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Ecologically or Biologically Significant Areas (EBSAs)

  published: 11 Jul 2017

Arabian Sea Oxygen Minimum Zone

General Information
The Arabian Sea is known to have a large oxygen minimum zone (OMZ) located between depths of 200 to 1000 m. Oxygen levels in this zone can be as low as 0.1 mg/l. The low oxygen zone contains nitrite maxima, suggesting active nitrate reduction and denitrification, which results in utilization of oxygen and thus oxygen-level drops. This low oxygen zone contains unique fauna predominantly consisting of lanternfishes (myctophids). Dominated by Benthosema pterotum, B. fibulatum and Diaphus spp. Bolinichthy spp., the mesopelagic animals exhibit diurnal vertical migration. Myctophids are believed to form an important food for large predators, including large squids, ribbonfishes, tuna and billfish. The oxygen minimum zone of the Arabian Sea is a unique ecosystem, with distinctive biological features.
The Arabian Sea has some of the most extreme climatic regimes due to seasonal fluctuations in air and water temperatures. Being blocked from the north, this sea is characterized by high tropical temperature regime, reversal of monsoon, which in turn results in reversal of the currents, presence of a large oxygen minimum zone (OMZ) and limited upwelling confined to Oman/Yemen and Somalia. The formation of the oxygen minimum is probably due to high organic production in the euphotic zone, sinking of a large amount of organic matter, the lack of horizontal advection due to the land-locked nature of the sea, and the presence of highly saline water in the upper layers (Bulow, 2010; Qasim, 1982). High oxygen at intermediate depths and the second oxygen minimum in the range 800 to 1500 m probably occurs as a result of physical processes peculiar to this part of the Arabian Sea. The flow pattern consists of several eddies and meanders. Inorganic phosphorus is high in the surface layer and still higher at greater depths. Nitrate-nitrogen is low at the surface and increases with depth. The low oxygen zone with nitrite maxima suggests active nitrate reduction and denitrification, which results in utilization of oxygen and thus oxygen levels dip below 0.1 mg/l. This low oxygen zone extends to almost the entire Arabian Sea beyond shelf waters, covering India, Pakistan, Iran, Oman and Yemen. The OMZ, despite being low in oxygen, is a highly diversified ecosystem and contains a large number of mesopelagic species dominated by lanternfish. Benthosema pterotum, B. fibulatum, Diaphus spp. Bolinichthy spp. are the dominant myctophids found in this zone. They exhibit distinct diurnal vertical migration, residing during daytime at depths of extremely low oxygen levels (<0.1 ml O2 1−1) and foraging in the oxygen-rich surface layer at night (Kinzer et al., 1993; Luo, et al., 2000; Qasim, 1982). All species appear to be opportunistic predators that prey on a comparatively narrow food spectrum consisting principally of small- to medium-sized copepods (Banse, 1994; Böttger-Schnack, 1994). Myctophids are believed to form an important food for large predators, including tuna, billfish, ribbonfish and large squids. The oxygen minimum zone of the Arabian Sea is a distinctive ecosystem. Myctophids are found in dense schools but have patchy distribution. The Oman Sea is relatively rich in fisheries resources, with considerable quantities of mesopelagic fish occurring on, and seaward, of the continental slope and throughout the water zone (Clark, 1973; Gartner, 1993). The importance of lantern fishes in the Oman Sea was first indicated by studies of eggs and larvae collected by R/V Anton Bruun from 1959 to 1965 (Gjosaeter and Kawaguchi, 1980). The most important myctophid species in the area is the skinnycheek lantern fish Benthosema pterotum, which is distributed throughout the Oman Sea (Nafpaktitis & Nafpaktitis, 1969; Gjosaeter & Tilseth, 1983; Johannesson &Valinassab, 1994). The oxygen minimum zone of the Arabian Sea and associated ecosystem has characteristics that make it unique and thus requires due attention.
