-

J. mar. biol. Ass. India, 2000, 42 (1~92): 1 20

Seasonal variations in the physico-chemical and biological

characteristics of the eastern Arabian Sea
V. N. Pillai, V. K. Pillai, C. P. Gopinathan and A. Nandakumar
Central Marine Fisheries Research Institute, Cochin - 682 014

Abstract
The entire area along the west coast of India, between Cape Comorin and Kandla within the
EEZ was covered in six research cruises of FORV Sagar Sampada, 2 each in pre-monsoon,
monsoon and post-monsoon periods during 1991-93. Data on the physical, chemical and bio-
logical parameters were collected at every station (one degree square) from surface to maxi-
mum depth of 500 m by using Rosette samplers along with CTD Unit. The results of the
investigationsindicated definite fluctuations in the physico-chemical and biological features in
different seasons. The influence of physico-chemical features on primary production in terms
of chlorophyll a and other relevant observations made by earlier workers were compared and
briefly discussed in order to get a comprehensive picture of the characterestics of the waters in
different seasons in the eastern Arabian Sea.

Introduction (1966,1968), the various reports published

~t is well known that the seasonal cli- by the erstwhile FAO/UNDP Pelagic Fish-
matic changes in the marine environment ery Project, relevant papers brought out
play a significant role in the ecological by NIO based on IIOE, R. V. Gaveshini
cycle of the Indian seas, especially the and ORV Sagar Kanya data, Darbyshire
Arabian sea. Observations made over the (1967), Wyrtki (1973), Pillai (1983, 1989,
years point to the fact that the seasonal 1991, 1993) Pillai et al. (1980), Longhurst
changes brought in through the pre-mon- and Wooster (1990) etc. are worth men-
soon, monsoon and post-monsoon phe- tioning in this context. Most of the obser-
nomena along with the resultant oceano- vations made in the past had limited
graphic changes influence the overall pro- coverage both in terms of area as well as
ductivity of the region significantly. The the number of parameters observed.
biological productivity of the seas in this The data presented here were collected
region is dependent on the complex physi- on board FORV Sagar Sampada
cal, chemical and biological processes during 1991-93. The authors sincerely
active in the medium and subsequently acknow-led-ge the help and co-
transferred to different trophic levels. operation extended by the participants
Observations made by Banse (1959, of the above cruises. The participants are
1968), Ramamirtham and Jayaraman K. G. Girijavallabhan, N. Kaliaperumal,
(1960), Edelman (1960), Ramasastry and M. Rajagopalan, G. S. D. Selvaraj,
Myrland (1960), Murthy (1965), Sharma S . Muthusamy, R. N. Misra,
2

S. Krishna Pillai, P. K. Krishna-

kumar, V. V. Singh, P. Kaladharan,
T. V. Sathianandan, K. Vijayakumaran,
T. S. Naomi, K. M. Santhosh,
K. M. Shivaraj, K. S. Leela Bhai,
P. M. Aboobaker, K. M. Muniyandi,
V. K. Balachandran, J. R. Ramalingam,
M. P. Sivadasan, L.R. Khambadkar,
L. Jayasankar, K. Diwakar, K. K. Valsala,
M. Shahul Hameed, R. Anilkumar,
K. Solomon, K. N. Pushkaran,
A. Vairamani, and V. K. Suresh.
Material and methods
The entire area along the west coast of
India, between Cape Comorin and Kandla
within the EEZ was covered in six re-
search cruises of FORV Sugar Sampada, 2 Fig. 1 . Location of stations covered
each in pre-monsoon (March-April-May),
monsoon (June-July-August-September) variation of about 2°C between south-
and postmonsoon (October-November- eastern and northeastern regions, the
December-January-February) during the lower temperature being observed to-
period 1991-93 (Fig. 1). wards northern latitudes. The water tem-
Data on the physical and chemical as- perature over the shelf and beyond re-
pects of the waters were collected at every mained at around 30.5"C in the southern
station (representing one degree square) latitudes (05'30'N). The thermocline was
from surface to maximum depth of 500 m detectable beyond the shelf at about 75m.
by using the Rosette samplers along with The isotherms showed an upward tilt from
CTD Unit. the deeper layers (75-100m) of the outer
Surface water temperature was recor- continental shelf towards shallow depths
ded using a bucket thermometer. Salinity (Fig. 2) showing the commencement of
was recorded from the CTD probe, and upwelling in the region.
dissolved oxygen estimated by standard Surfacewaters remained warmer (tem-
Winkler method. Nutrients were estimated perature >30°) in the coastal and oceanic
by standard methods using a UV/VIS region at 12"30'N latitude sector. In gen-
Spectrophotometer on board the vessel. eral, the sea surface temperature showed
Results an increasing trend towards oceanic re-
Premonsoon gion. The depth of discountinuity layer
Sea Surface Temperature: was observed below 100m. The upward
The sea surface temperature showed a tilting of isotherms from depths below
lOOm indicated that the process of up- trend was reported by the earlier workers
welling has already commenced in the during the pre-monsoon season for the
deeper areas. However, over the shelf the west coast of India. A well defined salin-
water temperature remained above 28°C. ity maximum was observed within a
Towards north, at lat. 15'30' the sur- tongue of high saline water at deeper levels
face waters were comparatively cooler beyond the shelf in the southern sections
compared to the south, with surface tem- (09"30' and 12"301N lat.). Higher salinity
perature ranging between 28.0" and pockets above 36%0 were observed
28.8"C. The thermocline was observed between depth zones 30-100 m in the
below 100 m but unlike in the southern middle latitudes (12"30' - 15"30'N) (Fig.
region the presence of a strong thermal 3). Salinity above 36%0 were observed
gradient was not noticed in this region. throughout the water column upto a
The sea surface temperature in the region depth of lOOm in the northernmost sec-
at 1B030'latitude sector and further north tion. In this region the water column
ranged between 27.5"C and 28°C and within the shelf was more or less
comparatively low temperature prevailed homohaline but not homothermal.
over the entire shelf area. Isotherms at Dissolved oxygen
depths did not indicate any upward tilt-
ing and they existed almost parallel to the In the north-eastern region
coast. oxygen values were generally high and
more or less uniform upto a depth of 50-
Salinity 75 m and below that there was consider-
Salinity showed a progressive increase able reduction in the dissolved oxygen
towards northern latitudes within the levels at deeper levels. The oxygen levels
coastal as well as oceanic region. The same in the surface layer were invariably high

