One summer morning in 2011, the smell of rotting eggs marred “another day in paradise” for residents of the eastern Caribbean nations. The distinctive smell signaled the beaching of mountains of Sargassum seaweed on their shores.
The unprecedented event represented just the beginning of an annually recurring and growing region-wide phenomenon. The largest quantity and frequency increase of Sargassum beachings to date occurred in 2015 (there have been fewer beachings so far in 2016). The algal masses covered popular tourist beaches, filled bays, and caused numerous environmental and economic problems.
The Sargassum beaching events in the Caribbean, West Africa, and other regions have received wide media attention, prompting action by regional governmental agencies and environmental groups seeking to understand this new phenomenon. In 2015, excessive Sargassum beachings along the western Caribbean coast threatened the coastal economy in Mexico—the incidents became a cabinet-level crisis, and the Mexican Navy was called to take action.
Long-term predictions of Sargassum blooms and beaching events will require a thorough biological and ecological understanding, as well as integration of observations of oceanographic and climate data into forecasting models. On the other hand, short-term predictions that rely on numerical models and timely information of the location and quantity of Sargassum at sea are vital for field survey planning and short-term management decisions, as well as for tourism and other business development.
Here we highlight recent activities regarding Sargassum observations. We also introduce a prototype integrated Sargassum Watch System to monitor and track Sargassum in near-real time using satellite imagery and numerical models and emphasize targeted research and monitoring needs.
Sargassum as a Habitat and a Marine Resource
Sargassum natans and Sargassum fluitans are mostly found in the Intra-Americas Sea (including the Gulf of Mexico and the Caribbean) and Atlantic Ocean. Unlike the more than 350 taxonomically identified ocean bottom-dwelling (benthic) Sargassum species worldwide, these are the only two holopelagic species of the Sargassum family, meaning that they spend their entire life cycle in surface water.
The benthic species reproduce sexually, but S. natans and S. fluitans reproduce solely by vegetative fragmentation. Generally, Sargassum seaweed are not differentiated into true leaves, stems, and roots; rather, they have tough, leathery, densely branched thalli, composed of axes, blades, and air bladders. Both S. natans and S. fluitans serve as unique and important habitats for a diverse group of marine animals, providing food, shade, and shelter to fish, shrimp, crabs, and turtles [Witherington et al., 2012; Lapointe et al., 2014].
Sargassum in the Atlantic Ocean is regarded as the “golden floating rainforest” by the Sargasso Sea Alliance [Laffoley et al., 2011]. The Gulf Coast Research Laboratory has identified 139 species of fish associated with Sargassum, which also represents a marine resource for food and fuel. Sargassum serves as fertilizer for the sand dune plant ecosystems that help protect shoreline stability and promote biodiversity of seashore birds, crabs, and other small invertebrates.
A Beach Nuisance
Although it’s an ecosystem boon to the open ocean, excessive amounts of Sargassum on beaches in populated areas represent a nuisance and must be physically removed.
Sargassum beaching events are new to many residents in the southern Caribbean, West Africa, and Brazil but are well known in the northern Gulf of Mexico (especially Texas), the Sargasso Sea, and the northern Caribbean islands. Sargassum emits hydrogen sulfide gas as it decomposes onshore, releasing an odor like rotting eggs. This decomposition causes many environmental problems by attracting insects, smothering turtle nesting sites, and killing sea turtles and fish [Feagin and Williams, 2010].
Sargassum beaching events cause economic problems as well [Franks et al., 2011]. A survey of 424 private companies by the Chamber of Commerce of Guadeloupe estimated that diminished tourism caused an economic loss of $5.5 million dollars for the first half of 2015. However, despite a handful of studies, the driving factors leading to recent increases in beaching events remain largely unknown. Addressing these unknowns will require coordinated research, but increased beaching events call for immediate and effective monitoring and forecasting systems.
Sargassum Watch from Space
The University of South Florida’s (USF) Optical Oceanography Laboratory (OOL) has been working on a virtual antenna system (VAS), which downloads and processes raw satellite data into customized images, since 2010. These images are distributed through a Web portal within 4–6 hours of the satellite overpass [Hu et al., 2014], providing the infrastructure to implement a Sargassum Watch System (SaWS).
Specifically, the USF OOL has used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua satellites to examine the red edge reflectance (enhanced reflectance in the near infrared) of floating vegetation, using a floating algae index (FAI) [Hu, 2009] or alternative FAI (AFAI) [Wang and Hu, 2016]. These indices are similar to the maximum chlorophyll index used for the first time by Gower et al.  to detect Sargassum from space.
