bladders, it plays a role in marine ecosystems. Its increasing prevalence and accumulation raise concerns about its persistence, impacting coastal communities and marine environments. Understanding its duration, both at sea and ashore, is crucial for addressing its widespread effects.
Sargassum’s Life at Sea
Sargassum floats without seafloor attachment, sustained by gas-filled bladders (pneumatocysts) providing buoyancy. These oxygen-filled structures allow it to remain at the ocean’s surface and grow to several meters. Two holopelagic species, Sargassum natans and Sargassum fluitans, are entirely free-floating and reproduce vegetatively.
Reproduction occurs through fragmentation, where pieces break off due to currents, waves, or other disturbances, growing into new plants. This continuous growth allows mats to persist for years in the open ocean. Nutrient availability, water temperature, and sunlight influence its growth and survival. Ocean currents significantly influence mat movement and aggregation, concentrating them into vast lines. These mats also serve as habitats, providing food, shelter, and breeding grounds for diverse marine life, including fish, crabs, shrimp, and juvenile sea turtles.
Sargassum on Shorelines
When sargassum washes ashore, its persistence changes dramatically as it decomposes. Decomposition rate is influenced by environmental factors like temperature, humidity, and tidal action. High temperatures accelerate decay, causing it to rot quickly on the beach. As it decomposes, sargassum releases hydrogen sulfide gas, producing a strong, unpleasant odor similar to rotten eggs.
Decomposition can cause localized poor water quality in shallow coastal areas, reducing dissolved oxygen and potentially altering pH. Accumulations can cover large stretches of sand, forming thick layers from inches to several feet high. If left undisturbed, these masses can remain on beaches for days to weeks, or even months, depending on volume and removal efforts. Decomposing sargassum can also alter beach ecosystems by changing sediment composition and nutrient cycling.
Why Sargassum’s Persistence Matters
Sargassum’s prolonged presence, both in the ocean and on shorelines, has environmental and economic implications. In the ocean, dense mats can block sunlight, harming ecosystems like coral reefs and seagrass beds by impeding photosynthesis. This reduction in light and oxygen can lead to marine species mortality and alter ecosystem balance. Additionally, mats can impede boat navigation and entangle marine animals, including sea turtles.
On beaches, decomposition creates challenges. Unpleasant hydrogen sulfide odors can deter tourists, decreasing tourism revenue. Studies indicate sargassum can decrease local economic output, with effects observed for up to a year. Accumulations can act as physical barriers for nesting sea turtles, making it difficult for females to reach nesting sites and for hatchlings to emerge and reach the ocean, increasing their vulnerability. Continuous influx and decomposition necessitate ongoing management and costly removal efforts to mitigate these impacts.