chapter ii hydrographic conditions of the...
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CHAPTER II
HYDROGRAPHIC CONDITIONS OF THE AKATHUMURI LAl<E
A TYPICAL HABITAT OF ISOPODS - ASEASONAL STUDY
CHAPTER II
HY DROGRAPHIC CON DITIONS OF
THE AKATHUMURI LAKE - A TYPICAL HABITAT OF ISOPODS
- A SEASONAL STU,DY
INTRODUCTION
The State of Kerala which occupies an area of 38,855 sq.kms is located
at the southern-most tip of Peninsular India on the West Coast. It is situa
ted between North latitude 8° 18' and 12° 48' and East longitude 74°
52' and 77° 2 '. This state owes its well balanced climatological conditions
to the presence of the Western Ghats on its east and the Arabian Sea
on its west.
The scenic beauty of Kerala is enhanced by its chain of backwaters
which extends to about 460 km from Badagara in the North to Trivan
drum in the South. These backwaters are a system of interconnected brack
ish water lagoons oriented parallel to the sea. It is separated from the
Arabian Sea by a narrow strip of land which varies in width at various
places. Apart from these backwaters, Kerala is endowed with bountiful
rivers, forty-one west-flowing and three east-flowing ones originating from
the Western Ghats. The backwaters are connected to the Arabian Sea
through openings of the sand bar at certain places. These openings may
be either permanent or temporary. During the monsoon seasons, large
influx of freshwater from rivers into the backwaters occurs and in those
backwaters with a temporary opening, the bar-mouth opens during these
seasons so that free access to the sea is possible as long as the fresh
water flooding continues. The forty-one west-flowing rivers empty their
waters into the backwaters which in turn open into the Arabian Sea.
On account of its peculiar physiography, Kerala enjoys two rainY
seasons a year, viz., the South-west and the North-east monsoons. Thus,
on the basis of rainfall, it can be inferred that Kerala encounters three
seasons a year - the South-west monsoon season extending from June
to September, the North-east monsoon season from October to January
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and the pre-monsoon season from February to May. The physico-chemical
parameters of the backwaters are affected by tile tidal influence of the
sea and the freshwater influx from the rivers. During the monsoon seasons
when the lake is flooded by the freshwater discharge the bar-mouth opens
and there is normally a decrease in the salinity of the lake water, provided
the freshwater influx is strong enough to overcome the tidal effect of
the sea. !Juring the pre-monsoon season evaporation due to exceSSive
so lar radia tion is grea t and consequent Iy the salinity is high. Hence, rainfall
with its attendant effects plays a major role in moulding the physico
chemical nature of the lake.
On the basis of surface salinity, George and Kartha (I 963)· catego
rized three periods a year - (a) the monsoon period from June to Sept
ember when the salinity IS low, (b) the post-monsoon season from Octo
ber to January when a general rise in salinity is observed and (c) the
pre-monsoon season from February to May when the surface salinity of
the backwater is comparable to that of adjacent inshore waters.
Certain backwaters of Kerala still remain largely unexplored from
the point of view of its hydrography and productivity while certain others
have been studied extensively. The hydrography of Cochin Backwaters
has received wide attention (George, 1958; George and Kartha, 1963;
Shetty, 1963; Rarnamritham and Jayaraman, 1963; Cheriyan, 1967; Qasim
~~, 1969; Qasi m and Gopinathan, 1969; Sankaranarayanan and Qasim,
1969; Menon et al., 1972; Haridas ~ .§l., 1973; Sreedharan and Salih, 1974;
Silas and Pillai, 1975; Lakshmanan ~~, 1982). The other backwaters
of Kerala that have been studied from the point of view of its physico
chemical characteristics are the Korapuzha estuary in North Kerala (Rao
and George, 19)<3; !<rishnamurthy and Vincent, 1975; Krishnamurthy et
al. 1975), Vembanad Lake (Ayyar, 1982), Akathumuri - Anchuthengu
Kadinamkularn backwater system (Nair ~~, 1983 b,c), Ashtamudi Lake
(George, 1973; Dharmaraj and Nair, 1981; Ayyar, 1982; Nair ~ al.,1983a;
1984), Paravur Lake (Azis, 1978) and Veli Lake (Krishnan, 1974; Sobhana,
1976; Ayyar, 1982).
Perusal of literature reveals that the importance of hydrography
has been driven home and many of the rivers, estuaries and backwaters
8° 44'~---------------------"'"Map of the Akathu muri lake show·lng
the station of field study
""
""
JI
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of India have been studied in detail. The hydrography of the Chilka Lake
was studied by Banerjee. and Roychoudhury (1966) and Jhingran and Nata
rajan (1966), that of Hoogly estuary by Dutta et al. (1954), Bose (1956)
and Gopalakrishnan (1968, 1969). The Godavari estuary has received the
attention of Ramasarma (1965), Ganapati (1969) and Chandramohan and
Rao (1972) while the Adyar River estuary was studied by Chacko (1954)
and Evangelin (1967); the Pulicat Lake by Kaliyamurthy (1973) and Rao
and Rao (1975) and the Vellar estuary by Chacko ~ al. (1954), Rangarajan
(1956), Dyer and Ramamurthy (1969) and Purushothaman and Venugopalan
(1972).
