BGD
7, 1751–1782, 2010
Formation of anoxia
and denitrification in
bottom waters
G. D. Martin et al.
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Biogeosciences Discuss., 7, 1751–1782, 2010
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Formation of anoxia and denitrification in
the bottom waters of a tropical estuary,
southwest coast of India
G. D. Martin
1,2
, K. R. Muraleedharan
1
, J. G. Vijay
1
, G. Rejomon
1
, N. V. Madhu
1
,
A. Shivaprasad
1
, C. K. Haridevi
1
, M. Nair
1
, K. K. Balachandran
1
,
C. Revichandran
1
, K. V. Jayalakshmy
1
, and N. Chandramohanakumar
2
1
National Institute of Oceanography, Regional Centre, Kochi-682018, India
2
Dept. of Chemical Oceanography, Cochin University of Science and Technology,
Kochi-682016, India
Received: 23 February 2010 – Accepted: 3 March 2010 – Published: 15 March 2010
Correspondence to: G. D. Martin (martinnio@gmail.com)
Published by Copernicus Publications on behalf of the European Geosciences Union.
1751
BGD
7, 1751–1782, 2010
Formation of anoxia
and denitrification in
bottom waters
G. D. Martin et al.
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Abstract Introduction
Conclusions References
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Abstract
Hydrographic characteristics of the southwest coast of India and its adjoining Cochin
backwaters (CBW) were studied during the summer monsoon period. Anomalous for-
mation of anoxia and denitrification were observed in the bottom layers of CBW, which
have not been previously reported elsewhere in any tropical estuarine systems. The5
prevalent upwelling in the Arabian Sea (AS) brought cool, high saline, oxygen deficient
and nutrient-rich waters towards the coastal zone and bottom layers of CBW during
the high tide. High freshwater discharge in the surface layers brought high amount of
nutrients and makes the CBW system highly productive. Intrusion of AS waters seems
to be stronger towards the upstream end (∼15 km), than had been previously reported,10
as a consequence of the lowering of river discharges and deepening of channels in the
estuary. Time series measurements in the lower reaches of CBW indicated a low mix-
ing zone with increased stratification, 3 h after the high tide (highest high tide) and high
variation in vertical mixing during the spring and neap phases. The upwelled waters
(O
2
≤ 40 µM) intruded into the estuary was found to lose more oxygen during the neap15
phase (suboxic O
2
≤ 4 µM) than spring phase (hypoxic O
2
≤ 10 µM). Increased stratifi-
cation coupled with low ventilation and presence of high organic matter have resulted
in an anoxic condition (O
2
= 0), 2–6 km away from barmouth of the estuary and leads
to the formation of hydrogen sulphide. The reduction of nitrate and formation of nitrite
within the oxygen deficient waters indicated strong denitrification intensity in the estu-20
ary. The expansion of oxygen deficient zone, denitrification and formation of hydrogen
sulphide may lead to a destruction of biodiversity and an increase of green house gas
emissions from this region.
1 Introduction
Globally, estuaries and coastal waters are increasingly exhibiting eutrophication due25
to the enrichment of nutrients caused by anthropogenic activities (Zill
´
en et al., 2008;
1752
BGD
7, 1751–1782, 2010
Formation of anoxia
and denitrification in
bottom waters
G. D. Martin et al.
Title Page
Abstract Introduction
Conclusions References
Tables Figures
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J I
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Cox et al., 2009). Increased algal production in the surface waters overlying within low
oxygen environments [hypoxic (O
2
< 22 µM) to suboxic (O
2
< 4.4 µM)] were transformed
into anoxic (O
2
= 0) zones with an emission of green house gases (Naqvi et al., 2000,
2005) and leads to shifts in both pelagic and benthic organisms. Thus, coastal waters
are considered to be a significant source of green house gases, contributing to more5
than 60% of the global oceanic flux (Zhang et al., 2006; Bange et al., 1996; Bouwman
et al., 1995; Nevison et al., 1995; Naqvi et al. 2005). Therefore, the contribution
of estuaries to increased production of green house gases depends strongly on the
intensity of oxygen depletion and denitrification caused by human-induced changes.
The Arabian Sea is one of the highly productive regions in the world, mainly due to10
natural processes (Naqvi et al., 2000). Enhanced biological production coupled with
sinking of organic matter leads to a high oxygen demand in the intermediate depths.
During summer monsoon, the existence of a strong density gradient prevents quick re-
newal of subsurface waters through horizontal advection (De Sousa et al., 1996). Con-
sequently, dissolved oxygen gets severely depleted (<4.4 µM) between 150–1200 m15
depths resulting in the reduction of oxidized nitrogen species to molecular nitrogen, by
denitrification (Naqvi and Qasim, 1983; Naqvi and Jayakumar, 2000a). The oxygen
minimum zone (OMZ) in the AS is intense and denitrification rate is about one-third of
the estimated global water column denitrification rate (Codispoti et al., 2001; Naqvi et
al., 2005). Hence, AS is considered to play a crucial role in regulating the atmospheric20
chemical composition and the earth’s climate.
