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Ecologically or Biologically Significant Areas (EBSAs)

  published:15 Jun 2015

The Sargasso Sea

General Information
The Sargasso Sea is a fundamentally important part of the world ocean, located within the North Atlantic sub-tropical gyre with its boundaries defined by the surrounding currents. Its importance derives from a combination of physical and oceanographic structure, complex pelagic ecosystems, and its role in global ocean and earth system processes. The Sargasso Sea is home to an iconic pelagic ecosystem with the floating Sargassum seaweeds, the world’s only holopelagic algae, as its cornerstone. It hosts a diverse community of associated organisms that includes ten endemic species, and provides essential habitat for key life stages of a wide diversity of species, many of which are endangered or threatened. The Sargasso Sea is the only breeding location for European and American eels, the former being listed as critically endangered, and is on the migration route of numerous other iconic and endangered species. A variety of oceanographic processes impact productivity and species diversity, and the area plays a disproportionately large role in global ocean processes of oxygen production and carbon sequestration. The sea floor has two large seamount chains, home to specialized, fragile and endemic communities, and models predict the presence of numerous other isolated seamounts. Both pelagic and benthic ecosystems are impacted by a range of human activities, and the currents of the gyre act to concentrate pollutants. The portion of the Sargasso Sea described here meets all of the criteria for ecologically orbiologically significant areas (EBSAs) under the Convention on Biological Diversity.
First recorded by Columbus and named after the characteristic floating brown algae, Sargassum spp., the Sargasso Sea is an area of open-ocean within the North Atlantic Subtropical Gyre, bounded on all sides by the clockwise flow of major ocean currents. It is one of the best known areas of the world’s ocean, studied since the 1870s and home to Hydrostation S, the longest running time series of oceanographic measurements, begun in 1954. Data from the Sargasso Sea are critical to our understanding of global ocean processes and global change. Water retention within the North Atlantic gyre is estimated to be up to 50 years (Maximenko et al. 2011), and a variety of oceanographic features and processes influence the ecology and biology of the Sargasso Sea on different spatial and temporal scales. Cyclonic or anticyclonic rings and eddies spun off the Gulf Stream may persist as distinct entities for many months to years (Richardson et al. 1978, Cornillon et al. 1986, McGillicuddy et al. 1999). There are also smaller mode water eddies that form in mid-water. The different types of eddies create localised upwelling and downwelling and impact the upper layers of the Sargasso Sea by mixing surface and deeper waters. This affects nutrients, heat and salinity which together create localised areas of high or low productivity (Volk and Hoffert 1985, Maurino-Carballido and McGillicuddy 2006, Glover et al 2008). The eddies impact biodiversity by “capturing” and bringing “foreign species” into the area, creating relic populations which may persist for months or, conversely, by spinning species out into the Gulf Stream (Boyd et al. 1978, Wiebe and Boyd 1978, Ring Group 1981). The Subtropical Convergence Zone (STCZ) occurs between 20° and 30°N in the western Sargasso Sea, where warm and cold water masses meet and create distinct thermal fronts in the upper 150 m of the ocean from fall through spring (Katz 1969, Weller 1991). Sargassum weed accumulates in this area, along with other organisms, so the fronts are important feeding areas for predatory pelagic fishes and migratory marine mammals in the Sargasso Sea. These fronts are also zoogeographic boundaries between warm and cold water masses (Backus et al 1969, Miller and McCleave 1994), and the presence of organisms from different biogeographic provinces enhances diversity in the area. Conventionally regarded as an area of low nutrients and low productivity enhanced by localized upwellings, the Sargasso Sea, per unit area, has a surprisingly high net annual primary production rate that matches levels found in some of the most productive regions in the global ocean (Steinberg et al. 2001, Rho and Whitledge 2007, Lomas et al. 2012). This is due to a complex combination of factors—the production of carbon in the surface waters by photosynthesis, the location of the Sargasso Sea in the sub-tropics, which results in a deep euphotic layer, and differences in phytoplankton communities and associated nitrogen fixation (Chisholm et al. 1988). The difference is that in the Sargasso Sea most of the production is recycled by bacteria in the so-called microbial loop rather than channelled into biomass of larger, harvestable organisms (Carlson et al. 1996, Steinberg et al. 2001). As a result of this high primary productivity, to which must be added the annual production of Sargassum weed, which is now being estimated from satellite measurements (Gower and King 2008, 2011), the Sargasso Sea plays a key role in both global oxygen production and ocean sequestration of carbon (IPCC 1996, 2001 and 2007, Ullman et al. 2009, Lomas et al. 2011 unpub). The bathymetry of the Sargasso Sea from west to east passes from the continental rise of the North American continental margin at around 2000m depth, descends gently into part of the Hatters, Nares and Sohm abyssal plains, with depths reaching over 4500m, then becomes progressively shallower toward the mid-Atlantic Ridge, where water depth is less than 2500m. This regional relief is modified dramatically by extinct volcanoes that form the Bermuda Islands, seamounts and associated Bermuda Rise, the Muir seamount chain, and the New England and Corner Rise seamount chains further north (Figure 1). Modelling also indicates that there are numerous smaller isolated seamounts distributed across the eastern part of this area. Several major fracture zones, the Atlantis, Northern, Kane and Blake Spur, cross the area (Figure 1). (See Uchupi et al. 1970, Vogt and Jung 2007, Parson and Edwards 2011 unpub, Yesson et al. 2011, Halpin et al. 2012 unpub).
