DISCUSSION

It has been pointed out in the introduction that the structure and dynamics of the

zooplanktop community in a waterbody is governed chiefly by the physico-chemical

conditions (water temperature, pH, oxygen distribution and turbidity), food quality and

quantity, and predation regime (Wetzel 1983). Since the majority of water bodies are

shallow or have a large proportion of their basins in shallow waters (Wetzel 1989), they

experience wide amplitude of seasonal fluctuations in the water level which often includes

a variable period of drying.

The zooplankton community in such shallow and temporary waters would consist of

species which possess appropriate strategies for survival during the dry phase. These

strategies may include production of resting stages and the ability to detect a wet phase

of sufficient duration for rapidly building up its population. When these waterbodies fill

up after the dry phase, the initial structure of the population is determined by emergence

from the sediments and subsequent growth of the organism. The latter would be governed

by the abiotic factors, f200d resources and predation (De Stasio 1990).

The present study, besides examining the zooplankton community structure and

dynamics has addressed some of these aspects by studying emergence patterns, growth,

reproductive rates and the seasonal dynamics of the population structure in the field.

Habitat Characteristics

The study has been conducted in two different habitats, a large ox-bow pond with wide

littoral zone which dries up during the summer leaving a small part of the permanent deep

water, and several shallow pools which dry before the onset of summer. Both the

waterbodies had alkaline and hard waterswith variable amounts of nutrients. Most of the

waters in the Yamuna floodplain are brackish and hard. The nitrate nitrogen concentrations

were found to be higher during the rainy season in these waterbodies. The sediments were

sandy loam found commonly in the Yamuna floodplain. These exposed sediments could

get heated' to very high temperatures during the hot and dry summer although soil

temperatures were not recorded in this study.

DISCUSSION 201

COMMUNITY STRUCTURE

Species Composition

The community structure is characterised by species composition and the species diversity

(species richness and the density of constituent species). The density of a species

population in turn is governed by natality (growth rate, reproductive rate, brood interval

and brood size), mortality (natural and predatory), competition for food resources, habitat

condition and migration within the habitats.

In the present study, a total of 30 species of zooplankton were recorded (13 species

of Cladocera, 15 species of Rotifera and two Copepods). The presence of a few

cosmopolitan species like Chydorus sphaericus and typical tropical rotifer species like

Brachionus patulus, Keratella tropica makes the zooplankton assemblage to be a mixture

of tropical and cosmopolitan components (pejler 1977).

The dominant Cladocera and Copepoda in the tropics are mainly eurytropic species

which commonly occur in all types of habitats (Fernando 1980, Seymour-Sewell 1934).

Species Richness

Of the above species recorded in this study, there were all occured at the Okhla sites

whereas only 22 occurred at Nizamuddin. The temporary pond fauna appears to be richer

than permanent waters as seen in an earlier study conducted in the same climatic zone

(Hajra 1991). As compared to 13 species of Cladocera and 15 species of rotifers, only 5

species ofCladocera and 3 species of rotifers were found in the permanent ponds. This

could be mainly attributed to the development of dense beds of submerged and floating

macrophytes with which Chydorids and rotifers are associated (Galindo et al 1994).

Patil and Gouder (1985) report 4 species of cladocerans, 5 copepods and 7 species

of rotifers from a subtropical pond in Karnataka, India, with a mean depth of 2 meters.

In another study from a deep lake (mean depth 10 m) in Calcutta Khan (1984) reports 4

cladocerans and four copepods. In the case of a deeper reservoir in Bihar (mean depth

14.5 m), there were 5 species of Cladocera, 4 species of Copepoda and 5 species of

Rotifera ~erma and Dutta Munshi 1987).

Disturbance can enhance species diversity by contributing to the elimination of

DISCUSSION 202

dominant species and favouring resource partitioning (Denslow 1985). Shallow temporary

waters can be more severely disturbed by fluctuating hydrological conditions than larger

ones (Walber 1989). In the present study both the sites are highly, equally disturbed hence

the role of disturbance in increasing species richness cannot be concluded with much

emphasis.

