Climate Adaptation and Environmental Specialist, ESSA in
Research Associate, Changing Ocean Research Unit, University of British Columbia
Sargassum blooms commonly occur and wash up on southern Caribbean beaches, but 2018 was different. The scale of the 2018 event was unprecedented: hundreds of beaches from French Guyana and the Atlantic coast of Trinidad in the south to Mexico and Florida in the north experienced the worst strandings of Sargassum in history. Some bays reportedly filled up in a matter of hours with tens to hundreds of thousands of tonnes of algae. At sea, Sargassum – a type of large brown seaweed – coverage in the Caribbean and West Atlantic in October 2018 was estimated at 323 km2, which is more than three times the historical average for the years
2011 to 2017. Of note, the Sargassum bloom in 2011 was itself 200-fold greater than the average of the previous eight years (Huffard et al. 2014).
2018 was also the first instance of year-round occurrence of Sargassum blooms in the Caribbean Sea. The Sargassum Watch System (SaWS) of the University of South Florida (USF) – a satellite-image based model – serves as an early warning system for Sargassum blooms (Wang and Hu 2017). Monthly outlooks of Sargassum occurrence, based on model outputs, are made available on its website. The model predicts bloom occurrence between May and August in the Eastern Caribbean by the end of February with 80% accuracy. As early as January 2018, SaWS reported unusually-large aggregations of Sargassum. By early spring, the Eastern Caribbean islands of Guadeloupe, Barbados, Montserrat and Dominica were already reporting major strandings. These events continued throughout the summer, impacting the Yucatan Peninsula by June. Sargassum coverage decreased in November and mostly disappeared by December. Should such patterns continue into the future, Sargassum blooms could become less of a seasonal phenomenon, instead impacting shorelines intermittently on an annual basis.
Speculations about triggers for the mass-blooms are varied, including nutrient loading, far-off dust storms, and the effects of climate change and variability. Very high nutrient loads flushed out to sea from agriculture and poor land-use through much of the Amazon Basin are among the triggers put forward for these mass blooms (Louime et al. 2017; Schell et al. 2015; Johnson et al. 2013; Huffard et al. 2014). A fertilizer effect due to increased iron and phosphate from Saharan dust fall over the mid-Atlantic has also been suggested as a possible cause (Johnson et al. 2013). In 2018, thick atmospheric plumes of dust from the Sahara and spreading right across the Atlantic to the Caribbean, as well as heavier than normal haziness and respiratory illness were reported (NASA 2018; Dolce 2018). This same pattern played out in 2015, a year also associated with a mass-bloom event (Destin 2015). Climate change and variability may also play a role. Higher water temperatures and increased flushing of terrestrial nutrients due to heavier rainfall events are among the climate change effects linked to mass Sargassum production (Johnson et al. 2013). Studies have attributed the transport of Sargassum mega blooms fertilized off the coast of Brazil to an Atlantic Equatorial Counter Current strengthened by warm sea-surface temperatures during El Niño events (Vos et al. 2016). Another climate-driven trigger could be rougher winter seas, which fragment the plants. This is important as pelagic Sargassum reproduces exclusively through vegetative means (Redmond 2014).
Regardless of the cause for the blooms, researchers now believe that the main nursery for Sargassum mega blooms is located off the West African coast (Schell et al. 2015; Vos et al. 2016; Putman et al. 2018), and not the Sargasso Sea, a well-known source area north of the Caribbean Sea (Hill et al. 2015).
Out at sea, large rafts of Sargassum provide important habitat to a wide range of species (Casazza & Ross 2008; CRFM 2016). However, when massive quantities of the seaweed drift close to shore the mats are deathtraps for many animals and can contribute to the degradation of important coastal habitats. Once Sargassum enters the foreshore, and their gas bladders that allow them to float burst in breaking waves, the plants lose their buoyancy, get propelled to shore by nearshore circulation patterns and masses of vegetation build up on the shoreline rapidly (e.g., Maurer et al. 2015).
The accumulating and beached algal mats have had a number of negative impacts on marine animals. Foraging adult turtles and marine mammals can become entangled and trapped in the mats. Stranded Sargassum can restrict access of nesting beaches to turtles, especially with peak nesting season coinciding with the peak depositional period (April to October). The mats may also prevent warming of nests by direct sunlight, skewing hatchlings’ sex ratio towards males (Maurer et al. 2015). Recent research on the Serranilla Bank, offshore of Caribbean Colombia, indicates that large accumulations of Sargassum also reduce the chances of hatchlings successfully reaching the open ocean (Gavio and Santos-Martinez 2018). Adverse impacts to inshore fish nurseries like mangroves (FAO 2017) and seagrass meadows (Gill et al. 2015) have also been documented.