Description of the location
Wider Caribbean and Western Mid-Atlantic
The Sargasso Sea is surrounded by the Gulf Stream to the west, the North Atlantic Drift to the north, the more diffuse Canary Current to the east, and the North Equatorial Current and the Antilles Current to the south (Figure 1). These currents vary, however, particularly the Canary Current, so the precise boundaries of the Sargasso Sea also vary (Ryther 1956, Butler et al. 1983, Coston-Clements et al. 1991). A map of the Sargasso Sea showing characteristics such as ocean current and eddy occurrence, remote sensing of Sargassum weed distributions and seabed topography (Ardron et al 2011, unpub) was commissioned by the Sargasso Sea Alliance in order to refine the area under consideration in a variety of proposals including the present one. It accounts for the variable eastern boundary current and the EEZs of adjacent countries, while ensuring that the full range of ecologically important features is included. For pragmatic reasons, the eastern boundary is considered to lie to the west of the mid-Atlantic Ridge in the western basin of the Atlantic Ocean, and boundaries were placed outside the current EEZs of all adjacent countries except for Bermuda. For the purposes of describing this area meeting EBSA criteria, the Sargasso Sea ecosystem is deemed to officially transition into the shallow water habitats associated with the Bermuda platform at the base of the Bermuda Rise. The resultant map agrees broadly with the overlap of previous delineations and is shown in Figure 2, with the area described in this proposal outlined in red at both its outer and inner limits. The area of the Sargasso Sea considered here occupies ~ 4,163,499 km2 and extends from 22o to 38oN and 76o to 43oW, centred on 30oN and 60oW.
DISCLAIMER: The designations employed and the presentation of material in this map do not imply the expression of any opinion whatsoever on the part of the Secretariat concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
Area Details
The features proposed here for international recognition are the diverse pelagic communities dependant on the Sargassum spp., the iconic and threatened pelagic species that migrates into or through the area, the lesser known mid-water communities and the specialised benthic communities that live on the seamounts. Together, these communities and species occupy the Sargasso Sea from the surface to the sea floor. Sargassum and Sargassum communities The Sargasso Sea has a characteristic surface ecosystem based upon Sargassum, which hosts its own unique communities, acts as a nursery and feeding area for many species, and serves as a migration route for others. It is a vital habitat for many species of economic importance to Bermuda and to countries on both sides of the Atlantic – the Sargasso Sea is the ecological cross-roads of the Atlantic Ocean. The surface ecosystem is based upon two species of floating Sargassum, both of which reproduce solely by fragmentation and are thus holopelagic and distinct from all other seaweeds (Deacon 1942, Stoner 1983). There is an estimated biomass of one million tonnes of Sargassum in the Sargasso Sea (Gower and King 2008, 2011), making it the only area of significant distribution for these species in the deep open-ocean, where they provide a rare and valuable structurally complex habitat. Although it is part of a broader tropical western Atlantic distribution of Sargassum, the Sargasso Sea contains the most northerly persistent ecosystem formed around this floating seaweed. Together with its persistence, it is the great area and thickness of the floating Sargassum, which in turn attracts a great density and diversity of associated organisms, that distinguishes this floating ecosystem from that of any other drift algae (Coston-Clements et al. 1991, Moser et al. 1998, Casazza and Ross 2008). As the Sargassum drifts round it collects “passengers” which increases the diversity of attached invertebrates that settle upon it; this biodiversity varies seasonally, as well as with location in the gyre and the age of the algae (Stoner and Greening 1984). Ten species are known to be endemic to floating Sargassum, most of which have quite specific adaptions and are camouflaged in some way. Perhaps the most iconic of these is the Sargassum angler fish (Histrio histrio) (Coston-Clements et al. 1991, South Atlantic Fishery Management Council 2002, Trott et al 2011). More than 145 invertebrate species have been recorded in association with floating Sargassum (Fine 1970, Morris and Mogelberg 1973, Butler et al. 1983, Coston-Clements et al. 1991, Sterrer 1992, Calder 1995, South Atlantic Fishery Management Council 2002, Trott et al. 2011), and it also provides a habitat for over 100 species of fish (Dooley 1972, Fedoryako 1980, Coston-Clements et al 1991, South Atlantic Fishery Management Council 2002, Casazza and Ross 2008, Sutton et al 2010). This diverse community of organisms living at the surface also interacts with the oceanic fauna, many of which migrate vertically up at night and down during the day, thus providing connectivity between the surface community and the deep-sea. The overall importance of Sargassum for fish has led the USA to recognise it as essential fish habitat (NMFS 2003). ICCAT has also recognized the importance of Sargassum as fish habitat and has requested that Contracting Parties assess the ecological status of Sargassum as habitat for tuna, billfish and sharks (ICCAT 2005, ICCAT 2011b). Sargassum and the Sargasso Sea as a resource for key life stages Oceanic fish that spawn in the Sargassum include flying fish (Exocoetidae) that build bubble nests for their eggs within the weed and have eggs with long extensions for attaching to the weed (Dooley 1972, Sterrer 1992). Other fish that spawn in the Sargasso Sea include white marlin (Tetrapturus albidus), blue marlin (Makaira nigricans) (South Atlantic Fishery Management Council 2002, Luckhurst et al 2006, White Marlin Biological Review Team 2007) and various species of eels, of which the European and American eels are the most iconic (Schmidt 1922, Schoth and Tesch 1982, Kleckner and McCleave 1988, McCleave and Miller 1994, Miller and McCleave 1994, Miller 2002, Miller and McCleave 2007). Juveniles of many fish species live within the Sargassum canopy (Fedoryako 1980, Coston-Clements et al 1991, South Atlantic Fishery Management Council 2002, Casazza and Ross 2008, Rudershausen et al 2010). Larger predatory species that feed