1 '7'9

CHAPTER-VII

PLANKTON

RESULTS

A. PHYTOPLANKTON

The seasonl variation in the numerical density of

phytoplankton are presented in the Tables. 65 to 69 figs. 41

to 45. Among the five stations in the Neyyar River

phytoplankton density showed maximum at st. I I I (36.95%) and

the minimum at st. IV (7.62%). The relative abundance

recorded at st.!. I I and V were 26.58%, 17.46% and 11.39%

respectively. The dominance of chlorophyceae was observed

in all the stations. (72.19%).

The ce I I number at st. I varied from 1866 to

150862.m- 3 during May and December respectivelY (Table. 65).

The bulk of the population was contributed by Chlorophyceae

(79.75%) and the rest consisted of Cyanophyceae (9.47%),

Bacillariophyceae (10.69%). The dinophycean members were

only spa.rsely. represented (0.09%). (Table. 65). The pea.k in

the total density during December was mainly due to the

bloom of Trioloceros sp.which constituted 98.27% of the

total phytoplankters during the period. The density of

phytoplankton was highest during post-monsoon period and

lowest during monsoon period. Cyanophyceae showed the

maximum during February (7860 m- 3 ) and the minimum during

Augus t (160m- 3 ) • The peak density during February was due

to the abundance of the species, Phormidium amblguum which

was dominated during nine months of the whole one year

Maximal density of Bacillariophyceaeperiod (Ta.ble.

wa.s observed

65) .

maximum during February ( 16200. m- 3 ) and a.

depletion was noted during November and December. The major

portion of the density during February was constituted by

Synedra. sp. Na. vic u I a. s p . wa.s the dominant s pec'i es

represented in eight months. Green algae (Chlorophyceae)

showed three maxima peaks, one in February {10800.m- 3 ,one in

November (10883. m- 3) a.nd another one in December (150595. m- 3 ).

Pedia.strum SP •• 2vgnema sp. and Staurastrum sp. were

constitued the bulk of the population during February, whdle

the other two peaks observed were mainly due to the bloom of

(Ta.ble.65). represented byTripoloceros sp.

Peridinium sp. wa.s present

D i nophycea.e

only during March. Both

cyanophyceans and diatoms showed the seasonal

pre-monsoon per iod a.nd lower density during

pea.ks dur i ng

post-monsoon

period. However, Chlorophyceae was dominated during post-

monsoon period and the dens i ty wa.s low during monsoon

pe r i od. The seasonal variation of representative species of

phytoplankton are given in the Table. 65.

Tho:! cell count a.t st. [ [ ra.nged from 366 to

62500.m- 3 during May and November respectivelY· (Table. 66

fi~. 42). The peak density during November was due to the

bloom of Triploceros.sp. The phytoplankton population showed

seasonal maximum during post-monsoon period (91933.m- 3) and

minimwn dlJ't'ing the pr'e-monsoon per iod (14633. m-:1 ). Among the

four taxa

79 . .5% of the tota.l.

and which constituted

The diatom population comprised of

12.61%, while cyanophyceae and Dinophyceae represanted 7.69%

a.od 0.2% respectively. The dinophycean members were only

spa.rsely represented with Peridinium sp. l.Ta.ble. 66). Blue

green algae was noted in high numbers during September and

was absent during October, November a.nd December.

Phormidium ambiguum was the dominant Cyanophycean and was

represented in -.:;ix months of the period. Cyanophycea.e wa.s

showed the maximum was during monsoon period and minimum

during the post-monsoon period. Bacillriophyceae showed

pea.k the occurrence during December and was completely

absent during May, July and November. Seasonal mean showed

the maximum during the post-monsoon and minimum during pre-

and Nitzschia sp.monsoon period.

domina.nt forms

Na.v icu I a. sp.

encountered which were

were the

represented half of

the period of the sampling. The density of cholrophyceae

ra.nged from 33 to 62500.m-;l; (May and November). The bloom of

Triploceros sp. a.nd Sta.urastrum sp. during November

coincided with the peak dansity. Spirogyra sp. and Zygnema

sp. were the other dominant forms and which were high in

September (Table. 661. Seasonal mean of Chlorophyceae showed

the maximum during post-monsoon (Table.66) period(77883.m-;l;

and minimum during the pre-monsoon period (7067.m- 3).