STATIONS 5

50

Fig. 2. Vertical temperature profile during the pre-monsoon period

(4.12 to 4.67 ml/l) and upto 70m depth anic region at lat. 17'30'N. Similar values
the trend was almost similar. However, were recorded in the coastal as well as
below 75m depth a rapid decrease was oceanic waters in the 21°30'N lat sector.
noted and below 100m the dissolved However, the highest values of phosphate
oxygen concentrations were less than 1.5 were observed in the mid shelf stations at
ml/l. lat. 19O30'N where the level ranged from
Nutrients 109-133 pg at/m2, while in the coastal
region between latitude 1B030' to 21°30'
The distribution of nutrients showed the phosphate concentrations recorded
that while phosphate showed a gradual were 50pg at/m2. The nitrite levels were
increase towards bottom, nitrate and ni- generally very low in the southern lati-
trite showed higher levels in the midwater tude stations (below 09O30'N) where the
layers. values recorded were less than 1 pg at/
Phosphate levels were mostly below 5pg m2.In the shallow coastal stations values
at/m2(integrated value for column of 75m) were below 10pg at/m2 in the southern
in the stations south of lat.15°30'N both latitude sector at 090301N.The nitrite con-
in the coastal and oceanic region, except centration showed an increasing trend
for an isolated high value (49.5 pg at/m2) towards dee-per stations in the area be-
in the nearshore station at lat.8"30fN. tween 10°30-130'N and values ranging
While comparatively higher levels were from 18-43 pg at/m2 were recorded in the
observed in the region north of lat. 15"30fN mid shelf and oceanic regions. In the
lowest values observed were 13-23pg at/ northern latitude sector north of 15'30'N
m2in the shallow coastal regions between pockets of high concentration (20-29 pg
15°30-16030'N and the coastal and oce- at/m2) were recorded mostly in the outer

Fig. 3. Vertical salinity profile during the pre-monsoon period.

 shelf stations between latitudes 15"30°-
CHLOROPHYLL A
18"3O1N and at 21°30'N. Rather low lev- mq/mz
COLUMN PRODUCTION
els (less than 10pg at/m2) were observed
in the coastal stations at 18°30'-19030'N
and at 21°30'N latitude sector. In general, EO M BAY

the nitrite levels were mostly between

10-20 pg at/m2 in the region north of RATNAGlRl

lat. 15"30tN. MARMAGOA

Chlorophyll-a concentration
MANGALORE

The chlorophyll -a distribution in the

euphotic zone indicated that the coastal
waters of S.W.region and n.w. region have > ;1
COCHlN

high rate of production. Along the south

west coast, the Wadge Bank area recorded
high rate of production (32.53 mg/m2) of
chlorophyll-a in the water column. Simi-
larly low to moderate levels of chloro-
phyll-~production (2.18 pg/m2) was re- Fig. 4. Column production of clzlorophyll a (mg/
corded in the oceanic regions of the south m2)from surface to 50m depth during the pre-
monsoon period.
west coast. The north-eastern Arabian sea
near Bombay and Veraval showed high
rate of production (50-70 mg/m2) (Fig. 4). area under observation.
However, low to moderate levels of chlo- Sea surface temperature : SST varied
rophyll-a were recorded in the oceanic between a minimum of 25°C and a maxi-
region off Ratnagiri and Kandla. mum of 293°C in the south eastern re-
Monsoon gion and the same varied between 26.08"C
and 28.6"C in the north eastern region.
Wind : Wind direction was predomi-
nan tly westerly, sometimes deviating to The main thermocline was observed
south westerly. Wind speed varied be- between a minimum depth of 20m (top of
tween a maximum of 40 knots to a mini- the thermocline) and a maximum of 100
mum of 7 knots. m (bottom of the thermocline).
Clouds : Clouds were predominantly Salinity:
Cumulonimbus and Nimbostratus, the In the southeastern region, sea surface
amount varying between 2/10 and 9/10. salinity varied between 30.45%0 and
Sea state : Heavy swells with white 37.53%0. In the northeastern region,
horses prevailed in the major part of the generally, high salinity was observed
6