Beached Sargassum has afflicted two key Caribbean industries, fisheries and tourism, resulting in lost revenues and unanticipated remediation costs. Fisheries impacts include nuisance to fishing vessel landings and nearshore navigation in general (Ramlogan et al. 2017) as well as damage to fishing gear, loss of time and opportunity, and changes in productivity, particularly of the more seasonal pelagic species such as flying fish and dolphinfish (FAO 2017). Impacts on tourism are yet untallied. Although visitation rates appear stable, the tourism industry is concerned about Sargassum’s negative aesthetic effects on beaches, the health effects of the noxious smell associated with the rotting seaweed (CAST and Caribbean Hotel and Tourism Association 2015, Resiere et al. 2018), the cost of removal and possible damage to reputation. The quality of visitors’ experience is also affected by the loss of beach and dive site access (Baker et al. 2015), which may impact the potential for repeat business and visitor satisfaction reports.
Several ideas and approaches to manage extreme accumulations of Sargassum on beaches have emerged in recent years. One regional response after the mass event in 2015 led to the creation of a protocol (CRFM, 2016), which promotes beach clearance as the main response to Sargassum beach strandings. No action is suggested for nearshore (in-water) pile-ups. The protocol recommends removal and burial, with tractors only to be employed for accumulations greater than 10 inches (almost always the case). Such a method is effective, but contributes to the destruction of sea turtle nesting habitat and beach erosion. Other ideas include letting nature run its course (Hinds et al. 2016), and allowing the mats to decompose on beaches. Frustration with the impacts associated with strandings and their clearance led a Guadeloupe company to develop an offshore system that uses a conveyor belt to collect the seaweed and load it into a boat (Fogarty 2018). An extensive system of oil-spill containment booms modified to prevent algal mats from coming ashore was also deployed in Mexico, Antigua and the Dominican Republic. Both strategies look promising, although clean-up success was highly localized because of the sheer scale of the blooms.
Considerable creativity has also gone into finding beneficial uses for stranded Sargassum. These include using it as raw material for chipboard (Hinds et al. 2016), mulch (CAST and Caribbean Hotel and Tourism Association 2015), fertilizer (GEF 2018), and adobe houses (in Mexico). Seaweed has been studied as a source of chemical compounds for pharmaceuticals and personal care, fish feed, fertilizer and fuel (Milledge and Harvey 2016). The suitability as compost/fertilizer is currently being assessed by France’s Centre National de la Recherche Scientifique and l’Université des Antilles (CRNS, 2018). Use for animal or human consumption may be limited as the species assimilate heavy metals and toxins. The Gulf and Caribbean Fisheries Institute recently released a factsheet outlining do’s and don’ts for responding to the influx on Sargassum on Caribbean beaches (see https://www.gcfi.org/EmergingIssues/Sargassum/ONLINE-Sargassum.png).
The exploration of beneficial uses for and sustainable clean-up options of stranded Sargassum are commendable. However, these measures do not address the cause underlying the massive accumulations and widespread consequences facing Caribbean resource managers.
Efforts are afoot to stimulate innovative and preventative approaches to dealing with the Sargassum problem. Antigua’s Department of the Environment has initiated a procurement process to find viable alternatives to collect Sargassum while at sea (Government of Antigua and Barbuda 2018). Mexico is similarly searching for solutions that build on the local success of the 27-km-barrier solution implemented in 2018 (Mexico News Daily 2018). In June 2018, the former French Environment Minister presented a €10 million plan to help combat the problem in Guadeloupe (Mufson 2018). The Inter-American Development Bank is exploring funding the economic evaluation of the impacts associated with the 2018 Sargassum mass-bloom and supporting creative solutions through its Blue-Tech Challenge Programme launched in September 2018 (Moscoso 2018). Also in 2018, the consortium CLS-NovaBlue Environment (NBE) was awarded a project with the European Space Agency to implement an innovative Earth Observation service to provide a state of the art early warning system for Sargassum blooms.
As the impacts of mass Sargassum blooms and future prospects become clearer so is the need for pooling resources and coordinating action. Controlling the blooms while the mats are still manageable in size at sea could be key. Yet, there are trade-offs, as stopping Sargassum accumulation while still in the mid-Atlantic would limit the many functions (including shelter and nursery) floating mats provide to a wide array of species with unknown consequences for biodiversity and ecosystem system services such as fish catches. Robust information on environmental impacts, disaster risks and economic costs of mass-blooms are needed and should guide support for research and development as well as the testing of new technologies and innovative approaches. With heightened global attention on the world’s oceans the time is now for joint collaborative Caribbean action on this shared risk before it becomes a recurring environmental disaster.
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