June 6, 2011

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The Sargasso Sea in the North Atlantic Ocean

One of Sylvia Earle’s Hope Spots is the Sargasso Sea. This area in the central North Atlantic Ocean is a clockwise circulating gyre which accumulates material from both sides of the North Atlantic. Maps and charts from the 1730s to 1840s show extensive areas of Sargassum, the free-floating brown macroalgae (Phaeophyte) Sylvia Earle refers to as “the golden rain forest of the sea.” The “floating reefs” of Sargassum give the Sargasso Sea its’ name. For many years Scientists and ocean advocates have been meeting to establish a marine protected area within the Sargasso Sea because of its unique biodiversity.

The endangered Bermuda Petrel

An update on progress toward protecting the Sargasso Sea was provided in an IMCC talk by Sheila McKenna of the SEAlliance (McKenna and Hemphill, 2011). She began by pointing out that the Sargasso Sea is home to a few signature species, one of which is the Endangered Bermuda Petrel or Cahow (Pterodroma cahow). This seabird was thought extinct by 1620 because of hunting and human expansion on Bermuda. However in 1951 eighteen pairs were discovered. With subsequent protection the Bermuda Petrel population recovered to 250 individuals by 2005, but remains severely threatened. A second signature species of the Sargasso Sea is the Porbeagle shark (Lamna nasus). This is a cold water shark critically endangered across the North Atlantic after years of fishing pressure. While normally living in cold water areas of the North Atlantic from Canada and Iceland, tagging studies surprisingly found the females travel to the warm southern Sargasso Sea as a critical habitat where they bear their young (Campana, et al., 2010). The Atlantic bluefin tuna is another signature species of the Sargasso Sea, which tagging confirms makes extensive use of seamounts in the northern Sargasso Sea (Block, et al., 2005; see also Slide 20 in Freestone, 2011).

Plastic density/km2 in the North Atlantic (from Wilber, 1987)

Threats to these and other Sargasso Sea species include over-fishing, and, as in the Pacific, a North Atlantic “garbage patch” of accumulated plastic (Law, et al., 2010). This plastic is eaten by sea turtles, seabirds, fish and other marine species, putting them at risk as a result of the long-term cumulative disposal of these materials by man. As a protected area, stronger regulations could be proposed to restrict the use and disposal of plastics around the North Atlantic and reduce their accumulation in Sargasso Sea.

Windrows in the ocean accumulate not only Sargassum and plastic, but also floating tar balls from oil spilled by ships or other accidents. Thankfully the amount of oil released at sea from ships has dropped greatly since regulations banning such disposal went into effect. However oil drilling accidents such as the Gulf of Mexico IXTOC-1 oil spill in 1979 sent tar balls up the east coast of Florida with the Gulf Stream and out into the Sargasso Sea. The size distribution of such tar balls is such that many are ingested by sea turtles with fatal results (Richardson & McGillivary, 1991). Such oil drilling accidents continue to pose a risk: if the Deepwater Horizon Oil Spill in the Gulf of Mexico had been caught by currents entering the Gulf Stream, it too would have sent tar balls out into the Sargasso Sea. Currents and winds spared the Sargasso Sea pollution from Deepwater Horizon tar balls, but drilling in the Gulf of Mexico continues, and has just begun off the coast of Cuba. Any accidents from such drilling would pose a serious threat to the ecology of the Sargasso Sea ecosystem.

As Dan Laffoley, Marine Vice Chair for IUCN World Commission on Protected Areas, noted in a Time magazine article in the fall of 2010 discussing her work (Walsh, 2010), it is Sylvia Earle’s goal to have the Sargasso Sea be the first high seas Marine Protected Area (MPA) to provide a roadmap for protection of open ocean areas elsewhere. To further this goal a workshop was held in Bermuda in February 2010. Sheila McKenna reviewed in her talk the formal mechanism for establishing such protection, which requires an area meet seven criteria for an MPA to be established by the UN (UN, 2008), including:

1. Uniqueness or rarity of habitat,
2. Special importance for life history of species,
3. Importance for threatened, endangered or declining species and/or habitats,
4. Vulnerability, fragility, sensitivity, slow recovery (fragile),
5. Biological productivity,
6. Biological diversity, and,
7. Naturalness.

The Sargasso Sea meets all these requirements, and can be considered a priority area for conservation (see nomination documents, McKenna and Hemphill, 2010). Partnering with the Pew Trusts and others, the government of Bermuda has recently hired Dr. David Freestone as Executive Director of the Sargasso Sea Alliance to assist with legal guidance for establishment of the Sargasso Sea as a transnational marine protected area by the end of 2013 under Articles 192 and 194(5) of the UN Convention on the Law of the Sea. He has reviewed some of the issues involved, including ship traffic (see slide 26 in Freestone, 2011).

