Ecologically or Biologically Significant Areas (EBSAs)
published: 12 Jun 2015
Subtropical Convergence Zone (STCZ)
The Subtropical Convergence Zone is delimited to the north by the subtropical gyres and to the south by the northernmost current band of the Antarctic Circumpolar Current. The area has high productivity compared with the oligotrophic waters to the north and supports a significant diversity of biota. The area supports species such as Southern Bluefin Tuna, Southern Right Whale and seabirds recognized as threatened by IUCN, including the critically endangered Tristan Albatross.
The Subtropical Convergence Zone (STCZ) is delimited to the north by the subtropical gyres and to the south by the northernmost current band of the Antarctic Circumpolar Current (ACC) (Dieter Wolf- Gladrow 2012). STCZ is a high productivity oceanographic feature in the South East Atlantic Ocean with distinct oceanographic signatures such as strong gradients in salinity, temperature and nutrients (strong surface nitrate gradient). The enhanced productivity supports a high diversity of epipelagic species, and the zone is inhabited by globally threatened species such as Southern Bluefin Tuna, Leatherback Turtle, and Tristan Albatross. In addition, it is likely that the enhanced and localized near-surface productivity supports a diverse benthic fauna.
The area is an elongated polygon from 9°–18°W to 36°–43°S and connects with the fringes of the Walvis Ridge and the Mid-Atlantic Ridge to the West. Specific elements of the feature extend the boundary up to 31° and down to 45.5°S. The oceanographic features of the STCZ continue to the west towards the South American continental margin. The national jurisdiction of the Tristan da Cunha is excluded from the westward end of the area. This area is located exclusively in marine areas beyond national jurisdiction (ABNJ). The Subtropical Convergence Zone borders to the north the subtropical gyres and to the south the northernmost current band of the Antarctic Circumpolar Current.
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.
STCZ is primarily a physical oceanographic feature. It is created by convergence of water masses and currents, creating a complex zonal structure (Perez et al. 2012). The feature straddles both the South Atlantic Gyral Province and the Westerlies Subtropical Convergence Province of the Longhurst biogeochemical classification (Longhurst 1998) described primarily on the basis of satellite observations of primary production. Mean surface productivity nearly doubles in the convergence zone compared with adjacent oceanic waters, and the production exhibits a moderate seasonal fluctuation pattern (Moore and Abbot 2000). The area furthermore recognizes ecological linkages and includes the Cape Basin and Vema Seamount, one of the few seamounts in this area that extends to the photic zone. Towards the western end of the feature, the area has been expanded longitudinally to encompass documented Important Bird Areas. The Cape Basin is an important feeding area for the Critically Endangered Tristan Albatross (Diomedea dabbenena).
This is a spatially fixed oceanographic feature. Its properties are likely to be affected by climate change and ocean acidification but otherwise it is unlikely to be changed by human activities in the near future. Fishing activities are regulated by competent international bodies (Regional Fisheries Management Organizations).
Anderson, O., Wanless, R., Small, C., 2009. Seabird Bycatch in IOTC Longline Fisheries. BirdLife International. IOTC-2009-SC-INF14. ACAP. 2009. ACAP Species Assessment: Tristan Albatross Diomedea dabbenena. Available at: #http://www.acap.aq/acap-species/download-document/1206-tristan-albatross. AS@S – Atlas of Seabirds at Sea. Available at: http://www.saeonmarine.co.za/seabirds/. Behrenfeld M.J., Falkowski P.G. (1997) Photosynthetic rates from satellite-based chlorophyll concentration, Limnology and Oceanography, Vol 42:1-20, Accessed 01/06/2011. Available online: http://www.science.oregonstate.edu/ocean.productivity/index.php. Boebel, Olaf; Johann Lutjeharms, Claudia Schmid, Walter Zenk, Tom Rossby, Charlie Barron (2003). The Cape Cauldron: a regime of turbulent inter-ocean exchange. Deep Sea Research Part II: Topical Studies in Oceanography. Volume 50, Issue 1, Pages 57–86. BirdLife International (2009) Designing networks of marine protected areas: exploring the linkages between Important Bird Areas and ecologically or biologically significant marine areas. Cambridge, UK: BirdLife International. www.cbd.int/doc/meetings/mar/ewbcsima- 01/other/ewbcsima-01-birdlife-02-en.pdf. BirdLife International (2010) Marine Important Bird Areas toolkit: standardised techniques for identifying priority sites for the conservation of seabirds at-sea. BirdLife International, Cambridge UK. Version 1.1: May 2010. www.birdlife.org/eu/pdfs/Marine_IBA_Toolkit_2010.pdf. Cuthbert and Wanless – tracking data held in the Global Procellariiform Tracking Database – www.seabirdtracking.org. Dencausse G., Arhan M, * and Speich S. (2010) Routes of Agulhas rings in the southeastern Cape Basin. Deep Sea Research Part I: Oceanographic Research Papers November 2010, Volume 57, Issue 11, Pages 1406-1421. Dieter Wolf-Gladrow (2012). ANT-XXVIII/3, Weekly Report No. 2. January 2012. Feldman, G.C., McClain, C.R., Ocean Color Web, MODIS and SeaWiFS monthly products averaged 2002-2010, NASA Goddard Space Flight Center. Eds. N., Kuring, S.W., Bailey, Accessed 01/06/2011. http://oceancolor.gsfc.nasa.gov/. IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 15 March 2013. IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 10 April 2013. GEBCO_08 Grid, Version 20100927, British Oceanographic Data Centre (BODC), Accessed 30/01/2010, Available online: http://www.gebco.net. McCANN, C. and TALBOT, F. H. (1964), The occurrence of True's beaked whale (Mesoplodon mirus True) in South African waters, with a key to South African species of the genus. Proceedings of the Linnean Society of London, 175: 137–144. Moore, J.K., and M.R. Abbot. (2000). Phytoplankton chlorophyll distributions and primary production in the Southern Ocean. Journal of Geophysical Research 15:28,709–28,722, http://dx.doi.org/ 10.1029/1999JC000043. Perez, J.A.A., E. dos Santos Alves, M.R. Clark, O. Aksel Bergstad, A. Gebruk, I. Azevedo Cardoso, and A. Rogacheva. 