Sargassum Inundation Events (SIEs): Impacts on Aquatic Life and Associated Ecosystems

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Aggregations of Sargassum in the open ocean are considered unique floating ecosystems that provide habitat, shelter, and support to food webs that include diverse assemblages of fish, invertebrates, sea turtles, and birds. Hence, Sargassum has been designated as Essential Fish Habitat for several open sea fish species in the Southeast U.S., Gulf of Mexico, and the U.S. Caribbean and as Critical Habitat for loggerhead sea turtles under the Endangered Species Act in two distinct areas in the U.S. South Atlantic and Gulf of Mexico. However, Sargassum inundation events (SIEs) of beaches and nearshore coastal waters can negatively impact aquatic life such as seagrasses, corals, fish, shrimp, crabs, and sea turtles, and even harm entire ecosystems.

SIEs can harm seagrasses and corals by blocking the light the grasses need for health, reproduction, and survival. This in turn impacts the availability of food and habitats for other aquatic species, reduces the amount of oxygen in the water (which can stress or kill plants and animals in the water), and makes it easier for invasive species to establish or grow. Sargassum may also affect aquatic species and their ecosystems by accumulating marine debris, including microplastics, and leaching heavy metals, pesticides, and other pollutants, some of which may be generated from its decomposition.

• Sargassum Inundation Events (SIEs): Impacts on Marine Ecosystems
• Sargassum Inundation Events (SIEs): Impacts on Seagrass Meadows
• Sargassum Inundation Events (SIEs): Impacts on Fish, Benthic Organisms, and Corals
• Sargassum Inundation Events (SIEs): Impacts on Sea Turtles and Other Marine Life

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Sargassum Inundation Events (SIEs): Impacts on Marine Ecosystems

Although Sargassum aggregations in the open ocean serve as habitat for many marine animals, the inundation of coastal areas with excessive Sargassum biomass has detrimental impacts to aquatic life and coastal marine ecosystems. The deposition and decomposition of excessive Sargassum in environments such as beaches and warm, shallow waters results in localized poor water quality.

When Sargassum decomposes in nearshore coastal waters, the water becomes brown and turbid (hence the name “golden” or “brown tides”), generating particles and dissolved organic matter that leaches from the decaying algae. The increased particles and dissolved organic matter reduce the light available for seagrasses and corals. The same particles and dissolved organic matter may absorb light and increase water temperatures. The decaying Sargassum also increases organic matter and nutrient concentrations that fuel decomposition in the water, decreasing the availability of dissolved oxygen, and potentially altering the pH. These changes can lead to animal deaths like those reported during the 2018 Sargassum inundation event along the Mexican Caribbean coast. Overall, these changes in chemistry contribute to alterations in a range of essential coastal habitats, the communities of organisms that live there, their food webs, and the ecosystem services they provide, although the impacts may depend on local factors such as landforms and the processes shaping them, currents, and wind patterns. Lastly, these impacts add to a range of already existing stressors on these ecosystems (e.g., climate change, ocean acidification, human waste, fishing pressure).

Sargassum Inundation Events (SIEs): Impacts on Seagrass Meadows

Seagrass meadows are essential components of many marine ecosystems due to their role in providing habitat for marine life, stabilizing sediment, increasing water clarity, and reducing wave energy. Excessive Sargassum from SIEs and its decaying biomass may negatively impact seagrass meadows by reducing the amount of sunlight available for photosynthesis, limiting oxygen availability, and increasing temperature by up to +2 to +3°C (+35.6 to +37.4°F). This can lead to changes in species coverage and composition of seagrass meadows, potentially helping invasive species disrupt the local ecosystem services provided by native species (e.g. nursery habitat, food, foraging habitat, sediment stabilization, etc.).

Excessive Sargassum from SIEs  that shade the water column, as well as the dissolved organic matter from decomposing Sargassum reduce water column light to levels below those required for growth, negatively impacting seagrasses. A recent study found significant growth rate and mass losses of the seagrass Thalassia testudinum in waters with significant biomass of Sargassum compared to waters with small amounts of Sargassum. This study also noted significant differences in dissolved oxygen between the areas with Sargassum and those without. Furthermore, the increase in organic matter and decrease in dissolved oxygen from decaying Sargassum may have caused sulfide intrusion in seagrass tissues, which can lead to decreased growth performance and death. Overall, the combined effect of these stressors from excessive Sargassum  and their decomposition increased the harmful effects to seagrass populations.

