Environmental Engineering Reference
In-Depth Information
NITROGEN CYCLING IN THE SUBOXIC WATERS
OF THE ARABIAN SEA
Allan H. Devol
1
, S. Wajih A. Naqvi
2
, Louis A. Codispoti
3
1
University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195-7940,
USA
2
National Institute of Oceanography, 403004 Dona Paula, Goa, India
3
University of Maryland Center for Environmental Science, Horn Point Laboratory, 2020 Horns
Point Rd, P.O. Box 775, Cambridge, MD 21613, USA
Abstract
Arabian Sea contains one of the world's three large oxygen deficient zones
(ODZ). Within the ODZ oxygen concentration is vanishingly small between
about 200 and 800 m depth and in this depth interval denitrification is the ma-
jor mode or organic matter oxidation. This makes the Arabian Sea a globally
important sink in the marine combined nitrogen cycle. Within the ODZ nitrate
concentration profiles typically show a local minimum near 250 m and nitrate is
depleted relative to phosphate. Stoichiometrically based estimates of the amount
of nitrate removed from the water column, or the nitrate deficit, suggest maxi-
mum values of
∼
12 µM. However, the amount of excess nitrogen gas, which is
presumably a result of denitrification, has been estimated to be almost twice this
value. Although the reason for this discrepancy between the nitrate deficit and the
quantity of excess nitrogen gas remains unresolved, possible causes include non-
Redfield stoichiometry for denitrification, contributions due to nitrogen fixation
either in the Arabian Sea or in the ODZ source waters, anammox, or sedimentary
denitrification. Nitrate deficit based estimates of overall denitrification rate are
about 40 Tg N a
−
1
, but if the larger excess nitrogen gas measurements are correct
they suggest the rate could be correspondingly higher. Nitrate is enriched in the
heavy N isotope within the denitrification zone due to denitrification. During
upwelling this enriched N is brought into the euphotic zone where it is take up
by phytoplankton. When remains of the phytoplankton sink and become incor-
porated into the sediment the resulting sediments are also enriched in the heavy
isotope relative to other areas of the ocean. However, during glacial periods the
N in the sediment was not enriched suggesting better ventilation of the ODZ
waters and a lack of denitrification. Nitrous oxide, an important green-house gas
and an intermediate in both denitrification and nitrification typically displays
local maxima at the upper and lower boundaries of the ODZ, while it is found
in very low concentrations within the ODZ. Low values in the ODZ are the
result denitrification and this is evidenced by enrichment in the heavy isotopes
of both N and O. Maxima at the boundaries of the ODZ may result from either
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