In temporary pools the number of species recorded in one sample was higher than

the total number of species collected during several cycles in permanent or semipermanent

ponds (Galindo et al 1994). Highest crustacean species richness and diversity have been

reported to occur in large waterbodies which because of their size and longer wet phase,

contain greater number of microhabitats (Fryer 1985).But this relationship was not so

simple as some species showed increase in percentage occurence froin larger to smaller

waterbodies.

Fernando (1980) who has reviewed the literature on zooplankton fauna of the

tropical regions, has concluded that the tropics have fewer zooplankton species as

compared to the temperate zone. Morton and Bayly (1977) report 60 species of

cladocerans and copepods from temporary pools in Victoria, Australia. From a study in

temporary ponds in South Carolina Taylor and Mahoney (1990), 19 species of

cladocerans, 12 species of rotifers and 10 species of copepods were recorded. According

to him, the uniform high temperature would have restricted niche diversity seasonally, thus

reducing the number of species. Food could be another important factor as the dominance

of blue green algae could restrict species of zooplankton. Predation by young stage of fish

is often quoted to be a factor in reducing diversity but none could apply to the whole of

the tropics.

The Cladocera, Copepoda and Rotifera are characterised by their low species rich­

ness in tropical freshwaters. There are fewer cladocera in a tropical region than in a

temperate one, the number of species in any tropical region is around 60 while in a temp­

erate region it is around 95 (Fernando 1980 a & b). Daphnia lumholtzi was present at all

the study sites. Although Daphnia is rarer in the tropics, it has been reported in the

subtropical and temperate regions of the Indian subcontinent (Arora 1931, Biswas 1971).

Other cladoceran species recorded in the study included Ceriodaphnia sp, Moina micrura and Diaphanosoma excisum which commonly occur in tropical waters (Fernando 1980).

DISCUSSION 203

In the case of copepods only one genus of Cyclops and Diaptomus was found in

the study. This could be due to the difficulty in identification of the organisms to the

species level, but it has been reported by Fernando (1980) that cyclopoids are restricted

to only two major species in the tropics. For calanoids too the number of genera is low.

The situation is not clear as regards to calanoids because of inadequate taxonomic studies.

Fifteen species of rotifers were recorded during this study out of which five species

belonged to the genus Brachionus. Keratella tropica, Lecane quadricomis and Monostylla sp. were also found. The rotifers of tropical regions include mainly species of Brachionus and Keratella tropica (pejler 1917).

In this study the species richness varied between the sites seasonally. At the Okhla

site, the species richness was initially high during the rains but soon after it decreased in

the post-rainy season. However the species richness continued to increase even during the

post-rainy season. The emergence from the sediments contributed to the initial increase

in the number of species. Subsequent decrease could have been due to the disappearence

of shortlived species which had emerged. Dispersion into the larger volume of water

during the post-rainy season could be responsible for the decrease in species richnes6s.

Factors like predation and competition for the same resource base could also have led to

this decline .

. Species Diversity

The diversity of the zooplankton community at Okhla and Nizamuddin ranged from

0.5-3.25 as indicated by the Shannon Weiner Index. Cisneros and Mangas (1991) studied

a lake under environmental stress due to increase in the trophic level in Nicaragua and

found the diversity index to vary from 0.83 to 2.20. Rao et al (1988) studied the species

diversity in a small eutrophic lake in Udaipur, Rajasthan, India and reported that the

diversity values as indicated by the Shannon Weiner Index to range from 1.5 to 2.8.

The diversity values in the shallow marginal and marginal sites of Okhla were

slightly higher than the deep water sites. This could be due to the abundance of aquatic

macrophytes in the littoral zones which offer a variety of microhabitats. The shallow pools

of the Nizamuddin site show higher diversity during the winter season (due to large

populations of Daphnia lumholtzl).

DISCUSSION 204

The factors which affect species diversity include time, spatial heterogenity,

competition, predation, climatic stability and productivity plus several combinations of

these (Menge and Sutherland 1976). In structurally simple environments, competition

reduces diversity through exclusions whereas in structurally complex environments

competition may increase diversity though habitat specialization. Refuges are few in a

structurally simple environment so predation first increase then decrease diversity. In

structurally complex environments, refuges are more, so they reduce predator effects.