(above 36%0) at the surface levels up to Chlorophll-a concentrations:

100 m depth. In the southeastern region, at surface
In the north eastern region salinity levels (the maximum depth of observation
values are more or less the same within was 200m) chlorophyll-a varied between
the surface 100 m and a decrease is found zero and 4.07 mg/m3. Horizontal varia-
only below 100-150 m depth. On the other tions of chlorophyll-a at surface levels in
hand, in the south eastern region salinity the south eastern region is plotted and
values, in gneral, were low at surface levels presented in Fig. 5 A which clearly indi-
with a gradual increase towards 100m. cate comparatively higher concentrations
Dissolved oxygen : in the coastal waters between Cochin and
Quilon.
In the south eastern region dissolved
oxygen values at sea surface varied be- In the northeastern region, chlorophyll-
tween 3.19 ml and 4.5 ml/l. It gradually a concentrations varied from traces to a
decreased to 0.5 ml/l at a depth of 100m. maximum of 4.03 mg/m3 in the coastal
In the north eastern region the dissolved waters near Bombay. Horizontal varia-
oxygen values at surface levels were uni- tions are presented in Fig. 5B.
formly high (4.98 ml to 5.03 ml/l). Low Post-monsoon
values were found only at depths of 50m Sea surface temperature
and below. The horizontal variations in
dissolved oxygen content of waters at a The surface water temperature during
depth of 50m clearly indicated a decrease October-November '91 from Cape to Goa
towards south near the coast between varied from 26.47 to 29.46"C and from
Ratnagiri and Goa where values as low as Goa to Kandla, it ranged from 27.0 to
0.5 ml/l were observed. 29.5"C. Surface temperature values were
higher (>29"C) in the renion between
Nutrients: Both in the south eastern as
well as north eastern regions phosphate
concentrations at surface levels varied
between 0.1 and 1.83 pg a t / l and gradu-
ally increased towards deeper waters.
Range values for the nutrients were as
follows:
Phosphate : 0-3.7 pg at/l
Nitrate : 0-7.12 pg at/l
Nitrite 0-5.28 pg at/l
Fig. 5. A. Horizontal variations of chlorophyll a
Silicate : 0-28.4 pg at/l (rng/m3)at sea surface level between Cochin and
Quilon.
 mixed layer is found deeper in the north-
ern region (lat. 18O30' to 2I030'N) than
the southern region. The bottom tempera-
ture values were below 17°C in the shelf
region at 120301N section while the cor-
responding values for the same depth zone
at 09O30'N were above 24°C. The iso-
therms showed an upward tilt towards
shallower areas of the shelf in the section
at 18"3O1N.The vertical temperature pro-
file indicated the presence of compartively
lower temperature water in the southern
Fig. 5. B. Horizontal variations of chloropkyll a section than the northern section. (Fig. 6).
(mg/m3)at sea surface in and around the area of
possible upwelling activity. Salinity
12'30' to 18"30'N, compared to the area The salinity of the column waters upto
south and north of it. Temperature values 500m depth ranged from 34.53 to 36.91
above 50m water column varied between ppt in the region between Cape to Goa
24.59 to 29.12"C in the northern latitude during October-November '91 while the
areas, between 15'30' to 21°30'N with salinity values from Goa to Kandla during
comparatively lower values in the south- October '92 was in the range of 31.41 to
ern part, while in the region below 37.07 ppt. There was not much difference
12"30'N, the temperature values in the between the values at the surface and
corresponding depth zone (0-30m) varied bottom levels in most of the stations
between 25.72 to 29.32"C. The depth of covered (Fig. 7).

STATION 5 3 2 1

Fig. 6. Verfical temperature profile during tlie post-monsoon period.