At st.ll! the tota.l phytoplankton eel Is ranged

from 4000 to 114334.m- 3 (June and Octoberl. (Ta.ble. 67

a.nd Diatoma sp.

The peak density during Octoberfig. 43) .

bloom of z."gnema. sp, Na.vicula. sp.

was due to the

The

population density was high during post-monsoon period and

the minima observed during the pre-monsoon period. Among the

4 major taxa ChlorophYceae maintained a dominance at this

sampling site also (77.58%1. Dinophyceae was represented by

Peridinium sp. The other taxa such as Cyanophyceae and

Bacillariophyceae constituted 6.69% and 15.23% repectively

of the total population. Cyanophyceae showed the maximum

February and the min i ma. during October a.nd

December. (Tab 1e. 671. The peak density during February was

mainly constituted by Oscillatoria sp. and Phormidium

ambi~uum. P. a.mb i guun was the dominant species which

represened almost all the months. The seasonal mean of

Cyanophyceae showed the maximum during pre-monsoon a.nd

minimum was during post-monsoon period. The list of species

in the Table. 67.seasonal variation are given-3.nd the i r

The pea.k occurrence of Bacillariophyceae was noted during

Oc tobe r a.nd the minimum was observed during December. The

bloom of Na.v icula. sp. and Diatoma sp. coincided with the

peak during October. Seasonal mean showed the maximum during

post-monsoon and minimum during pre-monsoon

period (9666.m-:s I. Na.vicula sp. and Nitzschia sp. were the

most domina.nt dia.toms ·represented ten a.nd e i g h t months

respectively. The density of Chlorophyceae ranged from 666

to 84667.m-:S (June and October). The increase in density

':lring 0 . ber was mainly due to the bloom of 2vgnema sp.

{Table. 67) .. ~~~~~o~r~a= sp., Spirogvra sp. and 2ygnema sp.

were . edam i nant b ue-green algae . Microspara sp.and

:oino~vra.Sp. showed the ma.ximum during Apr i 1 • while

owed .hree peaks one in August (38800.m- 3 t, one

in cep ember {34837. m-:: and one in Oc ober (83333.m-:: t

he dens i ty of phy top 1a.nk ton at s t. I V ra"nged from

2120 to 15033.m-:: (June a.nd September). The peak density

during Sep ember was due to the bloom of Triploceros.sp and

2v~ne a. sp. Cyanophyceae was represented three species of

wh"c Phormid"um ambiguum was abundant during March. The

de Ie ion of Cyanophyceae was noted during September and

Dc ober. { a.ble. 68l. The seasonal mean showed the maximum

duri g pre-monsoon and the minimum during post-monsoon

period. he re resen.atian of Bacillariophyceae also limited

.0 a few species and which were absent during October,

ovem er and December. Diatoms showed two peaks of abundance

one n July (236 • m- 3 ) and the other one during

'la.rc { 667. -3 t. itzschia sp. and Nav"cuia sp. were the

dom"nan diatom species obsevered. (Table. 68)

At st.V t e cell count ranged from 933 to 59999.m- z

(Sep.ember and May). Maximum densi y was observed during the

pre-monsoon (82368.m- Z ) and .he minimum during post-monsoon

e r - od ( 13 1 4- • - 3 ) • The bloom of avicula lanceolatae.

Soirogvra.• sp. 2ygne a. SP· , M icrospora. sp,

d C . du . g May resul edsp. an osmarlum.sp. r n in the

peak density. Cyanophyceae showed manimum occurrence during

May and the months of August a.nd in October a. total

depletion of the ta.xa wCl.S noted (Table. 69). The seasona.l

manimum was observed during the pre-monsoon and the minimum

during post-monsoon period. sp. was the most

dominant cyanophycean recorded during the period. Dia.toms

exhibited maximum occurrence during May and were absent from

August to December. The peak density during May was due to

the abundance of ~avicula sp. Seasonal mean showed maximum

during the pre-monsoon and the minimum during post-monsoon

The cryptophycean and dinophycean members were onlyperiod.

sparsely represented. Cryptophycea.e was represented by

Croomona.s s p. which was present March a.nd January.