in association with Sargassum include jacks (Caranx spp.), amberjacks (Seriola spp.), rainbow runners (Elagatis bipinnulata), dolphins (Coryphaenus spp.), barracudas (Sphyraenidae), various mackerels, wahoo and tunas (Scombridae), and billfishes (Istiophoridae) (Gibbs and Collette 1959, Stephens 1965, Dooley 1972, Fedoryako 1980, Manooch and Hogarth 1983, Manooch and Mason 1983, Manooch et al 1984, Manooch et al 1985, Coston-Clements et al 1991, South Atlantic Fishery Management Council 2002, Casazza and Ross 2008, Rudershausen et al 2010,Trott et al 2011). Green turtles (Chelonia mydas), hawksbill turtles (Eretmochelys imbricate), loggerhead turtles (Caretta caretta), and Kemp’s Ridley turtles (Lepidochelys kempii), all of which are threatened or endangered, use Sargassum as a nursery habitat (Carr and Meylan 1980, Carr 1987, Schwartz 1988, Manzella and Williams 1991, Luschi et al 2003). Hatchlings swim hundreds of miles to the Sargasso Sea, where they hide in the Sargassum to feed and grow in relative safety, spending their so called “lost years” amongst the weed (Carr and Meylan 1980, Carr 1987, Schwartz 1988, Luschi et al 2003). Haney (1986) observed seabirds foraging in association with Sargassum mats to the west of the Gulf Stream off the Georgia coast and recorded 26 different species, with bird densities being 32 to 43 times greater in areas with Sargassum than in areas without. Further offshore the diversity of bird species is lower, and this is reflected in Table 1, which combines Haney’s observations (1986) with those of Thomas (2005), who examined the oceanic bird habitat further from land. The endangered endemic Bermuda petrel, the cahow (Pterodroma cahow), protected under Appendix 1 of the Convention on Migratory Species (http://www.cms.int/documents/appendix/cms_app1.htm) travels across the northern Sargasso Sea and beyond (Hallett 2011, unpub). Tracking data from Birdlife International show that the western Sargasso Sea is an important feeding area for Audubon’s shearwater (Puffinus lherminieri) and white-tailed tropic birds, (Phaethon lepturus) from the Bahamas (B. Lascelles, pers. comm., www.seabirdtracking.org). The Sargasso Sea as a migration area A number of species of sharks and rays inhabit or migrate through the Sargasso Sea, including whale sharks, tiger sharks, manta rays and spotted eagle rays (Hallett 2011, unpub). New satellite tagging data has revealed that the Sargasso Sea is important habitat for several shark species that have only recently been reported to occur there. For instance, basking sharks (Cetorhinus maximus) make regular seasonal movements to the Sargasso Sea during winter months at depths of 200 to 1000m metres (Skomal et al 2009). Satellite tagging has also recently shown that large female porbeagle sharks (Lamna nasus) migrate over 2,000 km at depths of up to 500m from Canadian waters to the Sargasso Sea where they may be pupping (Dulvy et al 2008, Campana et al 2010). Most recently, a large female white shark (Carcharodon carcharias) was tracked from coastal Massachusetts to Sable Island on the Scotian Shelf and then down into the Sargasso Sea during winter months of 2010/2011 (G. Skomal and S. Thorrold 2011, pers. comm.). The observation of large, potentially pregnant females of several threatened shark species in the Sargasso Sea raises the intriguing possibility that this area represents critical nursery habitat for these species. Eastern and western populations of Atlantic bluefin tuna (Thunnus thynnus) migrate through or to the Sargasso Sea (Lutcavage et al 1999, Block et al 2001, Block et al 2005, Wilson and Block 2009). Both populations are in decline and are below 15% of the unfished, historical baseline (ICCAT 2008). Lutcavage et al (1999) noted that some of the giant bluefin tuna tagged in their study were in the Sargasso Sea at the same time as other giants were located in a known spawning ground in the Gulf of Mexico. This evidence suggested that the Sargasso Sea was, and may still be, a spawning location for bluefin tuna in the western Atlantic Ocean. Several other tuna species, including yellowfin (Thunnus albacares) and bigeye tuna (Thunnus obesus), also move through the Sargasso Sea, and further west into coastal U.S. waters, from spawning grounds in the eastern tropical Atlantic (ICCAT 2010). Yellowfin tuna appear to migrate through the Sargasso Sea to frontal boundaries along the Gulf Stream. Bigeye tuna may be residing for some time in the Sargasso Sea based on tagging and depth distribution data. Albacore tuna (Thunnus alalunga) are also regular visitors to the Sargasso Sea, and are believed to spawn in the Sargasso Sea (ICCAT 2010 and 2011a). The Sargasso Sea is also important habitat for Atlantic swordfish (Xiphias gladius), moving through the Sargasso Sea as part of a seasonal migration from the tropical Atlantic to temperate northwest Atlantic waters. They make diurnal vertical movements spanning at least 1,000m (Loefer et al 2007) that mirror those of mesopelagic fishes. These fishes undoubtedly transfer a significant fraction of primary production from the epipelagic (near-surface) zone of the Sargasso Sea to mesopelagic depths. In addition to the movements of the other turtle species within the Sargasso Sea, adult leatherback turtles (Dermochelys coriacea) migrate north through the Sargasso Sea from nesting sites in the Caribbean Sea (Ferraroli et al 2004, Hays et al 2004), and seasonally to the Sargasso Sea from foraging locations in coastal waters of New England and Nova Scotia (James et al 2005). Thirty cetacean species (whales and dolphins) have been recorded from the Sargasso Sea (Kaschner et al 2011). Of particular note are humpback whales (Megaptera novaeangliae) that pass through the Sargasso Sea during their annual migrations between the Caribbean and the northern North Atlantic (Martin et al 1984, Stone et al 1987, G. Donovan and R. Reeves 2011, pers. comm.). Adults are often seen within sight of the south shore of Bermuda during March and April with their newborn calves. The same individuals are seen year after year at Bermuda and further north in the Stellwagen Bank Marine Sanctuary off the east coast of the USA (Stevenson 2011, unpub). The importance of this connectivity was recognised in 2011 by the signing of a collaboration arrangement between the Government of Bermuda and the Stellwagen Bank National Marine Sanctuary. Some of these individuals have also been recorded at another area meeting EBSA criteria, the Marine Mammal