Some aspects of the oxygen-deficient conditions and denitrification in the Arabian Sea
Abstract:Utilizing a fairly large amount of recently collected data, some outstanding questions concerning the Arabian Sea denitrification problem are addressed. The true levels of dissolved oxygen, determined colorimetrically, are about an order of magnitude lower than those reported previously from the oxygen minimum zone. Lateral advection of waters from south into the oxygen-deficient layer is suggested by the presence of an intermediate oxygen maximum discernible even at very low oxygen levels.
An unusual minimum in nitrate and a corresponding maximum in nitrite are observed occasionally within the depth range 700–1,200 m at several stations, generally located in the northeastern Arabian Sea. These features probably represent the development of a deeper denitrifying layer, in addition to the main denitrifying layer invariably found in the northern Arabian Sea at shallower depths. The deeper layer appears to be related to an increase in particulate organic carbon, probably resulting from seasonal changes in primary production, lateral advection of waters from the slope region off the Indian coast, or an in-situ production of organic matter.
Reoccupation of a number of stations reveals large short-term variability in denitrification intensity. Associated with the temporal variability, the southern boundary of denitrification appears to oscillate between Lats. 12 and 14N, except in the western Arabian Sea where it might shift to 18N during the southwest monsoon. Peak values of the integrated deficits do not occur within or near zones of high biological productivity; i.e., along the eastern and western boundaries. This is attributed to a more intense renewal of waters along these margins, through the northerly flow of waters, relatively rich in oxygen, to compensate for the vertical advection (upwelling) off Arabia, and through a hitherto undetected undercurrent off the west coast of India. Well-defined tongues of high and low integrated deficits, alternately extending northward and southward, respectively, possibly reflect net transports within the oxygen-deficient layer.
Rate of denitrification in the Arabian Sea is estimated from the exports of nitrate deficits out of the denitrification zone. The results indicate that the horizontal processes are responsible for the removal of the bulk (>85%) of the deficits. The deduced rate (29.5 × 1012 gN y−1) is at least an order of magnitude higher than the previous estimates. Combining this value with the estimated "standing crop" of denitrified nitrogen, the renewal time of the oxygen-deficient layer is deduced as ∼4 y. The short renewal time, supported by tritium data, is consistent with the observed short-term variability in denitrification intensity. The high rate of denitrification deduced in the present study appears to conform to global trends. It is suggested that currently accepted estimates of oceanic water column denitrification should be scaled-up by 40 to 100% in view of the present results.
Document Type: Research Article
Publication date: 1987-11-01
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