A striking feature of limnetic zooplankton, besides their fewer species, is their

small size. The commonest Daphnia species in Asia, Daphnia lwnholtzi is below 1.5 mm

in total length. This trend is reflected in copepod species also (Burgis et alI973). As one

moves from tropical to subtropical reg~ons larger zooplanktonic cladocerans and copepods

become more numerous as reported by Arora (1931), Biswas (1966), Swar & Fernando

(1979 a, b). In this study Daphnia was represented by Daphnia lwnholtzi which attained

a size of upto 1.5 mm, Cyclops sp. of 3 mm and Diaptomus sp. of upto 3mm.

Seasonal Changes in the Dominance of Groups

In the present study copepods dominated the zooplankton assemblage for a major part of

the year. The cladocerans dominated during the winter season. The rotifers showed overall

low density and they were most abundant during the rainy and summer season at different

sites.

In a majority of waterbodies studied in the tropics, Rotifera is the most abundant

group (Nasar 1977, Michael 1969, Fernando & Kanduru 1984 and Yousuf 1989). Joshi

and Adoni (1993) from a study on the Sagar Lake in Madhya Pradesh report a

predominance of rotifer species (47 to 97 %) and have related this to increase in the trophic

level in the waterbody. In some typical wetlands of Kashmir cladocerans and copepods

dominated during the summer months and declined by the rains when rotifers became

dominant and remained so throughout the autumn (Kaul and Handoo 1993). However in

certain deeper lakes and reservoirs which have a lesser degree of nutrient enrichment,

copepods are more dominant. Singh et al 1982 in a study of the lakes in the Kumaun

region found copepods to be dominant. Similarly Tarnot and Bhatnagar (1988) reported

dominance of copepods over rotifers in the Upper Lake in Bhopal, Madhya Pradesh.

DISCUSSION 205

The low density of rotifers could be attributed to a variety of reasons. Copepods

including copepodite stages of cyclopoids and diaptomids prey upon rotifers ( Anderson

1970 and Williamson 1987). The low rotifer density during the winter season could be due

to competition with increased cladocerans for the same resource base.

The relative contribution of different groups has also been shown to be influenced by

the trophic level of the water. Nutrient enrichment leads to phytoplankton blooms, change

in predator abundances and other physico-chemical changes. The total zooplankton

biomass increases with increase in lake productivity and is accompanied both by species

and group replacements within the macrozooplankton - Cladocera and Copepoda and there

is an increase in microzooplankton population consisting of rotifers and ciliate protozoa

(Bays & Crisman 1983). Rotifers are indicators of higher trophic levels ( Arora 1966,

Saxena 1987). Waters with abundance of copepods are said to be at a lower trophic stage

(Yousuf 1989).

In this study the cladoceran assemblage during the rainy season consisted of species

capable of feeding on both detritus and algae (Fryer 1985) and could possibly switch from

one mode to the other. During the post-rainy season predatory invertebrates including

hemipterans, odonates, dipterans and some other insect groups became abundant which in

turn could influence cladoceran densities (Fernando et al. 1990).

Daphnia lumholtzi was the most abundant cladoceran during the winter season at all

the sites. This species with pronounced helmets and long tail spines could avoid predation

by larger invertebrates and young fish (O'Brien & Vinyard 1978). Lewis and Maki (1981)

have found a direct relationship between increased hardness of water and daphnid

productivity. The total hardness of water during winters was found to be quite high (200-

300 mg/I) in the present study.

Moina micrura was the dominant species during the early summer season. Similar

peaks in the density of Moina micrura have been observed from other Indian studies also

(Murugan 1989).

In the present study Diaptomus and Cyclops alternate in being the major part of the

copepod community. Several studies made on the seasonal variation of copepods show that

greater abundance during the rainy and post-rainy season. These studies have been made

in a variety of habitats, Sharma and Saxena (1983) from a perennial tank in Gwalior,

DISCUSSION 206

India, Piyasiri & Jayakody (1991) from a new resevoir in Sri Lanka. Cyclopids have been

reported to be all the year round taxa whereas calanoids occur more frequently (Sharma

& Saxena 1983).

In the present study, large amount of detritus was available during the rainy season

when Diaptomus sp. was the dominant copepod. Cyclops sp. was dominant during the

post-rainy, winter and summer season. They seem to be adapted to high fluctuations in

environmental conditions like the quality and quantity of food and low dissolved oxygen

levels (Roa 1994).

