STUDIES ON THE POPULATION STRUCTURE OF ZOOPLANKTON IN THE KOTMALE RESERVOIR SWARNA PIYASIRI 1

Planlrto~i samples were collected fromKotmale reservoir, a t approximately monthly intervals from August 1990 to August 1991. Investigations werc foc~ised on species composition, seasonal variation, vertical and horizontal distrjbution ancl the size class distribution in the population structure of' zooplanlcton. Two species of cyclopoids, two species of calanoids, nine species of claclocerans ancl sixteen species of rotifers were recorded. Among these, the most abundant species during the study period wereMesocyclops sp,Plzyllodiaptonrus UIL,LCL~, Ccriodc~phl~ia col.nc~ta, Kcratella tl-opica, Filina lo~zgiseta, B~-nchio~z~rs c n r r d n t z r s a n d TJ-icocer-cc' sinrilis. A location at the deepest region of the reservoir closer to the dam was selectei as a representative station to investigate the vertical distribution of zooplanlcton. At this location, the vertical distribution patterns of all the zooplanlcton werc almost similar throughout the year. As this location is situated a t the down stream region of the river, i t was expected to contain most of the zooplanlrton of the reservoir. The horizontal distribution of zooplankton dicl not show much variation. The nauplii stages and cyclopoids showed highest abundance in the population of zooplanlrton. However,during most of the months, the adult stages of calanoids and cladocerans were highly abunclant. Kcy words: Cyclopoids, calanoids, claclocerans, rotifers, seasonal variation, population structure, zooplanlrton.


INTRODUCTION
Kotmale reservoir is located at an elevation of 640 to 762m, 2100 feet above sea level with a geograpl.lica1position of 7O 03' to 7 O 05' N and 80° 36' to 800 41' E. The morpliometric features of the reservoir could be summarized as follows: surface area = 6.5 km2, maximum depth = 90 m, volume = 174mcm, mean depth = 26.8m,maximum length = 6.8 km and maximum width = 1.41 krn.
Icotmale reservoir is the uppermost reservoir of a n interconnected reservoir chain which is fed by a large number of tributaries from its catchment.The inlets of the catchment could therefore often influence the species diversity, seasonal variation and the population structure of ,the zooplankton in the reservoir.
Much work has been already done on zooplankton in the water bodies of Sri Lanka.According to Fernando1" thc limnetic zooplankton of Sri Lalika are typically tropical and poor in species d i v e r s i t y k d those with large bod37 sizes were absent or rare in tropical waters.3peciescomposition in various water bodies of Sri Lanka is markedly different during different seasons ofthe year,' which may be both short and long-term.*.Work on the composition of zooplallkton species in Mahaweli reservoirs, "lo indicated that the zooplanlctoa component was representcd by copepods, cladocerans & rotifers in the upland Mahaweli I-cservoirs and that the species richness of copepods was less than that of the rotifers and cladocerans.
The objective of the present work is to construct a complete picture of the species composition, seasolla1 fluctuations and the vertical and horizontal distribution patterns of the zooplankton and to describe the population of the zooplai~kton in the reservoir during the period ofinvestigation from August 1990 to August 1991.

METHODS AND MATERIALS
Snnzplitzgpl-oced~~re: Fig. 1 illustrates the locations ofthe sampling stations.St1 closer to the dam, is the deepest region in the reservoir and it was selected as a suitable location to investigate the vertical distribution pattern of zooplankto11 due to the following reasons: (1) As this location is situated at the down stream region, due to the direction ofwater flow, it was expected that this location would contain most of the representative zoopla~lkton types ofthe reservoir.(2)As this is the deepest part of the reservoir its water level remains a t a considerably higher level, even during severe droughts.(3) Therefore it allowed the comparison of the vertical distribution patterns of zooplankton during both high and low water levels.(4) Thus the results were comparable throughout the study.
For horizontal sampling, 10 locations were selected from the dam site to the upstream region as illustrated in Fig. 1.
Vertical distribution: At major station 1, zooplankton samples were collected by filtering vertical columns of water from varying depth intervals from 0lorn, 10 -20m, 20 -30m etc. using a closing type net with 5 0 p mesh size which filtered 0.2011 m3 of water.
Sampling was done from 0900h to 1400 h, a t monthly intervals for a period of one year from August 1990 to August 1991.Samples collected were transferred into sampling bottles and fixed in 5% formaline and transported to the laboratory for further investigations.

