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The Sargasso Sea

submitted by Sheila McKenna, Arlo Hemphill

Figure 1. The Sargasso Sea is defined by the meandering currents of the North Atlantic Subtropical Gyre.

Alone in supporting the center of distribution for a holopelagic (continuously pelagic) drift algae (Sargassum spp.) community, the Sargasso Sea is a globally unique marine ecosystem whose entire water column provides a range of critical services (e.g. habitats, migratory routes, spawning and feeding grounds) to a multitude of species including endemic, endangered and commercially important ones. To illustrate how such an area can meet the EBSA criterion for uniqueness, information on the biological, ecological and oceanographic features of the Sargasso Sea from peer reviewed literature, technical reports and data sets were examined and compared to the four other similar regions of the ocean found within subtropical gyres.

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Scientific Background

The Sargasso Sea is unique in being the only area in the world to function as a center of distribution for a self-sustaining community of holopelagic (continuously pelagic) drift algae, consisting of massive amounts of floating Sargassum dominated by two species (S. natans and S. fluitans) (Conover and Sieburth 1964, Algaebase, Hemphill 2005). Globally, the paramount importance of such drift algae is its provision of structural habitat for a range of fauna, including endemic, threatened and commercially important species. In other regions of the world this habitat is rare and ephemeral, whereas in the Sargasso it is close to ubiquitous.

Defined by the currents that envelope it, the Sargasso Sea is seasonally dynamic as well as the only “sea” without coastlines (Figure 1).  Sitting on the North Atlantic Subtropical Gyre and correlated roughly with the Azores High Pressure System, the Sargasso Sea at any given time is approximately 1126 kilometers wide and 3219 kilometers long (Conover and Seiburth 1964).  A considerable wealth of information about the Sargasso Sea exists, dating back to the times of Columbus and documented in scientific publications from at least the year 1854 (Stevenson 1910). Only key aspects most pertinent to CBD uniqueness criteria are covered here, including key habitats, species and oceanographic features.

The greater subtropical and tropical North Atlantic are the only places in the world where drift algae have become holopelagic, vegetatively reproducing independent of the benthos. Sargassum algae occur in individual clumps, small patches, large rafts and weedlines. Sargassum and other flotsam can form long linear or meandering rows collectively termed “windrows” as a result of Langmuir circulations, internal waves, and convergence zones along fronts.

Many known as well as uncounted species have life-history patterns and ecology adapted to habitats unique to the floating Sargassum mats. For example, Sargassum provides habitat to the endemic Sargassum pipefish (Syngnathus pelagicus), Sargassum snail (Litiopa melanostoma), Sargassum angler fish (Histrio histrio), shrimp (Latreutes fucorum) and the Sargassum crab (Planes minutes). Many of these species have adapted characteristics to blend in with the algae.  Beyond this, many species of commercially important adult fishes (e.g. tuna, dolphin, wahoo and billfish) associate regularly to nearly exclusively with Sargassum, as do several species of sea turtle and sea bird (e.g. shearwaters, tropic birds and boobies), which use the Sargassum for foraging and roosting sites (Haney 1986). Sargassum weed is crucial to the survival of hatchling and post-hatchling hawksbill, green and loggerhead turtles. These hatchlings spend the first year or more of their lives drifting with the Sargassum, and the floating mats provide them with food and cover, increasing their chances of survival at a life stage when they are very vulnerable to predation (Carr and Meylan, 1980).

Numerous studies have shown that the size, biomass, growth stage and amount of epibiont colonization on the Sargassum can affect the distribution and abundance of a range of species including fishes. Not surprisingly, Sargassum clumps with more area and biomass tend to have a higher abundance and diversity of species.

Regionally, holopelagic Sargassum (S. natans and S. fluitans) and the vast array of species associated with their habitat are also found in the Gulf of Mexico and off the eastern continental shelf of the Unites States. The oceanographic features of the Sargasso Sea, in particular its western boundary current, the Gulf Stream (Figure 2a) and its eddies, play a key role in transporting Sargassum from the Gulf of Mexico and off the continental shelf of the United States to the Sargasso Sea (Gower and King 2008).  The fauna associated with the Sargassum drift algae habitat is less diverse before crossing the Gulf Stream and converging in the Sargasso Sea. Once in the Sargasso Sea, the species from the West Antillean region of the Caribbean associate, recruit or utilize the floating algae, hence increasing the biodiversity of organisms associated with the Sargassum.