The rotifers were present in low densities during the entire wet phase in this study.

Members of the family Brachionidae were dominant during the rainy season. Temperature

and ionic concentrations determine suitable habitats for Brachionus species. Its influence

may be indirect, intensifying or delaying development and cooperating with other biotic

and abiotic factors (pejler 1977). Arora (1966) observed the response of certain rotifers

to temperature and species like Brachionus calycijlorus and Keratella tropica could tolerate

high tempe:atures upto 37°C.

The seasonal succession among zooplankton species has also been related with a

difference in their temperature adaptation (Allan 1977), and the nature of the available

food which influences their growth and reproduction (Hall 1964, Vijverberg 1976).

Zooplankton prefer green algae over blue green filamentous algae (Straskraba 1966,

Lampert 1982). Size selective predation by a variety of vertebrate and invertebrate

predators also plays an important role in the organisation and dynamics of Zooplankton

communities (Lynch 1980, Taylor & Gabriel 1992). However, these aspects were not the

subject of the present study.

Total density

The total density of zooplankton in the present study ranges from 300-1250 ind/l. In the

deep water sites studied maxima was observed in September - October. The population .

maxima in the shallow marginal site occurred in March and April before they were

exposed. The zooplankton are restricted to the shallow pools in the early summer season

but during rains dispersion occurs in the overall volume.

Maximum zooplankton density of2180 ind/l was found in the Bhopal lakes (Tamot

DISCUSSION 207

and Bhatnagar 1988). Verma and Dutta Munshi (1987) report the total zooplankton density

to be between 500 to 2500 ind/l in a resevoir in Bihar. Piyasiri & Jayakody (1991) report

the zoopankton density to vary from 100 to 500 ind/l in a reservoir in Sri Lanka and

observed a population maxima during May - June, Cisneros & Mangas (1991) from a

study on a eutrophic lake in Nicaragua found it to range between 200 to 1200 ind/l and

observed a peak in the zooplankton density during the rainy season.

In a shallow waterbody situated in Bihar, population maxima was observed in

summer and again in autumn - early winter season (Nasar 1977). In the Bhopal lakes the

population maxima was observed during February (ramot and Bhatnagar 1988). In a

reservoir in Bihar the population showed a bimodal peak in December and February (

Verma and Duttamunshi 1987).

EGG BANK ("SEED BANK") AND EMERGENCE

Organisms living in a variable environment are -often exposed to abiotic and biotic

conditions that threaten their survival. Dormancy and dispersal are two major strategies

that allow these organisms to avoid these harsh environmental conditions (Harper 1977).

Examples of dormancy in seasonal environments include seasonal diapause in insects,

dormant seed production by terrestrial and aquatic plants (Harper 1977) and dormant

stages of organisms living in both freshwater (Wiggins et al. 1980) and marine habitats.

The cladoceran family (Macrothricidae, Chydoridae, Bosminidae and Daphnidae)

produce epihippial eggs or resting eggs which settle down on the sediments where they

overcome unpredictable conditions (Elbom 1966, Barclay 1966, Wiggins et al 1980).

Cyclopoid copepods undergo diapause in advanced cope-podite stages and encyst in bottom

sediments (Wiggins et al1980, Morton and Bayly 1977) and in some cases are known to

produce resting eggs as well (Rzoska 1961). The calanoids produce resting eggs which

can remain in the sediments for considerable periods (De Stasio 1990, Taylor et alI990).

Not much literature is available for rotifers, but they are known to undergo diapause as

dehydrated individuals and encysted adults in the sediments (pennak 1953).

Light, temperature, oxygen and osmotic conditions have been shown to be

important factors influencing release from diapause and the development of resting eggs

DISCUSSION 208

(Stross 1971, Brewer 1964). Moritz (1987) has shown hatching to be dependent on the

depth in the sediment at which the resting eggs are found.

Viability

In this study all the three groups emerged from the sedimets. In this study total

zooplankton emergence over a period of 40-45 days ranged between 450-38000 ind/m2.

Similar densities have been reported by Wyngaard et al (1991) and Lloyd and Boulton

1992. The total numbers of zooplankton that emerge are a direct measure of the viability

of the resting stages and the availability of conditions favourable for hatching.