Because floating vegetation has enhanced reflectance in the near-infrared spectral bands, these products clearly show Sargassum aggregations as elongated slicks in AFAI imagery (Figure 1) that can be easily identified even with an untrained eye. The system covers the Intra-Americas Sea in subdivided regions (Figure 2), with the capacity to extend as far as West Africa and Brazil.
Tracking Sargassum Rafts
In addition to the AFAI imagery, SaWS also uses nightly updated surface current data from the Hybrid Coordinate Ocean Model (HYCOM), via the VAS. All data products (AFAI, HYCOM currents, sea surface temperature, and surface color patterns, among others) can be displayed in Google Earth with a simple mouse click, thereby facilitating visualization and navigation (Figure 3).
Within Google Earth, once a Sargassum raft is identified, its latitude and longitude, combined with current speed and direction near the raft, can be used to predict the movement of the raft and a possible beaching time—in essence, forming an early warning system. This method has actually been used by a company called Nova Blue Environment, which has released Sargassum beaching risk alerts every week to local authorities in Guadeloupe (French West Indies) since May 2014. Several alerts were correlated to actual Sargassum beaching sites identified by helicopter surveys.
Thanks to the watch system, many local groups have been able to monitor and track large Sargassum aggregations on an almost daily basis. In Puerto Rico, researchers have been using it routinely to detect Sargassum aggregations and conduct research, finding excellent accuracy of the system when tracking large rafts and ocean eddies (J. Morell, University of Puerto Rico, personal communication, 2015).
Other Sargassum Tracking Remote Sensing Methods
The same red edge concept used on MODIS has been extended to Landsat sensors. Unlike MODIS’s daily revisit cycle, Landsat passes a given region every 16 days. However, Landsat sensors have much higher (30-meter) resolution than MODIS, which can reveal small aggregations that are invisible in the 1-kilometer-resolution MODIS imagery. Currently, SaWS produces high-resolution FAI imagery from Landsat 8 in several selected regions in the Caribbean. Similarly, Landsat instruments have been used to provide early alerts on possible Sargassum beaching events along the Texas coast through a Sargassum Early Advisory System (SEAS) [Webster and Linton, 2013].
Likewise, local groups have used aircraft to assess the extent and severity of Sargassum beachings in Guadeloupe (F. Mazéas and M. Laurent, DEAL Guadeloupe, personal communication, 2015). Between 2011 and 2015, eight 2-hour Civil Security helicopter flights were completed to document the most affected areas of Guadeloupe Island, with a comprehensive report issued from each airborne survey.
A combination of the various systems may maximize the value of both spaceborne and airborne observations through coordinated efforts.
Sargassum Watch from Ships
Observing pelagic Sargassum is not new—it started with the voyage of Columbus and other early explorers and became a quantitative science almost a century ago [Parr, 1939]. Shipboard studies in the 1980s and 1990s were the first to suggest that with a reproduction rate that doubles or triples biomass every 25 days [Lapointe et al., 2014]), this vegetation was more productive than previously thought [Lapointe, 1995]. Systematic Sargassum surveys have been conducted by the Sea Education Association since the 1970s during annually repeated 6-week cruises, resulting in biomass per unit area data for more than 7000 net tows between 1992 and 2015 (Figure 4).
Data synthesis is still underway, but preliminary results show distinct distribution patterns across time and space for each Sargassum species, as well as proliferation of a previously rare morphological form (Sargassum natans VIII) and decreased abundance and diversity of associated fauna during the 2015 Caribbean inundation event [Schell et al., 2015].
Other researchers and volunteers have also made field observations, including Sargassum beaching times and locations. The Gulf Coast Research Laboratory manages a Web portal specifically designed for ad hoc reporting of Sargassum sightings made during at-sea encounters or coastal observations. Such information complements that obtained from systematic ship surveys to provide a more complete picture of Sargassum distributions.
Both remote sensing and ship surveys may provide information on the distribution, relative abundance, and long-term trend of this important marine habitat. Wang and Hu  used MODIS data to provide a long-term time series for the central west Atlantic region (Figure 5) that showed dramatic increases in Sargassum abundance from 2013 to 2015. However, a full understanding of Sargassum biology and ecology [e.g., Carpenter and Cox, 1974; Lapointe, 1995] requires efforts far beyond those outlined here.