Sanctuary – Banco de la Plata y Banco de la Navidad, off the Dominican Republic (Omar Reynoso, pers. comm., http://www.intec.edu.do/ballenas). The other large whale seen regularly in the Sargasso Sea is the sperm whale (Physeter catodon). Groups including calves are commonly seen (Antunes 2009), and it is likely that they feed in the frontal convergence, around the boundaries of Gulf Stream rings and above the seamounts (National Marine Fisheries Service 2010). Tracking data from Birdlife International show eight species of seabirds migrating through the Sargasso Sea in significant numbers (B. Lascelles, pers. comm., www.seabirdtracking.org). Sargassum patches can be dense enough for some birds, notably bridled and sooty terns (Sterna anaethetus and Sterna fuscata) to roost upon (Haney 1986). Threatened species Many of the species utilising the Sargasso Sea are of considerable global conservation interest, appearing on the International Union for the Conservation of Nature (IUCN) Red List of endangered species, and/or under the Convention on International Trade in Endangered Species (CITES) (Hallett 2011, unpub) (see Table 2). They feature in the annexes of the 1990 Specially Protected Areas and Wildlife Protocol to the Convention on the Protection and Development of the Marine Environment of the Wider Caribbean Region (SPAW Protocol, http://www.cep.unep.org/cartagena-convention/spaw-protocol). Although the geographical area of the Protocol does not extend to the Sargasso Sea, the Protocol does require countries in the Caribbean region to implement conservation measures to protect and recover and, where relevant, to maintain populations of these species at optimal levels. Species of relevance to the Sargasso Sea include seabirds in the air above, turtles in the floating Sargassum, large pelagic fishes and cetaceans in the waters below, and a wide variety of corals on seamounts rising from the seabed (Table 3). Tables 2 and 3 list examples of threatened species, but they are only an indication of the numbers of endangered species that occur in the Sargasso Sea. European and American eels (Anguilla spp) The Sargasso Sea is of considerable international importance for the economically valuable American and European eels, Anguilla rostrata and A. anguilla, which spend their adult lives in freshwater and migrate thousands of miles to the Sargasso Sea to spawn (Schmidt 1922, Kleckner et al 1983, Friedland et al 2007). The larvae of both species develop in the Sargasso Sea and then take seven to 24 months to follow the Gulf Stream back to their respective freshwater habitats in North America and Europe, where they metamorphose into juvenile “glass eels” (ICES 2010). Both species of eel are the subjects of important fisheries, as both “glass eels” and as adults (Wirth and Bernatchez 2003). The exact location and circumstances of eel spawning in the SW Sargasso Sea remain unknown although there is evidence that oceanographic features such as thermal fronts may direct eels to spawning locations (Kleckner and McCleave 1988). The small larvae, or leptocephali, of both species have broad distributions in overlapping areas of the Sargasso Sea. Recruitment and populations of both species are in significant decline, with the European eel listed by CITES and classified by IUCN as Critically Endangered and at increasing risk of global extinction. Tentative links have been proposed between changes in the Sargasso Sea and the decline of both American and European eel species (Friedland et al 2007). These include changes in location of their spawning areas, changes in wind-driven currents that transport eel larvae to adult habitats in Europe and North America, and potential changes to feeding success for eel larvae (Miller et al 2009). A range of other causes has been suggested to affect the juveniles and adults, including pollution (particularly substances like PCBs) (Robinett and Feunteun 2002, Pierron et al 2007), the effects of the swimbladder parasite (Anguillicoloides crassus) (Gollock et al 2005, Gollock 2011, unpub), the poor condition of migrating silver eels (Svedäng and Wickström 1997), and the destruction of their freshwater habitat (Haro et al 2000). The evidence for some of these factors is greater than others (often varying with region), but it is unlikely that there is one single cause. In 2007 the European Union adopted an eel recovery plan (EC 2007). This plan directs European member states to reduce eel fishing efforts by at least 50% relative to average efforts deployed from 2004 to 2006. Similarly in Canada, the American eel has been identified as an Endangered Species under Ontario’s Endangered Species Act of 2007. Quebec and Newfoundland and Labrador have also introduced measures to regulate eel fishing and eel escapement back to the sea. Recognising the importance of the Sargasso Sea is a first step towards international measures to complement these regulations. Deep water pelagic species Beneath the Sargassum layer, the Sargasso Sea descends to depths of around 4500m and is populated throughout by deep ocean animals. Sargassum, once it sinks, contributes to the food webs of these deepwater communities, as well as providing up to 10% of the energy inputs to communities on the seabed (Schoener and Rowe 1970, Rowe and Staresinic 1979 in Angel and Boxshall 1990, Butler et al 1983). The Sargasso Sea has been sampled intensively by deep-sea biologists and oceanographers for over a century, and numerous accounts show that Sargasso Sea assemblages of species are generally similar to those found throughout the subtropical Atlantic (e.g. Marshall 1979, Herring 2002). However, a study of the bathypelagic fish family Stomiidae found a suite of sub-tropical endemic species in the northern Sargasso Sea, primarily from the genus Eustomias but also in the genera Photonectes and Bathophilus, and determined that the area exhibited higher levels of endemicity for this group than other North Atlantic biogeographic provinces (Porteiro 2005). Other recent studies have focused on fish during the 2006 Census of Marine Life Programme (Sutton et al 2010), amphipod crustaceans (Gasca 2007), and chaetognaths (Pierrot-Bults and Nair 2010), whilst a selection of older work includes decapod crustacean (Donaldson 1975), general zooplankton (Deevey 1971), biomass profiles (Angel and Baker 1982, Angel and Hargreaves 1992), and ostracod crustaceans (Angel 1979). Gelatinous zooplankton are well-represented in the Sargasso Sea, being a heterogeneous