Pelagic Sargassum is a macroalgae that drifts at the ocean surface, creating a unique and ecologically-significant marine ecosystem. Accumulations of Sargassum, whether they be isolated clumps or aggregated long windrows or mats covering 100’s of square meters, serve as a food source, nursery for juveniles, spawning ground and/or protective habitat for diverse invertebrate, fish, turtle and seabird species at various points of their life cycle.
For over 45 years, SEA has been documenting Sargassum species, abundance and distribution along repeated cruise tracks through the Sargasso Sea, Caribbean Sea and Gulf of Mexico. Recent investigations include the associated epibiont and mobile fauna communities, seasonal and interannual variability in distribution, and extraordinary Caribbean inundation events. SEA Semester students, faculty and collaborators use a variety of tools to examine Sargassum itself and its hosted organisms, from field observations to morphological metrics to molecular analyses of diversity. Marine spatial planning and policy initiatives working to conserve the Sargasso Sea ecosystem also benefit from SEA students’ contributions and creative ideas. Seeking to share Sargassum research beyond the scientific community, SEA faculty scientists have taken part in many outreach events and were featured on the Meet the Ocean podcast.
Historically, only two species of pelagic Sargassum were recognized in the North Atlantic: S. natans and S. fluitans. However, field collections via dip net and surface net tow combined with morphometric and genetic analyses by SEA scientists have identified distinct forms of both species. Presence or absence of thorns on the stem distinguishes between species: S. natans has smooth stems while S. fluitans has thorns. Within a species, blade and float attributes differ widely among forms – S. natans I has long, narrow blades and floats frequently adorned with an apical spine whereas S. natans VIII has long, wide blades with floats that rarely exhibit a spine. Preliminary observations indicate that a third form, S. natans II, can be identified by blade width between that of S. natans I and S. natans VIII and floats adorned with a spine. S. fluitans III has short, compact blades and no spines on the floats. Because Sargassum forms have discrete but overlapping ranges, correct field identification through visual study is critical for addressing questions of distribution, evolution, and connectivity between geographic regions. Genetic tools developed by SEA researchers distinguish morphotypes and further resolve phylogenetic relationships among Sargassum specimens.
SEA faculty and collaborators: Jeff Schell (SEA), Deb Goodwin (SEA), Kerry Whittaker (SEA), Amy Siuda (Eckerd College)
Selected Taxonomy papers and publications
SEA’s 45-year plankton net tow data set indicates that distinct seasonal and interannual distribution patterns exist for each Sargassum species and morphological form. S. natans I is most commonly found in the central Sargasso Sea while S. natans VIII is often seen in the tropical Atlantic and Caribbean, and S. natans II is most frequent in the western Caribbean. S. fluitans III is broadly distributed across the Gulf Stream, North Equatorial Current, and Caribbean in addition to the Sargasso Sea. SEA field observations complement satellite detection methods by providing species identification and finer spatial resolution.
SEA faculty and collaborators: Jeff Schell (SEA), Deb Goodwin (SEA), Amy Siuda (Eckerd College)
Selected Distribution patterns papers and publications
Community of organisms
A uniquely adapted pseudo-benthic community of organisms lives among the dense vegetation of each pelagic Sargassum clump, including common epifauna such as hydroids and mobile fauna such as shrimp, snails and crabs. SEA scientists and students seek to understand factors affecting the diversity and composition of this community, such as host species, genetic connectivity, geographic location, clump size and clump aggregation/dispersion pattern. Both visual/microscope-aided identification and genetic population analyses are utilized. Preliminary studies reveal that each Sargassum species and morphological form harbors a unique assemblage of organisms. Further, emerging SEA research suggests that the genetic structure of epibiont species could help explain questions of Sargassum origin and distribution.
SEA faculty and collaborators: Jeff Schell (SEA), Deb Goodwin (SEA), Kerry Whittaker (SEA), Amy Siuda (Eckerd College), Annette Govindarajan (WHOI), Lindsay Martin, Madelyn Taylor
Selected Community of organisms papers and publications
During 2011-2012, 2014-2015, and again in 2018, pelagic Sargassum washed ashore in unprecedented quantities throughout the tropical Atlantic, including on many Caribbean islands. Once-pristine tourist beaches were covered by meters of stranded seaweed. Researchers at SEA discovered the Sargassum that inundated the Caribbean in 2014-2015 was the previously-rare form S. natans VIII, originating in the northern tropical Atlantic region. Inundation events have ecological consequences at multiple scales, such as impacts to Sargassum mobile fauna communities, dependent fisheries and iconic species, and coastal ecosystem function.
Selected Inundation events papers and publications
SEA Faculty, Crew and Alumni Help Unlock Mysteries of Pelagic Sargassum
July 08, 2021
Doug Karlson, Marketing & Communications Coordinator
Sea Education Association alumni, faculty, and crew are the co-authors of a new research paper, Pelagic Sargassum morphotypes support different rafting motile epifauna communities, published June 29th in Marine Biology, that sheds new light on Sargassum ecology.
SEA Semester’s Dr. Deb Goodwin Comments on Sargassum Inundation
July 08, 2019
SEA Semester in the NEWS
“Why Waves of Seaweed Have Been Smothering Caribbean Beaches”
By Ed Yong
Since 2011, blooms of Sargassum have wreaked havoc on tropical shores. A new study explains why this is likely a new normal.
Student Researchers Dive Deep to Better Understand Sargassum and Its Impact on Coastal Communities
May 22, 2019
Alexandra Reilinger, Cecilia Howard, Gail Johnson, Vassar College, Johns Hopkins University, Oberlin College
The seaweed appears as if out of nowhere, vast swaths suddenly blanketing the beaches of Caribbean islands, yet little is known about the many various forms of the Sargassum seaweed. Student researchers set out to study the genetics of the pervasive weed, a critical building block of the ecologically rich Sargasso Sea, to better understand the role it plays in the dynamic ocean environment.
The Ocean as Classroom
May 16, 2019
Doug Karlson, firstname.lastname@example.org
An in-depth conversation with SEA Professor of Oceanography Jeff Schell on teaching at SEA, the health of coral reefs, and the mysteries of the Sargasso Sea
Professor of Oceanography Jeff Schell is the former director for SEA’s Colonization to Conservation in the Caribbean program and led the creation of SEA’s Reef Expedition programs. A graduate of College of the Holy Cross (BA), SUNY Stony Brook (MS) and University of Wisconsin at Madison (PhD), his areas of interest include the ecology of marine and freshwater habitats with a focus on distribution, diversity, and species composition of plankton communities, the ecology of pelagic Sargassum and its associated community, marine environmental history, interdisciplinary approaches to teaching, science illustration and storytelling.
Deploying the Neuston Net
February 28, 2019
With nearly 50 years of surface neuston tow data, SEA’s archives offer the unique opportunity to examine biological response to global change. SEA Semester leverages the opportunities presented by its remote, open ocean cruise tracks and repeated annual sampling to build valuable datasets in poorly studied areas of the world.