While the Sargasso Sea is primarily unique for being the center of distribution (Conover and Sieburth 1964) of Sargassum drift algae, it is also important as a spawning site and migratory route for several species. The deep waters of the Sargasso Sea provide critical spawning sites for two species of catadromous eels, the American eel Anguilla rostrata and the Red-listed critically endangered European eel A. anguilla. The larvae of both species will drift, develop and swim in the Gulf Stream back to their respective freshwater habitats. As adults, each species of eel will migrate back to the Sargasso Sea to spawn. Populations of both these species are in decline and research shows a potential link to changes in the oceanic conditions of the Sargasso Sea (Friedland et al.  2007). Also spawning within these waters are dolphinfish (Coryphaena hippurus), jack fish, and the white marlin (Tetrapturus albidus).

Migratory routes of the yellowfin tuna (Thunnus albacores), Atlantic bluefin tuna (Thunnus thynnus - threatened), Albacore tuna (Thunnus alalunga), loggerhead sea turtles (Caretta caretta - threatened), and the humpback whale (Magaptera novaeangliae - endangered) occur in the Sargasso Sea. 

The pelagic and benthic features of the Sargasso Sea provide conditions important for biodiversity. Productivity in this otherwise low-nutrient area is increased by Gulf Stream eddies, which transport colder, nutrient-rich water into the Sargasso Sea (figure 2a). The Sargasso Sea also has several seamounts. The New England Seamount chain (Figure 2b) transverses through the northwest section while in the eastern region, the Corner seamounts (Figure 2c) are found. The Corner seamounts are known as an important deep water fish species aggregating and spawning area (Vinnichenko 1997). Along the Bermuda rise, several seamounts occur. Our knowledge of the seamounts occurring in the Sargasso Sea especially those near Bermuda is limited.

Figure2aFigure2bFigure2c

Figure 2(a) Satellite image of sea surface temperature (SST) within the North West Atlantic. The bright red color depicts the warmer water of the Gulf Stream that defines the western boundary of the Sargasso Sea. Eddy/wind interactions have been shown to pump deep nutrient rich water up to fuel plankton blooms in the Sargasso Sea increasing primary productivity (McGillicuddy et al. 2007). 2(b) Image depicts seamounts occurring in the Northwest Atlantic.  The northwestern portion of the Sargasso Sea is bisected by the New England Seamount Chain.  The dashed line denotes approximate area within Sargasso Sea. White arrow notes location of Bermuda. (Lamont Doherty Earth Observatory, Earth Institute at Colombia University, original image generated with Bill Haxby's GeoMapApp and slightly modified with dashed line and arrow here). 2(c) Map depicting the ocean floor with geological features of the Sargasso Sea (www.sitesatlas.com).

 

How the criterion was applied

This illustration is an example of how peer reviewed literature, technical reports and data sets can be used to identify areas meeting the EBSA uniqueness criterion. Here, information on the biological, ecological and oceanographic features of the Sargasso Sea was examined and compiled. We then identified similar regions and compared the defining features of the Sargasso Sea with four other similar regions of the ocean found within subtropical gyres. Many similarities were found in terms of the oceanographic features or patterns of subtropical gyres and the waters they surround. For example, oligotrophic waters are usually found sitting within all the major subtropical gyres of the oceans (i.e. North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Oceans).  The Sargasso Sea was found to be the only area in the world within a subtropical gyre that is a mass epicenter for the accumulation of vast amounts of Sargassum dominated by two holopelagic species, and important for a wide variety of endemic, threatened and commercially important species.

Sources of data

A variety of data sources were used to develop information on this unique region. These included Algal Base http://www.algaebase.org/, Bermuda Atlantic Time-series Study (BATS), European Station for Time Series (ESTOC) with associated peer reviewed publications, Fish Base http://fishbase.org/. Finally, other peer reviewed literature and technical reports were also used. It is important to note that the uniqueness criterion is broad in scope and does not lend itself to one discrete data set (for example, the home range of one species). Instead the distribution, ecology and life history of several species, populations and communities were examined as well as their habitats in tandem with the oceanographic features of the Sargasso Sea.

Important considerations

Widely recognized, the North Atlantic subtropical gyre and the Sargasso Sea is arguably one of the most well studied subtropical gyres and regions in the ocean (Mouriño-Carballido and Neuer 2008).  Hence more information on this area (the gyre and the waters lying within) and lack of information on the other less studied subtropical gyres could lead to perceived uniqueness. It should be noted, though, that the uniqueness criterion can be applied at any scale, from global to regional and local, and is dependent on best available scientific information. In the case of the Sargasso Sea, the uniqueness criterion is met on the global scale because no other area of the world’s oceans have, thus far, been identified as centers of distribution of holopelagic Sargassum drift algae. Given the amount of global monitoring now in place, through satellites and other remote technologies, it is highly unlikely any other similar places of the scale and importance of the Sargasso Sea exist, awaiting discovery.

In accordance with the rationale for the CBD uniqueness criteria (annex 1, decision IX/20), the loss of Sargassum weed habitat would likely mean the loss of, or reduction in, important endemic species and threatened sea turtles.


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