Dinophyceae was represented by Ceratium sp.was noted during

Februa.r y only. A decreasing trend in the density of

Chlorophyceae was noted when compared to the other stations;

however, the taxa was dominant(56.38%). The density of green

Spirogvra. sp. was most dominant chlorophycean

algae ranged from 0

the

to 44167.m-:S (.J a.nuar Y a.nd. May) .

which

represented nine months. Seasona I mea.n showed maximum

The list of representative species of phytoplankton

during

period.

the pre-monsoon a.nd minimum during the monsoon

at st.V are given in the Table. 69.

RESULT O~ STATISTICAL ANALYSIS

The correlation coefficient values of phyto-

plankton with ten hydrographical parameters are given in

Ta.b l e. 77. phytoplankton density showed a. positive

relationship with mean oxygen content,rainfal I, BOD and TDS

in almost a I I the sampling stations. A I I the other

pa.ramsters such as water temperature, a.1 ka lin i ty, to ta.l

ha.rdness, phosphate, nitrate and silcate contents showed

inverse pattern of influence on phytoplankton.

of the medium had a negative correlation in the

Temperature

last four

sta.tions. Maximum correlation was noted at st. IV (-0.351).

Mea.n oxygen content showed a positive correlation

(signif ica.nt at 5% I eve I ) with phytoplankton at st. IV.

There existed a negative correlation between alkalinity and

phytoplankton density in the first four stations which were

signif ica.nt at 5% level at st.ll and IV. The level of

correlation between akalinity and phytoplankton was highly

significant at st.V (0.931). A simila.r trend like alkalinity

has been observed with hardness. Like alkalinity, hardness

showed a positive correlation

sta.tions exhibited inverse

>1% level at st.V.

relationship with

Last four

phosphate.

Phytoplankton density showed the lowest level of positive

correlation with phosphate at st. I (0.068). Except a.t st. IV

nitrate content showed negative correlation. At st. I V

phytoplankton showed strong correlation with nitrate. The

206

relationship of silicate content with phytoplankton density

vary with the stations. The negative correlation showed

significant a.t 10% level at sLV. Both st.I and IV showed

positive correlation with silicate. The influence of

on phytoplankton density noted was positive atra.infa.ll

st. I I to IV and was negative at st. I and V. Positive

correlation at 2% level was noted at st.V with BOD while the

values were showed inverse trend at st. I and I I 1. TDS

showed a positve correlation with phytoplankton density in

the last four stations.

are given in(ED)

The dissimilarity

between five stations

index va.lues of phytoplankton

Table. 78. The

Euclidean Distance, (ED.) between sta.tion I I I and V showed

maximum (0.60) and minimum was observed between st. IV and V

(0.23). Among the five stations the seasonal mean value

between st. I I I a.nd V recorded the maximum during pre-monsoon

a.nct the lowest value was noted between st. IV and V during

post-monsoon period. The dissimilarity value between st. I

and II ran~ed from 0.004 to 1.00 (May and July) and the mean

ED showed the maximum (0.69) during post-monsoon and the

minimum during pre-monsoon period. The Euclidean Distance

between st. I and I I I varied from 0.02 to 1.0 (November and

Apr i I ) a.nd the sea.sonal mean exhiited a maximum during

monsoon(0.63) and the minimum during the pre-monsoon period

(0.50). The highest dissimilarity between st. I a.nd IV has

been observed during December ( 1. 0) and the lowest during

Apr i I (0.005). The index value ranged from 0.02 to 1. 00

between st. [ and V and the higest mean was noted during the

pre-monsoon and lowest during monsoon season. The Euclidean

Dis tan,::e be tween

and January(1.0)

st. II a.nd 1[1 showed maximum during March

and the minimum was noted during December

(o.OOS). The values of dissimilarity index ranged from 0.009

density between

to 0.97 (April and November)

st. I I and IV.

in the case of phytoplankton

Mean density between st. I I

and [V exhibited ma~imum during post-monsoon and minimum

during pre-monSoon. The dissimi larity between st. I I and V

recorded maximum during November (1.0) and the minimum

during February (0.004) • The Euclidean Distance values

between st. I I I and IV varied form 0.02 to 0.99 (May,December

a.nd Apr i 1 ) • The dissimilarity index values between st. III

and V showed three peak values in the consecutive months of

August,September and October and the minimum was noted

during December (0.02).

st. IV and V was wide.