assemblage of generally large-bodied, jellyfish-like species including medusae, siphonophores, ctenophores, thaliaceans and some polychaetes and pteropods. Salps are diverse and frequently abundant in the vicinity of Bermuda (Madin et al 1996). Pyrosoma, a colonial thaliacean, also migrates over similar distances. The only really different feature of deep-sea animals in the Sargasso Sea is the impact of Gulf Stream rings and mesoscale eddies upon their distribution described earlier. The impact of Gulf Stream rings on groups ranging from protozoa to fish has been well documented (Weibe and Boyd 1978, Fairbanks et al 1980, Backus and Craddock 1982, Wiebe and Flierl 1983). In addition to these conventional studies, the Sargasso Sea is one of the few ocean areas that has been studied using cutting edge intensive DNA barcoding efforts to document the biodiversity of its inhabitants at all different depths (Bucklin et al 2010; Sutton et al 2010). Despite the apparent overall similarity between the midwater communities in the Sargasso Sea and elsewhere, an inventory of planktonic ostracods for the Atlantic Ocean found that 10% of the species caught below 2000m were new to science and speculated that, if the sampling had reached the benthopelagic zone near the seabed, the novel component would have soared (Angel 2010). The ostracods are likely to be an indicator of the potential numbers of new invertebrate species yet to be discovered in the deep ocean. Benthic communities The biology of the abyssal plains is best known through work done on a repeated transect between Bermuda and Gay Head in the USA in the 1960s and 70s (Sanders et al 1965)—work that remains a milestone in our knowledge of deep-ocean bottom faunas. Very recently a new observational programme on the larger bottom fauna has started using baited cameras (MBARI Sargasso Sea Expedition 2011 www.mbari.org). The best known seamounts below the Sargasso Sea are the New England Seamount chain and the Corner Rise seamounts, with peaks rising as much as 4000m from the abyssal plain. These seamounts support complex coral and sponge communities, including numerous endemics, which provide habitat for diverse invertebrate communities that include some highly dependent commensal species (Watling 2007, Watling et al 2007, Cho 2008, Simpson and Watling 2011, Pante and Watling 2011, ICES 2011, Shank 2010). These seamounts also host abundant populations of deep-water fish, which have been heavily exploited commercially since 1976 (Vinnichenko 1997), but despite this they remain important as aggregating and spawning areas for the alfonsino (Beryx splendens). Deep-sea and seamount fish stocks are particularly vulnerable to exploitation because the fish are very long lived, take many years to reach sexual maturity, and have very low fecundities (Norse et al 2012). Limited observations of the Muir seamount chain within the Bermuda EEZ show that its surface is covered with abundant hydroids, sponges, calcareous algae, and rubble (Pratt 1962).
Despite its remote location, the Sargasso Sea does not remain totally natural. A recent global analysis of human impacts of marine ecosystems concluded that the area has sustained moderate to high impacts over time (Halpern et al 2008). Fisheries landings for many species in the North Central Atlantic have declined significantly in the last 50 years, indicative of impacts on those populations (Sumaila et al 2011, unpub, Pauly and Watson 2005, Pauly et al 2005). Regulatory actions by ICCAT aim to address this. Bottom trawling between 1976 and 1995 on the Corner Rise seamounts caused extensive destruction of the benthic fauna (Vinnichenko 1997, Waller et al 2007, Shank 2010). As a precautionary management measure, 13 fishable seamounts, including 25 peaks shallower than 2,000 m on the New England and Corner Rise seamounts were closed to demersal fishing by the Northwest Atlantic Fishery Organization (NAFO) from 1 January 2007. This closure was recently extended until 31 December 2014 (NAFO 2011). The recovery of these habitats in the coming years should be monitored. Floating plastic particles were reported in the Sargasso Sea as early as 1972 (Carpenter and Smith 1972), and today the currents of the North Atlantic gyre have trapped floating debris on a scale similar to the more infamous North Pacific garbage patch with concentrations of plastic particles reaching in excess of 100,000 pieces km-2 in some places (Law et al 2010). This clearly impacts the naturalness of the area, and the negative impacts of plastic debris on organisms such as turtles and seabirds are well documented (Witherington 1994, Rios et al 2007). There are also numerous submarine communications cables that have a minor effect on the naturalness of the seabed (Telegeography 2011), and this is likely to be an ongoing issue. The Sargasso Sea lies within one of the world’s busiest international shipping areas and is crossed by a large number of vessels each year (Roberts 2011, unpub). This affects the naturalness of the area, but impacts on condition are unclear as appropriate research is lacking (GESAMP 2009). Areas of concern include the possible introduction of “foreign” organisms via ballast water (South Atlantic Fishery Management Council 2002, Halpern et al 2008, Roberts 2011, unpub), the potential impact of underwater noise generated by ships on marine mammals (Wright et al 2009), and the risk of collision with whales, dolphins and turtles (Laist et al 2001, Jensen and Silber 2003, Panigada et al 2008). Shipping transiting the Sargasso Sea may also have a direct physical impact on the Sargassum mats, destroying the integrity of the floating community. Research is clearly needed to quantify the degree of pressure that shipping exerts on the Sargasso Sea. Despite these concerns regarding the condition of the Sargasso Sea, the ecological and biological functionality of the ecosystem remain intact, allowing this unique area to still fulfil its role as a home and an essential resource for a great diversity of species, many of which are of considerable conservation interest. Actions to address some of these concerns can only serve to improve the future outlook for the area. Concerns for the future include the potential for commercial extraction of Sargassum seaweed and seabed mining activities (Parson and Edwards 2011, UN, 2009). Application of the precautionary approach is vital to ensure the continued ecological and biological importance of this area into the future.
