Sargassum: The Benefits and Ecological Importance 

Sargassum is best known for its namesake, the Sargasso Sea. Located within the Northern Atlantic Subtropical Gyre, the Sargasso Sea is thought to harbor ten million tons of Sargassum seaweed. This area is a unique and complex ecosystem, characterized by the golden-brown, free-floating seaweed called Sargassum. The warm tropical waters and low winds allow for the equatorial current to bring mass volumes of the seaweed into the eastern Caribbean. While uncertain until recently, it has been identified that sargassum seaweed may also be arriving to the Caribbean in larger volumes from the equatorial Atlantic area via the North Brazil Current and is also affecting West Africa (Brooks et al. 2018, Wang et al. 2019, Johns et al. 2020, Putman et al. 2018, Wang and Hu 2016, Wang and Hu 2017, Wang et al. 2019). 

The two species of Sargassum in the Caribbean area are unique to the other 300+ types of sargassum seaweed in the world as they never anchor to the bottom of the seafloor during any part of their life stage (Hinds, Oxenford, Cumberbatch, Fardin, Doyle, Cashman 2016). Sargassum seaweed creates a floating ecosystem moved by currents and surface winds that serves as a refuge for a diverse range of marine life. Many species of fish, crabs, shrimp, and turtles find shelter and food within its intricate structure. Furthermore, the floating mats of Sargassum provide a safe haven for the reproduction and development of numerous marine species, serving as a crucial nursery for juvenile fish and other organisms that benefit from its protective environment. This golden rainforest harbors biodiversity in much need of protection. 

As a safe haven for marine creatures, sargassum has been seen as a natural and healthy part of large marine ecosystems until more recently. Sargassum also sequesters carbon by absorbing carbon dioxide from the atmosphere, helping mitigate the impacts of climate change by reducing greenhouse gas levels in the air. 

Impacts, Disturbance and Losses 

While Sargassum provides habitat for many marine species, an excessive influx can have multiple negative effects. The excessive accumulation of sargassum on coastlines can cause a range of issues, including fisheries disruptions, nearshore coastal ecosystem damage, major losses in tourism, and thus impacted livelihoods in coastal communities. Since these increases in sargassum quantity coming ashore were first noted in 2011, the cause has been uncertain, but more recently is being unequivocally linked to climate change and eutrophication- nutrient enrichment from agricultural runoff and human activities. Excessive nutrients, particularly nitrogen and phosphorus, create favorable conditions for sargassum proliferation (Hinds, Oxenford, Cumberbatch, Fardin, Doyle, Cashman 2016). As we observe changing climate conditions, like sea surface temperatures, surface winds and currents, the volume of sargassum washing ashore on windward shorelines has been increasing dramatically resulting in mass strandings (Franks, Johnson, and Ko 2016, Wang et al. 2019). 

Oxygen depletion in water under thick mats of Sargassum can harm marine life, leading to fish kills and other ecological imbalances. Coastal communities heavily dependent on tourism and fisheries may experience economic setbacks due to the unsightly, odorous and potentially harmful nature of large Sargassum blooms. Disruptions in fisheries include issues with gear and tangled lines, as well as worsened conditions of visibility and access to the fish themselves beneath these floating mats. This combined with loss in tourism has created the need for alternative livelihoods in impacted areas. 

Dune ecosystems are considered essential coastal ecosystems, providing habitat for unique flora and fauna and are necessary coastal buffers to storm surges, flooding and wave action. Preventing beach erosion and acting as stabilizers, they protect inshore infrastructure providing numerous ecosystem services (Sigren, Figlus, and Armitage 2014). As sargassum strandings increase, what was once beneficial to dune ecosystems, providing nutrients and stabilization, is now becoming a suffocating health hazard to coastal animals and plants as well as humans. 

Mangroves, seagrass and coral reefs also provide ecosystem services that are valued in the hundreds of billions of dollars annually and are essential for climate resilience.These large marine ecosystems are becoming increasingly compromised by the amassing presence of high sargassum volumes. All three ecosystems are experiencing reduced oxygen, suffocation and limitations to photosynthesis when these large dense seaweed mats are coming in. This not only decreases species' own ability to survive but alters the entire food chain and environment. 

The Solution(s) 

Many of the current strategies to mitigate mass sargassum strandings are either very costly, inefficient, or extremely damaging to shorelines; destabilizing dune ecosystems, disturbing sea turtle nests and causing a loss of sand and thus beaches for tourism and recreation. The sargassum is then deposited in landfills where it releases large amounts of methane, exacerbating the original problem. New approaches are urgently needed. 

Here at The Ocean Foundation (TOF) we have been working in Sargassum mitigation and processing since 2019 across the Caribbean. Recognizing the environmental and socio-economic impact of these seaweed blooms, TOF has strategically engaged in projects aimed at understanding, mitigating, and managing the challenges posed by sargassum with site specific and adaptive strategies, and long term partnerships that retain the leadership and priorities in the hands of local communities. 

Our main strategy is harvesting the fresh sargassum and diluting it at a rate of 1:1 with other organic material, often provided by the local forestry service or agricultural community. This is then turned into highly fertile, nutrient dense compost that can be used on local farms facing degraded soil, erosion and fractured local food systems. In our sites in the Dominican Republic, farmers have reported 200% increases in yields and higher quality produce. We now have sites in St. Kitts and Nevis, Cuba, and soon to be in Puerto Rico and Mexico. We also create biochar from the sargassum, through a process of slow pyrolysis under limited oxygen. This is then used as a soil amendment to increase water retention, disease resilience, nutrient uptake, and fertility for degraded soils. Biochar is also considered as long term carbon sequestration. 

We are able to address concerns from the use of seaweed composts, primarily that of high salinity and possible heavy metal accumulation (Ocean Harvest Technology 2016, Tirolien 2019), through our compost methodology. The dilution of the sargassum with organic materials is at a ratio that allows for the heavy metals, primarily Arsenic, to decrease to safe levels for agricultural consumption. We are also exploring bacterial, enzymatic and fungal remediation strategies to remediate heavy metals. Through mandatory and regular analysis of soil both for heavy metals, contaminants and general composition of minerals and nutrients we are collecting a dataset to assure that this will be the case long term and that we can produce a replicable model that can be used throughout the Caribbean. High salinity can cause soils to become unproductive over time so it is necessary to have a washout phase of the composting process, which happens naturally if there is regular rainfall while the sargassum is breaking down, and otherwise requires somewhat regular watering and turning during dry periods. 

This sargassum project allows for the removal of nuisance and harmful excessive seaweed strandings, the conversion of this material into a resource rather than refuse, creates jobs for multiple sectors, produces an agricultural commodity that can be used on local farms or sold to generate local income, and protects essential coastal ecosystems that are needed for long term climate resilience and biodiversity. 

Sargassum seaweed, with both its ecological benefits and harmful impacts, embodies the complexity of nature's delicate balance. While it plays a vital role in supporting marine life, the increasing presence of sargassum on coastlines necessitates a deeper understanding of the underlying causes. Balancing human activities, mitigating nutrient runoff, and addressing climate change are crucial steps towards preserving 

the ecological integrity of our oceans and the delicate ecosystems within. Deforestation, urbanization, and the release of pollutants into coastal waters exacerbate the conditions that promote sargassum growth. Human activities play a significant role in altering the natural balance of marine ecosystems.