The oxygen minimum zone in the Arabian Sea during 1995

J. M. Morrison, L. A. Codispoti, Sharon L Smith, Karen Wishner, Charles Flagg, Wilford D. Gardner, Steve Gaurin, S. W A Naqvi, Vijayakumar Manghnani, Linda Prosperie, Jan S. Gundersen

Research output: Contribution to journalArticle

182 Citations (Scopus)

Abstract

This paper focuses on the characteristics of the oxygen minimum zone (OMZ) as observed in the Arabian Sea over the complete monsoon cycle of 1995. Dissolved oxygen, nitrite, nitrate and density values are used to delineate the OMZ, as well as identify regions where denitrification is observed. The suboxic conditions within the northern Arabian Sea are documented, as well as biological and chemical consequences of this phenomenon. Overall, the conditions found in the suboxic portion of the water column in the Arabian Sea were not greatly different from what has been reported in the literature with respect to oxygen, nitrate and nitrite distributions. Within the main thermocline, portions of the OMZ were found that were suboxic (oxygen less than ~ 4.5 μM) and contained secondary nitrite maxima with concentrations that sometimes exceeded 6.0 μM, suggesting active nitrate reduction and denitrification. Although there may have been a reduction in the degree of suboxia during the Southwest monsoon, a dramatic seasonality was not observed, as has been suggested by some previous work. In particular, there was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 μM. Waters in the northern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower, as evident in the reduced nitrite concentrations in the northern part of the basin. Organism distributions showed string relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but the biological responses to the OMZ varied with type of organism. The regional extent of intermediate nepheloid layers in our data corresponds well with the region of the secondary nitrite maximum. This is a region of denitrification, and the presence and activities of bacteria are assumed to cause the increase in particles. ADCP acoustic backscatter measurements show diel vertical migration of plankton or nekton and movement into the OMZ. Daytime acoustic returns from depth were strong, and the dawn sinking and dusk rise of the fauna were obvious. However, at night the biomass remaining in the suboxic zone was so low that no ADCP signal was detectable at these depths. There are at least two groups of organisms, one that stays in the upper mixed layer and another that makes daily excursions. A subsurface zooplankton peak in the lower OMZ (near the lower 4.5 [μM oxycline) was also typically present; these animals occurred day and night and did not vertically migrate.

Original languageEnglish
Pages (from-to)1903-1931
Number of pages29
JournalDeep-Sea Research Part II: Topical Studies in Oceanography
Volume46
Issue number8-9
DOIs
StatePublished - Aug 1 1999

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Arabian Sea
oxygen
nitrites
nitrite
denitrification
nitrate
Acoustic Doppler Current Profiler
nitrates
acoustics
organisms
monsoon
sea
suboxic conditions
nepheloid layer
nekton
nitrate reduction
vertical migration
thermocline
backscatter
dissolved oxygen

ASJC Scopus subject areas

  • Aquatic Science
  • Geology
  • Oceanography

Cite this

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 Research Part II: Topical Studies in Oceanography, 46(8-9), 1903-1931. https://doi.org/10.1016/S0967-0645(99)00048-X

The oxygen minimum zone in the Arabian Sea during 1995. / Morrison, J. M.; Codispoti, L. A.; Smith, Sharon L; Wishner, Karen; Flagg, Charles; Gardner, Wilford D.; Gaurin, Steve; Naqvi, S. W A; Manghnani, Vijayakumar; Prosperie, Linda; Gundersen, Jan S.

In: Deep-Sea Research Part II: Topical Studies in Oceanography, Vol. 46, No. 8-9, 01.08.1999, p. 1903-1931.

Research output: Contribution to journalArticle

Morrison, JM, Codispoti, LA, Smith, SL, Wishner, K, Flagg, C, Gardner, WD, Gaurin, S, Naqvi, SWA, Manghnani, V, Prosperie, L & Gundersen, JS 1999, 'The oxygen minimum zone in the Arabian Sea during 1995', Deep-Sea Research Part II: Topical Studies in Oceanography, vol. 46, no. 8-9, pp. 1903-1931. https://doi.org/10.1016/S0967-0645(99)00048-X
Morrison JM, Codispoti LA, Smith SL, Wishner K, Flagg C, Gardner WD et al. The oxygen minimum zone in the Arabian Sea during 1995. Deep-Sea Research Part II: Topical Studies in Oceanography. 1999 Aug 1;46(8-9):1903-1931. https://doi.org/10.1016/S0967-0645(99)00048-X
Morrison, J. M. ; Codispoti, L. A. ; Smith, Sharon L ; Wishner, Karen ; Flagg, Charles ; Gardner, Wilford D. ; Gaurin, Steve ; Naqvi, S. W A ; Manghnani, Vijayakumar ; Prosperie, Linda ; Gundersen, Jan S. / The oxygen minimum zone in the Arabian Sea during 1995. In: Deep-Sea Research Part II: Topical Studies in Oceanography. 1999 ; Vol. 46, No. 8-9. pp. 1903-1931.
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