The variation of ED values between

The minimum distance was observed

during December and the maximum during June.

The indices of Shannon-Weaner(H), evenness(J) and

Margalef(d) for five stations are given in Table. 79 a,b,c

a.nd d. The range of diversity index and the coefficient of

va.r iat ions a.re given in Table. 79 a. and d. The Shannon-

Weaner diversity index exhibited high variation in almot all

the stations except at st.IV. The peak of var ia.t ion wa.s

noted at st.V (CV-45.98%) and the next higher value of

variation W3.S

2()E;

at st. 1 (CV-44 . 37%) . At st. 1 the H va.lue

ranged between 0.10 and 2.32 (December and Feburary). The

diversity index ranged from 0.35 to 2.07 (November and

Apri I) at st. 11 and from 0.81 to 3.00 at st. I I I (September

and February). The range of variation at st. IV noted lowest

<a.64 to 2.03). The indel< value varied form 0.56 to 3.02 at

st.V {August and MarchI.

decreasing trend from pre

The Shannon-Weaner indel< showed a

to post-monsoon period in all the

stations and the mal<imUm diversity has been observed during

the pre-monsoon period. {Ta.ble.7.9al.

The ranges of eVenness indel< and the coefficient of

variations are given in Table. 79 b&d. The Pielou's evenness

indel< showed high variation in the first three stations and

the maximum variation was noted at st. {34.69%1 and the

nel<t higher value of variation wa.s at st. III {c.v.30.98%1.

The coefficient of variation of the indel< showed the lowest

3. t bo t h st. I V a.nd V {T a.b Ie. 79.dl. Enenness index values

el<hibited the manimum during pre-monsoon ina I 1 the

stations. Seasonal mean showed the lowest during post­

monsoon at St. I, I I I and V whi Ie at St. I I a.nd IV the minimum

was noted during monsoon period (Table. 79b). The higest

evenness was noted during June and lowest during December

3.t st.!. At st. I I both maximum and minimum values were

recorded during the consecutive months of October and

November {0.99 and 0.311. The evenness index r~nged between

O. 3 9a n d 1. 2 6 (September and J 1.1 I Y I at 9 t. I I 1 . At 9 t. I V three

peak values wefe noted. one d uri n g Feb r ua I" y ,one d uri n g

Apr i I and the other one during June (0.97) and the minimum

was observed during November (0.74) . The evenness index

varied from 0.70 to 0.96 at st. V (F e b I" ua. I" y , December and

November) and the range of va.riation noted lowest of the

total.

The diversity index of Margalef exhibited high

variation in al'most all the stations except at st. IV. The

peak coefficient of variation wa.s noted at st.V (e.V.85.

74%) a.nd the lowest at st. IV (G.V. 39.18%) (Ta.ble. 79d).

Like Shannon Wiener index the Margalef index ~xhibited the

maximum during the pre-monsoon and the minimun during post-

The highest coefficient ofmonsoon in all the stations.

variation was also noted at st. a.nd III. The index value

ranged from 0.52 to 3.01 (Ja.nuary and April) at st. 1. At st.

I I the highest value was noted during April (2.77) a.nd

lowest during May (0.35).

0.98 to 5.30

The Margalef index

(December a.nd Februa.ry) at st. I I I.

va.r ied from

The index

value showed lowest va.riation a.t st. IV and the va.lues ranged

from 0.25 to 2.36 (October and August). The Margalef index

exhibited wide fluctuations at st.V and the values noted

highest (7.46> during March and lowest during August (0.29).

(Table. 79c).

:21.U

and II intermediate values of mean density were

by

by

the

Kallada

formed

a.nd in

is shared

is

Lakshminarayana,

rest

the second minimum

in the Neyya.r

the minimum at st. IV

(36.95%)

Dinophyceae and Crypto-

not fol low a pattern of

In the present investigation

it was

only the

phytoplankton

v

pattern with

DlSCUSSlON

in contrast tothe condition noticed

(Part-I of the thesis) and the other

the other lotic systems viz.,

the total

(72.19 %) and

is' much

phytoplankton density

Bacillariophyceae,

the maximum at st. [II

of

Unlike

The

a.nd

rivers (Chakraba.rty et a.l, 1959;

the Neyyar River did

bulk

[nd ia.n

(7.62% )

the

heterotrophy to autotrophy.

recorded and at station

River,

from the River Kallada

exhibited an irregular

Ch I orophycea.e

Cyanophycea.e,

phyceae which

statons

(11.39%).