DissoIved oxygen the inorganic phosphate values were also
The dissolved oxygen content at the found to be less, showing negative corre-
surface between Cape and Goa during lation with primary production. The val-
October-November '91 ranged from 2.84 ues upto lOOm depth were found to be
to 4.95 ml/l and upto 50m depth, the trace or low and below 100ml the values
values ranged from 1.80 to 2.40 ml/l; 50- were observed to be slightly higher. The
lOOm depth, values ranged from 0.27 to area between Cape and Goa upto 100 m
1.91ml/l. The oxygen values within 100m depth indicated values ranging from trace
to 500m depths were very low (0.01 to to 3.0 yg at/l below 100 to 500 m depths,
0.21 ml/l) and sometimes negative values the values were 3-7 pg at/l. Similarly, in
were also obtained below 250m depth. the region between Goa and Kandla the
values upto 100 m depth were 0.025 to
The dissolved oxygen values between 0.295 pg at/l and below 100-500 m depths,
Goa and Kandla at the surface ranged the values being 0.140 to 0.375 pg at/l.
from 3.04 to 5.76 ml/l, showing slightly This revealed that the inorganic phosphate
higher values compared to the southern was slightly higher between Cape and
half. The values upto 50m depth indi- Goa compared to the northern region.
cated a range of 1.27 to 3.05 ml/l and 50-
The column production of the phos-
lOOm depth indicated a range of 0.32 to
phate indicated high values (220-250 pg
2.62 ml/l. Within lOOm to 500m depths,
at/l) in the shallow nearshore stations off
the values were found to be low to neg-
Trivandrum, off Calicut and at the oce-
ligible levels (0.02 to 1.77 ml/l).
anic regions off Mangalore all in the south-
Nutrients eastern region. Moderately high values
Similar to the nitrogenous compounds, (100-150 pg at/l) were observed in the

Fig. 7. Vertical salinity profile during the post monsoon period

oceanic regions between lat. 09O30'- pg at/l. Similarly, the area between Goa
12"0O1N, in the Quilon-Kasargod area. and Kandla upto 50m depth showed
Higher values were also observed in the traces or very low values (0.1 to 1.0 pg at/
deeper stations, lat. 10-llONand 13-14"N 1) and from 50-100 m depth indicated low
(110-190 pg at/l). Comparatively low val- to moderate levels (1-10. 3 pg at/l) and
ues were recorded (30-90 pg at/l) in the below 100-500 m depths, very high values
north eastern area with certain pockets of (3 to 23 pg at/l).
high values (150 pg at/]), off Veraval and In contrast to the values of nitrate and
in the area between Marmagoa and phosphate, the silicate values were found
Ratnagiri (100-180 pg at/l). to be very high in the water column of the
The column production of phosphates eastern Arabian sea; especially the area
revealed a close negative correlation with below 100m depth. The region between
values of chlorophyll-a in the post-mon- Cape and Goa upto lOOm showed values
soon season along the west coast of India. ranging from nil to 40pg at/l and high
values were noted below lOOm to 500 m
The nitrite content of the water col- depths (50-200 pg at/l). However, the
umn upto 100m depth in the area be- region between Goa and Kandla upto 50m
tween Cape and Goa indicated trace values depth showed low values (1-45 pg at/l)
to a maximum of 3.91 pg at/l and from with an increasing trend upto 100m. Very
100-500m depths, the values ranged from high values were observed below 100-
0.006 to 0.428 pg at/l. Similarly, the area 500m depths (50-212 pg at/l).
between Goa and Kandla showed values Primary production
ranging from 0.002 to 0.315 pg at/l upto
100m depth and from 100-500m depths, The data related to the primary pro-
the values were in the order of 0.008 to duction estimated by 14Ctechnique indi-
0.484 1-18 at/l revealing that the nitrog- cated that generally the rate of produc-
enous materials are slightly at higher level tion was low to moderate levels (0.02 to
between Cape and Goa during October- 0.93 gC/m2/day) throughout the area
November period. from Cape to Goa during the period of
observations. Moderate to slightly high
In contrast to the nitrite values, the values were observed off Cochin (0.82 gC/
values of nitrates were found to be slightly m2/day) and in the lat. 08'30'N and long
higher throughout the west coast during 75"301E off Wadge Bank area (0.93 gC/
the study period. The area between Cape m2/day). However, the rate of primary
and Goa upto 50m depth indicated nil to production estimated by oxygen technique,
trace values and 50-100 m depths, the from Marmagoa to Kandla, especially off
values were ranging from 0.39 to 14.80 pg Bombay, Veraval and Kandla regions have
at/l. However, the values of nitrate from low to moderate levels of production (0.1
100-500 m depths, showed 1.87 to 23.85 to 0.19 gC/m2/day).
10