For example, how much total biomass is in a certain region? What is the chlorophyll, carbon, nitrogen, phosphorus, and iron content of this biomass? How does the associated fauna community vary between geographic regions and Sargassum species or morphological forms? How does Sargassum influence the oceanic carbon and nutrient cycles? What environmental factors drive its productivity, growth, and distribution? How does Sargassum respond to such global changes as increasing reactive nitrogen and carbon dioxide? How do ocean warming and changes in ocean currents affect Sargassum?
Without addressing these research questions, it is difficult to explain the sudden increases in tropical Atlantic and Caribbean Sargassum blooms after 2011. In situ observations of abundant S. natans VIII suggest that recent Sargassum blooms in the Caribbean did not originate from the Sargasso Sea [Schell et al., 2015]. Results from Gower et al.  and our preliminary analysis also indicates a tropical Atlantic origin. However, we still don’t know why the blooms tend to originate there.
Furthermore, even though Sargassum biomass per area may be comparable to that of water column phytoplankton, Sargassum has generally been considered insignificant to marine productivity and is rarely addressed in oceanic biogeochemical studies. Clearly, because only a handful of studies focusing on its biogeochemistry and ecology are available, more coordinated research is urgently needed. The unprecedented beaching events in recent years may promote opportunities to conduct dedicated research to fully understand this ecologically and economically important marine algae.
Massive Sargassum beaching events are an entirely new phenomenon for many residents in the Caribbean and West Africa. The seaweed has caused many negative effects, but at the same time it may also provide raw materials for commercial use. Local residents need a system to forecast Sargassum blooms and Sargassum beaching in both the long and short terms. This is essential for investment decisions in cleanup strategies and technologies and in development of value-added commercial products and manufacturing infrastructure.
At present, no one can predict the long-term trend of Sargassum blooms, but it is unlikely that the beaching events of the past 5 years will stop. Continuous and improved monitoring and forecasting systems at regional and local scales are necessary for management and decision making. The Visible Infrared Imaging Radiometer Suite (VIIRS, 2012 to present) is equipped with spectral bands similar to those of MODIS, and currently, VAS generates and distributes VIIRS AFAI imagery in the same manner as MODIS (Figure 1). NASA and the European Space Agency are also planning several advanced ocean color missions (Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE), Geostationary Coastal and Air Pollution Events (GEO-CAPE), Sentinel-3) with improved spectral and temporal resolutions to enhance SaWS capabilities. Additionally, high-resolution sensors such as the MultiSpectral Instrument (MSI; 10–60 meters) on Sentinel-2 (2015 to present) and follow-on sensors can also be used to track small Sargassum aggregations, similar to the use of the Landsat 8 Operational Land Imager (OLI).
Various governmental agencies are taking actions to facilitate research, monitoring, and forecasting. For example, a Gulf Coast Sargassum Symposium in April 2015 brought together various stakeholders concerned with the effects of Sargassum on coastal states of the Gulf of Mexico to discuss the current status and future directions in Sargassum research, monitoring, and forecasting. The Predicting Sargassum Blooms (PSB-CARIB) project has been recently funded by the French Environment and Energy Management Agency, Regional Council Martinique, and Direction de L’environnement de L’aménagement et du Logement Guadeloupe to study Sargassum blooms in the central Atlantic and the Caribbean. Similar projects in French Guiana, Mexico, and other countries or regions are also underway.
SaWS received direct support from NASA to improve and enhance its capacity for near-real-time monitoring and tracking of Sargassum blooms (Figures 1–3). Further, NASA’s Jet Propulsion Laboratory has partnered with the Sargasso Sea Commission to develop a prototype data portal to compile all relevant data (e.g., Sargassum location and abundance, sea surface temperature, sea surface height, ocean currents, winds), which will help us understand spatial and temporal Sargassum distributions.
Collaboration and Coordination
With increased funding and research activities to study Sargassum ecology, biogeochemistry, and how Sargassum responds to environmental changes, our knowledge of Sargassum and our short- and long-term predictive capabilities are expected to improve significantly in the near future. At the same time, better collaboration and coordination will help research and monitoring efforts to develop long- and short-term forecasting. Moreover, improved public access to data products will assist with management decisions and economic development.
Most of the satellite-based Sargassum watch has been supported by NASA through its Ocean Biology and Biogeochemistry program and Gulf of Mexico Program. We thank NASA, the National Oceanic and Atmospheric Administration’s National Environmental Satellite, Data, and Information Service, and the U.S. Geological Survey for providing MODIS, VIIRS, and Landsat data. We also thank the HYCOM consortium for providing numerically modeled ocean currents. We are grateful to three anonymous reviewers, the editor of Eos, and the staff members of the editorial office for helping to improve the presentation of this article.