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