Know ledge of the hydrographic condition of lakes is essential to under
stand the anthropogenic stress on it, the measures that are to be taken
to curb over-exploitation of these natural resources by man and how an
increase in the productivity of the lakes can be brought about.
Venkatesan (1969) opined that the backwaters and coastal waters
of India ex tend a promising future for aquacultural practices. The back
waters of Kerala are especially fertile for aquaculture but small-scale
coir processing units on the banks of this system have polluted the water
to a large extent. Azis (1978) has dealt at length on the problems caused
by the retting of coconut husks in certain backwaters of Kerala. If these
lakes can be maintained well enough theirproductiv ity can be increased
considerably.
PHYSIOGRAPHY OF THE STU DY AREA
The area of the present study, the Akathumuri Lake, is situated in
the southern part of Kerala, along the South-west coast of India, 34 km
north of Trivandrum (latitude 8° 41' and 8° 44'N and longitude 76° 45'
and 76° 47' E). This lake is connected with the Anchuthengu Lake at
its South-western tip. The Anchuthengu Lake is connected to the Edava
Nadayara Backwaters on the north by the Varkala Canal and on the South
to the Kadinamkularn Lake which is a temporary estuary in this region.
The backwater remains open to the sea at Perumathura during the period
of the monsoons. The Varnanapuram River flows into the Anchuthengu
Lake before it joins the Kadinamkulam Backwater.
A view of the Akathumuri Lake
Part of the embankment of the lake built of laterite stones
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The Akathumuri Lake is situated away from both the Vamanapuram
River and the Perumathura bar-mouth and hence, there is less tidal influ
ence. This zone is comparatively broad with an average depth of 2.45
m.
The shallow areas of the lake are used for the retting of coconut
husks by small-scale coir processors. Therefore, though the lake is gene
rally free frolll industrial pollution, retting of coconut husks along the
banks contributes to organic pollution. The embankments of the lake are
built of laterite stones which is the habitat of Cirolana willeyi Stebbing,
the subject matter of this investigation and the boring sphaeromatids,
Sphaeroma terebrans Bate and Sphaeroma annandalei Stebbing.
Cirolana willeyi was sampled from the area where they were found
In abundance. In this area, the embankment had crumbled so that many
loose laterite blocks were found scattered submerged in the shallow water
of the lake, facilitating easy access for collection. The isopods were found
to congregate more on that face of the stones that were being continually
washed by water.
tvlATERIALS AN D METHODS
Water samples, both surface and bottom, were collected monthly
from the chosen site of the Akathumuri Lake, for a period of one year
extending from October 1982 to September 1983. The water samples were
collected during the first week of every month between 8.00 and 8.30
a.m. I.S.T. and the temperature of the samples recorded immediately.
The rainfall data was procured from the Meteorological Station, Triva
ndrum, this being the nearest to the collection site.
The transparency of the water was· measured usmg a Secchi-disc.
The hydrogen-ion concentration (pH) of the water was estimated
with the help of an 'Elico' pH meter.
For the estimation of dissolved oxygen, \vater samples were colle
cted in 250 ml bOD bottles, taking care not to trap any air bubbles. The
samples were fi:xed on the spot using Manganous sulphate and alkaline
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Potassium iodide and analysed in the laboratory by the modified Winkler
method (Martin, 1968).
For the estimation of salinity, samples were collected in polythene
bottles and titrimetrical1y estimated in the laboratory by the_ Mohr-Knudsen
method as described by Martin ([ 968).
For the estimation of nutrients, water samples were collected in
polythene bottles, filtered, preserved with Chloroform and stored in the
refrigerator til1 the time of analysis which was always within 48 hr of
collection. Inorganic nitrate, phosphate and silicate were estimated follow
ing the method given by Strickland and Parsons ([972) with the modifica
tions given by Grasshoff ([983) and Koroleff ([983 a,b). Nitrate was esti
mated by the method of Mul1in -and Riley ([955) as described by Barnes
([959).
OBSERVATIONS
The monthly fluctuations of hydrographic parameters such as water
temperature, water transparency, pH, rainfall, salinity, dissolved oxygen
and the nutrients such as phosphate, nitrate, nitrite and silicate are given
in Table I and illustrated in Plates XII and XIII.
Water temperature :
Monthly variations in the water temperature, both surface and bottom,
have been illustrated in Plate XII, Figure l. Vertical thermal stratification
was small throughout the period of study. The maximum surface water
temperature was recorded in November and April (32.6° C) and the mini
mum in January (28.2°C) while the maximum bottom water temperature
of 32.0°C was observed during March and April and the minimum of 28.8°C
in August. The atmospheric temperature was recorded to be slightly lower
than the temperature of the surface water during most of the year. How
ever, In December, January and March, the atmospheric temperature
was slightly higher than that of the surface water. The bottom water
temperature was almost equal to or slightly lower than that of the surface
water. During the months of December, January, May and August, the
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bottom water had a slightly higher temperature than the surface water.
