Post on 14-May-2018
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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.