Chlorophyll-a concentration bian Sea was 3.730 mg/m3 in the oceanic

waters off Calicut. Southern part of the
Fig. 8 indicate the chlorophyll-a distri-
south eastern Arabian sea is found to be
bution from surface to 50 m depths of the
more productive during the post mon-
euphotic zone along the west coast dur-
soon period.
ing the post-monsoon period.
The distribution of chlorophyll-a at 10m
In the surface waters, pockets of high
depth indicated pockets of high values
values exceeding 2 mg/m3 of chlorophyll-
exceeding 2-4 mg/m3 in the coastal areas
a were observed at lat. 11°30'N and long.
between Mangalore and Marmagoa and
72'30' and 740301E; off Calicut and at
also on the southern side of the Wadge
09'30' and 07'30' south of Cochin. High
Bank area, while chlorophyll-a distribu-
values were also observed in the Wadge
tion in the north showed comparatively
Bank area, (06O30' to 07'30'N and long.
low values ( 4 . 6 mg/m3), especially be-
70°30'E). In the north, surface chlorophyll-
tween Marmagoa and Ratnagiri (<0.267
a values ranged between 0-1.335 mg/m3
in the entire area. The highest chlorophyll mg/m3>.
a value observed in the south eastern Ara- The concentration of chlorophyll-a at
20 m depth indicated pockets of high
values (>5 mg/m3) in the oceanic regions
south of Cochin and high concentrations
COLUMN PRODUCTION
of chlorophyll-a on the southern side of
Wadge Bank (>4 mg/m3).Moderately high
values of chlorophyll-a (>2 mg/m3) were
observed in the coastal stations between
Mangalore and Marmagoa. Low values
as in the case of 10 m depth have been
noticed ( ~ 0 . mg/m3)
3 between Marmagoa
and Ratnagiri.
The distribution of chlorophyll-a at 30m
depth indicated high values in the sta-
tions between Mangalore and Marmagoa
(2-4 mg/m3). Pockets of high values were
also observed in the oceanic regions at lat.
08"30fN and long. 73"30fE (>3 mg/m3).
Very low values were observed between
Marmagoa and Ratnagiri (<0.3 mg/m3).
In the north, chlorophyll-a values ranged
Fig. 8. Column production of dtlorophyll a (mgl between 0.3 and 0.5 mg/m3 with certain
mZ)from the su$ac to 50m depth. pockets of high values (1-1.6 mg/m3)
 High concentration of chlorophyll-a water column within the shelf and be-
ranging from 2-4 mg/m3 were observed yond compared to southern region. How-
at lat. 08"30'N in the 50m depth. Other ever, the sea surface temperature varied
areas along the south-west coast revealed between 27.8"C and 28.6"C in the region.
0.3-1.4 mg/m3. The north-west coast The temperature discontinuity layer was
showed 0.3 to 1 mg/m3 with pockets of not pronounced in this section and the
high values in the coastal waters off isotherms showed stable summer condi-
Ratnagiri (2.14 mg/m3) and in the oce- tions. Banse (1968) observed the presence
anic regions off Bombay (3.8 mg/m3). of a weak thermocline in the northern
Column production of chlorophyll-a in- latitudes (18'30' - 21°30'N) during the
dicated that high values were noticed in same season. He has also observed a ther-
the southern part lat. 08'30'N (>I50 mg/ mal stratification in the waters during this
m2)in the oceanic region, 100-123 mg/m2 period.
at lat. 09"301N, south of Cochin and lat. In general, the sea surface tempera-
11°20'N off Calicut. In the area between tures were comparatively higher in the
Mangalore and Marmagoa, in the coastal southern latitudes (09"30' to 12'30') when
waters, the column chlorophyll-a was 50- compared to the temperature distribution
80 mg/m2. In the north eastern Arabian in the northern region. The upward tilt-
sea more than 10 mg/m2 were observed ing of isotherms at depths observed in the
in the oceanic regions off Bombay (lat. southern latitudes were not seen in the
19"301N).Low values (<25 mg/m2) were northern sector. The 25°C isotherm was
recorded in the area between Marmagoa observed within 100-125m depth zone
and Ratnagiri. throughout the area while it was present
Phaeo-pigments at 30m depth in the shallow coastal sta-
tion off Kandla. A low temperature pocket
Moderately low fractions of phaeo- (below 28°C) was observed in the shallow
pigme-nts were observed in the column regions off Veraval.
waters of the oceanic regions between
Ramamirtham and Patil (1966) while
Cape and Goa (0-96 mg/m2). The maxi-
describing the hydrography of the west
mum phaeo-pigment content was ob-
coast for the period January to May stated
served at lat. 11°30'N and long. 71°30'E.
that a salinity transition zone is evident at
Comparatively very low phaeo-pigments
about lat. 12-18"N in which the salinity
were observed in the north the maximum
increased rapidly. They also observed that
off Veraval (61 mg/m2).
an increase was found in the salinity val-
Discussion ues in the section off Mangalore. Even at
Premonsoon surface values as high as 35.2 %O occurred
In the northernmost sector at 21°30'N at some of the stations and the vertical
latitude the water temperature showed extent of the high salinity zone is found
reduction at the surface and in the entire between 50 and 200m depths. They obser-
12