The vertical thermal stratification was observed to be very slight,' during
the pre-monsoon season. During most of the pre-monsoon months and
the early part of the South-west monsoon, a comparatively higher surface
water temperature was recorded.
Water transparency :
Plate XII, Figure 2 depicts the monthly variations in water transp
arency. The water transparency was noted to be high during the North-east
monsoon season, the water visibility being maximum in January (147.0
cm). During February and March, the transparency was low (63.0 and
47.0 cm respectively), it increased slightly In April (58.0 cm) and from
then onwards an ascending trend was noticed so that the water was clearer
during the South-west monsoon season. The water was observed to be
less turbid during the North-east monsoon season with the values of 94.5
cm, 76.0 cm, 106.0 cm and 147.0 cm in October, November December
and January respectively. Thus surprisingly, the turbidity of the water
was low during the monsoon seasons and high during the pre-monsoon
season .
. Hydrogen ion concentration (pH) :
The Hydrogen-ion concentration 'of the water during most of the
months remained on the alkaline side (Pl. XII, Fig. 3). The pH of the sur
face water varied between 6.89 in September and 7.48 in January. The, '
pH of the bottom water ranged between 6.60 in May and 7.66 in January •
The bottom water usually had a lower pH than the surface water, but
during the months ot December, January, March and April, the bottom
water was observed to have a higher pH. The bottom water during October,
May, June, July and September and the surface water in March and Septem
ber had a slightly lower pH. No seasonal pattern of fluctuation in pH
was noticeable.
Rainfall :
Rainfall was one of the major hydrographical parameters which show
ed· distinct seasonal variations (PI.XII, Fig. 4). The North-east monsoon
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season, which usually extends from October to January, fell short by a
month, causing a short spell of dryness. January to March, was a compl
etely dry period with no measurable quantity 01 rainfall. The heaviest
shower of the South-west monsoon season was observed in August (237.9
mm).
Salinity :
Plate XII, Figure 4 shows the monthly fluctuations in salinity. A pro
nounced seasonal variation in salinity was observed during the period of
study. An increasing gradient of salinity was noticed from January to
July. From July to December, the salinity was comparatively low. The
maximum surface water and bottom water salinities were 17.73 x 10-3
in March and 17.79 x lO- 3 in January respectively. The minimum surface
water and bottom water salinities recorded were 5.55 x 10-3 in Septem';)er
and 5.94 x lO- 3 in October respectively. With the commencement of the
pre-monsoon in April, both the surface and the bottom waters were consi
derably diluted. The low level of salinity in North-east monsoon season
was followed by a hike during the pre-monsoon season. The salinity was
modera tely high during the early South-west monsoon, decreasing later
on. The lake being quite shallow, vertical gradient in salinity was very
slight.
Dissolved oxygen
No pronounced seasonal change" in dissolved oxygen was noticed (Pl.
XII, Fig.5). The dissolved oxygen was observed to be comparatively high
during the North-east monsoon season with the peak in the surface water
being 7.3l ml.l- l recorded in November. The minimum dissolved oxygen
in the surface water recorded was 3.00 mI. (1 in June. The dissolved
oxygen of the bottom water was also observed to be high during the North
east monsoon with the maximum value of 6.33 mI. l-l recorded in Nove--1mber· and the minimum value of 2.57 mI. l observed in September.
The dissolved oxygen content of the surface water showed a declining
trend during the period December to March, when the rainfall was scarce.
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With the advent of the pre-monsoon showers in April and May, the dissolved
oxygen content showed a slight increase but again decreased to reach
its minimum in June. In July, a c<?mparatively high content of dissolved
oxygen was observed which again declined slightly in August, in spite
of low temperature, moderate salinity and quite heavy rainfall. A moderate
level of dissolved oxygen was recorded in September. The surface water
was almost always conceivably more oxygenated than the bottom water.
However, March and June were the two months when the bottom water
was observed to have a higher level of dissolved oxygen.
Phosphate:
Monthly fluctuations In the phosphate concentration of the water
are illustrated in Plate XIII, Figure 1. Low values of phosphate were obser
ved during the period April to July, the lowest recorded was 1.34 J.l mol.C 1
in May and the highest was 4.25 J.l mol.C I in March. The bottom water
was usually observed to be richer in phosphate than the surface water
except.in November, April, June and August. Phosphate content of the
surface water exhibited an ascending trend from October to March and
from April onwards a descending trend was discernible but in August
and September, a step-up in concentration was noticed. Thus the phosphate
concentra tion was low during the early· South-west monsoon season and
comparatively high during the pre-monsoon.