1965 ; Pahwa and Mehrotra, 1966; NautiYal Prakash, 1985)

however, the pea.k of .chlorophycea.n members has been noted in

North Carol ina. strea.ms (Whiteford a.nd Schuma.cher, 1963) and

in Saudi Arabian streams (Whitton, 1986). The construction

of dam across the stream had a profound influence on the

ecology of

community in

the river

pa r tic u I a. r.

in general and the phytoplankton

Ward and stanford (1983) visualised

the effect of relatively high dams as producing a shift in

ecosystem structure and function. Accqrding to Payne (1986)

the impoundment of a river generally encou'rages phyto-

plankton gr-o\"th par-tlY due to mor-e stable conditions with

~nhanced nutr-ient concentrations and par-tly due to incr-eased

light ava.ilabl ity foll.owing sedimentation of the r-iver-'s

suspended loa.d.

and II proba.bly

Relative increase of phytoplankton at Sts. I

due to the stable conditions and incr-eased

greatly releated to the relase of water

light availability

qua.ntity at st. I I I

of the reservoir. The increase in

from the reservoir as it is located in one of the man made

ca.nal. The pea.k occurrence at st. I I I might be contributed

by the community in the reservoir discharged into the canal.

Downstream stations (station IV and VI of the river showed a

decr-easing trend of phytoplankton density and may be due to

the unstable conditons of the system.

The density of chlorophyceans were noted prominent

at s t. [ a.nd I I while diatom population reduced considerably

when compared to the lower reaches of the river. Such a.

condition can be explained in view of the fact that diatoms

wi th hea.vy si 1 ica. frustules sink fa.ster than other groups.

Diatoms, however, because of their heavy silica frustules

a.re more prone to sinking and may do so at an average of

2.5 m. day -1 compared to the much lighter green algae, for

Similarly on depleted nutrient conditions

example, where

et a.l, 1975).

the rate may only be 0.8 m. da.y -1 (Lehman

green a.lga.e tend to become common (Payne, 1986 ) . The

dominance of green algae on lower concentration of phosphate

s.nd nitrate has been reported form lakes of Pokhara. and

Ka.thmandu valley, Nepal (Lohman et a.1 1988). The increa.se of

algal population due to stream regulation was observed by

Obeng (198l> in his studies on Volta. Lake. In the resevoir

station. {st. I I) a.mong dia.toms, Na.vicula. sp. and Nitzschia.

sp. showed dominance and similar kind of observa.tion was

made by Gandhi (1967) in the Fountain Reservoir, Ahmadabad.

The results of correlation coefficient showed that

the physio-chemical conditions have less impact on plankton

of the Neyyar River than that of Kallada River. Tempera.ture

tends to have lesser impact on phytoplankton density as the

seasonal temperature differences are too low to account for

any variability. The nega.tive values at st. I and II with

rainfall indica.ted low production during monsoon. The upper

portion of the reservoir (st. I) exhibited low density of

phytoplankton during monsoon may be due to the rise of

susoended materials contributed by the ra.in. Welch (1952),

Roy (1955), Lega.re (1957) and Ashton (1985) observed the

adverse effect of rain water on plankton production. Similar

studies conducted in Volta Lake by Biswas(1975) observed

consistant depression of phytoplankton during the floods.

The relationshp between dissolved oxygen and phytoplankton

density was noted positive a.nd significa.nt (at 5% level) in

st. IV • Griffiths (1923), Gonsalves and Joshi (1946), Zafar

(1964) and Munawar (1970) made similar observations in other

I n i a.n R i ve r s • Both a.lka.linity a.nd ha.rdness had diret

influence on phytoplankton production in 9 t. V which

indicated marine influence.

2L3

The influence of alkalinity on

phytoplankton at st. [V is significant (a.t 5% level) might be

The concentration

phytoplankton atthe reason for

station.

lower abunda.nce of

of TDS and density

this

of

phytoplankton are significantly 1 inked in st. I [ and V.