ved that this marked increase in the The clockwise circulation which develops
salinity occurred at about 13" N which in the Arabian Sea during this season
can primarily be taken as a transition re- would have contributed to the higher
gion for salinity. They stated that this furt- salinity values observed in the northern
her divides the whole of the west coast stations. The southerly current which de-
into two regions from Cape Comorin to velops in May is known to continue till
Ratnagiri, a third zone probably being October. The effect of the spreading of the
north of Ratnagiri in the northern Ara- high-saline Arabian Sea water towards
bian Sea. south is neutralised by the south west
For the pre-monsoon season Rama- monsoon rains and river runoff. Hence
mirtham and Patil(1966) observed that in the horizontal variation of salinity at sur-
general, in the mixed layer the waters are face levels is dependent on the onset of
nearly 80% saturated in respect of dis- the south west monsoon and also the
solved oxygen concentrations and near direction, velocity and duration of the
the coast values as high as 4.8 ml/l have southerly current mentioned above.
been noted. They also observed a rapid The seawater temperature within the
decrease within the temperature discon- area under study shows spatial fluctations.
tinuity layer and below 150m depth the When the northern stations exhibit com-
values are never greater than 1.0 ml/l. paratively high temperatures, the situa-
For the north-eastern region (Ratnagiri to tion prevailing in the central and south-
Veraval) Patil et a1 (1964) reported that ern stations is entirely different with com-
along these sections surface values of paratively lower temperatures.
dissolved oxygen varied between 3.95 and In general, the shelf waters were poorly
4.8 ml/l. Down below the surface oxygen aerated during the south west monsoon,
values were greater than 4.0 ml/l upto a especialy towards south.
depth of 75 m, maximum values being
observed at varying depths from 10 to 50 Detailed analysis of sea water tempera-
m depth at individual stations. They also ture, salinity and dissolved oxygen data
observed that below 75 m depth a sharp collected along 6 sections in the south
gradient in oxygen content occurred and eastern region between Cape Comorin
this extended down to 150 m depth. and Goa revealed the presence of up-
welling in this area between lat. 08O30'N
Monsoon
(off Trivandrum) and 11°30'N (off Calicut)
The period of coverage (July and Au- near the coast. In this area the 23°C
gust) in the eastern Arabian Sea repre- isothern was found at comparatively shal-
sented the peak of southwest monsoon lower depths (18 to 25 m), the movement
season in this region characterised by com- of which on the vertical plane could be
paratively higher wind velocities, rough taken as an indicator for the presence of
sea conditions and relatively high rainfall. upwelling or sinking, as the case may be.
 The present study also confirmed that the
vertical oscillation of 23°C isothern on a
vertical time section can be taken as an
indicator for the commencement, intensi-
fication and cessation of the process of
upwelling along the southwest coast of
India which was first revealed from a
detailed analysis of similar data collected
onboard Research vessels attached to the
erstwhile FAO/UNDP Pelagic Fishery
Project, Cochin between 1971 and 1978
Fig. 9. Vertical movement of 23°C isotherm be-
(Pillai et a1 1980). tween 6g030'E and south west coast of India
In the present study upwelling activity during July, 1991 at different latitudes.
was observed in the area between lat
levels clearly revealed higher concentra-
08"301Nand 11°30'N lat. Fig. 9 shows the
tions in the coastal waters between 9" and
vertical movement of 23°C isotherm be-
10°N latitudes in the south eastern region
tween long. 69"30fE and the south west
and also between 18" and 21°N latitudes
coast of India during July, 1991 at differ-
ent latitudes viz. 07"30fN, 0S030'N,
09"301N, 10°30'N, 11"30'N, 13"30'N and
14"30rN.The vertical extent of oscillation
of 23°C isotherm at the different latitudes
is clearly brought out in Fig. 9.
Occurrence of 23°C isotherm at differ-
ent stations along the above mentioned 6
sections located south off Goa, Karwar,
Calicut, south off Ponnani, Alleppey and
Trivandrum are presented in Fig. 10which
can be used as a possible indicator for the
presence of upwelling, especially the hori-
zontal spread in space. In the north east-
ern region the 23°C isotherm occured
between depths of 50m and 131 m
whereas in the south eastern region, es-
pecially in areas of possible upwelling ac-
tivity the isotherm moved to shallower
depths of 18 to 25 m. Fig. 10. Depth of occurrence (m) of 23°C isotherm
at different stations in the south eastern Arabian
Horizontal variations observed in the Sea during July 1992 as a possible indicator of
Chlorophyll-a concentrations at surface the process of upwelling
14

near the coast in the north eastern region. induce upward motion near the coast.
Zooplankton biomass also showed higher Sharma (1966) while reviewing the
concentrations in the same areas. How- opinion given by earlier workers (Bense,
ever, this cannot be attributed to the effect 1959; Ramasastry and Myrland, 1960;
of upwelling observed especially in the Ramamirtham and Jayaraman, 1960)
southern region because the 'effect' of based on the work carried out during the
upwelling is to be looked at a distance late fifties and early sixties expressed the
away from the point where the upwelled view that owing to limitations of facilities,
water reached surface levels since the none of the reports are based on continu-
upwelled water is carried away from the ous studies for at least one year. Accord-
area of surfacing by the prevailing surface ing to him unless the continuous variation
currents which are southerly during July of any one of the relevant parameters is
- August. Hence higher concentrations of considered, it is not possible to get a clear
nutrients, phytoplankton and zooplank- picture of the commencement and cessa-
ton observed in the coastal areas during tion of upwelling and sinking. Sharma
the south west monsoon season should be (1966) after analysing hydrographic data
interpreted with care especially with re- (in the absence of adequate Bathythermo-
gard to it's origin. There are few occasions graph data) for the period March, 1964 to
where in the absence of strong surface August, 1965 and constructing topo-
currents, the 'cause' and 'effect' can be graphic maps for the top of the ther-
observed side by side. mocline and vertical time sections for sea
According to Banse (1959, 1968), the water temperature for a coastal station
prevailing current system and not the wind near Cochin noticed that in the month of
is to be regarded as the main cause gen- February the prevailing winds being north-
erating and maintaining the upwelling. erly to north easterly and with offshore
According to him even if a uniform cur- transport of the surface water, the condi-
rent velocity is considered all along the tions are favourable for upwelling. In the
coast, the rise of denser, deep water will month of August the winds are south
be stronger in the north farther away from westerly to westerly and the surface flow
the equator. He expressed the view that few miles away from the coast being
off the south west coast of India, upwelling easterly to south easterly, turns parallel to
starts with the onset of the south west the coast owing to boundary conditions
monsoon and reached the maximum in- giving rise to a southerly component.
tensity during July-August, established by Further, the precipitation and river dis-
late September and ends by mid October. charge near the coast stratify the surface
Varadachari (1961) found that a north- layers opposing any tendency for up-
erly wind driven current on the east coast welling. Except for a slight southerly com-
and a southerly wind driven current on ponent in the current, in general, all other
the west coast in the northern hemisphere conditions are unfavourable for upwelling
 15

