The oxygen minimum zone in the Arabian Sea during 1995

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, Dis solved oxygen, nitrite, nitrate and density values are used to delineate th e OMZ, as well as identify regions where denitrification is observed. The s uboxic conditions within the northern Arabian Sea are documented, as well a s biological and chemical consequences of this phenomenon. Overall, the con ditions 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 w ith respect to oxygen, nitrate and nitrite distributions. Within the main t hermocline, portions of the OMZ were found that were suboxic (oxygen less t han similar to 4.5 mu M) and contained secondary nitrite maxima with concen trations that sometimes exceeded 6.0 mu M, suggesting active nitrate reduct ion and denitrification. Although there may have been a reduction in the de gree of suboxia during the Southwest monsoon, a dramatic seasonality was no t observed, as has been suggested by some previous work. In particular, the re was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 mu M. Waters in the nort hern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower. as ev ident in the reduced nitrite concentrations in the northern part of the bas in. Organism distributions showed string relationships to the oxygen profil es, especially in locations where the OMZ was pronounced, but the biologica l 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 mig ration of plankton or nekton and movement into the OMZ. Daytime acoustic re turns from depth were strong, and the dawn sinking and dusk rise of the fau na were obvious. However, at night the biomass remaining in the suboxic zon e 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 a nd another that makes daily excursions. A subsurface zooplankton peak in th e lower OMZ (near the lower 4.5 mu M oxycline) was also typically present; these animals occurred day and night and did not vertically migrate. (C) 19 99 Elsevier Science Ltd. All rights reserved.