The biological productivity in the AS is strongly influenced by seasonal changes
associated with the monsoon. During the summer monsoon per iod, the wind driven
upwelling along the southwest coast of India is vigorous when compared to that of
Somalia, Yemen and Oman (Naqvi et al., 2006). The upwelled water is located just25
off the shelf break (Shetye et al., 1990), where the coastal circulation and tides has
a dominant role in advecting these waters into the estuaries. The estuaries of the
northwest coast (Mumbai) have a mean tidal height (∼3 m) higher than (Srinivas et al.,
2006) that of southwest coast (∼1 m). The low tidal amplitudes and increasing number
1753
BGD
7, 1751–1782, 2010
Formation of anoxia
and denitrification in
bottom waters
G. D. Martin et al.
Title Page
Abstract Introduction
Conclusions References
Tables Figures
J I
J I
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of oscillations in the SW coast may lead to small inter-tidal expanses, which reduce
flushing (Qasim, 2003).
The CBW is one of the largest estuarine systems along the west coast of India
(Fig. 1), running parallel to the SW coast with a relatively lower (∼1 m) tidal amplitude
(Srinivas et al., 2003). The system is influenced by monsoonal rains (annual average5
rain fall of 3200 mm), contributing to 75% of the annual rainfall during the summer mon-
soon season (June–September). River discharge (from 7 rivers) brings high nutrients
to make CBW highly productive (Madhu et al., 2007). Increased anthropogenic activi-
ties of this region generates 104×10
3
m
3
of industrial and 260 m
3
of domestic wastes
per day, which are being released directly into the estuary without any treatment (Bal-10
achandran et al., 2005; Martin et al., 2008). The Cochin backwaters are known for their
rich bio-resources, but in the pr istine conditions have suffered severe depletion due to
human intervention (Menon et al., 2000; Martin et al., 2008).
Major interventions in the CBW started from 1920’s for different developmental ac-
tivities such as construction of harbor, industrial areas, expansion of Wellington Island15
and intensive reclamations for human settlement. Due to these activities in the begin-
ning of the 19
th
century, the total area of CBW has shrunken from 365 km
2
to 256 km
2
(Gopalan et al., 1983; Balachandran et al., 2005). Apart from this, a removal of 2.5
million cubic yards of silt associated with periodic dredging every year to maintain ship-
ping channel (Gopalan et al., 1983) will drastically influence channel cross section and20
depth (Joseph and Kurup, 1989). This could lead to an increase in tidal storage, slug-
gish water movements and decreased flushing rates, which may expedite stratification
and sedimentation process (Joseph et al., 2009). In this paper, the influence of coastal
hydrography on the adjacent CBW has examined during summer monsoon period to
identify the causative factors responsible for the development of anoxia and denitrifica-25
tion process.
1754
BGD
7, 1751–1782, 2010
Formation of anoxia
and denitrification in
bottom waters
G. D. Martin et al.
Title Page
Abstract Introduction
Conclusions References
Tables Figures
J I
J I
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2 Materials and methods
2.1 Study area
The Arabian Sea experiences biannually reversing winds and associated circulations
during the summer and winter monsoons. Western India has a divergent and passive
continental margin (Ramalingeswara Rao and Veerayya, 2000) from Cape Comor in5
(8
◦
N) to Kutch (22
◦
N). Many small rivers like the Mandovi and Zuari in Goa, the Kali-
nadi in north Kanara and the Netravati in south Kanara flow across the coastal plain
and opens into the AS. CBW (also known as Vembanad lake) and the Ashtamudi lake
are the two major estuar ies debouching freshwater into the Arabian Sea (Purnachan-
dra Rao et al., 1983) from the south.10
The CBW (Lat. 9
◦
30
0
–10
◦
10
0
N and Lon. 76
◦
15
0
–76
◦
25
0
E) constitutes a complex
estuarine system (Fig. 1), characterized by an ox-bow shape, running parallel to Ara-
bian Sea (Soman, 1997). Due to its peculiar topography, the circulation patterns in
the northern and southern arms of the CBW are found to be different (Ramamirtham
and Muthusamy, 1986). Hence, the high freshwater flow during summer monsoon su-15
presses the tidal charateristics and increases stratification in the lower estuary (Qasim
and Gopinathan, 1969). CBW is connected to the AS through two inlets, one at Cochin
and other at Azhikode, the former being wider (450 m) than the latter (250 m). The
CBW is generally wide (0.8–1.5 km) and deep (4–13 m) towards south but becomes
narrow (0.05–0.5 km) and shallow (0.5–3.0 m) towards north. It is a bar-built microtidal20
estuary (with a tidal height of ∼0.9 m at spring tide and ∼0.5 m at neap tide), receiving
large volume of fresh water runoff (22×10
9
m
3
year
−1
) from seven rivers (Fig. 2a and
b) mainly during the summer monsoon (July). Monthly river discharge data (Fig. 2a)
showed a high seasonality, 60–70% of the total discharge occurring during the south-
west monsoon (June–September). The yearly wise river discharge (mean) data (ob-25
tained from Central water commission, India) from 7 major rivers indicates a decrease
in discharge during the last few decades especially after 1995. The drastic decrease
in discharge (Fig. 2b) was noticed from the southern rivers than the north. The major
1755