Nitrate:
Plate XIII, Figure 2 illustrates the monthly variations in the nitrate
content of the surface and bottom waters. Comparatively low values
of nitrate was recorded during May to September and October. The maxi
mum nitrate concentration found in the surface water was 6.25 ]J mol.I- 1
in April and the minimum was 0.22 jJ mol. C 1 in June. Except during
January, April, June and August, the bottom had a richer concentration
of nitrate when compared to the surface water. The maximum concen
tration of nitrate in the bottom water was 4.21 r mol.l- 1 in December
and the minimum was 0.07 fJ mol. (1 in June. Moderate levels of nitrate
was present during November and December, however, by January, a slight
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deCline In the concentration was noticed. The declining trend continued
until an abrupt and sharp increase was noticed in April such that nitrate
content of the surface water reached its peak during this rT]onth.. From
May onwards, a declining trend was noticed and the minimum nitrate
was recorded in June in both the surface and bottom waters.
Nitrite:
The available data reveals that nitrite was at a considerably low
level throughout the period of study (PI.XIII, Fig.3) Seasonal variations
suggest that the nitrite in the surface water was low during the South-west
and the North-east monsoon seasons. It attained its peak value of 2.41
}J mol. (l in the surface water in February and 5.50 Jl mol. (1 in the
bottom water during the same month. With the progress of the South-west
monsoon, the nitrite concentration was seen to decline and reached its
minimum value in both the surface and bottom waters in June (0.04 JJmol.l- l ). Hence, the nitri te concentration was perceptibly high during
the pre-monsoon and low during the subsequent monsoons. The surface
water was almost always richer in nitrite except during November, January,
February, August and September, when the bottom water was found to
be richer in nitrite.
Silicate:
The monthly pattern of fluctuation of silicate is depicted in Plate
XIII, Figure 4. Low levels of silicate was recorded during the pre-mon
soon season, though the monsoon seasons, too, did not exhibit much escala
tion in silicate concentration. The cumulative effect of the North-east
monsoon was probably noticed In January, when the silicate attained its
peak of l2 0.69 ? mol. (l. A sharp decline in the concentration was noticed
during the subsequent period of February to May and in May, the minimum
concentration of silicate in the surface water (43.0 Jl mol.l-l) was recorded.
Except in January, April, August and September, the bottom water was
richer in silicate content tha.n the surface water. The maximum concentra
tion of silicate in the bottom water was 87.65 }J mol.l-l
in December
and the rnlnlmUrTl was recorded in May (43.34 }J rnol.l-l). This nutrient
exhibited an inverse relationship, tt)ough not a significant one, with salinity.
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Analysis of correlation coefficients (Table II) exhibited significan1
(P <-'0.01) inverse relationships between salinity and dissolved oxygen anc
nitrite and rainfall. No significant relationships were observed betweer
any of the other parameters. pH was observed to be negatively relatec
to temperature and rainfall. Salinity also showed a negative relationshi~
to rainfall. The dissolved oxygen was inversely related to temperature,
pH and rainfall. A negative, though not significant, relationship was noticec
between concentration of phosphate and water temperature, dissolved
oxygen, salinity, rainfall and pH. A negative relationship was observed
between nitrite and water temperature, dissolved oxygen, rainfall and
phosphate. Nitrate exhibited an inverse relationship with salinity, rainfall,
pH and phosphate. Silicate exhibited a negative relationship with water
temperature, salinity and rainfall.
DISCUSSION
The results obtained during the course of the investigation regarding
the hydrography of the Akathumuri Lake reveals that the monsoon rain
is the major factor regulating either directly or indirectly the water tempe
rature, the dissolved oxygen content, the salinity of the medium and the
nutrient contents of the water.
No great thermal stratification was observed in the lake which has
a depth not exceeding five metres. The temperature of the surface water
was either slightly above or equal to that of the bottom water during
most of the months. However, the higher temperature of. the bottom water
during late North-east and pre-monsoon and mid South-west monsoon
can probably be attributed to the turbulence caused by strong winds which
probably brought the colder bottom water to the surface and carried the
warmer surface water to the sub-surface level. Welch ([952) has put forth
the explanation that bottom water gets heated up by convection. Sahai
and Sinha (1969), Mllnawar (1970) and Azis ([978) observed an intimate
relationship between atmospheric and water temperature. The atmospheric
temperature was observed to be lower than the surface water during all
the months except December, January and March. This was probably beca
use the sarn?ling was always done during the early hours of the day when
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the atmosphere had just started warming up. Lower surface water tempera
ture when compared to the atmospheric temperature was recorded by
Munawar (1970) in the freshwater ponds of Hyderabad, India and Swarup
and Singh (1979) in the Suraha Lake, Uttar Pradesh, India. The higher
water temperature recorded during the pre-monsoon season can· be attri
buted to the excessive solar radiation and insolation of heat energy from
the clear sky. Low temperature during the monsoon season may be due
to the lower atmospheric temperature and the cooling effect of rainfall.