Decaying Sargassum can also sink, damage native seagrasses, and indirectly trigger rapid growth of invasive species. Increases of nutrients in the water released from the decomposition of Sargassum accumulate in seagrass meadows, fueling the growth of other species that can replace valuable seagrasses. Repeated or extended SIEs can reduce the cover of native seagrass beds, making them more vulnerable to erosion and rapid colonization by invasive algal species due to increased exposure of the sea floor. If the colonizing species have shallower root systems, these habitats become more susceptible to future erosion, suspended sediments, and changes in water quality, potentially leading to cascading impacts on the coastal ecosystem.

Sargassum Inundation Events (SIEs): Impacts on Fish, Benthic Organisms, and Corals

Although Sargassum aggregations in the open ocean, like those historically found in the Sargasso Sea, serve as habitat for many marine animals, SIEs in nearshore coastal waters have detrimental impacts on aquatic life and marine ecosystems. The decomposition of Sargassum in environments such as beaches and warm, shallow waters, results in localized poor water quality (e.g., low dissolved oxygen, low pH, elevated hydrogen sulfide and ammonia). The changes in water quality associated with decomposing Sargassum can kill fish, benthic (bottom-dwelling) organisms, and corals. In 2015, Tobago declared a natural disaster due to the smell of decomposing excessive Sargassum biomass and the associated dead fish and turtles. Between May and September 2018, the decay of an extensive SIE along the Mexican Caribbean coast, was linked to a massive die-off event that included a combination of 78 animal species, mainly benthic organisms and fishes due to changes in water quality, specifically hypoxia. While most mortality was seen in benthic nearshore fish and crustacean species, impacts extended as far as 480 m from the shoreline.

In addition to massive die-off events, the chemical and habitat changes  produced by SIEs and its biomass decomposition (e.g., low dissolved oxygen, low pH, and elevated hydrogen sulfide and ammonia) can cascade through the ecosystem, impacting the availability of food and other habitat resources to aquatic animals, including fishes. Loss of seagrasses and coral reef species, for instance, will impact aquatic animals dependent on these for food (directly or via their food web) and shelter. Excessive Sargassum may have similar effects in mangroves, which also serve as natural fish nurseries and feeding areas. Additionally, Sargassum carries invasive species that may be exotic to regional ecosystems, affecting the ecological balance and food web of these systems. During SIEs, excessive live Sargassum entering coral reefs have been shown to reduce sunlight available to these communities for photosynthesis. When impacted by lower light, corals become stressed and lose their symbiotic algae, which leads to coral bleaching. Reduced light can also impact the recovery from coral bleaching caused by other stresses. Hypoxia from decomposing excessive Sargassum represents an added stress on corals, favoring epiphyte growth, and leading to coral mortality. For instance, in Puerto Morelos, Mexico, decomposing SIEs have impacted hard corals in reef lagoons, where colonies experienced partial to complete mortality. Like corals, other coastal benthic communities are impacted by the anoxic conditions, smothering, and other habitat alterations caused by deposition of excessive Sargassum. For example, lower species diversity and abundance of benthic organisms have been observed in areas under excessive Sargassum compared to adjacent areas.

Sargassum Inundation Events (SIEs): Impacts to Sea Turtles and Other Marine Life

SIEs can also affect other marine life, including sea turtles and marine mammals. The build-up of Sargassum in coastal areas blocks the access of sea turtles to beaches to lay their eggs and impedes turtle hatchlings from reaching the ocean. In addition, the presence of Sargassum on beaches has been found to impede the warming of nests by direct sunlight, which causes the sex ratios of hatchlings to shift towards more males. In other instances, decomposing excessive Sargassum biomass may lower nesting success in sea turtles because the decomposing biomass creates lethal high temperatures for developing embryos.

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