2012. Patterns of life on the southern Mid-Atlantic Ridge: Compiling what is known and addressing future research. Oceanography 25(4):16–31, http://dx.doi.org/10.5670/ oceanog.2012.102. Pitman, R. L., Van Helden, A. L., Best, P. B. and Pym, A. 2006. Shepherd's beaked whale (Tasmacetus shepherdi): Information on appearance and biology based on strandings and at-sea observations. Marine Mammal Science 22(3): 744-755. Richardson P.L, J.R.E Lutjeharms, O Boebel (2003). Introduction to the “Inter-ocean exchange around southern Africa”. Deep Sea Research Part II: Topical Studies in Oceanography. Volume 50, Issue 1, Pages 1–12. Schmid, C., G. Siedler, and W. Zenk. 2000. Dynamics of intermediate water circulation in the subtropical South Atlantic. Journal of Physical Oceanography 30:3,191–3,211, http:// dx.doi.org/10.1175/1520-0485(2000)030<3191:DOIWCI>2.0.CO;2. Ssalto/Duacs, Sea Level Anomaly monthly products averaged 2002-2010 and Mean Surface Wave Height and Wind Speed one-day products averaged 2009-2010, Aviso with support from Cnes, Accessed 01/06/2011, Available online: http://www.aviso.oceanobs.com/duacs/ Stramma, L., and R.G. Peterson. 1990. The South Atlantic Current. Journal of Physical Oceanography 20:846–859.
Areas described as meeting EBSA criteria that were considered by the Conference of the Parties
C1: Uniqueness or rarity Medium
This is a pronounced persistent high productivity oceanographic feature in the South East Atlantic Ocean (Stramma and Peterson 1990; Schmid et al., 2000) and thus unique to the area. It has strong gradients in salinity, temperature and nutrients (strong surface nitrate gradient). The area includes Vema seamount, one of the few seamounts with summits reaching high into the photic zone. The Cape Basin in the northeast end includes a discrete and specific foraging area of the Critically Endangered Tristan Albatross .
C2: Special importance for life-history stages of species High
The area is one of only two known main feeding areas used extensively by Tristan Albatross during the non-breeding season. Tracking data shows that the area is known to be of global significance for life history stages of seabirds (www.seabirdtracking.org). Whaling records generated from OBIS from the 19th century indicate that this was and probably continues to be an area with high concentrations of Southern Right Whales .
C3: Importance for threatened, endangered or declining species and/or habitats High
Critically Endangered species (according to IUCN RedList 2012) that occur in the area include the Southern Bluefin Tuna (Thunnus maccoyii), Tristan Albatross, and Leatherback Turtle (Dermochelys coriacea). Endangered or near endangered species (according to IUCN RedList 2012) include Atlantic Yellow-nosed Albatross (Thalassarche chlororhynchos), Sooty Albatross (Phoebetria fusca) and the Sooty Shearwater (Puffinus gravis). Species that occur in the area that are listed as Vulnerable (according to IUCN RedList 2012) include Sperm Whale (Physeter macrocephalus), as well as Black-browed Albatross (Thalassarche melanophris), Wandering Albatross (Diomedea exulans), Southern Rockhopper Penguin (Eudyptes chrysocome), and the White-chinned Petrel (Procellaria aequinoctialis) which have been recorded in the area (according to OBIS data) A number of species listed as data deficient also occur in the area, including True’s Beaked Whale (Mesoplodon mirus), and Shepherd's beaked whale (Tasmacetus shepherdi).
C4: Vulnerability, fragility, sensitivity, or slow recovery Medium
The endangered seabird species associated with this feature are long-lived and slow at reproducing, leaving them prone to slow recovery after population declines, which often occur as a result of fisheries bycatch (Anderson et al. 2009). Other taxa such as mammals, turtles and certain benthic invertebrates (e.g., cold-water corals likely to occur on the shallower parts of the seafloor underneath the frontal zone) also have extended life cycles and are vulnerable to human impacts.
C5: Biological productivity Medium
Compared to the productivity of eastern boundary current systems, the productivity of this feature is moderate. However, as an oceanic feature, it has high productivity. Average chlorophyll concentration across the site is given as 0.25 mg m-3, and primary productivity was calculated at 546.2 mg C/m2/day (Behrenfeld and Falkowski, 1997). The Cape Basin is characterised by a gradient that is observed in both chlorophyll concentration and primary productivity with low values in the north-west corner increasing to maximums in the south-east corner. The site has high levels of water mixing with sea level anomalies across the area, deviating from 4 cm below to 11 cm above the average sea surface height (Ssalto/Duacs, 2011). These factors are associated with the Aghulas rings that result in localized upwelling and enhanced productivity.
C6: Biological diversity Medium
Information on biodiversity is limited (e.g., review by Perez et al. 2012). The area is important for seabirds, epipelagic fish species and cetaceans (see above).
C7: Naturalness Low
The area has an extensive history of whaling, and in recent decades fishing has taken place. However, the area remains naturally highly productive.
Additional criteria BirdLife Important Bird Areas Criteria (BirdLife 2009, 2010) A1 Regular presence of threatened species A4ii >1% of the global population of a seabird