[ n the comparison of phytoplankton abundance

showed close ED (dissimilarity index) between station [V and

v. The compositon of phytoplankton species might be similar

in the downstream stations which indicated by the low value

of ED (dissimilarity index). The percentage compositon of

Chlorophyceae, Bacillariophyceae, and Cyanophyceae in both

the stations were almost similar. According to Jongman et

a.l. (1987) the sites with simila.r species compositon occupy

nearby Dositons in species spa.ce and therefore, the

Eu.cl idean Distane (ED) between two sites is an obvious

measure of dissimilarity. However, the occurrence of species

greatly differ in each of these stations. At st. IV the

whole sample was represented by 21 species whereas Cl.t st. V

the sample constituted by 59 species. Similar trend was

observed between sts. I and I V and between s ts. [ and V a.[so

exhibited close values of ED. Relatively close ED in the

compa.r is ons cited above may be due to the low numerica.1

density than the species compositon.

to the view of Jongman et a1.(1987)

This is contradictory

that the sites with

simila.r species composition occupy nearly positions in

9p.9.ce. The highest numerical density of

phytoolankton at st. (I ( was

21 ....

noted among the five stations

and fa i r I y this station exhibited wide ED with other

sta.tions. (n g e n era lin the presen t .5 t ud Y the dis 5 i mil a r i t Y

index is more sensitive to the density of phytoplankton than

do the species composition.

Except at st. IV, the diversity index of Shannon

Weaner exhibited high variation. The variations in the

stations can be explained on the basis that the distribution

of species is always disturbed by the alteration in the

structure and function. At st.V the influence of saline

water and the opening and closure of the bar-mouth plays a

ma.j or role in changing the patterns of diversity. The

f I uc tua t i on at st. I can be viewed from the point of

headwater region where the influence of terrestrial setting

and the allochthonous input into the stream are of

considerable significance. The less variation a.t st. I [ is

due to the impoundment of water and it is nO longer a lotic

system. The highest diversity index at st. I during February

revea I s tha.t the species are uniformly distributed through

out the period. The peak density during December was the

t "b t" f I 5 " d hence th~- lowest value ofcon.rl u Ion 0 on y speCIes an -

Shannon Wiener diversity and the evenness index also showed

a. low key. The lowest value of equitability index during

the period clearly indicates that the species are not

equally distributed or equally abund~nt.In station II, the

peak density was obtained during November andthe bulk was

shared only by 3 species resulted in the lowest diversity

and evenness indices. The maximum value of diversity index

a t St. I I I d uri n g Feb r ua. r y is implicit in tha.t the a.bundance

is shared by 24 species and this has been further evidenced

by the highest diversity at st. V during March when the

population was represented by 31 species. This indicated the

dependence of diversity index on the species richness as

reported by Edgar {19831.

B. ZOOPLANKTON.

RESULTS

In the Neyyar River, the reservoir site (st. II)

recorded the highest density of zooplankton(27.54%) compared

to the other sampling stations and the lowest was at Pool/ar

(12.24%) (st.V). Yet, the density of zooplankton was evenly

distributed and the variations among the sampling sites were

small. In st. I the dominance of zooplankton was noted during

the post-monsoon and the minimum during the monsoon, when

the seasonal basis was considered~ The numerical density

ranged from 90 to 24801 m- 3 (June and October). A depletion

of zooplankton was noted during May and December. Wh i Ie

copepods and copepod nauplii predominated during the pre-

monsoon, and rotifers and cladocerans dominated in the post-

monsoon period. Ostracoda represented only during November.

The density at St. I I (Neyyar reservoir), fluctuated from 167

to 27000.m- 3 (July and October). Monsoon season recorded the

maximum density (30400.m- 3 ) and post-monsoon period recorded

mimimum ( 3100. m- 3 ). A de pIe t ion 0 f zooplankton was noted

only during November and December at st. I I to IV. The main

ta.xa. a.t st. I I were cope pods and cladocerans among which

copepods occurred 9 months and cladocermans represented 6

months of the period. Peak occurrence of cladocera was

not8d during September (Table. 66). Rotifers and ostracods

were recorded only during September. The numerical density

in st.III varied from 667 to 8000 m- 1 (October and March).