in August and September along the south started as early as February at the bottom
west coast of India. levels. It started at the different sections
at different times each year. The com-
Darbyshire (1967) concluded that there
mencement of the process in February was
is no system of wind generated upwelling
possibly initiated by the northerly winds
during the south west monsoon period
which would transport the surface water
along the west coast of India and the
away from the coast there by initiating a
dense bottom waters approached the
vertical ascending motion from below.
surface because sf the immediate inter-
Perhaps the depth at which the motion
play of the current with the tilting of the
sea surface and the thermocline. Wyrtki gets started, would to a great extent,
(1973) concluded that upwelling is noth- depends upon the velocity, direction and
ing more than the shoaling of nearshore duration of the prevailing wind system in
isopleths, a consequence of baroclinic a specific area, the bottom topograpy, the
prevailing current system at the surface
adjustment to the anticyclonic monsoon
circulation and hence is remotely forced. levels and also the vertical stability of the
water column. The speed of the ascend-
Sharma (1968) after conducting a study ing motion would also depend on the con-
of the seasonal variations of some hydro- tinuance of the above mentioned
graphic parameters of the shelf waters off favourable factors with more or less the
the west coast of India concluded that same intensity. A closer examination of
upwelling along the west coast of India the prevailing wind system during the
starts earlier in the south and slowly south west monsoon season revealed the
extends towards north. The process com- presence of favourable northerly and
menced at deeper depths earlier in Febm- north westerly components in certain
ary and reached the surface by May. localities where upwelling intensity also
Upwelling comes to an end by July-Au- showed correspondingly higher values.
gust when the top of the thermocline
reaches the surface layers. The influence According to Longhurst and Wooster
of the river run off and rain stratify the (1990) sea level can be an indicator of
surface layers from July onwards thereby upwelling. The authors compared varia-
opposing the process. tions in oil sardine abundance with sea
level as an indicator of recruitment suc-
Studies made by one of the authors cess. One has to consider the invasion of
(Pillai, 1983) revealed that none of the the shelf with oxygen poor waters which
above mentioned theories is directly ap- occurs during Malabar coast upwelling,
plicable to the south west coast of India would tend to exclude oil sardine from
as a whole. The causative factors which the coastal region where diatom blooms
bring up the subsurface waters to surface are most intense. In such an event it is oil
levels vary in space and time. The studies sardine rather than mackerel whose
revealed that the process of upwelling spawning strategy will place at risk that
year's recruitment. According to them, be observed side by side. The results of the
this is the most likely explanation of sta- 1991 monsoon survey in the same area
tistical relationship between sardine re- revealed very interesting features in sup-
cruitment failure and unusally early re- port of the above claims.
motely forced upwelling. They also opined
that the abundance of oil sardine on the
Malabar coast is highly variable in the It is well known that three factors such
decadal scale. 0- group recruitment to as light, nutrients and primary produc-
the fishery begins towards the end of tion are of importance for determining
summer monsoon. At this time sea level the biological productivity of an area. Light
indicates remote forcing of upwelling penetration of the waters determine the
(caused by the geostrophic upsloping of depth of the euphotic zone while the
the isopleths towards the coastal) rather nutrients, especially nitrates and phos-
than the wind driven upwelling that oc- phates indicate the fertility of the waters
curs during the monsoon. Unusually early to promote productivity and the availabil-
remote forcing appears to inhibit subse- ity of phytoplankton reveal the produc-
quent recruitment perhaps through the tion at the primary level. Light penetra-
exclusion of spawning fish from the ner- tion in the sea largely depends on surface
itic zone by oxygen deficient upwelled irradiance and the type of water mass in
water. a region.
Very often a higher zooplankton bio- From the present investigation, it was
mass is observed away from the point found that the average depth of the eu-
where the upwelled water reached sur- photic zone in the west coast is 60 m.
face levels. It also varied from year to According to Radhakrishna et a1 (1978)
year. The upwelled water is carried away and Qasim (1982) the depth of the eu-
from the point of surfacing by wind gen- photic zone in the northern Arabian Sea
erated or density currents which change was 40 m and that of the southern part
its direction from place to place and from 60m. However, in the present investiga-
time to time. Hence the 'effect' of up- tion during the post monsoon period in-
welling is to be looked at a distance from dicated 75 m depth of photic zone along