The influence of the South-west and the North-east monsoons on the water
temperature, in the various lakes of Kerala, during the different seasons
have been pointed out by George (1958), Nair (1965), Haridas ~ al. (1973)
and PiJJai (1974) in the Cochin Backwaters; Rao and George (1959) in
the Korapuzha estuary; Krishnan (1974) in the Veli Lake and Azis (1978)
in the Edava-Nadayara Lake.
UsuaJJy the turbidity of the water column is seen to be high during
the monsoon seasons on account of land drainage which happens to bring
in large quantities of suspended sediment, winds which stir up the bottom
sedirnent and growth of plankton. Similar observations were made by Kall
yamurthy (1973) in the Pulicat Lake and Qasirn (J 973) in the Cochin Back
waters. However, in the course of the present study, a totally different
pattern emerged with the water transparency increasing during the monsoon
months and decreasing during the pre-monsoon months. The reason for
this may be that during the pre-monsoon season when retting was in progr
ess, the poJJuted water from the retting pits lent turbidity to the open
lake water. The bar-mouth at Perumathura also remained closed during
these months and so the water remained stagnant and highly turbid, but
during the South-west and North-east monsoon seasons when the bar-mouth
opened, the highly turbid water was flushed out into the sea causing the
turbidity to drop considerably.
The hydrogen-ion concentration (pH) of the water varies in response
to the photosynthetic release of oxygen and absorption of carbon-dioxide,
respiratory processes of animals and plants and rainfaU. Gonzalves and
Joshi (1946), Rao (1955), Zafar (1964) and Swarup and Singh (1979) reported
an inverse relationship between pH and free carbon-dioxide content of
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the water. Azis (1978) dealt at length on the effect of retting on the
pH of the medium. It was observed that positive relationship exists between
pH and concentration of hydrogen sulphide and that under oxygen depleted
conditions higher pH values were recorded. A higher pH value, during
the monsoon season when the oxygen value was high, was also observed.
During the present study, low pH values were observed when the dissolved
oxygen concentration was comparatively low. It is now well known that
poorly aerated water receiving much decaying organic matter are chara
cterised by low oxygen concentration and relatively low pH. This probably
accounts for the slightly acidic water noticed at the bottom during the
monsoon. The low pH observed in the surface water during March and
September may be accounted for by the rise In temperature during these
months leading to rapid decomposition of organic matter liberating surplus
carbon-dioxide into the medium.
Salinity fluctuations depend mainly on precIpItation, land drainage
and tidal effect in an estuary (Balakrishnan, 1957; Rao and George 1959;
Qasim and Gopinathan, 1969). The low values observed during the North
east monsoon may be due to dilution of the lake water by precipitation.
During the pre-monsoon season, the salinity was observed to increase
considerably OWIng to the evaporation of water resulting from excessive
solar radiation. A similar pattern of seasonal variation was observed in
many of the lakes of Kerala by Rao and George (L 959) in the Korapuzha
estuary, Ramamritham and Jayaraman (1963) in the Cochin Backwaters,
Krishnan (1974) in the Veli Lake, Azis (1978) in the Paravur Lake and
Ayyar (1982) in the Veli, Vembanad and Ashtamudi Lakes. During the
present study the effect of heavy rains during the South-west monsoon
was seen to be felt from August onwards when there was a sudden sharp
decline in sdlinity. Vertical stratification in sdlinity was very less indicating
a homogeneous composition of water.
Dissolved oxygen does not seem to become a limiting factor for isopods
in the Akathumuri Lake since the average value of dissolved oxygen for
the whole year was 4.76 mi. (I and this concentration is well within
the survival limit of the isopods. Dissolved oxygen content was generally
high during the monsoon season especially the North-east monsoon in
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the Akathumuri Lake, in the course of the present investigation. Such
a relationship between the dissolved oxygen content and rainfall has been
generally noticed during hydrographic studies. Sankaranarayanan and Jaya
raman ([972) while working on the hydrography of the Mandovi-Zuari
estuaries at Goa; Rajan (1972) and George ([973) in the Ashtamudi Lake;
Haridas ~~. (1973) in the Backwaters of Cochin; Sobhana ([976) in the
Veli Lake and Azis ([978) in the Paravur Lake, all these situated in Kerala,
India, have reported such a positive relationship. The solubility of the
atmospheric oxygen on account of the agitation of the water masses during
the monsoon may be the reason for high dissolved oxygen concentration
during the months of monsoon. Sreenivasan ([966), Sahai and Sinha ([969)
and Swarup and Singh ([979) have ascribed the high oxygen content during
the monsoon season not only to greater mixing of atmospheric oxygen,
but also to the prolific growth of photosynthetic algae during these months.