CONTMJRS IN FEET
F i g u r e 1: Kotmale reservciir w i t h t h e locations of sampling.S t l , ~ndicates the location where the sampling was done along the vertical profile of the water colun~u.At S t 2 , St 3 , A , B, C, D, E, F & G stations, sampling was done a t the surface & a t loin depths.
Horizontal distribution: As the euphotic limit of the reservoir was less than 10 m as indicated in Piyasiri,11J2 for investigations on horizontal distribution, sampling was done at the top l0m-epilimneticregion where the zooplanlrton was highly abundant.
Samples collected in the field were transferred into sampling bottles and fixed in 5% formaline and transported to the laboratory for identification and enumeration.
Analysis of samples: All the plankton samples were diluted upto 1001111.Then 5 random sub sampleskach representing l m l with a coefficient of variation of 0.1 -0.35) were used to adalyse the zooplankton.For identification of zooplankton in the samples, keys of Streble & ~r a u t e r l ~~w e r e used.Different types of zooplankton in each sub sample were countedusing a Sedgewick Rafter counting chamber.

Life cycle stages and size class distribution:
To study the population structure of the zooplankton community, the total lengths of different stages were measured in 80% of the counted individuals using a n eye piece micrometer.
There were Nauplii of various stages with body lengths ranging from 0.09 to 0.33mm.N1& N6 stages were identifiable and belonged to the size ranges of 0.09 to 0.12mm (Nl) & 0.31 to 0.33mm (N6) respectively.As it was difficult to distinguish N3 to N5 stages, they were collectively considered in the size range of 0.13 to 0.31mm in the population structure.The copepodite stages (C, to C,) were identified using their morphological features and were categorized into different size classes accordingly.Ceriodaphnia cornuta were categorized into three size classes, which represented the embryonic stage (the smallest size class), the nymphs and the adults.For Rotifera, it was impossible to identify the stages and therefore analysis of life cycle stages was not attempted.For data analysis on size class distribution of plankton, related to their stages, zooplankton collected from epilimnetic waters along the horizontal axis of the reservoir was considered.

Seasonal fluctuation of zooplankton
Table 1 illustrates the richness of species in Kotmale reservoir.Sixteen species of rotifers, nine species of cladocerans, and three species of copepods were recorded during the present investigation.Fig. 2 illustrates the monthly variation in percentage occurrence of Cyclopoida, Calanoida, Cladocera & Rotifera of the epilimnetic waters of the reservoir.Copepods made a significant contribution to zooplankton population (Fig. 2).Calanoids were the least abundant and cyclopoids and rotifera dominated the population throughout the investigation.Cladocera occurred in fairly high percentages throughout the investigation.Mesocyclops sp.

Thermocyclops crassus Tropodiapton~us australis
Fig. 3 illustrates the monthly fluctuations in the abundance of different species of Cladocera, Rotifera and Copepoda a$ the surface epilimnetic waters of the reservoir.C. cornzrta was the dominant species of the Cladocera population during most months whereas all the other cladoceran species appeared in low densities.There were two morphological forms in C. cornuta p~pulation.~A thick bloom of Microcystis aeruginosa covered the Kotmale reservoir during the latter part of 1991 from Sept. to December 199i.12During this period, it was found that the C. cornuta with small body sizes were dominating the Cladocera population of the reservoir.
D. lumholtzi was the largest Cladocera (mean length = 1.38mmSD f 0.12) in the range of 1.98 mm to 2.34 mm.It had two morpholo~cal foiins.The less common one Was with a thin long spine-like process on the head region and a thin body while the common form had much shorter and stouter process on the head and its body was stout.C. eurynotus species was generally abundant from December 1990 to March 1991.Other Cladocera species:.Moina species, D. aspinosum and D. modigliani were found in low numbers during this study.
As shown in Fig. 3 the most abundant rotifer species was B. caudatus, F. opoliensis, K. tropica, T. birostris and Asplanchna species.The peaks in the rotifer population were observed from Aug. to Sept. 1990, April -May 1991 & July to Sept. 1991.
As illustrated in Fig. 3 the copepods population was dominated by Nauplii stages throughout the investigation.Copepodite stages of cyclopoids (Cyclo S) also made a sigmficant contribution.Considering the adults, cyclopoids (Cyclo A) dominated over the calanoids (Cala A).Copepodid stages of calanoids (Cala S) were low in number throughout the investigation period.