217

The mean density recorded its maximum during the pre-monsoon

(22400m- 3) and the minimum during the post-monsoon period

(,5667 m- 3 ). Copepods and copepod na.upl i i formed the bul k of

the zooplankton and other groups such a.s Rotifera.

C I adoce ra. I a.rvae of Plecoptera and Hydracarina appeared

sparingly. The copepod density was the highest during March

and the copepod nauplii recorded maximum during May

( Ta.b Ie. 57 ) • The range of density in st. LV was between

333 m-:S and 12833 m- 3 {July and September}. The mean density

showed the maximum during monsoon (15220. m- 3 ) and the

minimum during post-monsoon period (3917.m- 3 ). The ma.in taxa

at st. IV were copepods and copepod nauplii. The maximum

density of copepods was during February and the density of

cope pod nauplii was during September. Rotifera represented

four months of the period and the maximum density was noted

d'Jring September. The

noted during September.

peak occurrence of Cladocera was

The rest of the taxa, Hydiacarina,

Odonata larvae and chironomid larvae represented only once

during the period of sampling {Table.58}. The variation in

the density ins t. V wa.s between 300 __ 3 and 6709 m-:S

(December and March) and a faunal depletion was noted during

Seasonal mean showed the maximum duringNovember.

monsoon (10541 a. nd min i ma I d1.lr i ng monsoon

pre­

period

(4465. m-:S ). Copepod and copepod nauplii though dominated in

the bulk, it showed lean densities during post-monsoon.

Both taxa recorded maximum during March. Pea.k dens i ties of

218

Rotifera and Cladocera was noted during October. Chironomus

larvae. coleopteran and hydra~ar;~es appeared once

ITable. 69).Copepod~ ~8re r~p~esented by Paradiaptomus sp;

Allodiaotomus so: Mesocyclops sp. and Macrocyclops sp. and

the cladoceran species were Alona and Chydorus.

219

DlSCUSSION

The density of zooplankton in Neyyar River durini

the presen t 5tudy was generally higher than in KalladQ

River, however, the abundance noted is comparatively low.

The less abundance of zooplankton in rivers can be justified

on the basis of the reports from other tropical riverslOdum.

1959; fJelcomme, 1979; Sanchez et a.l. 1985; Pa.yne, 1986).

The abundance of zooplankton is known to be low in rivers as

the fact tha.t the flowing water is unfavourable fo~

zoopla.nkton. Among the five stations, the reservoir station

(st I I ) recorded the highest numerical density

zooplankton and this may be due to the physical stability o~

the water column which is considered to be favourable for

zooplankton abundance. The downstream stations at Poovar anj

Neyyattinkara recorded lowest zooplankton ab0ndance and this

might probably be attributed to the unstable conditions.

Water movement and human activities downstream inclined to

promote.

turbidity and there by reduce light penetration.

Turbidity caused by suspended matter which inturn reduce

light penetration thought to be unfavourable for zooplankton

a.bunda.nc e. Genera.l I y in rivers, f low of wa.ter tend to

produce unstability of the s ys tem a.nd the stability

increa.ses with lower flow ra.te. The higher abundance of

zooplankton in the reservoir may be attributed to this fact.

Shiel (19SS1 observed similar blocking effect during lOiN

flow effected the increase

22<)

in density and diversity of

plankton in the Darling River system, Australia. According

to Ferna.ndo. (1980 a) the diversity of zooplankton does vary

on a. more local scale; for example, rivers and streams in

Sri Lanka carried half the number of species of nearby ponds

a.nd reservoirs.

and this

The density of phytopla.nkton at st. I I I noted high

may also be the reason for the increase of

zooplankton. As the zooplankton rely a good deal on the

phytoplankton as a source of food, the zooplankton biomass

is broa.dly linked to phytopla.nkton density (Hawkins, 1988).

S IJC h 5 i mil a. r linkage has been observed in the first three

stations during the prese~t study. High density of copepodes

and copepod nauplii during was noted mostly during the pre-

monsoon and early monsoon period and such a trend was more

pronounced at st. I I I and v. Holden a.nd Green, (1960) made

similar observation in the Sokoto River,where they observed

the production of nauplius larvae from March to July.