the point where the upwelled water the north west coast in the oceanic wa-
reached surface levels. Higher concentra- ters. The photic zone in the Wadge bank
tion of nutrients, phytoplankton and zoop- area along the south west coast indicated
lankton observed in the coastal areas 45-50 m depth only.
during the upwelling season should be The source of the supply of nutrients
interpreted with care especially with re- in the topmost lOOm layer in the west
gard to it's origin. There are rare occa- coast is largely due to upwelling and river
sions where in the sbsence of strong sur- run off and also drainage from the land.
face currents the 'cause' and 'effect' can Very often the nutrients brought to the
surface are not fully utilized and signifi- m2/day in the shelf and 607 mgC/m2/
cant concentrations are detected in the day in the offshore waters of the Arabian
surface and column waters, even during Sea. Further, Qasim et al (1978) have
the period when there is no upwelling. reported a maximum production value of
The nitrogenous products upto lOOm 0.75 gC/m2/day in the coastal waters off
depth did not show any significant value Karwar and Calicut. Radhakrishna (1989)
but indicated a negative correlation with has reported that during October '67, the
the .primary production.However, the production from Cochin to Quilon varied
inorganic phosphate measured in the from 0.38 to 1.11 gC/m2/day. Further,
present investigation also indicated Radhakrishna (1989) has stated that high
a close negative correlation with the phytoplankton production during post
chlorophyll-a values revealing that most monsoon months extending upto March-
of the nutrients have been utilized by the April and low in the south-west monsoon
phytoplankton during the post-monsoon from the coast off Maharashtra. In the
period. The values of silicates in the col- present investigation, conducted during
umn waters of the west coast during the the post-monsoon season, the shelf wa-
postmonsoon period showed a higher level ters of the south-west coast of India indi-
with increase in depth. Upto 100 m depth, cated 0.19 gC/m2/day and the offshore
the values generally showed a lower level waters an average value of 0.14 gC/m2/
and below 100 m, the values were slightly day.
higher, revealing that the silicates upto Based on the present investigation, it
100 m depth have been utilized by the was observed that the regions of high con-
diatoms for their cell wall formation. centrations of chlorophyll-a are the coastal
According to Qasim (1977) and waters of Gujarat and Bombay along the
Radhakrishna et a1 (1978) the west coast north-west coast and Wadge Bank area
of India is an area of wide temporal and along the south-west coast. Integrated
spatial fluctuations from the point of pro- values of chlorophyll-a between 0-75 m
ductivity. Recently Nair and Pillai (1983) depth of the west coast showed that much
have reviewed the productivity of the of the chlorophyll-a occurs below the
Indian seas. Bhargava et a1 (1978) while surface. The concentration of photosyn-
studying the productivity of the Arabian thetic pigment was found to exist between
Sea mentioned average values ranging 20-30 m. Column production of chloro-
from 76-806 mgC/m2/day in different phyll-~along the west coast indicated that
months. Silas (1977) reported the produc- the southern region has higher values than
tivity on the shelf waters between 50-200 northern regions. However, according to
m along the west coast of India as 470 Radhakrishna et a1 (1978) and Qasim
mgC/m2/day and for the off-shore as 180 (1982) the average production in the
mgC/m2/day. However, Radhakrishna et northern Arabian sea is higher than the
a1 (1978) reported high values of 875 mgC/ average for the entire west coast implying
that the northern region is by and large In conclusion, it could be stated that (i)
more fertile than the southern region. the level of primary production and chlo-
Recent studies by Balachandran et a1 rophyll-~content of the waters during the
(1989) in the inshore surface waters of post-monsoon along the south-west coast
Cochin have pointed out the role of pho- is generally high; that the waters off
tosynthetic pigments as indices of biologi- Cochin and Wadge Bank area off Cape
cal productivity along the south west coast are highly fertile with column production
of India. of >I00 mg/m2 of chlorophyll-a and pri-
mary production of 0.8 gC/m2/day (ii)
It is a general belief that upwelling the maximum productivity occurs at sub-
along the south west coast during the surface layers (10-30 m) implying
south west monsoon has considerable in- photoinhibition at the surface levels (iii)
fluence on the coastal production. Ac- there is no direct relationship between
cording to Subrahmanyan (1959), along chlorophyll production and availability of
the west coast maximum production of inorganic nutrients and (iv) the eastern
phytoplankton takes place during the Arabian sea is a region of great contrast
south-west monsoon months of May-Sep- constituting both very rich and very infer-
tember after which there is a decline in tile pockets of photosynthetic materials.
the productivity. Comparatively low to
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