Distribution
Vertical distrib~~tion during high water level Fig. 4 A illustrates the general vertical distribution pattern of zooplankton at St1 recorded in August 1990 (when reservoir water level was high closer to the dam).When the reservoir water level was high, most of the zooplankton types were distributed throughout the water column.B. caudatus and F. opoliensis densities were high even a t 30 to 40m depths.All the other rotifer species showed more or less uniform distribution along the water column.
Out of the cladocerans, C. cornuta was dominant from 30 to 40m depth range whereas D. aspinosum density was high upto 10 to 20m. C. eurynotes and D. modigliani were more or less uniformly distiibuted along the vertiCal profile.
Out of the copepods, Cyclopoid copepodites indicated highest densities from 20 to 40m whereas all the other copepods were approximately uniform along the water column.Nauplii stages were strikingly high from 20 to 40m.
Eggs and nymph stages of cladocerans did not show any similarity to the vertical distribution pattern indicated by the adults and other stages of zooplankton.The highest accumulation of zooplankton from 20m to 40m depth range was the general tendency ofthe vertical distribution pattern of zooplankton in Kotmale reservoir.was limited).This pattern shows that the zooplankton was high in abundance from the surface to 10m depths and decreases in density with increasing depth.This was the general trend foundin the vertical distribution pattern of zooplankton at St1 during the other months of the investigation period during low reservoir water levels.

Vertical distribution during low water levels
Fig. 5 illustrates the horizontal di'stribution of copepods, cyclopoids, C. cornzctn and Keratella sp. at the epilimnetic water layer from the dam site to the upstream region along the longitudinal axis of the reservoir.Population densities of all these forms were generally low a t the dam site.However at the middle part of the reservoir, highest abundance ofthese types were recorded.At the upstream region their abundance was generally high.There was no variation in horizontal distribution pattern of Nauplii at the surface and a t lorn depths.For all the samples collected during the investigation period a t ten sites, the mean number of'Nauplii per litre present a t the surface was 32.2 SD -t 13.6, CV 0.42 and at 10m depths it was 35.3, SD -t 17.95, CV 50.As illustrated in Fig. 5, the cyclopoids were observed throughout the reservoir with a mean abundance oi'12.78 per litre, SD f 9.5, CV 0.74.During the investigation period, they were more abundant in St2, St3, and a t F locations.There was no difference in the horizontal distribution of C. conzz~ta, but their abundance was slightly higher in St3 than the ot11cr stations un most o f the sampling occasions.Data from all sites a t surface to 10m depths, (horizontal sampling locations) were utilized to compare these results.Plankton population was conlposed of Nauplii ofvarious stages and their body lengths varied between 0.09 to 0.33mm.The smallest size range (N1 stage) was 0.09 -0.12mm which comprised 34% -67% (mean of 50.6%, SD f 0.84, CV = 0.017) of the Nauplii stages.The total number of individuals decreased gradually with increase of the size range (with the stage).Comparatively few numbers of individuals were present in the higher size classes of 0.31 -0.33mm whichrepresent the N6 stage (Fig. 6).From Oct. -Nov.1990 & Feb .-March1991 large Nauplii (N6) with length classes of 0.31 to 0.33mm were present in the population, but were very few in number and negligible compared to other stages as indicated in Fig. 6.