Beyond the direct threats, one must now consider the effect of global climate change and ocean acidification on the Sargasso Sea. In addition to the many intriguing creatures that are full-time residents of the Sargassum community, nearly 1500 fish species are visitors that lay their eggs in Sargassum, including more than 100 species of commercially harvested fish (Butler, et al., 1983; Casazza and Ross, 2008). As the Sargassum circulates around the Sargasso Sea the juvenile fish associated with it are returned to the Caribbean, Gulf of Mexico and US east coast to settle out and sustain these fisheries. The effects climate changes and ocean acidification will have on the pelagic Sargassum community and the fish species that make use of it as a nursery remain unclear at this time. Expeditions begun in February 2011 have started to address this question (see http://www.mbari.org/expeditions/Sargasso/crew.htm and http://www.mbari.org/pelagic-benthic/deepsea.htm ). However, given the importance of Sargassum as a keystone species structuring life in the North Atlantic, setting aside an area of the Sargasso Sea for protection and conservation will help preserve not only sea turtles, but also the fish and eels that use these waters for their juvenile stages. Given the food security concerns of the US, which imports the majority of its seafood, there is reason enough to protect and conserve the Sargasso Sea solely on the basis of its’ role in fisheries productivity and food security. Conservation of the Sargasso Sea will protect sustainable fisheries resources we cannot afford to lose, while helping preserve the biodiversity of this unique high seas ecosystem.

Written by Dr. Philip McGillivary

References:
Block, B., et al.. 2005. Electronic tagging and population structure of Atlantic bluefin tuna. Nature 434, 1121-1127. (Online at: http://www.nature.com/nature/journal/v434/n7037/abs/nature03463.html )

Butler, J.N., B.F. Morris, J. Cadwaller and A.W. Stoner. 1983. Studies of Sargassum and the Sargassum Community. Bermuda Biological Station, Specl. Publ. No. 22, 307pp.

Campana, S.E., W. Joyce and M. Fowler. 2010. Subtropical pupping ground of a cold-water shark. Can. J. Fish. Aquat. Sci. 67:769-773. (Online at: http://marinebiodiversity.ca/shark/english/document/campana%20et%20al%20porbeagle%202010.pdf )

Casazza, T.L. and S.W. Ross. 2008. Fishes associated with pelagic Sargassum and open water lacking Sargassum in the Gulf Stream off North Carolina. Fish. Bull. 106: 348-363.

Freestone, D. 2011. High-Seas Protected Areas. Some Pragmatic Perspectives. Feb. 28, 2011, online at: http://www.lib.noaa.gov/about/news/Freestone_02282011.pdf

Law, K.L., S. Morét-Ferguson, N.A. Maximenko, G. Proskurowski, E.E. Peacock, J. Hafner, and C.M. Reddy. 2010. Plastic accumulation in the North Atlantic subtropical gyre. Science 329, 1185-1188 (Online at: doi:10.1126/science.1192321).

McKenna, S. and A. Hemphill. 2010. Nomination for Sargasso Sea MPA Status. Online at: www.gobi.org/Our%20Work/rare-2/at_download/pdf and http://www.gobi.org/candidate-ebsas/the-sargasso-sea, with more on organizations participating at: http://iwlearn.net/publications/gefpolicies/areas-beyond-national-jurisdiction/the-sargasso-sea-alliance
Ibid., 2011. The Sargasso Sea Initiative: Promoting open ocean conservation. Intl. Mar. Conserv. Congress, Victoria, B.C. (Abstract online at: http://birenheide.com/scbmarine2011/program/singlesession.php3?sessid=MPA%2011).

Richardson, J. and P. McGillivary. 1991. Post-hatchling Loggerhead Turtles Eat Insects in Sargassum Community. Marine Turtle Newsletter 55:2-5.

United Nations. 2008. Conference on Biological Diversity COP (Parties to the Convention). Criteria for Ecologically or Biologically Significant Areas (EBSAs). (Online at: http://data.unep-wcmc.org/decisions/2).

Walsh, B. 2010. Code Blue. Time, Sept. 23 (Online at: (http://www.time.com/time/specials/packages/article/0,28804,2020806_2020805_2020796-1,00.html).

Wilber, R.J. 1987. Plastic in the North Atlantic. Oceanus 30:61-68.

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