References
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Status of submission
Areas described as meeting EBSA criteria that were considered by the Conference of the Parties
  • dec-COP-11-DEC-17
Assessment of the area against CBD EBSA criteria
C1: Uniqueness or rarityHigh
The two species of floating Sargassum are the world’s only holopelagic macroalgae, distinct from all other complex seaweeds in not having an attached benthic stage (Deacon 1942, Stoner 1983). Although these species occur in the Gulf of Mexico and Caribbean, the extent of their occurrence in the Sargasso Sea provides a unique and valuable structurally complex habitat in deep, open- ocean waters. Here, the areal extent of the Sargassum and the thickness of the mats it forms, along with their persistence, attract and retain a great density and diversity of associated organisms, distinguishing the Sargasso Sea ecosystem from other drift algal habitats (Stoner and Greening 1984, Coston-Clements et al 1991, Moser et al 1998, Casazza and Ross 2008). The Sargasso Sea is also the northerly limit of persistent Sargassum presence. The Sargasso Sea is home to numerous endemic species that are, by definition, rare. The floating Sargassum community hosts ten endemic species from a broad range of taxa (Coston-Clements et al 1991, SAFMC 2002). The mid-water fish community of the Sargasso Sea includes a suite of sub-tropical endemics from three genera in the family Stomiidae (Porteiro 2005). On the sea floor, the New England seamount chain and the Corner Sea Rise seamounts are known to host endemic species and specialised communities, and models indicate that other isolated seamounts occur throughout the area (Figure 1).
C2: Special importance for life-history stages of speciesHigh
The Sargasso Sea is of considerable international importance as the only spawning area for American and European eels, Anguilla rostrata and A.anguilla. Both species spend their adult lives in freshwater but migrate thousands of miles to the Sargasso Sea to spawn (Schmidt 1922, Kleckner et al 1983, Friedland et al 2007). The larvae of both species develop in the Sargasso Sea and migrate along the Gulf Stream back to their respective freshwater habitats in North America and Europe. Satellite tagging has also recently shown that large female porbeagle sharks (Lamna nasus) migrate over 2,000 km at depths of up to 500m from Canadian waters to the Sargasso Sea, where they may be pupping (Dulvy et al 2008, Campana et al 2010). The Sargassum anglerfish (Histrio histrio) and pipefish (Syngnathus pelagicus) lay their eggs in the Sargassum mats, and oceanic flying fish (Exocoetidae) build bubble nests for their eggs within the weed and have eggs with long extensions for attaching to the weed (Dooley 1972, Sterrer 1992). Other fish that spawn in the Sargasso Sea include white marlin (Tetrapturus albidus) and blue marlin (Makaira nigricans) (South Atlantic Fishery Management Council 2002, Luckhurst et al 2006, White Marlin Biological Review Team 2007) and various additional species of eels (Schoth and Tesch 1982, McCleave and Miller 1994, Miller and McCleave 1994, Miller 2002, Miller and McCleave 2007). Albacore tuna are also believed to spawn in the Sargasso Sea (ICCAT 2011a). The Corner Rise and New England Seamounts host abundant populations of deep-water fish and, despite heavy commercial exploitation, remain important as aggregating and spawning areas for the alfonsino (Beryx splendens). The mats of Sargassum and their associated communities are essential as nursery habitats and feeding areas for many species of fish, seabirds, and turtles. Fish living within the Sargassum canopy include juvenile swordfish (Xiphius gladius), juvenile and sub-adult jacks (Carangidae), juvenile and sub-adult dolphinfish (Coryphaenidae), filefish and triggerfish (Balistidae) of all life stages, and driftfish (Stromateidae) (Fedoryako 1980, Coston-Clements et al 1991, South Atlantic Fishery Management Council 2002, Casazza and Ross 2008, Rudershausen et al 2010). These fish are in turn a food resource for larger predatory species. Green turtles (Chelonia mydas), hawksbill turtles (Eretmochelys imbricate), loggerhead turtles (Caretta caretta), and Kemp’s ridley turtles (Lepidochelys kempii), all of which are threatened or endangered, use Sargassum as a nursery habitat (Carr and Meylan 1980, Carr 1987, Schwartz 1988, Manzella and Williams 1991, Luschi et al 2003). Hatchlings swim hundreds of miles to the Sargasso Sea, where they hide in the Sargassum to feed and grow in relative safety, spending their so called “lost years” amongst the weed (Carr and Meylan 1980, Carr 1987, Schwartz 1988, Luschi et al 2003). A variety of seabirds feed in association with Sargassum in the Sargasso Sea (Table 1) (Haney 1986, Thomas 2005). Tracking data from Birdlife International show that the western Sargasso Sea is an important feeding area for Audubon’s shearwater (Puffinus lherminieri) and white-tailed tropic birds (Phaethon lepturus) from the Bahamas (B. Lascelles, pers. comm., www.seabirdtracking.org). The Sargasso Sea provides critical food and shelter for a variety of organisms on migratory routes between the tropical and temperate Atlantic. Nearly all the large tunas and tuna-like species managed by ICCAT, including the bluefin tuna (Thunnus thynnus), migrate through the Sargasso Sea (Lutcavage et al 1999, Block et al 2001, Block et al 2005, Wilson and Block 2009, ICCAT 2010). Basking sharks (Cetorhinus maximus) make regular seasonal movements to the Sargasso Sea during winter months at depths of 200-1000m metres (Skomal et al 2009). Adult leatherback turtles (Dermochelys coriacea) migrate north through the Sargasso Sea from nesting sites in the Caribbean Sea (Ferraroli et al 2004, Hays et al 2004), and seasonally to the Sargasso Sea from foraging locations in coastal waters of New England and Nova Scotia (James et al 2005). Humpback whales (Megaptera novaeangliae) pass through the Sargasso Sea during their annual migrations between the Caribbean and the northern North Atlantic (Martin et al 1984, Stone et al 1987, G. Donovan and R. Reeves 2011, pers. comm.). The same individuals are seen year after year off Bermuda and further north in the Stellwagen Bank Marine Sanctuary off the east coast of the USA (Stevenson 2011, unpub). Some of these individuals have also been recorded at another area meeting EBSA criteria, the Marine Mammal Sanctuary – Banco de la Plata y Banco de la Navidad, off the Dominican Republic (Omar Reynoso, pers. comm., http://www.intec.edu.do/ballenas). Tracking data from Birdlife International show eight species of seabirds migrating through the Sargasso Sea in significant numbers (B. Lascelles, pers. comm., www.seabirdtracking.org). Sargassum patches can be dense enough for some birds, notably bridled and sooty terns (Sterna anaethetus and Sterna fuscata) to roost upon (Haney 1986).