Copepodite stages of cyclopoids
In the copepod population, the copepodite stages ofcyclopoids and calanoids were identifiable.The smallest Cyclopoid copepodite stage (C1 stage) recorded had a body length of 0.27mm and the smallest calanoid copepodite stage had a length of 0.36mm.
Fig 6 illustrates the distribution of different copepodite stages of cyclopoids in different months collected from epilimnetic waters ofthe entire reservoir.The increase in their body length is a measure of their stage.Different copepodite stages belonged to different size class ranges as C1: 0.27 -0.35mm, C2: 0.36 -0.44mm, C3: 0.45 -0.53mm, C4: 0.54 -0.62, and C5: 0.63 -0.71mm and the Adults: 0.72 -1.34.Early copepodite stages were high in number compared to that of Nauplii of the final stages, which indicate molting of Nauplii to the copepodite stages.In May 1991, the lowest size class (0.27 to 0.35mm) became most abundant and in the, same month lowest water level (35m a t dam site) was recorded with high temperature which could be considered as favourable conditions for hatching of eggs.During this period, abundance of Nauplii and copepodite stages were also fairly high.However the density of the adult cyclopoids decreased to low levels.The population ofcopepodite stages decreased gradually with the increase in body size (stage).The adults (body size > 0.63 mm) were always numerically lower than the copepodite stages.During high water levels, the adults were numerically high in the population.
In Aug. to Sept. 1990, Oct. to Dec. 1990and in March 1991, the adults of calanoid population were more numerous.This situation was different from cyclopoids population where copepodites dominated throughout the investiga-tion period.However both copepodite stages and adults were lower in numbers in calanoids than that of cyclopoids throughout the year.According to Fig. 6 there was a fluctuation in percentage contribution of different stages with different size classes to the population of calanoids.Percentage of C4, C5 and adults of calanoids were less than the smaller copepodite stages of the population.

Cladocera
As illustrated in Fig. 2, cladocerans contributed significantly to the zooplankton community of the epilimnetic waters of the reservoir during most months of the year.
C. cornzcta dominated the samples throughout the study (Fig. 3) except in Feb .1991.As illustrated in Fig. 6, Cladocera community of the epilimnetic waters of the reservoir containedvarious stages of C. cornuta (with different size class ranges).There were rare occasions where largcr individuals with a body length between 0.87 to 0.90mm were recorded.The individuals belonging to the first size class range were considered as embryos (not properly developed) and gencrally they were present in low numbers.The second size range (0.33 to 0.41mm) which was considered as nymphs, were abundant in the epilimnetic waters.The percentage abundance of C. cornuta adults was higher in the population than the nymphs and the embryos in the limnetic zone.The population structure and the dominant size class varied monthly.

