Environmental Engineering Reference
In-Depth Information
SEASONAL OXYGEN DEFICIENCY OVER THE
WESTERN CONTINENTAL SHELF OF INDIA
1
, H.Naik
1
, D.A. Jayakumar
1,2
, M.S. Shailaja
1
,and
S. Wajih
A. Naqvi
P.V.
Narvekar
1
1
National Institute of Oceanography, Dona Paula, Goa 403 004, India
2
Princeton University, Department of Geosciences, Princeton, NJ 08544-1003, USA
Abstract
The North Indian Ocean contains about two-third of the global continental-
margin area affected by natural oxygen deficiency (O
2
<
0.2 mL L
−
1
)inthe
water column. Also, the littoral countries of this semi-enclosed basin account
for a quarter of the world's population, making the sensitive O
2
-depleted envi-
ronment especially vulnerable to anthropogenic perturbations. We describe here
factors responsible for the occurrence of O
2
deficient conditions, their evolution
over the annual cycle, and their impact on biology and chemistry off the west
coast of India. The O
2
deficiency in this region, associated with the seasonal
(southwest monsoon) upwelling, seems to have intensified in recent years, pre-
sumably in response to enhanced nitrogen loading from land. The O
2
deficiency
affects patterns of organic production and distribution of organisms including
commercially important fishes, and modifies chemical fluxes through microbial
reduction of polyvalent elements especially nitrogen (denitrification). While the
extent of water-column denitrification over the shelf is modest (1.3-3.8 Tg N
y
−
1
), a very substantial fraction of the nitrate undergoing reduction appears to
end up as nitrous oxide, which accumulates to levels rarely seen elsewhere in
the ocean. Relative changes in dissolved inorganic nitrogen (DIN) and dissolved
inorganic phosphorus (DIP) closely conform to those predicted by the Redfield-
Ketchum-Richards stoichiometry in the oxic and suboxic waters. However, a
higher-than-expected buildup of DIP occurs in anoxic waters, probably due to
dissolution of the iron-oxyhydroxo-phosphate complex from sediments. This
DIP may support nitrogen fixation after the cessation of upwelling.
Keywords:
Arabian Sea, Indian shelf, upwelling, primary production, suboxia, anoxia, ni-
trogen cycle, denitrification, nitrous oxide, remineralization ratios
1. INTRODUCTION
Photosynthetic production of oxygen (O
2
), its exchange with the atmosphere,
and turbulence caused by winds, waves and tides combine to keep seawater
generally well oxygenated in all open-coastal areas except those affected by
195