C3: Importance for threatened, endangered or declining species and/or habitatsHigh
Many of the species utilising the Sargasso Sea are of considerable global conservation interest, appearing on the IUCN Red List of endangered species, and/or under CITES (Hallett 2011, unpub) (see Table 2). They feature in the annexes of the 1990 SPAW Protocol of the Cartagena Convention (http://www.cep.unep.org/cartagena-convention/spaw-protocol), which requires countries in the Caribbean region to implement conservation measures for these species (Table 3). Tables 2 and 3 list examples of threatened species, but they are only an indication of the numbers of endangered species that occur in the Sargasso Sea. Important examples include European and American eels, porbeagle sharks and four species of turtle— Kemp’s ridley, hawksbill, loggerhead and green—which are heavily dependent on the Sargasso Sea and/or Sargassum. Some 30 cetacean species live in or migrate through the Sargasso Sea, as do several species of endangered or threatened tuna and sharks. Leatherback turtles also migrate through the area. The distinct seamount communities are rare habitats that are home to a variety of endemic species with very limited distribution that are thus at high risk of extinction. Threatened and endangered species utilising the Sargasso Sea include seabirds in the air above, turtles in the floating Sargassum, large pelagic fishes and cetaceans in the waters below, and a wide variety of corals on seamounts rising from the seabed.
C4: Vulnerability, fragility, sensitivity, or slow recoveryHigh
Deep-sea organisms and their communities are considered extremely vulnerable because such species are generally long-lived, slow to mature and have low reproductive rates, leading to slow recovery times if their populations are perturbed (Norse et al 2012). In addition, many organisms in the deep sea are physically fragile and easily damaged. The benthic communities on seamounts are particularly vulnerable as they may be isolated from external sources of replenishment and they often contain endemic species with very limited distributions. Within this area meeting EBSA criteria, the Corner Rise, New England and Muir seamount chains support complex coral and sponge communities, including numerous endemics, which provide habitat for diverse invertebrate communities that include some highly dependent commensal species (2007, Watling et al 2007, Cho 2008, Simpson and Watling 2011, Pante and Watling 2011, ICES 2011, Shank 2010). These seamounts also host abundant populations of deep-water fish. Deep-sea and seamount fish stocks are particularly vulnerable to exploitation because the fish are very long-lived, take many years to reach sexual maturity, and have very low fecundities (Norse et al 2012). These fish stocks have been heavily exploited commercially since 1976 (Vinnichenko 1997), and the fragile seamount ecosystems of the Corner Rise and New England chains have already sustained considered damage from these deep-sea trawling activities. Nevertheless, the seamounts remain important as aggregating and spawning areas for the alfonsino (Beryx splendens). The communities dependent on the Sargassum mats are also vulnerable, although to a lesser degree, as their existence relies on the presence of the complex physical structure provided by the seaweed.
C5: Biological productivityHigh
The discovery of Prochlorococcus and the development of techniques able to evaluate the role of picoplankton in primary production measurements have revolutionized our perceptions of productivity in the Sargasso Sea and subsequently of the global ocean (Chisholm et al 1988). Conventionally the Sargasso Sea was regarded as an area of low nutrients and low productivity, enhanced by localized upwellings. Yet despite having low nutrient levels and therefore being officially classed as oligotrophic, the Sargasso Sea, per unit area, has a surprisingly high net annual primary production rate that matches levels found in some of the most productive regions in the global ocean (Steinberg et al 2001, Rho and Whitledge 2007, Lomas et al 2012). This is due to a complex combination of factors—the production of carbon in the surface waters by photosynthesis, the location of the Sargasso Sea in the sub-tropics which results in a deep euphotic layer, and differences in phytoplankton communities and associated nitrogen fixation. Integrated over the entire area of the Sargasso Sea the annual net primary production, the balance of primary production and plankton respiration, is estimated to be some three times higher than in the Bering Sea (Steinberg et al 2001, Lomas et al 2012), conventionally regarded as one of the world’s most productive seas. The difference is that in the Sargasso Sea most of the production is recycled by bacteria in the so-called microbial loop rather than channelled into biomass of larger, harvestable organisms (Carlson et al 1996, Steinberg et al 2001). As a result of this high primary productivity, to which must be added the annual production of Sargassum weed, which is now being estimated from satellite measurements (Gower and King 2008, 2011), the Sargasso Sea plays a key role in both global oxygen production and ocean sequestration of carbon (IPCC1996, 2001 and 2007, Ullman et al 2009, Lomas et al 2011 unpub). Production in the upper ocean, particularly of Sargassum, contributes to food webs in mid-water and benthic regions, providing up to 10% of the energy inputs to communities on the seabed (Schoener and Rowe 1970, Rowe and Staresinic 1979 in Angel and Boxshall 1990, Butler et al 1983).