DISCUSSION
During the present investigation, twenty eight zooplankton species were found in the epilimnetic waters of the Kotmale reservoir.The highest number of species was found in Rotifera in which sixteen species were recorded, whereas it was low in Copepoda and Cladocera.Fernando14has also found similar results for three taxonomic groups of zooplankton present in the water bodies of Sri Lanka.Out of the three major taxonomic groups, the Copepoda and Rotifera have dominated the zooplankton population at the epilimnetic waters of the reservoir.Piyasiri & JayakodyYor Victoria reservoir & Pathmalal & Piyasii-i8 for Randenigala have ,also found copepods as the most dominant zooplankton type as in the present case.Rajapakse & Fecnando15 have found copepods in high percentages in the littoral zones of the water bodies of Sri Lanka.The calanoids population was comparatively smaller than the cyclopoids population in the epilimnetic waters of the reservoir.Large number of copepodite stages than the adults represented calanoids and cyclopoids.VijverberglG has found copepodite I and I1 stages and the adults as the most abundant stages in the zooplankton population of temperate lakes.In the present study, as illustrated in fig.3, copepodite I, I1 & I11 stages and the adults were the most abundant in the calanoids population of the epilimnetic waters.In cyclopoids population (Fig. 3), the copepodite stage I1 was the most dominant compared to other stages.
Nauplii of the crustaceans have dominated the copepod population of the Kotmale r e ~e r v o i r .~ The crustaceans Nauplii were most noticeable among zooplankton in Colombo lake.17 The Nauplii density found in the epilimnetic waters of the reservoir indicated very low densities of late Nauplii stages (N6) compared to the smaller stages.This could be related to many reasons.The development time of the 6th Nauplii stage to adult may be very fast under warmer conditions and also due to their large sizes, they may become a prey for larger invertebrates and fish larvae which cause the high mortality rates.
C. cornuta was the most abundant Cladoceran type in the epilimnetic waters of the Kotmale reservoir.Fernando1 also found C. conzz~ta as a dominant liinnetic species and according to Rajapakse & Fernando,15 it is found in every habitat type.C. cornuta is the most dominant Cladocerain the Victoria reservoir as in the present study." Presence of two morphological forms in C. cornz~ta has been previously recordcd.lSI" Both had equal body sizes but one was with tiny protuberances on the head region and a small eye and the otherwas without any protuberance but with a large eye.lS.'Wajapakse & Fernando15 also described a cyclomorpl~osis in C. c o r n ~~t a with horn like projections on the head and pointed beak like rostrum with similarities to the forms found in the Kotmale reservoir.The unhelmeted form of D. l ~m l z o l t z i ~~ was not recorded in the Kotmale reservoir and the two morphological types found in the Kotmale reservoir were with processes on the head.
As mentioned earlier, it is a known fact that the general trend of the body sizes of zooplankton in tropical waters is smaller sizes than that of temperate waters.Vidal" based on his detailed ecopl~ysiological experimental work has explained body size related patterns of geographical and vertical distribution in species of zooplankton.That is, oligotrophic tropical and sub tropical regions with deep thermoclines are characterized by the dominance of small sized species, while richer temperate and boreal regions with shallower thermoclines are noted for the abundance of larger species.In a fresh water lake in the Netherlands,16 the length of the copepods was inversely related to the temperature.This caused relatively small sized copepods in summer.lGPiya~iri'~ found similar results for a tropical Calanoid, Plzyllodiaptomzrs annae, which suggests that the body sizes of the zooplankton are smaller in tropical waters, compared to that of temperate waters.D. lzr?nlzoltzi found in the present study was the largest Cladocera of the reservoir with a mean length of 1.38mm SD + 0.12 in the range of 1.98 to 2.34 mm.
According to Brooks,23 the species found in the temperate zones reached body lengths of 2.5 to 5.0mm.The mean body length recorded for cyclopoids in the Kotmale reservoir was well within the length range recorded by Fernando.14However C. cornuta found in the Kotmale reservoir was fairly big (0.36 to 0.84mm) compared to body lengths recorded for C. cornuta in the Netherlands by Zaret 20 and the body lengths (0.32 -0.34mm) recorded by Rajapakse & Fernando15 in Sri Lanka.When the Kotmale reservoir was covered with a thick bloom of M. aertginosa in Sept. to Dec. 1991, C. cornuta with smaller body size dominated the zooplankton population.There is no scientific explanation for this.However a similar situation has been recorded in a sub tropical lake in South Africa.24 In the present investigation different zooplankton types have shown differences in their vertical distribution pattern (Fig. 4).However most of the zooplankton experienced high densities from 20 to 40m region.At the Stl, when the water level dropped during drought and when the power generation was limited, the densities of most of the zooplankton decreased with increasing depth.Piyasiri & Jayakody6 found similar patterns for the Victoria reservoir.Accumulation of most zooplankton types in the water column from 20m to 40m indicates that they may have found the thermocline establishedl1at 20m depth as a barrier for upward migration.Welcl~~Vescribed a similar situation for many other water bodies.However it is difficult to generalize the vertical distribution of zooplankton as a whole."The controlling circumstances being so involved and so variable a general statement is not possible. .As described by Welch," the distribution behaviour of zooplankton may be very different in different kinds of water.I t may be influenced by combined f'actors such as (i) physic0 chemical stratification, (ii) light, (iii) food (iv) dissolved oxygen (v) temperature (vi) wind, (vii) gravity and (viii) age of individuals of a species.The Kotmale reservoir is thermally stratified" and dissolved oxygen is low in the deeper hypolimnion compared to the epilimnetic region.These factors together with high density of phytoplankton and better light conditions might have influenced high abundance of certain zooplankton types in the surface water layers of the Kotmale reservoir.

Figure 3 :
Figure 3: Seasonal variation and abundanceof different species of ~alanoida,~btifera & Copepoda (as Number per litre) at the epilimnetic waters of the Kotmale reservoir.

Fig
Fig. 4B illustrates the vertical distribution pattern of zooplankton at the dam region in October 1990, (at low reservoir water level when power generation

Figure 4A :
Figure 4A: General vertical distribution pattern of zooplankton at St1 recorded in August 1990, when the water level of the reservoir was high closer to the dam.

Figure 5 :Figure 6 :
Figure 5: Abu~ltla~icc of'lioralolla, C. CO~IILIL~I, Cylopoitls, copcpods (as no. per litre).& Nauplii (n~rrnber per litre) along the horizontal axis of the reservoir.Analysis was based on the samples collected from all the stations a t the surface to lorn depths.

Table 1 : Zooplankton in Kotmale reservoir during 1990 to 1992 investigation period.
All the samples collected during the study period were analyzed.