However, in the course of the present investigation, it was observed that
during the beginning of monsoon, in spite of moderate rainfall, the dissolved
oxygen was comparatively low. This was probably because during very
heavy rainfall and strong winds, the water from the retting pits flowed
to mix with the main body of water. The hydrogen sulphide present in
the water has been reported to result in the depletion of dissolved oxygen
(Azis, 1978). This can be attributed as the reason for the lowering of
oxygen concentration noticed in August In the Akathumuri Lake. The
utilisation of oxygen for decomposition of organic matter lowers the level
of oxygen in the bottom water. Reactive phosphate content above approxi
mately 3.0 flg at-II IS a sign of eutrophication (Ketchum, 1967), which
ultimately leads to low dissolved oxygen content. This might probably
be one of the reasons for the minimum oxygen concentration in the bottom
water during late monsoon period, because during this period the concen
tration of dissolved phosphate recorded was 4-.95 fl mol. Cl .
It was observed during the present investigation that the phosphate
concentration decreased slightly during the South-west monsoon season
and this is in contradiction to the generalisation that phosphate concen
tration is high during the monsoon seasons. This may be because during
the pre-monsoon or summer months when retting was in progress and
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the bar-mouth remained closed, the concentration of nutrients was high
in the stagnant water, but as the monsoon advanced, the bar-mouth opened
and the lake water was flushed out into the sea carrying the nutrients
along with it. Since, hardly any tidal effect was noticed, the nutrients
could recuperate only through decomposition of organic matter or fresh
river discharge. Moreover, heavy rainfall and river discharge cause the
amount of suspended sediments in estuarine waters to increase. According
to Jitts (1959), these sediments trap 80-90% of the phosphate-phosphorus.
Based on his observations in English freshwater lakes, Mortimer (1941,
1942) concluded that the bottom muds of these lakes have a remarkbale
impact on the phosphorus cycle of the region. Llss (1976) was of the opinion
that estuarine sediments are capable of both removing phosphate from
phosphate-rich water and adding it to water of low phosphate content.
Smith and Longmore (l980) opined that soil disturbance and disposal of
sewage and other wastes tended to increas~ considerably the phosphate
content of water.
The maximum phosphate concentration observed during pre-monsoon
in the present study may be due to the release of soluble inorganic phos
phate from the bottom muds under the influence of turbulence and is
probably not due to any active transport of phosphate leached from the
soil. The phosphate values were seen to increase from the surface to
the bottom during most of the months. If the phosphate concentration
was dependent on land drainage and freshwater run off then the surface
water should naturally contain a higher phosphate content. Sankaranara
yanan and Qasim (1969) have deduced that the higher concentration of
inorganic phosphorus at the bottom than at the surface may be due to
the decomposition of organic phosphorus at deeper layers, where water
becomes stagnant and anaerobic condi tons prevail, into inorganic phosphorus
which generally moves up to the surface. If that is the case, then fresh
water discharge can be related to phosphate content since influx of fresh
wa ter laden with sil t containing large quanti ties of dissolved organic matter
may be decomposed into inorganic phosphorus at the bottom. During late
North-east and pre-monsoon seasons and early South-west monsoon season,
the surface water was observed to be richer in phosphate than the bottom
and this was probably due to freshwater run off following heavy rains.
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Reid and Wood (976) stated that the concentration of total exchange
able phosphorus in natural waters is dependent primarily on the basin
morphometry, chemical composition of the surrounding terrain, organic
metabolism within the water and the rate at which phosphorus is lost
to the sediments. The main inputs of lakes are from inflowing rivers and
precipitation, though rainfall is a much less important source of phosphorus
than in the case of nitrogen. Seasonal fluctuations in reactive phosphate
content has been observed generally with phosphate reaching its peak
during the monsoon months. Sreedharan and Salih (974) found that the
concentration of nutrients was high in the Cochin Backwaters during the
monsoon season. Ayyar (982) observed high values of phosphate during
the South-west monsoon in the Vembanad Lake, Kerala. But, the role
of river water as a rich source of nutrients is a controversial issue. Rao
and George (1959) are of the view that the Korapuzha River, North Kerala,
does not transport any appreciable quantity of phosphate into the Kora
puzha estuary. Rochford (1951) and Ramamurthy (963) stated that the
phosphate contribution by rivers is negligible. At the same time, it was
suggested by the former that a geochemical study of the nature of the
drainage area would provide a solution to these contradictory statements.
During the present investigation, the nitrate content was seen to
exhibit an increasing trend during North-east and pre-monsoon periods,
a ttainir:g its peak concentration in April. During the heav y rains of the
South-west monsoon, the nitrate content declined. Even if the unprece
dented increase in the nitrate content during April could be accounted
for by freshwater discharge and land run off caused by the advent of
monsoon (Ewins and Spencer, 1967), this reason will not hold good in the
case of the increase noted during late North-east monsoon and the early
pre-monsoon seasons because this period was almost dry. Thus the pattern
of seasonal cycle of nitrate and its vertical distribution with higher values
at the bottom is suggestive of the fact that freshwater discharge is not
the main factor controlling the concentration of this nutrient in this area.