Description of the location
North-West Indian Ocean and Adjacent Gulf Areas
The area is in the Arabian Sea spreading along India, Pakistan, Iran, Oman and Yemen. It is also present in Gulf of Oman between Iran and Pakistan.
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 Arabian Sea OMZ is an extremely important and unique ecosystem with distinctive ecological and biological features. This low oxygen zone spreads through almost the entire Arabian Sea beyond shelf waters, at depths of between 200 to 1000 m,. Despite being low in oxygen, the OMZ is a highly diversified ecosystem and contains a large number of mesopelagic species dominated by lanternfish. Benthosema pterotum, B. fibulatum, Diaphus spp. Bolinichthy spp. are the dominating myctophids found in this zone. They exhibit distinct diurnal vertical migration, residing during daytime at depths of extremely low oxygen levels (<0.1 ml O2 1−1) and foraging in the oxygen-rich surface layer at night (Kinzer et al., 1993; Luo, et al., 2000; Qasim, 1982). All species appear to be opportunistic predators that prey on a comparatively narrow food spectrum consisting principally of small- to medium-sized copepods (Banse, 1994; Böttger-Schnack, 1994). Myctophids are believed to form an important food for large predators, including tuna, billfish, ribbonfish and large squidsMyctophids are found in dense schools but have patchy distribution. The Oman Sea is relatively rich in fisheries resources, with considerable quantities of mesopelagic fish occurring on, and seaward of, the continental slope and throughout the water column (Clark, 1973; Gartner, 1993). The importance of lantern fishes in the Oman Sea was first indicated by studies on eggs and larvae collected by R/V Anton Bruun during 1959-1965 (Gjosaeter and Kawaguchi, 1980). The most important myctophid species in the area is the skinnycheek lantern fish (Benthosema pterotum), which is distributed throughout the Oman Sea (Nafpaktitis & Nafpaktitis, 1969; Gjosaeter & Tilseth, 1983; Johannesson & Valinassab, 1994).
Exploitation of the mesopelagic resources has already been started by Iran. The biomass of mesopelagic fish was estimated to be about 100 million tonnes (range 60 to 150 million tonnes). The estimates from spring are higher than those from summer and autumn. Other regional countries, including Oman, have already started exploring the possibility of starting commercial fisheries. Although a number of studies have been carried out in order to increase understanding of the low oxygen phenomenon in the Arabian Sea, many facets of the ecology and biology are not well understood. Further studies are needed.
References
Abildgaard, N.L., Khan, M.W., Khaliluddin, M., Qureshi, S. and van Zalinge, N.P. 1986. Stock assessment of demersal fish in Pakistan waters (Results of bottom trawl surveys carried out in 1983– 1985) FI:PAK/77/033 Field Document No. 4. FAO. Rome. 85 p. Ahmad, J., Moazzam, M., and Imad, A. 1988. A Report of the Demersal Fish Resource Survey in Waters of Pakistan Carried out by R/V “Tehkik” in 1987-1988. Marine Fisheries Department, Karachi. 69p. Banse, K. 1994. On the coupling of hydrography, phytoplankton, zooplankton, and settling organic particles offshore in the Arabian Sea. Proc. Indian Acad. Sci.