In general the total numbers decreased with increase in drying durations. This

could be a result of loss of viable eggs. From the Okhla site sample maximum emergence

could be observed from a month dried sediments whereas from the Nizamuddin Site

sample the five month dry sediments guided the maximum number of zooplankton. These

two sites varied, in the time of drying and it is possible that the resting stages at the latter

site required a longer diapause.

There is not enough information available as to how long the resting eggs maintain

their viability in nature. Moghraby (1977) raised Macrothrix sp. from the sediments of a

55 year old mud wall in Sudan. The existence of pools of dormant stages that persist for

long peri<><!s has been observed by some (De Stasio 1989, Boulton & Lloyd 1992) who

report it to be three to seven years for different groups. The abundance of viable eggs in

the egg bank of a population will depend upon the relative rates of input to and loss from

the pool of stored eggs. Addition to the pool is made through egg deposition by active

individuals and sediment mixing and movement. The mixing of sediments by bioturbation

and water movements will lead to recruitment from eggs/esting stages produced at

different times. Losses would occur due to emergence, degradation by bacterial action,

predation and loss of viability. Each of these processes in tum vary as a function of

location within the habitat and depth in the sediment.

Temporal variations in Emergence

The study shows variability in the temporal pattern of emergence with the duration of

drying. However, there is little information available on this aspect from other temporary

DISCUSSION 209

waterbodies. Zooplankton emergence from the wetted sediments was staggered over the

total observation period. Organisms usually emerged only after a few days of inn un dation.

More than one peak in emergence was observed (Lloyd and Boulton 1992). These peaks

differed in timing between various drying durations and between the sites. No direct

relationship could be found between the duration of drying and the peaks of emergence.

Microtoprgraphica1 variations in the habitat determines the resting stage settlement and

could reflect in the variations seen between emergences from different sites, when the

water recedes from these sites at different times the timing of egg deposition could also

differ, and the duration for which the sediments are exposed will also differ. The

sediments which are covered with vegetation would be less susceptible to erosion than

uncovered sediments where loss of viable eggs could occur. Activities like digging and

grazing could alter the pattern of resting stages in the sediments, while some would get

burried deeper and the rest could be moved to more superficial layers. The emergence of

organisms after a minimum period of inundation could be a requirement for the resting

stages to break their diapause.

The zooplankton assemblage which emerged from the sediments was dominated by

cladocerans. From the marginal site sediments which had vegetal cover, rotifers formed

the second most dominant group. This could be due to the association of the resting

stages of rotifers which the aquatic vegetation (Galindo et al. 1994). Copepods were the

second most dominant group to emerge out of deep water sediments.

The cladoceran dominance in the experimental studies was in contrast to an overall

dominance by copepods in the field. The only explanation lies in the fact that the

experimental conditions provided the required stimulus for hatching of cladoceran resting

eggs and did not provide the same for the copepods.

Reactivation of diapausing eggs of cladocera has been found to depend upon the

photoperiod, with temperature and CO2 tension also involved (Stross 1971). The hatching

stimulus for eggs of a Diaptomus sp. was shown to be reduction in oxygen level brought

about by bacterial action in the organic material of the bottom mud. Cole (1953), while

studying encystment of a Cyclops sp. concluded that complex environmental factors play

a role in terminating diapause. Newly deposited resting eggs require a minimum period

of diapause to go through an annual cycle of thermal conditions to complete

DISCUSSION 210

embryogenesis. These developed eggs are sensitive to the hatching stimulus. However

mortality of these eggs occurs due to dessication (Brewer 1964)

The sediments in the laboratory were totally dried and kept at nearly constant

temperatures. The field conditions were different as the dry sediments were exposed to

diurnal variations of temperatures cladoceran species such as Daphnia lumholtzi failed to

emerg~ immediately after filling up of the water body, but emerged in the experiments

after inundation of the sediments for a few days: It probably required the stimulus of not

being exposed to durnal variations in temperature. The converse is probably true for the

calanoid and the cyclopoid species which showed very low emergence in the experiments

but were abundant in the field just after filling up of the water body.