C6: Biological diversityHigh
The biodiversity of the Sargasso Sea is exemplified by the diverse population of species that live on or amongst the Sargassum. More than 145 species of invertebrates have been recorded, including a variety of gastropod and nudibranch molluscs, portunid and amphipod crustaceans, pycnogonids, serpulid and nereid polychaetes, flatworms, bryozoans and hydroids (Fine 1970, Morris and Mogelberg 1973, Butler et al 1983, Coston-Clements et al 1991, Sterrer 1992, Calder 1995, South Atlantic Fishery Management Council 2002, Trott et al 2011). In addition, over 100 species of fish associate with floating Sargassum offshore (Dooley 1972, Fedoryako 1980, Coston-Clements et al 1991, Casazza and Ross 2008, Sutton et al 2010). Most of these species would not live in the open ocean without the floating algae. Ten of these species are endemic to floating Sargassum: the Sargassum anglerfish (Histrio histrio), Sargassum pipefish (Syngnathus pelagicus), Sargassum slug (Scyllea pelagica), Sargassum snail (Litiopa melanostoma), Sargassum crab (Planes minutes), Sargassum shrimp (Latreutes fucorum), the amphipods Sunampithoe pelagica and Biancolina brassicacephala, the platyhelminth Hoploplana grubei and the Sargassum anemone (Anemonia sargassensis). The diversity of oceanic plankton and micronekton within the Sargasso Sea is enhanced as species are drawn in from the surrounding currents via rings and eddies (Boyd et al 1978, Wiebe and Boyd 1978, Ring Group 1981). Overall, mesopelagic diversity is not considered to be higher than other similar locations, but a variety of species from a wide range of taxa have been documented from this deeper part of the ocean. A study of the bathypelagic fish family Stomiidae found a suite of sub-tropical endemic species in the northern Sargasso Sea, primarily from the genus Eustomias but also in the genera Photonectes and Bathophilus, and determined that the area exhibited higher levels of endemicity for this group than other North Atlantic biogeographic provinces (Porteiro 2005). In addition, it is worth noting that Angel (2010), in drawing up an inventory of planktonic ostracods for the Atlantic Ocean, observed that 10% of the species caught below 2000m were new to science and that if the sampling had reached the benthopelagic zone within a few metres of the seabed the novel component would have soared. The ostracods are likely to be an indicator of the potential numbers of new species yet to be discovered in the deep ocean. Benthic diversity is very high on the Corner Rise and New England seamount chains, where there are numerous endemic and novel species of coral that host specific commensal invertebrates, and some 670 species have been found (Watling 2007, Watling et al 2007, Cho 2008, Simpson and Watling 2011, Pante and Watling 2011, ICES 2011, Shank 2010). Less is known about the Muir seamount chain, but limited observations have shown it is covered with abundant hydroids, sponges, calcareous algae and rubble (Pratt 1962). The diversity of fauna living on and within the abyssal plain of the Sargasso Sea may well be amongst the highest on the planet (Sanders et al 1965, MBARI Sargasso Sea Expedition 2011 www.mbari.org).
C7: Naturalness Medium
Despite its remote location, the Sargasso Sea does not remain totally natural. A recent global analysis of human impacts of marine ecosystems concluded that the area has sustained moderate to high impacts over time (Halpern et al 2008). Fisheries landings for many species in the North Central Atlantic have declined significantly in the last 50 years, indicative of impacts on those populations (Sumaila et al 2011, unpub, Pauly and Watson 2005, Pauly et al 2005). Deep pelagic and bottom trawling between 1976 and 1995 on the Corner Rise seamounts caused extensive destruction of the benthic fauna (Vinnichenko, 1997, Waller et al 2007, Shank 2010). As a precautionary management measure, 13 fishable seamounts, including 25 peaks shallower than 2,000 m on the New England and Corner Rise seamounts were closed to demersal fishing by the Northwest Atlantic Fishery Organization (NAFO] from 1 January 2007. This closure was recently extended until 31 December 2014 (NAFO 2011). The recovery of these habitats in the coming years should be monitored. Floating plastic particles were reported in the Sargasso Sea as early as 1972 (Carpenter and Smith 1972), and today the North Atlantic gyre has a patch of floating debris akin to the more famous North Pacific garbage patch with concentrations of plastic particles reaching in excess of 100,000 pieces km-2 in some places (Law et al 2010). This clearly impacts the naturalness of the area, and the negative impacts of plastic debris on organisms such as turtles and seabirds, are well documented (Witherington 1994, Rios et al 2007). There are also numerous submarine communications cables that have a minor effect on the naturalness of the seabed (Telegeography 2011). The Sargasso Sea lies within one of the world’s busiest international shipping areas and is crossed by a large number of vessels each year (Roberts 2011, unpub). This affects the naturalness of the area, but precise impacts are unclear as appropriate research is lacking (GESAMP 2009) and this absence of data should be addressed. Despite these impacts, the ecological and biological functionality of the Sargasso Sea ecosystem remain intact, allowing this unique area to fulfil its role as a home and an essential resource for a great diversity of species.
Additional Information
Table 1. Seabird species known to be associated with Sargassum and the Sargasso Sea (composite list developed from Haney 1986 and Thomas 2005). Species Common Name Pterodroma hasitata Black-capped petrel Calonectris diomedea Cory's shearwater Puffinus gravis Greater shearwater Puffinus Iherminieri Audubon's shearwater Oceanites oceanicus Wilson's storm-petrel Oceanodroma leucorhoa Leach's storm-petrel Puffinus griseus Sooty shearwater Phaethon lepturus White-tailed tropicbird Phaethon aethereus Red-billed tropicbird Sula dactylatra Masked booby Sula leucogaster Brown booby Phalaropus lobatus Red-necked phalarope Phalaropus fulicaria Red phalarope Phalaropus sp. Phalarope sp. Stercorarius pomarinus Pomarine jaeger Stercorarius parasiticus Parasitic jaeger Stercorarius longicaudis Long-tailed jaeger sp. Larus argentatus Herring gull Sterna maxima Royal tern Sterna hirundo Common tern Sterna paradisaea Arctic tern Sterna antillarum Least tern Sterna anaethetus Bridled tern Sterna fuscata Sooty tern