Some other factors like ni trifica tion and rapid. biological uptake also play
an impressive role in regulating the nitrate concentration. A. factor that
might probably affect the nitrate concentration is same as that explained
-:ll8:-
for phosphate depletion during the monsoon season. During the summer
months when the bar-mouth was closed and retting was in progress, the
nitrate content tended to increase in the stagnant water. However, during
the monsoon season when the bar-mouth opened, outflow of the lake water
into the sea removed the amassed nitrate. This lowered the nitrate content
of the water during the monsoon season. Again, in December with the
closing of the bar-mouth and no rains, the nitrate started accumulating
in the lake water. The higher concentration of nitrate in the bottom water
during most of the months points to the possibility of nitrate regeneration
from sediments. Dharmaraj ~ al. (1977) have reported that nitrate and
nitri te may be produced by enzymatic reactions of free enzymes present
in the sediments.
According to Vaccaro ([965), the biological factors affecting the
distribution of molecular and combined nitrogen are nitrogen fixation,
denitrification, nitrogen assimilation by marine phytoplankton, the decompo
sition of organic nitrogen and the oxidation of inorganic nitrogen. High
concentra tions of ni tra te during the monsoon season has been reported
by Rao and George (1959) in the Korapuzha estuary, Rajan (l972) in the
Ashtamudi Lake, Qasim (1973) and Sreedharan and Salih (l974) in the
Cochin Backwaters and Azis (1978) in the Paravur Lake which are all
situated in Kerala, India. The monsoon rains and the resultant land drainage
seem to control the nitrate concentration during the monsoon months.
Menzel· and Spaeth (1962) reported that the ammonia content increases
during rainfall and the oxidation of this to nitrite and nitrate may probably
be the causative factor for the increase of these nutrients during rainfall.
Nitrite occurs in negligible quantities in unpolluted waters. Nash
(1947), Jayaralllan (l~5l, 1951+) and l~J.rnarnurthy (1953) observed that
polluted river water is rich in silicates and nitrites. Braarud and F¢yn
(195l) observed a direct relationship between the concentration of nitrite
in coastal waters and the polluted water flowing in. Large amounts of
nitrite is an indication of pollution by sewage (Reid and Wood, (976).
Since nitrite is an intermediary product formed during both the processes
of nitrification and denitrification, a definite seasonal cycle is not distinct.
Nevertheless, Sankaranarayanan and Qasim (1969) have observed high
-:119:-
concentrations of nitrite during a period when the system remains fresh
water dominated.
During the present study, a decrease in the nitrite concentration
was observed during the monsoon season with the maximum value in Febru
ary. As propounded for phosphate and nitrate, the flushing out of the
nutrient-rich polluted water from the retting area into the sea during
the monsoon months may result in the low concentration of this nutrient
during this season. The higher concentration of nitrite in the surface water
during the late North-east monsoon, late pre-monsoon and mid South-west
monsoon was probably because freshwater drainage tended to increase
the concentration of nitrite at a faster rate than oxidation of nitrite
to nitrate could take place. Orr (926), Zobell (935), Rakestraw (936),
Hutchinson (957), Kessler (1957, 1959) and Vaccaro and Ryther (960)
stated that the excretion of extracellular nitrite by phytoplankton influe
nces the distribution of nitrite within surface layers of natural waters.
The progressive decrease of the nitrite from bottom to surface suggests
the possible conversion of nitrite into nitrate (Sankaranarayanan and Qasim,
1969). Vaccaro (965) is of the view that oxidation of organic nitrogen
within the sediments may cause significant nitrite concentrations to appear
periodically in the bottom water. The oxidation of nitrite to nitrate and
its biological uptake may be one of the factors that cause a depression
In the nitrite level of the surface water.
It is a well known fact that freshwater IS richer in silicate than sea
water and freshwater discharge during. the monsoon season has been gene
rally accepted as a source of silicate, causing an increase in this nutrient
during the monsoon season. Liss (976) has mentioned that mixing of fresh
water with sea water may lead to a reduction of 40% in the dissolved
silicon of the Jor mer. Sankaranarayanan and Qasim (l969) in the Cochin
Backwaters and Farrell et .§l. (979) in a tropical lake in the North-western
Queensland reported that the silicate concentration was high during the
monsoon months. The impact of freshwater discharge on the silicate con
centra tion 01' lakes is shown by Sankaranarayanan and Qasim (969) and
Dharmaraj and Nair Onl) who reported a progressive decline in the con
centration of silicate from the surface to the bottom water.
During the present study, silicate concentration was low during the
-:120:-
pre-monsoon season and moderate during the monsoon seasons. Purusho
thaman and Venugopalan (1972) reported that much of the dissolved sili
cates in water is removed by inorganic precipitation and biological uptake.
However, silicate was seen to be higher in the bottom water than in the
surface water during most of the months. This goes to preclude freshwater
discharge as the main source of silicate. Atkins (1926) considered tempera
ture as one of the factors affecting the silicate cycle, an increase in
temperature favouring the solution of silicates. Growth of plankton has
been generally accepted to play a major role in regulating the silicate
concentration (Atkins, 1926; 1929-30; Armstrong, 1965; Ewins and Spencer,
1967; Dharmaraj ~ al., 1980). At the region where sampling was conducted,
the bottom was observed to be clayey and it is possible that dissolution
of clay, as suggested by Dharmaraj et ale (1980), caused considerable
concentration of silicate in the bottom water.