-Earth and Planetary Sci., 103: 125-161. Bulow, S. E., Rich, J. J., Naik, H. S., Pratihary, A. K., & Ward, B. B. 2010. Denitrification exceeds anammox as a nitrogen loss pathway in the Arabian Sea oxygen minimum zone. Deep Sea Res. Part I: Oceanogr. Res. Papers, 57: 384-393. Fanning L.P., Khan, M.W., Kidwai, S. and Macauley, G.J. 2011. Surveys of the offshore fisheries resources of Pakistan – 2010. FAO Fisheries and Aquaculture Circular. No. 1065. Karachi, FAO. 87 p.Gjøsæter, J. 1981. Abundance and production of lanternfish (Myctophidae) in the western and northern Arabian Sea. FiskDir. Skr. Scr. NavUnders., 17: 2 15 -25. Gjosaeter, J. and Tilseth, S., 1983. Survey on mesopelagic fish resources in the Gulf of Oman. February 1983. Reports on surveys with R.V. “Dr. Fridtjof Nansen” Institute of Marine Research, Bergen. NORAD/FAO/UNDP project GLO/82/001, 1-28. Johannesson, K., 1991. Stock assessment of myctophid resources in the Sultanate of Oman waters of the Oman Sea. Final report (Ministry of Agriculture and Fisheries). Johannesson, K., & Valinassab,T. 1994. Survey of mesopelagic fish resources within the Iranian exclusive economic zone of the Oman Sea. Final report (Govt/FAO project: UTF-IRA-020/IRA). 184 p. Kinzer, J., Böttger-Schnack, R., and Schulz, K. 1993. Aspects of horizontal distribution and diet of myctophid fish in the Arabian Sea with reference to the deep water oxygen deficiency. Deep Sea Res. Part II: Topical Stud. Oceanogr., 40: 783-800. Luo, J., Ortner, P. B., Forcucci, D., & Cummings, S. R. 2000. Diel vertical migration of zooplankton and mesopelagic fish in the Arabian Sea. Deep Sea Res. Part II: Topical Stud. Oceanogr., 47: 1451-1473. Mara J. and Barber R.T. 2005. Primary productivity in Arabian Sea. A synthesis of JGOFS data. Progr. Oceanogr. 65: 159–175. Moazzam, M., and Ziaullah, 2001. Some observations on the ornithological fauna of the offshore waters of Pakistan. Pakistan Jour. Mar. Biol. 7: 113-134. Morrison, J. M., Codispoti, L. A., Smith, S. L., Wishner, K., Flagg, C., Gardner, W. D., Gundersen, J. S. 1999. The oxygen minimum zone in the Arabian Sea during 1995. Deep Sea Res. Part II: Topical Stud. Oceanogr., 46: 1903-193.1. Nafpaktitis, B.G. and Nafpaktitis M., 1969. Lantern fishes (Family Myctophidae) collected during cruises 3 and 6 of the R/V “Anton Bruun” in the Indian Ocean. Bulletin of the Los Angeles County Museum of Natural History, Science 5, 1-79. Valinassab, T., 1998. Trial fishing for lantern fishes (myctophids) in the Gulf of Oman (1989-1990). FAO Fisheries Circular, no. 935. Qasim, S. Z. 1982. Oceanography of the northern Arabian Sea. Deep Sea Res. Part A. Oceanogr. Res. Pap., 29:1041-1068. Böttger-Schnack, R. 1994. The microcopepod fauna in the Eastern Mediterranean and Arabian Seas: a comparison with the Red Sea fauna. In Ecology and Morphology of Copepods (pp. 271-282). Springer Netherlands. Wiggert J.D., Jones B.H., Dickey T.D., Brink K.H., Weller R.A., Marra J. and L.A. Codispoti, 2000. The Northeast Monsoon’s impact on mixing, phytoplankton biomass and nutrient cycling in the Arabian Sea. Deep-Sea Res. II 47: 1353–1385.