Mote than five species of cladocerans emerged in our experiments. Moina micrura was the most abundant species to emerge and was always the first to appear in the

inundated samples. Lloyd and Boulton (1992), also recorded its appearence around third

to the seventh day of inundation. The rest of the species followed in succession but not

in any specific order. Daphnia lumholtzi dominated the emergent cladocerans from sedi­

ments which had been dried for a longer duration and it even emerged from one year dried

sediments. Several studies have been conducted on conditions required for hatching of

epihippial eggs (Wood and Banta 1933, Moritz 1987) which have found the temperature,

photoperiod and carbon dioxide tension to be among various factors affecting the reactiva­

tion of resting eggs. Shan (1970) has shown that chydorid eggs exposed to longer photo­

period and higher light intensity, hatch earlier. He also showed that the pigmentation of

the epihippia constitutes a mechanism for retarding or delaying development until hatching

can be accomplished under favourable environmental conditions. Schwartz & Hebert

(1987) found differences in hatching cues between Daphnia species and also between

populations of the same species. Banta (1939) emphasises that in addition to genetic

variability, maternal diet during epihippial egg formation, and the number of partheno­

genetic generations undergone may also affect the hatchability of epihippial eggs.

Populations which die out frequently may produce resting eggs with a higher hatchability

than those of permanent populations (Carvalho & Wolf 1989).

In the case of the group copepoda, Cyclops sp emerged within seven days of

inundation from the lesser dried sediments and all the emergence took place in a single

DISCUSSION 211

flush. Diaptomus species emerged only from 5 months dried sediments. Wyngaard et al

(1991) report the emergence of a number of cyclopoid species from a temporary pond

sediment and they emerge immediately after the pond fills up. However the cues that

stimulate entering or emerging from dormancy are not well understood but temperature

and photoperiod are important for some species (Watson and Smallman 1971).

The rotifer emergence in the experiments was characterised by single peaks which

differed in time between sites. More than four species emerged out of which Asplanchna

sp. and Tetramastix sp. were predominant. The colonial rotifer Conochillus sp. emerged

only from one month dried sediments, but failed to emerge from sediments dried longer.

This could be due to unavailability of suitable hatching conditions or loss of viability of

encysted stages. Tetramastix sp. was the only species to emerge from one year dried

sediments.

This indicates that the different taxa may retain viability for different periods or

that their hatching requirements change with the duration of dry period.

Growth and Reproduction

Many life history parameters such as life span, age specific survivorship, fecundity, age

at first reproduction and frequency of reproduction are generally viewed as coadapted

traits evolved to increase the rate of increase of a population in a given environment

(Stearns 1977). Temperature influences the egg development time, growth rate, brood

size and mortality of zooplankton (Hall 1964, Vijverberg 1980, Orcutt & Porter 1984) .

Zooplankton communities readily respond to changes in environmental conditions with

appropriate changes in their life history traits (Allan 1976). The study of life history

parameters is essential for a clearer understanding of zooplankton dynamics in a

waterbody ..

In the present study growth and reproduction in Daphnia lumholtzi was observed. The study reveals that Daphnia !umholtzi has six preadult instars at a temperature range

of 17.5-27.5 °C under laboratory conditions. 1983). The second and third generations had

only two to three preadult in stars. Venkataraman & Job (1980) reported that Daphnia carinata had two preadult instars at 35°C and seven preadult instars at 15°C. Variations

from four to eight preadult instars have been recorded in Daphnia magna by Anderson &

DISCUSSION 212

Jenkins (1942), who suggested that hereditary factors, temperature and differences in the

culture meaium may cause variation in the number of instars. The total life span of

Daphnia lumholtzi was 45 days during which it produced 80 eggs. This is some what low

as compared to other tropical species like Daphnia carinata (Navaneethakrishnan &

Michael 1971) and Simocephalus acutirostris (Murugan & Sivaramakrishnan 1973) who

conducted their experiment under higher temperature ranges.

Two peaks in the egg production were observed between the sixth and ninth instars

2and 16th and 19th instars for the first generation individuals, and similar trends were

observed in the other generations too. In an earlier study by Navaneethakrishnan and

Michael (1971), it was seen that the number of eggs produced increased from the first

adult instar to the last instar. Various factors are known to influence egg production like

food and temperature (Anderson and Jenkins 1942).