An inverse relationship between salinity and silicate content of the
wa ter was seen to exist. Such a relationship has been reported in the
Cochin Backwaters by Sankaranarayanan and Qasim (1969) and by Ayyar
(1982) in the Ashtamudi and Vembanad Lakes of Kerala. They claimed
that freshwater discharge into the lakes caused a fall in salinity and an
increase in the silicate concentration, thus causing an inverse relation
ship between these two parameters'.
This study has thus explicitly revealed that pollution dye to retting
of coconut husks influences to a great extent the hydrographic parameters
of this lake.
TABLE I
Hydrographic parameters of the Akathumuri Lake during October 1982 to September 1983
Water Temperature (oC) Water Rainfall Salinity Dissolved Phosphate Nitrate Nitrite Silicate'Ionths Sample Atmospheric Water Tra?spa)ency pH (mm) (x 10- 3) oXYl?yn -I -I -I -I
cm. (m 1.1 )0mol •1 ) (;1mol.1 ) (f-lm9J.l ) (pmol.l )
.
Surface 31.8 31.8 94.5 7.17 164.8 5.58 5.63 2.47 0.89 0.30 44.38
'ctober Bottom 31.8 6.87 5.94 4.69 2.47 1.01 0.24 50.00
<0\ e;;lber Surface 31.4 32.6 76.0 7.15 . 131.0 6.61 7.31 2.36 2.16 0.29 73.53
Bottom 30.8 7.10 6.61 6.33 2.13 3.62 0.95 80.88
December Surface 31.8 29.6 106.0 7.10 23.3 6.39 6.14 2.60 3.72 0.71 80.25Bottom 30.0 7.40 6.86 3.48 3.16 4.21 0.50 87.65
Surface 29.2 28.2 147.0 7.48 10.54 5.31 .'.anuary 0 2.35 2.43 1.40 120.69 -Bottom 31.0 7.66 17.79 3.25 3.1 3 2.23 1.45 86.21
N-..I
Februan Surface 29.4 30.6 63.0 7.30 0 14.28 4.12 2.45 2.05 2.41 50.85Bottom 30.4 7.30 14.40 3.53 2.63 2.59 5.50 51.71
.' aTch Surface 33.6 32.0 47.0 6.95 Trace 17.73 3.26 4.25 1.86 1.01 41.35Bottom 32.0 7.25 17.73 3.53 4.80 2.27 0.79 5'g-.34 .
i\pril Surface 32.5 32.6 58.0 7.10 119.9 11.32 4.27 1.45 6.25 1.64 49.69Bottom 32.0
~
7.16 11.32 3.26 1.40 3.63 1.34 45.00
'vlay Surface 30.0 31.3 84.0 7.20 106.1 16.48 4.36 1.34 1.83 0.97 43.00Bottom 31.8 6.60 16.12 3.79 2.22 1.83 0.97 43.34
June Surface 30.0 32.0 89.0 7.31 229.8 15.95 3.00 1.41 0.22 0.04 50.00Bottom 31.5 6.94 16.21 3.64 l.23 0.07 0.04 60.00
July Surface 28.4 30.4 76.0 7.20 100.3 17.43 4.69 1.94 1.37 0.45 50.00Bottom 29.8 6.74 17.43 4.02 2.12 1.83 0.41 59.00
August Surface 28.6 28.6 93.0 7.22 237.9 11.45 3.99 3.27 1.37 0.22 51.67Bottom 28.8 7.15 11.34 3.76 3.07 0.82 0.33 40.00
September Surface 30.4 30.6 74.0 6.89 226.3 5.55 5.03 . 3.72 0.55 0.09 80.67Bottom 30.6 6.69 6.09 2.57 4.95 0.70 0.34 75.00
TABLE II
Correlation coefficients of the hydrographic factors of the Akathumuri Lake
Temperature pH Rainfall Salinity Oxygen Phosphate Nitrate Nitrite Silicate
Temperature - -0.4123 0.1558 0.0935 -0.0416 -0.2281 0.1201 -0.0589 -0.5604
pH - - -0.1837 0.2152 -0.0334 -0.5531 -0.0539 0.2900 0.2751
Rainfall - - - -0.2342 -0.1015 -0.0806 -0.4073 -0.7319** -0.2789
Salini ty -0.7564** -0.2051 -0.L051 0.2805 -0.4594.'.
- - - - ~
NN
Oxygen -0.0203 0.1617 -0.1789 0.4848..
- - - - - I.Phosphate - - - - - - -0.2683 -0.1506 0.. 1578
Nitrite - - - - - - 0.5556 - 0.0433
Nitrate - - - - - - - - 0.0955
** p.(O.01