  • NWIO_14_EBSA.zip (/api/v2013/documents/284A2039-64C9-6E1C-DA20-43328CAD456D/attachments/NWIO_14_EBSA.zip)
  • Arabian Sea Oxygen Minimum Zone.pdf (/api/v2013/documents/284A2039-64C9-6E1C-DA20-43328CAD456D/attachments/Arabian%20Sea%20Oxygen%20Minimum%20Zone.pdf)
Status of submission
Areas described as meeting EBSA criteria that were considered by the Conference of the Parties
  • dec-COP-13-DEC-12
Assessment of the area against CBD EBSA criteria
C1: Uniqueness or rarity High
The presence of large oxygen minimum zone (OMZ) in the Arabian Sea is a unique feature of the area. The zone, which is located at depths of between 200 to 1000 m, is characterized by very low oxygen levels (> 0.2 mg/l in some area) and its unique ecology, dominated by lanternfishes (Benthosema pterotum, B. fibulatum Diaphus spp. and Bolinichthy spp.). All recorded myctophid fish species migrate in a diel pattern, residing during daytime at depths of extremely low oxygen levels (<0.1 ml O2 1−1) and foraging in the oxygen-rich surface layer at night (Kinzer et al., 1993; Luo, et al., 2000; Qasim, 1982). All species appear to be opportunistic predators that prey on a comparatively narrow food spectrum consisting principally of small- to medium-sized copepods (Banse, 1994; Böttger-Schnack, 1994). Myctophids are believed to form an important food for large predators, including tuna, billfish, ribbonfish and large squids. The OMZ of the Arabian Sea is a distinctive ecosystem.
C2: Special importance for life-history stages of species No information
The role of the OMZ of the Arabian sea in the life history of marine animals is not well understood. However, having a short life span (less than a year), myctophids offers a unique conditions that are not shared by many species (Johannesson and Valinassab, 1994). More information is needed regarding the life history of predators dependent on the biota of the OMZ, such as ribbonfish and large squids, in particular regarding their role in life history tactic of the species living this area.
C3: Importance for threatened, endangered or declining species and/or habitats Low
A number of large predators (especially cetaceans) directly or indirectly depend on the food chain of the OMZ of the Arabian Sea. However, no threatened, endangered or declining species is known to inhabit this zone. The biota of the Arabian Sea OMZ is not well documented, either.
C4: Vulnerability, fragility, sensitivity, or slow recovery Low
The Arabian Sea OMZ, being located in deeper layers, is not easily affected by any human-induced changes and also by phenomenon of the surface area. Commercial utilization of the myctophids and other animals of the Arabian Sea OMZ has recently started by Iran, however, considering the scale of operation, there seems to be no impact on the ecosystem functioning. Considering short life span of myctophids, their harvesting may not have bearings on the stocks of mesopelagic which are considered to be very high (> 100 million m. tons) of virgin biomass (Valinassab, 1998).
C5: Biological productivity High
The Arabian Sea OMZ is considered to be highly productive. No estimates of the biomass of mesopelagic fishes of the entire Arabian Sea has been made but the biomass in the Northern Arabian Sea between Pakistan, Iran and Oman is estimated to be > 100 million tonnes Gjøsæter (1981), Gjøsæter, and Tilseth (1983) and Johannesson (1991). Although the production mechanism is not well understood, it is generally believed that the high primary productivity of the Arabian Sea, results in increased production of zooplankton in the surface waters as well as mesozooplankton that support an extremely rich biomass of mesopelagics. The amount of biomass of lanternfishes in the Oman Sea has been estimated at about 2.3 milllion tonnes in Iranian waters (Valinassab, 1998), with more or less the same values for the southern Oman Sea (Omani waters) (Johannesson, 1991).
C6: Biological diversity Medium
Despite being low in oxygen, the OMZ hosts a highly diversified ecosystem and contains a large number of mesopelagic species dominated by lanternfish. Benthosema pterotum, B. fibulatum, Diaphus spp. Bolinichthy spp. are the dominating myctophids found in this zone. All species appear to be opportunistic predators that prey on a comparatively narrow food spectrum consisting principally of small- to medium-sized copepods (Banse, 1994; Böttger-Schnack, 1994). Myctophids are believed to form an important food for large predators, including tuna, billfish, ribbonfish and large squidsMyctophids are found in dense schools but have patchy distribution.
C7: Naturalness High
Despite being highly productive, no exploitation of the resources of Arabian Sea OMZ is being done (except for recent harvesting of lanternfish by Iran on small scale) mainly because of technological reasons, therefore, ecosystem and habitat remain intact.