The growth rate was higher during the early phase of the life cycle in all the three

generations. This rapid preadult growth seems to be a common feature for the arctic,

temperate and tropical species of daphnids irrespective of climatic differences (Murugan

and Sivaramakrishnan 1973).

The egg development time was longer in the early adult instars (3 days) which was

reduced to 2.5 days in the later instars. An analysis of the egg development time in

temperate and tropical cladocerans shows shorter egg development time for tropical

cladocerans (Murugan and Sivaramakrishnan 1973).

In the present study epihippial eggs were produced either in a long continuous

stretch in some generations or over shorter discontinuous flushes. The production of

epihippial eggs was intermittent with parthenogenetic eggs. In this study these epihippial

eggs were produced parthenogenetically. This phenomenon has not been reported by other

studies.

For Daphnia lumholtzi with an average brood size of 4 to 6 eggs/female, an egg

developme~t time of 2.5-3 days and organisms reaching sexual maturity (adult instars) in

3-4 days, very large populations could build up in a short period of time. The rate of

development is relatively faster in tropical species than their temperate allies, which may

be a device to build up the population rapidly before the onset of adverse environmental

conditions (Murugan and Sivaramakrishnan 1973).

DISCUSSION 213

CONCLUSION

This study shows that the zooplankton community of temporary waters is fairly rich in

zooplankton species but most of these occur only for a short period specially during the

rainy season. The Okhla and the Nizamuddin sites varied in the overall species richness.

The Okhla had a large number of submerged, floating and emergent macrophytes with

which a greater number of microcrustaceans and rotifers are associated (Galindo et. al

1994). The Nizamuddin sites for a major part of the year did not have any macrophytic

. vegetation. A relatively sharp decline in the species richness at the Okhla sites during the

post-rainy season could possibly be explained by the presence of more shortlived species

than at Nizamuddin (see Table 10).

In temporary pools like those at Nizamuddin the development of a zooplankton

community depends entirely upon hatching of the eggs or resumption of growth by the

resting stages in the sediment. The observations on the emergence of organisms from the

sediments dried for upto one year are submerged under water, confirm the fact that the

sediment ~rve as a large "Bank" for those resting stages and eggs.

The study also shows that at least in the study area around Delhi the viability of

the copepods is lost earlier than that of the cladocerans under dry conditions. Although th~

monsoons do not fail completely between any two successive years, a delayed rain after

a year of deficient rainfall may significantly reduce the copepod population. On the other

hand the cladocerans like Daphnia ensures its dominant position in the zooplankton

community by staggered hatching and rapid reproduction during the wet phase. The

frequent production of epihippial eggs in between parthenogenetic reproduction also

appears to be a strategy to ensure survival in fluctuating and unpredictable environment

or it could be a response to other factors as building up of metabolites, food, temperature

or photoperiod (Banta 1939, Stross 1971).

Survival of resting stages of Diaptomus sp. was found to be greater at the deeper

sediments of Okhla. The mixing of sediments causes deposition of eggs produced at

different times and which are at different stages of development at the deeper portions,

thereby increasing the chances of its success to emerge when conditions are suitable.

The egg development time and the growth rates has also been observed to vary

DISCUSSION 214

between the generations. Various environmental factors like food (Anderson and Jenkins

1942) have been reported to cause this variability. It is likdy that the experimental

conditions in the present study were not uniform because of the length of the study period

and the fact that the zooplankton were fed on microbes developing in the manure extracts.

It may however be possible that such behaviour provides an adaptive advantage to the

species in fluctuating and unpredictable environments.

The population structure study suggests that the survival strategy of the different

taxa of zooplankton varied. Daphnia lumholtzi and Diaptomus sp. produced resting eggs

at various times during their life cycle in the wet phase and, left them behind in the

sediment to form a seed Bank. Later continuous recruitment into the populations occurred

from the sediments. Cyclops sp. produced a large propotion of copepodites during the

early summer season before the sites were exposed to leave behind encysted inactive

individuals in the sediments, which resumed growth after conditions became favourable.

It may be concluded that different groups and species of zooplankton adopt

different strategies to overcome the dry period and build up their populations during the

favourable conditions. Further, they differ in their ability to withstand dry period of

different durations. And, interestingly, the shallow seasonal pools also serve as important

habitats for a rich diversity of zooplankton.